Guitar Tube Amp

Want to build your own tube amplifier for guitar? There are many options: build a kit, build from an existing schematic, or branch off like I did, and try something different.

Maybe, like me, you'll design and build from scratch...

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Check out the last steps-- information's been added since this guide was first published.

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Among the goals for this build:

--Build an amp with that MMM-good tube sound...
--Design it myself.
--Reuse salvaged and vintage components whenever possible, and save good stuff from the landfill.
--Make something unusual (6DG6GT's in a parallel single-ended configuration qualifies as unusual...as does the tone control....)

A whole lot of tweaking later, I've got an amp that pleases me. A small, but surprisingly LOUD amp that outputs something in the neighborhood of 8 watts (see the Power Amp Stage step for more info.) And the combination of 12AX7 and 6DG6GT tubes, though unusual, works quite well...

Oh, and this is a fairly hi-gain amp--i.e., it has a good amount of natural tube clipping and distortion, and a decently "dirty" sound. However, hi-gain and high volume are not the same....this amp is loud for it's wattage, but it's not a Marshall stack. It remains a studio type amp, but it is louder than all those Valve Jrs., Champs, Blackhearts, etc. which are so popular today....

Clean signal, no F/X.
Settings: volume 50%, tone 60%, presence 30% :

Clean signal, no F/X
Settings near max :
_{(Some "ghosting" on the highs is a resonating glass-door china cabinet about 5 feet from the amp...)}

In fact, there's a little too much gain...

One thing's for sure...tackling such a project means many happy hours pouring over data sheets, studying schematics, checking output transformer specs, and tracking down NOS tubes....

Noteworthy: there's a certain aspect to this build.... I wanted to retain the feel and budget of the radio-amateurs and home-builders of the past. You can easily spend in excess of $1000 USD for a small tube amp kit alone (nothing but the best audiophile components.) There's an elitism about modern tube amps I tried to avoid (or maybe I'm just cheap ;0)

Step 1 Danger, Will Robinson, Danger!

Here's the standard disclaimer:

This is dangerous, high voltage stuff. OK, it's not "High Voltage," technically, but it's high enough to kill you. The power supply in this projects kicks out 200V, which is plenty, with startup spikes near 240V or more...

Don't believe it when they say "it's not the voltage, it's the amperage that kills you"--because it's both. Amps AND volts together dictate the danger level. If it were amps alone, then even a AA battery can supply many times what's needed to stop a human heart. The volts do the "pushing," and overcome the natural resistance of your skin. And there's plenty of current available to harm you in any tube audio amp...

Remember:
--Always drain the power supply filter caps before touching the circuitry.
--Always unplug the mains cord before working.
--Double-check (with a VOM) to be sure the filter caps are drained.
--DON'T mess with this stuff unless you have a decent understanding of the dangers.
--DON'T mess with this if you believe you know EVERYTHING about high voltage, and think that makes you immune to electrical shocks.

Comments

[OldMacTech]

Cool project.

The white boxes with numbers on the side, what are those and what purpose do they serve?

Thanks.

[gmoon]

Thanks...

The white things are power resistors, and are part of the power supply. It's necessary to drop voltage for various stages in a tube amp, and it's almost always done via "brute force" (resistors).

[OldMacTech]

Thanks, do these fail or get out of spec? I have a 1481 Silvertone (cousin of your project) and the volume seems low. Did a re-cap, swapped speakers with some improvement-what should I look for as a possible cause of decreased volume?
Thanks.

JB

[gmoon]

Yeah, resistors can "drift" over time, especially the old carbon comp type. Some of those you can test in-circuit, but others will have to have one leg lifted for an accurate test.

Swap out the tubes for known good ones? I've learned that lesson from bitter experience (sometimes my backup tubes weren't any good either, and I wasted weeks replacing stuff that was fine).

[OldMacTech]

Thanks, I have a 1482 that is a beast-I have swapped tubes in/out from it. This 1481 has good tone and the replacement speaker increased the volume a good bit but is just seems to come up a bit short on volume. Maybe it's correct but If I can swap a $2 resisitor out and make progress a cheap price to pay. If I wanted to get the correct resistor to replace this, what would I ask for at the electronic store.
I really appreciate you help.
JB

Oh, and can describe the test I can run for this and the other resistors with a DMM?

[gmoon]

Comments seem to be back...

You can easily test those resistors that can be isolated from the circuit by pulling the tubes--the cathode bias resistors. Most of the other will need to have one leg disconnected.

Old resistors can still test good when cold, but may drift more when they get hot....

