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If you've ever thought about building your own hand wired amp, but weren't sure where to start, this replay of our live build-along series has all the tips you need.

In this 5-part series we show you step-by-step how to build our '57 Mini Tweed Amp Kit, with the famous 5f1 circuit. Even if you're a complete beginner don't worry! In this fifth episode we go over safety, installing the chassis into the cab, using a multimeter, testing your electrical components, installing the final hardware, and firing up the amp for the first time.

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Video Transcription

Pete: Hi, folks. Thank you for joining us. We are live here again in StewMac Studio. My name is Pete and I would like to welcome you and thank you for joining us for episode five, our finale. You know what? I think I'm not muted here. Excuse me. Let me get that taken care of. Sorry about that, everyone. Like I said, my name is Pete. Welcome to our episode five, the finale of our live build along series where we've been building our '57 Mini Tweed 5W Amp Kit. And last episode, we installed the eyelet board into our chassis to have something like this here. In this episode, we're going to be taking that chassis and installing it into the cabinet that we prepared in episode one.

But before we get into that, if this is your first time joining us, welcome. If you've been here all along, welcome back. We're really excited to get into today's episode... Just a little nervous. Hopefully, everything turns out well. But before we get into that, I'd like to thank you, Tommy Stumpp-

Tommy: Thanks, Pete.

Pete: For joining us again, lead us all through this. Of course, we have our videographer, Rachel.

Rachel: Hey, everybody.

Pete: And our wonderful director, Susan.

Susan: Hello.

Pete: And as we've been doing all along throughout the series, we're going to go ahead and start off with some important safety tips to follow. So Tommy, if you're ready to take it away.

Safety steps: Amp voltages are seriously dangerous! 

Tommy: Yeah. So in this episode, at the end, we're going to be working in a live amplifier circuit. Amplifiers, tube amplifiers in particular, have extremely high voltages sometimes. So we need to be able to understand how to deal with that properly in case we need to get in there and make any adjustments or repairs [on-screen text reads: Amp voltages are seriously dangerous!]. So this is the amp I was just playing, it's a live circuit. It's had tubes in it, which means it has the potential to have 600 volts of DC stored in the filter capacitors. The filter caps just to show you are these larger blue ones over here next to the power transformer. So your power transformer takes your wall voltage, which in the US is around 120, sometimes it can be 125, and it steps that up and then the rectifier tube turns that high AC voltage into DC. And it can be over 500 volts in this particular amplifier. So what I'm going to do is show you that voltage and then show you how to properly discharge it using a SnufferStick.

So what I'm going to do is turn this amplifier back on with just the rectifier and test the voltage at this first filter cap point. It'll be north of 500 volts. These other tubes will pull that DC voltage down. They'll also drain your voltage when your amplifier is turned off if your circuit is working properly, but you don't want to rely on that ever because at any time, something could fail and you could be working on an amp with lethal voltage possibly. So it's just really good to have good habits just to begin with when you're working on amplifiers.

Pete: Right. One of our safety tips is stay suspicious. Obviously, check power or check before powering it on and always unplug it when you're not working on it.

Tommy: Yeah, so I'm going to be working with this multimeter. There's a couple tests that I'm going to be doing throughout this episode. One of those is continuity represented by this diode symbol in the little speaker symbol. That's because on most multimeters, when its set to continuity, if the leads touch or if the leads are making an electrical connection with each other through your circuit, you'll hear a beep. And on this meter, it lights up as well, letting you know that you have continuity without having to look at the meter. The other test we're going to do is DC voltage. Some multimeters have a range where you set it for what you expect to be testing. Other multimeters are what's called auto ranging, where you don't need to set what range you're going to look for. You just set it to DC or AC. AC is represented by a sine wave, as you can see here. DC is represented as a solid line with dots. And again, those are very standard electrical symbols.

So what I'm going to do is clip a test lead onto my multimeter's negative probe, the black one. You can see that. What I'm going to do is connect it to a known ground point. Ground is connected to this green AC cord and essentially, it's anywhere in this chassis that's got a mounting bolt. These terminal strips are ground. There are a lot of different ground points that you can choose. For our purposes, and so we can see what I'm doing on camera. I'm going to clip it onto one of these mounting bolts up here through the cabinet. I'm going to choose this one because it's the most out of the way as far as camera work.

Pete: And for anyone who has been following along with us throughout the entire series, this is a review of the safety tips we went over in episode one where we did demonstrate the SnufferStick.

Tommy: Right.

Pete: This is just a little review, refresher, a little more in depth.

Tommy: So I'm going to check continuity just to make sure that this black test lead is indeed grounded. I'm going to touch a few different places on the chassis and one of my terminal strips. Cool. So I know that this is grounded. That's a good sign because when we test DC voltage, we need to test it against ground. You have your negative lead on ground, you have your positive lead set to the test point you're looking at. So what I'm going to do is set my multimeter to my 600 DC range. I'm using a test lead because I never want to have both of my hands inside the chassis when there's a possibility that there's DC voltage present. So what I'm going to do for demonstration purposes is turn this back on just for a couple seconds. And as I said, I'm going to that first test point. Do you want me to show you that again, Rachel?

Rachel: Yeah, let's see one more time.

Tommy: Okay. So that first test point where the red lead meets that blue resistor is where I'm going to test first. That's going to be the highest DC voltage in this amp. And you can see on that multimeter, 520. So it's a lot of voltage. Now what I'm going to do is clip my SnufferStick to another ground point on the chassis, and I'm going to show you how this thing will drain that voltage. So I've got that test probe on there. You can see that tube is slowly draining it, but I want to quickly drain it. So I'm going to place my SnufferStick on that same lead and you can see it just drop like a rock there.

A SnufferStick in general is two resistors wired in series that are clipped to ground. The resistors stop the voltage from immediately all jumping into ground and it's got a non-conductive wooden handle. So you can see, I'm down to four volts on my multimeter, that is safe to work on. You can see these other two filter caps that I'm testing are just a little bit higher. Anything below 10 volts DC is considered safe to work on.

Pete: And if you're following along in the instructions, all of these safety tips and the directions on how to use the SnufferStick are on page six.

Tommy: Right on. So again, why do we have to keep harping on this? If we were building a Ukulele Kit, we would not be talking about this. Even a pedal kit, you're only working with roughly nine volts at most. In this circuit, as you can see, we're working with 500 volts of DC which is enough to stop your heart. So it's just really important that we talk about it and go over it anytime we're going to be working in a live amplifier, which we will at the end of this episode.

Pete: All right, so yeah. Thank you, Tommy, for taking us through that again. Very important stuff to pay attention to. But now, we're so close to the fun part. I think we should just get right into it, right?

Step 63: Mount the chassis

Tommy: Absolutely. So right off the bat, step 63. So mounting our chassis. Now there's one thing I like to do before we mount the chassis. Just for ease, I like to mount this chicken head knob first just because it can be tough to get it mounted on there properly with the chassis in the cabinet. So there's just a small brass set screw. Can you see that, Rachel?

Rachel: I can.

Tommy: Far out. Make sure that's backed out enough using a small screwdriver, slot head so that you can allow the brass ring to slide on over that pot control shaft. I point the knob just past one moving counterclockwise on my chassis, if you can see that. And then just spin that set screw until it grabs on. Don't need to bear down. Again, it's just so it doesn't spin free. Now, I can hear it.

Now, I turned it a little too far. I didn't have my knob all the way off. So what I'm going to do is turn that all the way off. It's just a little mistake there. Tighten that again. So now, when I turn it all the way up, which you're going to want to do, it's on 12. All right. So now, we're going to mount our chassis.

Pete: Yeah, essentially what Tommy's done there is just moved step 66 ahead of 63 to just simplify making sure you can align that knob.

Tommy: Yeah, it can just be a little bit of a trial and error, just a little bit of a pain if you don't mount that knob first.

Pete: Hey Tommy, while you're getting that ready, we've got our first question here from Sassy Kat. Sassy Kat, thank you for joining us again.

Tommy: Welcome, Sassy Kat.

Pete: Bit of an odd question, how much electricity does the human body contain?

Tommy: I think it depends on the body. I don't really know. I'm an amp guy.

Pete: And then follow up hotter question, how much can it safely absorb?

Tommy: I think that also depends on the body, but generally, you don't want to put yourself in a position where you're trying to absorb extra electricity. It's one of my rules of life. So I've got these mounting bolts. They should pass right through that tweed cabinet. Sometimes, this tweed can overlap those holes. Not a big deal. You can see here, I just folded it back on this one, it's not going to cause any problems. And I'm going to just pass the chassis over those mounting bolts. I'm going to make sure these white wires on the left hand side, this white and black lead are out of the way because this bolt, if you can see that, Rachel-

Rachel: I can't see the angle of that, but yeah.

Tommy: This mounting bolt over here is very close to these AC wires. And if the nut squishes those wires, you can cause a problem. Just want to be extra careful that you don't damage any of those wires. So I'm going to get those locking nuts threaded on to the bolts. Let's get that guy out of there.

Pete: All right, and so while you're doing that, we've got a couple more questions coming in, which everyone who's joining us, please feel free to post your questions in the chat. That's why we're doing this live so we can answer your questions. Santiago Colina asks, "Now that we're coming to an end, you honestly think anyone could make this?" And they say, "Thanks and happy end of year, you guys. You rock."

Tommy: Yeah, we've broken the instructions down into simple step by step, the best way we know how. And as you can see, nothing I've done is way out there as far as complicated techniques or anything like that. We're going to leave this video up for the foreseeable future. So if you ever decide you want to build this, you can go back and check and make sure you're doing it how we like to do it, but I really do think this is a great place to start for anyone who wants to get into amp building. So you can see, I've got those mounting nuts on there. I'm just going to go ahead and tighten those. And once they're close, it's a good idea to pull that chassis up so it's pretty far up. So it'll be flush with where the back panel mounts when we get to the end.