If it were me, I'd check the voltages in the PS, and the tube plates. This is DANGEROUS! Deadly high voltage! Don't do it unless you're confident working with HV.

Don't forget to clean and re-tension all the tube sockets.

[aaronadennis]

Hello,

It comes to mind would a 6W6GT also be suited for this project. I think they also have a higher plate voltage and maximum power dissipation. I'm certain they are as available as a 6DG6GT as well.

cheers

[gmoon]

Yep, no doubt.

(I also kind built this around the transformer--that had something to do with the power tube choice.)

[pddonovan2011]

One really great project
Literally, the best sounding instrument I ever heard was an experimental Hammond Model that used BATTERIES to purify the electricity flowing into the instrument. This guy had taken apart a (I think it was a B3) Hammond Organ that had a huge speaker box and Tubes for amplification. I tried to find out why, exactly, but all I could get out of him was something about the electricity is cooler and the tubes lasted much longer. This chap was nearly 70 and could he ROCK that Organ! He would sit and listen to on record after another and play right along with then all! Uh! Record, the kind that you drop a needle on to listen to them. OH! And the turn table sat on a cement piling that passed through the floor into the ground and must have weighed close to 10 tons! He said that was the only way to make sure the needle never skipped! Oh! And this Organ ran 12 volts D/C exactly.

[aaronadennis]

Of coarse I understand, it would be very simple to modify your implementation for 6W6GTs and use any variety of more commonly available power transformers. Two 6W6GTs in parallel would yield 20 watts easy.

I like the nature of this website. But it shames me to notice there is not more tube enthusiasm !
I am working on a revision of your design with 6W6GTs and all Edcor transformers looks like its gonna be about $100 in transformers alone. And i respect the nature of your project completely. I am hoping to do a follow up instruction set on how add active bass and treble to your design as well as modify it for 6W6GTs if you have no objections ill start work on that soon.

I suck at making cabinets. So i am grateful for you added that.

[pfred2]

Sorry I can't get into tubes personally. I like solid state!

[gmoon]

Yes, this was purposefully a "ghetto" tube amp with as many recycled parts as I could get.

Use or abuse the info in the project as you please. I stole ideas from other circuits, so have at it.

I look forward to seeing your amp...

I looked closely at the 6W6GT datasheet, and you'll never get 20 watts from a pair in parallel (or even push-pull), but rock on! 

[aaronadennis]

I see i'm looking at 8 watts wit both of them.
Ill note i have seen plate dissapation form 10 watts GE to around 14 for these tubes.. depending on manufacturer.

But your right i grossly over estimated the power output.

[Lenny24]

Cool Instructable, but due to Electrical safety you should connect one lead of the Secondary side of the Output Transformer to ground. In case it fails, the Fuse will be blown instead of your Body.

[jjunglas]

I'm wondering, where does a person learn the beginnings of things like schematics and electronic? I don't want to be a noob any more. I want to learn. Where can I learn about electronics and wiring and voltage?

[Kidogo]

google. wikipedia is a good place to start, then use google to help understand the bits that are over-complecated. when youve starred at it enough in loads of different layouts, it should suddenly make sense:)

[aaronadennis]

I started out with old radio shack project books. Implementing 555 timers making simple logic circuits. In the front or back of any respectable electronics handbook should be a list of all the symbols and what they represent. If your not into dusty old books I'm positive the Internet has it.

You should definitely learn about voltage and current flow before you attempt anything with tubes.
As an electrical engineer i would suggest trying to reverse engineer simple populated PCBs and create schematics to them try to understand how they work. Avoid surface mounted devices and more than 2 layer boards for the beginning.

If you want to use a very useful piece of software that has no tubes, but can get you started understanding schematics and laying out PCBs.

http://www.cadsoftusa.com/
its cross platform. OSX Linux and Windows.

PS if you don't understand what I'm talking about Google can answer faster than I can. Also a very powerful resource for electronics questions is stack exchanges electrical engineering forum.

[gmoon]

Analog electronics are a little more slippery than the digital stuff. I think "learning by doing" is a great approach.

Try some simple stuff first--like a guitar effect. There are lots of designs and discussions over at diystompboxes.org.

[14jpm]

Hi!!
Congratulation for your proyect!!, i like me a lot. Before start I need you that I don´t speak well English I speak Spanish, so, perhaps you find a lot of mistakes in the comment :). I write for two comments, the first, I want share a program with you, its name is ‘TDSL Personal Edition 1.0.1 Duncan Amplification’, it´s a database of vacumm valves. look at!