Pete: All right. And Santiago Colina, thank you for your question. You rock. We've got more questions coming in. Metal 571 asks, "Do you think we can get a circuit analysis with the math behind how the component values were selected and the function of each component would love that?" Just a quick short answer while Tommy's doing that, we do have the schematic available in the instructions on page 29. As far as how the components were selected, I would assume, Tommy, you can correct me if I'm incorrect here, that they were selected based on the vintage 5F1 circuit that Fender designed.

Tommy: Well, I think what he's asking is for a circuit analysis, meaning where the voltages are at each point in the circuit and what each component does. This is the simplest amp circuit you're probably going to see as far as the tube amp. So the circuit analysis is pretty basic. There are a lot of resources on the web. I would particularly shout out Rob Robinette's website. He has a great website full of awesome information, especially regarding this amplifier. He uses this as the basis for his How Amps Work article. So it goes through every step of the way what each component does. We don't really have the time to do it here live on YouTube, but if you guys want something like that, that is something where we could absolutely make some kind of follow up where I talk about how the signal's routed, how the power's routed, what each component does and why certain values are what they are. So if that's something people are interested in, we can definitely try to make some type of followup episode.

Pete: Yeah. So leave your suggestions or your requests in the comments. And just to answer Gregory Hill Seniores, will we be replaying this series? It is going to stay up on our channels archive, so it's going to be available for the foreseeable future. I don't see why we would take it down for any reason.

Step 64: Install the fuse

Tommy: All right, so moving along. This next step is installing our fuse. So one amp slow-blow fuse, one of the most common for this style of amplifier. That just drops right into that socket. And you can see on that fuse cap, it's got little nubs on the side. I don't know if you can see that well, Rachel?

Rachel: Oh yeah.

Tommy: Okay. And there's notches in the socket as well. So I'm just going to line those up with that socket. And then it just turns a quarter turn once you just push it down a little bit, should click right in, lock that in place, and then we know our fuse is in there solid because this guy, if you remember in the previous episode, on this fuse socket, this tab was spinning free, and now it's got a good amount of pressure on it. So we know our fuse is properly installed there.

Pete: And again, that's just a safety feature. In case something goes wrong in your amp, it'll help protect some of the other components. And for everyone following along in the instructions, that is the end of step 64 on page 25.

Step 65: Install the pilot lamp

Tommy: Yah. Now we're onto step 65, installing the pilot lamp. So I remove that jewel lens.

Rachel: Let's see that real quick.

Tommy: And again, that just threads into this socket, the lamp socket. I'm going to get my number 47 pilot lamp and drop it in. And if you can see in the socket, there's a right angle here.

Rachel: You move that white wire up.

Tommy: Yeah.

Rachel: There it is.

Tommy: There's a right angle. And again, there's notches on the lamp. Can you see those, Rachel?

Rachel: Mm-hmm.

Tommy: Those notches should drop right in, give it a little twist and it'll hold it in those little, that is a retainer notch there. So that looks good. I'm going to reinstall my jewel lens.

And if you're not a fan of this classic red, we do sell a lot of other colors. I have one with amber at my house. It's classy.

Pete: Hey Tommy, can you say that link one more time? I think it was Rob Robinette.

Tommy: Rob Robinette, R-O-B, no, R-O-B-I-N-E-T-T-E, Robinette. If you search how amps work, it'll likely take you to his page. And again, this is called the 5F1 circuit. He has a ton of info about all the most popular amp circuits throughout the years, especially the formative years between, say 1955 and 1965.

Step 67: Clamp the power cord

So we are onto step 67 since we already installed our volume knob. So for this, we're going to clamp our power cord. And if you remember, we installed this in episode one. We're just going to unscrew that screw.

Rachel: Just a little bit again. There it is. Cool.

Tommy: So just a Phillips head, and I don't know if you can see that anymore, Rachel.

Rachel: I'm looking. We know where it is. You go for it.

Pete: Yeah, we'll show them where it is after.

Tommy: So just unscrew that screw, pull it out of the little clamp and then clamp it on your power cable. Doesn't really matter how much slack you have, I like to have just a little bit at least just so I can route that cable out of the way so it's not banjo tight right up top to the chassis.

Pete: Yeah, I think we say six inches in the instructions. It's really just the basics.

Tommy: Yeah, that's about a good recommendation. So I'm going to get that screw back through that clamp. Pretty simple stuff here. And screw that guy back in. No need to go crazy with that, but as you can see, my cable's not going anywhere and it's held up against the side of the cabinet there.

Rachel: There where it is...

Tommy: Cool. All right. And we did it.

Pete: Right. That's it.

Tommy: That is the end of the assembly part. The last thing we have to do is test just to make sure everything is what we want to see before we actually crank this thing up and hear how it sounds.

Pete: Right. Before we get to that, everyone out there who's just joining us for a good show or building along with us, give yourself a big round of applause. And we know you want to hop right in and test it and get right to playing. But like it says in the instructions, there's a big stop sign. Do not install the tubes yet. Don't plug in the amp yet.

Step 68: Perform a safe power-up

Tommy: And don't plug in your speaker yet either. So what I'm going to do is clip a test lead back onto a known ground point. I'm going to use that same one just so the camera can see everything I'm doing. The first test is a safe power up, no tubes installed. We're going to check AC voltage. For AC voltage, you test across both leads not to ground. If you test to ground, you get half of what your voltage is. So you can still do that, it's just not as accurate. So I'm going to disconnect my test lead from my meter and set my meter to AC voltage. And what we're going to do is check the AC voltage at pin nine and four and five on our 12AX7 socket. So I'm going to get plugged in here.

I'm sorry, I'm going to turn my volume knob down to three so that technically, the amp is turned on but still no power. This is so you don't need to reach in or around your chassis in case there is some type of severe problem. If anything is sparking, you're better off unplugging it than reaching in near the circuit.

Pete: Right. And we do have a couple questions, but this is some pretty important safety stuff so I'm going to hold onto that for a minute, but we will get to your question.

Tommy: So again, we're going to just perform a safe power up. No need to test anything at this point. I just plug my amp in. I can see my pilot lamp is lit and I don't hear or see or smell anything out of the ordinary. This is basically just checking to see that we didn't wire anything improperly in regards to this green twisted pair that lights our lamp and also provides the heater voltage for our circuit. I can see everything's looking good, so I'm happy with that.

Pete: Tommy, what are you specifically looking for that would indicate there would be a problem?

Tommy: Sparks, smoke, if this lamp didn't light. Those are the big ones. At this point, if you've wired everything properly, you are not going to see anything massively fail unless you have a defective component, which at this point, the only real components that are receiving voltage in the circuit are the sockets for the heaters and also the AC coming out of the transformer. But again, we don't normally expect to see much happen at this point.

Step 69: Test the standard AC voltage

So what we're going to do, just to make sure everything is still hunky dory, is check our AC voltage at our 12AX7 socket. So the 12AX7 socket gets this AC voltage through this socket as well as the lamp. So essentially, we can confirm with a good voltage check here that all this wiring is good to go. So what I'm going to do is tip this up so Rachel can see. Is it a good shot, Rachel?

Pete: And for anyone who is along in the instructions-

Rachel: It is. Good.

Pete: This does say step 67 on page 26, we realized there was a little bit of an error. This is actually step 69. It depends on when you're watching this. We may even have the instructions corrected online, just like I said, depending on when you're watching this.

Tommy: So I'm going to check from each of these green leads to each other to see my AC voltage. Can you see that voltage on the camera, Susan?

Susan: Yeah.

Tommy: Cool. And I can see I've got around nine volt... Oh no, I don't. There we go. 7.1 is acceptable. So that is just a little higher than it says in the instructions. The instructions say four to five volts or five to seven volts AC. That's a range because there's a lot of variation in components, tolerances as well as wall voltage. I'm assuming my wall voltage is probably somewhere around 125 so I'm guessing, just from that, that most of these readings are just going to be a little bit higher than it says in the instructions, but it's certainly no cause for concern. If it was somewhere in the range of four volts, our tubes wouldn't work. If it was somewhere in the range of 10 volts or something, then we might be looking at a real problem. But as long as it's within 10% or so, probably good to go.

Pete: All right.

Tommy: So we're good. We're going to unplug our amp and we're going to install our 5Y3 rectifier tube.

Pete: And while you're getting ready with that, let me get back to this question. JDS asks, "Hi guys, I have a Fender tweed amp and I noticed that it is quite noisy when turned on compared to my Vox AC-15. What level of white noise is normal for an amp to have and is there a way to avoid it?"

Tommy: So normal versus acceptable, all amps are going to hum.

Rachel: I think it should.

Tommy: Yeah, sorry. That's our 5Y3 rectifier tube. And you can see it only has five pins. Also on these octal-based tubes, there's a registration key. It's a ridge here between pins one and eight and that will line up with the tube socket. Very important that that's lined up properly. I've seen people force these in quarter turn off or something, it's not going to work properly and it could damage the tube or the circuit if you do that. So really important that you pay attention to that registration notch there. Did you get all that, Rachel?

Rachel: Just stick the chassis behind it the way you had it real quick and then-

Pete: Right here?

Rachel: There, it is cool. All right.

Tommy: All right, so I'm just going to visually locate that notch on the tube socket, slide that guy right in. So a vox AC15, depending on when it was made, depending on whether it's a PCB amp or hand wired amp could have lower hum than a fender tweed. A lot of fender tweed amps have a different circuit than they have a different circuit than AC 15 and might have inherently more noise. But if it's something you're worried about and you don't have a friendly amp tech to help, you can certainly check back on our previous episodes about lead dressing. That can be a big help when you're trying to get rid of hum. You can also isolate your circuit with copper tape like we did in episode one. That can help fight exterior noise, but if your amp is overly noisy, you might have bad filter caps. Those need changed every so often.

So I'd recommend if you're watching, we talk about filter caps a bit, you could definitely check those and they're not too expensive to replace.

Pete: And JDS, Tommy mentioned that it was episode four, I believe that we were discussing lead dressing. So if you want a refresher on that, check out episode four.

Step 70: Test the dangerous DC voltage

Tommy: So I'm going to go ahead and get my multi-meter set back up for this step 68 in these instructions. But it's probably actually step 70, is that right? Yes. So for this step, when we're testing the dangerous DC voltage, I'm going to hook my test lead back up. I'm going to get everything prepped before I plug the amp in. This step can cause stress on these filtered capacitors. Normally when your amp is functioning, there's going to be all your tubes installed and they're going to pull this voltage down that we're going to check in this next step with just your rectifier tube.

These filter caps are getting more voltage than they're really supposed to. And in some cases like what I expect to see, we're going to be over 500 volts. These are rated for 475, that's an operating rating, meaning they're safe to operate at 475 volts. If they go above that, there's a definite chance they could fail. So we do this check, this next test as quickly as possible just to make sure our rectifier tube is working and that this wiring here is all good. Again, I recommend getting everything as close to ready as you can before you plug the amp back in. So I've got my meter set to 600 DC.

I've got my test lead ready and I'm going to go ahead and plug it in. So I can see my light bulb is on and I can see just barely that the filament in my rectifier tube is lit. So I'm going to go ahead and check that first point. And I see 510 volts. That is what I expect to see. So I'm going to go ahead and unplug this. It's a quick check just to make sure that stuff is working properly. The longer you leave that on there, the more stress it's going to cause to those filter caps. And the longer you run it without any other tubes at this high voltage, the more likely these are to fail maybe prematurely. Again, nothing wrong with that 510 volts.

Rachel: Can you point out the spot where you just touched it?

Tommy: Yeah, here at eyelet 16 where this resistor and red lead meet this first filter cap. And again, in that step you're looking for smoke, smells, you're checking your voltage and getting in and out as quick as you can. So what we're going to do next is install our 12AX7 I believe.

Step 71: Install and test the 12AX7 preamp tube

Pete: Tommy, do you have time for another audience question?

Tommy: Yeah, so let me just answer it while I'm installing this. Yeah, this 12AX7 tube has nine pins. You can only install it in the socket one way, so there's no registration pin or anything. Just need to be sure that you get these pretty much lined up before you start pushing it in. These pins can bend and then you need to contact service@stewmac.com for a replacement. So go ahead, Pete, it's actually kind of related to what you're doing right now.

Pete: Eric Metal EC says, I've always heard that you are not supposed to touch tubes with your bare fingers, that the oils in your skin and fingertips could damage the tubes. Is that true? I think we discussed this briefly in a pedal build video, but can you speak on this again?

Tommy: I have yet to destroy a tube by touching it. I'm not saying it's not possible. I'd say don't eat a grilled cheese sandwich while you're working on your amp. But otherwise having a fingerprint or two on these glass tubes isn't likely going to destroy them. Now if they're really cranking and you touch them, you can burn yourself. And I have heard of people grabbing a hot tube and having it blow up, but it's not like say a halogen or Tungsten lamp that you'd see in a big expensive light where a fingerprint can absolutely cause it to blow. I touch tubes all the time and I have yet to have one explode because of it. I explode them for other reasons.

So we're going to go ahead and check our DC voltage at eyelet 26. This is a lower voltage, we'll expect to see it between one and two volts probably. So I'm down on my six volt range on my meter. I'm going to go ahead and plug our amp back in.

So again, not seeing any smoke, anything like that. And eyelet 26, all the way at the end here is where we're going to test. And so you can see just under 2.1 volts. Again, we're expecting to see everything a little bit higher than in the instructions just because of our little bit higher wall voltage. So I'm happy with that reading.

Pete: Hey Tommy, before we continue on, I think this is in reference to step 70. Kreenol B says, I have about 420 volts there. Is that still okay? We're looking to be around 479.

Tommy: Yes, that's okay. You might end up with a little bit more crunch a little earlier in your circuit with a little bit lower B plus, but that's still within reason for sure.

Pete: All right.

Tommy: So our next step is checking the DC voltage at eyelets 22 and 24.

Pete: And this is on step?

Tommy: This is still step 71 or 69 in my instructions.

Pete: Yeah. So on page 27, first step.

Tommy: So I'm going to check 24 here. I don't know if you can see that, Rachel?

Rachel: Give me a little tilt then we can see a bit. We do that after too.

Tommy: Okay. Yeah, if you can see it from above. So I'm going to move my meter up to a higher voltage, 600 volts DC and I can see I've got 270 approximately. Again, higher than what it says in the instructions, but not too much cause for concern. And at 22 I've got 267, those are going to be a little bit different. 22 and 24, they should be close, I'd say within 10 or 15 volts. And they should be within reason close to what the instructions say. So again, I was just testing at eyelets 22 and 24. All right. And that is our plate voltage for the 12AX7 tube.

Pete: And Sassy Cat asks, do we have a mockup of the circuit board prior to installation available to look at when we're reference, excuse me, when we're referencing points?

We don't have one here physically besides this spare, but it's a little too crowdy-

Tommy: There's completed wiring diagrams in the instruction.

Pete: Exactly. Yeah, you can find the completed diagram on page 24 of the instructions right before where we started for this episode.

Tommy: So at the end of our step that we just completed, there are some recommendations. If you find low voltage for your plate voltage for the 12AX7s at 22 and 24, this is supplied from this filter cap via behind the board jumper. A common problem when building this circuit is that jumper falling out, but as we saw, we have proper voltage there. So we're good to move on and test our 12, our 6V6 power tube.

Step 72: Test the 6V6 power tube

Another thing we need to do in this step is connect the speaker. If you don't have a speaker connected and you start playing the amplifier, especially in driving the output transformer, you could damage the output transformer.

So it's really important with two amps especially, but really all amps that you have a speaker or what we call a load connected at all times while you're operating it.

Pete: We've got plenty of questions coming in. Thank you folks.

Tommy: So let me just talk about this and then we'll install it.

Pete: Perfect.

Tommy: Same deal. Octobase tube. This one has six pins, that's our 6V6 tube has the same registration pin between eight and where pin one would be. And it's the same type of socket. So we've got a registration slot in the socket and a registration pin on our 6V6. Go ahead Pete.

Pete: All right, so A Minor Threat asks, how much of a gain difference is there between a 12AX7 and a 12AY7 in the 5F1 circuit?

Tommy: A 12AY7 I believe is a gain factor of 30 A 12AX7 is a gain factor of 100. So you're going to get a lot less drive from a 12AY7. It'll be pretty clean until you start over driving your power tube. Normally 12AY7s are used more for driving an effect or used for a phase inverter. This amp doesn't have a phase inverter.

Oops, sorry, I pulled that test lead off prematurely. So I'm going to get my amp plugged back in. And this is our last real testing stage. I can see my pilot lamp turned on. If at this stage your pilot lamp doesn't come on, you may have a bad power tube and it may have caused the fuse to fail, but I can see that everything looks good. I'm starting to hear that pleasing, familiar low hum. That is a good sign. So I'm going to get my meter back out and do these last few tests. We're going to check in our low range at eyelet 12. This should be around 1.65. It's likely going to be closer to two for our purposes as we have a higher voltage, but it's not. It's 1.5, that's totally acceptable.

And you can see that at eyelet 12. Did you catch that, Rachel?

Rachel: Oh yeah.

Tommy: Sweet. Okay, so I'm happy with that. Again, that's well within range we're going to check a little bit higher range at volt at eyelet 19. Sorry, eyelet 20. So eyelet 20 is down here where this cap and resistor are. That is the cathode voltage for our 6V6 tube.

This is also known as bias. And we're looking at 22.3. That's good by me. And now we're going to check eyelet 17. B plus voltage supply for our power tube. Go ahead Pete. Before we move on so quickly, that's where we're going to check next.

Pete: You mentioned bias at eyelet 20. What exactly is that setting? Can you speak on bias a little bit?

Tommy: Yeah. So first I'm just going to show you. We've got 353, in the instructions it says 325, that's within range. I'm happy with that. That's under 10%. So I'm calling all of our voltages correct. If you find significantly higher or lower voltages, you need to go back through and make sure everything's wired properly and you need to maybe double check all of your solder connections.

The voltages could be higher or lower if you're missing a b plus filtering stage in one of these capacitors, these filter caps. If one of these connections isn't right, if you lost this behind the board ground back here, or if your ground bus isn't properly connected, those can all cause voltage problems in what we were just checking, which is all our power side of our DC system. So bias is the difference between the grid and the cathode. The cathode supplies all the electrons that flow through a tube and the grid is what your signal is connected to and changes depending on the wave form from your guitar pickup. So when the grid goes positive, it pulls more electrons. When the grid goes negative, it stops the electrons and that's how amplification in a tube actually works. So you need the bias to be set for each tube. And you can see the bias for this guy is set by this 1.5 resistor. And the bias over here is set by this 4.70 resistor.

Pete: All right, we have a ton of questions rolling in here. All right, Jamie Kemp, I believe the pit, excuse me, the tele you're seeing in the background with the full pickguard is one of our Mini-T Style Kits, which you can just search mini tele or mini T style on stewmac.com. That should come up. Let's see. Okay, A Minor Threat has a follow up question. Tommy, they say since I have a 12AY7 in there amp, can they just simply take it out and switch it with a 12AX7 with no modifications?

Tommy: It depends on your amp. It depends on what the tube is doing. It'll fit, but they're not really all the same. So I believe the only one that really probably won't work is a 12AU7, if you swap that for a 12AX7. But I would definitely check your user manual and see why that's a 12AY7 in your circuit.

So folks, what I'm doing, since all of my voltages are checked out, I'm going to get this thing ready to play. I've still got it on at three volts on volume three. What I'm going to do is turn it off and put my back panel on. Before I put my back panel on, I'm going to drain those caps one last time just to be sure since I'm going to be working in and around the circuit. So I've got my SnufferStick clip to ground and I'm going to get in there at eyelet 16.

You don't need to get directly onto the eyelet, you can get on the lead of the filter cap. I don't know if you can see that, Rachel?

Rachel: Can you cut some wires?

Tommy: Yeah, there's a lot of wires in the way.

Rachel: There we go. That's great.

Tommy: Yeah, so anywhere on this lead is fine. You can drain all of them. Probably don't need to. Again, since we have a functioning amplifier circuit, these tubes probably drained all that B plus voltage anyway. Really good to just be doubly sure when you're working with high voltage.

Install the back panel

Pete: Absolutely. So Tommy, while you're setting that up and we're getting ready to test here. Well I guess like you said, you're going to install that back panel first. Want to make sure to get back. Santiago Colina is wondering how long it would take to ship an order to them in Spain. Depends on when we receive it in the day, but generally it will arrive depending on your shipping method, three to five business days if you're paying for express, after it ships or about two to four weeks if you're sending it just standard air mail.

Here's a question better suited for Tommy though.

Tommy: The amp kit probably isn't going to go air mail because it's heavier.

Pete: Right. That'd probably go economy. JDS asks, when tubes need to be changed. What is the difference between matched and unmatched tubes? Will it really make a difference?

Tommy: Let me screw these screws in and then we'll talk about matched tubes. So I've got my drill set on a low torque setting so I don't overdrive these screws. You want to overdrive your amp, not your screws. So pre amp tubes are not matched, power tubes are often matched. The manufacturer or supplier or testing house, test them for a few different values.

They test trans conductants and plate current mostly, those are the two big ones for a power tube. Matching tubes make setting your bias easier in a fixed adjustable bias amp. In an amp like this, we have a cathode bias, meaning it's not going to be something you're changing when you change a power tube. There are a lot of cathode bias tubes out there or amps out there, Supro amps, Fender amps, a lot of them are really popular. A cathode biased amp is a pretty inefficient amp as far as design goes, but they often just sound great. So it really depends on your amp. And then as far as matching, some mismatching is actually kind of good. You can get these nice harmonics when they start to break up that you can't get with a perfectly matched set of tubes. But again, the most common process is to get as close as you can as far as matching your power tubes.

And there's a lot of discussion on both sides about what's right or wrong, but I've heard amps that have mismatched tubes that sound absolutely fantastic.

Pete: And sorry to kind of delay, I know the tension's really building and I want to get to actually playing through this amp, but I think this question kind of follows along. Mike Collins asks, if the bias is not intolerance, how do you adjust it?

Tommy: In this amp you would be swapping a resistor. In adjustable fixed bias amps, there's often a trim pot that you can change that value up or down to adjust it. And again, this amp is extremely simple so it doesn't have any adjustment and it is just another reason why I love it so much because I can just pop a new 6V6 in there and see how it sounds different.

It's finally time to let it rip!

So folks having confirmed all of my voltages and the final assembly of my amp, we're going to go ahead and turn this thing on and see how it sounds. We're going to start at a low volume.

Pete: Fingers crossed.

Tommy: Yeah, fingers crossed. We're going to start at a low volume. I'm going to turn it to about three.

Pete: And reminder, we are live here so we would love to test this beforehand, but sort of the name of the game, we're live so we can take your questions and also I guess put ourselves out there and hopefully don't fall flat on our faces. Yeah, so drum roll.

Tommy: Yeah, so again, it takes a minute for your amp to warm up after just about 10 seconds or so you should hear noise and that's good. So again, at a very low volume and I'm plugged into input two, which is our low jack. You should have a very clean tone. Perfect for bedroom playing, practicing. I'm going to go ahead and turn it up a little bit, maybe around six. You should start here in just a little bit of breakup. Nothing too crazy yet. So what I'm listening for is oscillation, pops cracks, static, anything that isn't just a nice quiet low hum in my guitar signal. Sounds good. Sounds like I've got all the frequencies, I'm not missing anything. I'm going to go ahead and turn it up a bit more now. We're going to go up to nine. This should be pretty close to breakup. It should be a nice crunchy sound. Great. Sounded good. Now we'll give a little crunch.

That's what we want to hear folks. We've got a tube amp. Congratulations. 

Pete: Give yourselves a hand. Nice job, Tommy.

Tommy: So if you had any problems in that testing section or if you've turned it on and it just doesn't sound right, absolutely go back through the testing with a really suspicious eye. It's really easy to make a small mistake that can just cause your amp not to work. If you're not able to figure it out, please email us at service@stewmac.com. We're always happy to help. I'll probably ask you to send me some photos of your circuit. But if you did get this far and it sounds great, just give yourself a round of applause for building a point to point hand wired tube amp because that's no small feat.

Pete: And the greatest part about this is just the beginning. All of these skills that you've been learning and developing can all be applied to pretty much every hand to hand or point to point amp build and-

Tommy: Yeah, this is foundational for sure. These same techniques carry through to any kind of eyelet board build and they translate pretty well to our turret board builds as well. That's the other popular kind of hand wired amp, which we have a couple of those at this point too.

Pete: So we chose to demonstrate the build of the 57 mini tweed amp kit, but that's just one of five amp kits we have. So if you've built this or you're interested in the other ones, we also have a '59 Tweed 15 Watt, a '62 Brit-Plex 45 Watt, a '65 P-Reverb 15 Watt and a '66 D-Reverb 22 Watt. Yeah. Before we sign off here, I do want to say we've got a couple last questions. Again, please feel free to email service@stewmac.com if you have any issues or need help troubleshooting. Let's see.

Susan: [inaudible 00:55:58] List.

Pete: Oh wow.

Susan: Yeah, And you can skip through the ones that we've already done.

Pete: Okay, thank you Susan. Let's see. So the first one I have here is Anthony or Xeno says, why no vent on the back panel?

Tommy: It's not how they did it in the fifties. Most of the heat in this amp is on the outside with those chassis mounting components, the transformers and the tubes. And as you can see, this amp has an open back so all that heat can escape. If you are really cranking this thing for say an hour, this metal chassis may get a little hot, but I think those vents really came in when everything was kind of inside of a head with no real way to get out. And I just think vent would ruin the aesthetic.

Pete: You could put a vent in if you want, if you've got some carpentry skills and you're good with redoing tweed. And let's see, we have a question also. Oscillation Overdrive asks, how can I add treble middle and gain knobs?

Tommy: You get a different amp, it's not all going to fit in this amp. If you wanted to add a tone knob, you probably could, but you would definitely need to find a different chassis if you were going to add any more than one knob. As you can see on this control panel, there's just not space for three more pots. And again, inside of that circuit, this chassis is so cramped. And really the beauty of this amp is simplicity and how connected you are to the circuit, to the tubes without all those other controls. Part of the reason it's a great studio amp is because there's one setting. If you have it on six, it's going to sound like that every time no matter how many takes or how many days later it is that you're going back to punch in and fix an overdub or something.

Pete: All right, and a few more here. Well first Santiago Kalina gave you a shout out, Tommy. He was pretty pumped and he played Ooh La La by The Faces and-

Susan: We got a request for Christmas music.

Pete: We did. They wanted us to carol afterwards. I can't remember carols.

Tommy: That's all I got.

Pete: Also, hold on. Susan, go ahead and stand up there in front of the camera. They want to meet you after hearing your voice for almost a month.

Susan: What's up?

Pete: Hi Susan.

Susan: I don't know.

Pete: Just cleaning up.

Susan: Cleaning up Tommy's mess.

Pete: As always. I appreciate it. All right, Gary Alexander asks, is this amp a push pull or a class A? And if it is a class A, can I run 6L6s or EL84s?

Tommy: It is a Class A. I don't know that there's such a thing as a push pull amp with only one power tube, but if you wanted to use different tubes, you'd need to probably change your transformers as well. This amp is certainly modable, but its small form factor and very simple design, doesn't lend itself to as much modding as some of our other circuits, like the P-Reverb based on the AA 1164 circuit, which is really modable. So if you're into that kind of thing, I would definitely recommend doing a bit of research and seeing what kind of mods you were looking to do. For this amp you're not going to get more output by going up to a 6L6. It's only going to get as loud as it gets. It's got pretty undersized output transformer and a low wattage speaker.

Pete: All right, well, I just want to say thank you to everyone joining us. Really, without you all logging in viewing this we wouldn't be doing it. So a big, big thank you from everyone here. And please, please feel free to leave comments on these videos, what you'd like us to do next. We love making these videos for you guys, so please sound off in the comments and let us know what you'd like to see. I want to give a huge, huge thank you to Tommy Stump. Thank you for leading us through this.

Tommy: Thanks Pete for hosting. Thanks for Rachel getting those great closeups and thanks Susan for making sure I don't get off track and keeping everything going smoothly.

Pete: All right, so again, huge thank you to everyone and we hope to be doing more of these videos in the future for you. These will all stay up on our YouTube channel archive, so you can go back through and review them at any time. Tommy, why don't you play us out.

Tommy: Oh-oh.

Pete: Have a good night everyone.



Tommy Stump

StewMac Guitar Builder and Tech