The second comment, about technical questions , I´m beginner in electronic topics. So I don´t understand which is the capacitance unit (µF,mF,nF,pF,etc.) of red circles, neither resistance unit (kΩ,MΩ,etc) in blue line and finality about audio transformer, I sought its datasheet (http://www.edcorusa.com/products/522-gxse15-4-1_7k.aspx) and I don´t understand which is the correct wiring. So please help me with my questions. Thanks for your time (I image the time that you spent to understand me):)

[gmoon]

Hola--I understand you just fine!

My bad...the coupling caps are within the typical ranges for tube amps; I just assumed most people would savvy. That's sloppy on my part. So the capacitors circled in red are "uF" (microFarads).

The resistors in the blue boxes have no suffix--that's the actual value. So 180 = 180 ohms, and 150 = 150 ohms. The wattage is noted.

Looking at the Edcor site, I see the link I provided no longer works. Also, the transformers themselves have changed--no solder tabs, and now have end-bells. They are also a little more expensive. Here's a current link for the equivalent transformer.
http://edcorusa.com/products/522-gxse15-4-1_7k.aspx

The 10 watt version should also work fine (and is cheaper).
http://edcorusa.com/products/500-gxse10-4-1_7k.aspx

The hookups should be fairly obvious from the diagrams on each page... If not, let me know, and I'll see if I can help.

Note: I didn't include exact links before, simply because not everyone wants a 4 ohm output impedance for their amplifier (like if you have an 8 ohm cabinet)...

[smartrobot]

This person had a bunch of boxes on the curb it said free so I took a look, and one of the boxes was full of vacuum tubes. some of them are not labeled so how can I find out what they are?

[daniboy79]

an old trick a buddy of mine told me works is to stick the tubes in a freezer for a few minutes, then take em out and look for the condensation, it usually doesn't condense on the letters/numbers of the tubes designation. Try it and see if it works ;)

[gmoon]

Almost all tubes are marked in some way. Sometimes the labeling is hard to see, but try looking at different angles and varying light conditions.

Be careful when cleaning tubes; you can rub off the markings...

[smartrobot]

I googled the tubes that had letters on them it looks like they came out of a old tv so it does not matter.

[cnludwig]

Cool project. I like how you used otherwise unwanted parts (power tubes and transformer) and created a design around it.

[gmoon]

:-)

[eridan]

Very interesting. Thanx a lot!

[katatonicdean]

Alright, thanks for all your help!

[gmoon]

Antec--that was the company that makes a 150V toroidal transformer with filament windings too, at a reasonable price. It's the AN-1T150 model.

Find their stuff here. I've never used it, but that's the closest I've seen to the transformer I used, and it's not very expensive.

[katatonicdean]

Hey,
First off, I was just wondering if this can be converted into a bass amp with an added bass knob.
Secondly, do you know of a place that I can get a similar choke, power transformer, and output transformer?
And thirdly, what pins were used for the tubes? It would be really helpful to know. (I notices that you used the 12ax7 twice, but on the power supply you only had one of the filament pins powered?)

Thanks for all your help!

[gmoon]

Converting to a bass amp? Probably by using somewhat lower value coupling capacitors. Where this amp uses 0.02 uF caps, you could go with 0.033 or 0.047 uF coupling caps.

That would be more "bassy". If you need a separate bass knob, then use a different tone stack in place of the existing one. This website will be helpful.

Edcor makes the output transformer. You can find similar voltage power transformers, but that's a bit harder. I had a nice link to a 150V toroidal PT, but I lost it when my computer died. If I find it I'll post it later. Chokes are easier, Hammond makes those.

You'll have to look up the pin outs yourself, they are on the datasheet I linked (and you can find these online)--in fact, if you've never built a tube amp before, I'd recommend you do a lot more research. You'll figure it out...

The 12ax7 is listed twice, but it's a dual triode, so two triodes are enclosed in the same tube. Hence, only one filament is needed (note: 12ax7s can be wired as either 12.6V or 6.3V filaments).

[katatonicdean]

Oh, and can i use a higher ohm speaker. I was thinking an 8 ohm or 16 ohm speaker. What would I need to do?

[gmoon]

Choose an output transformer with whatever impedance you need.

I.E., if you have a 16 ohm cabinet, get a transformer with a 16 ohm output impedance. I'm sure Edcor makes this trannie in both 8 and 16 ohm versions.

Some OTs have multiple coil taps. You'd need to special order that from Edcor...

[ski4jesus]

DUDE. that is sooo sweet haha hey btw, i have that sammmeee guitar haha great job!

[reinovator]

Thank you for keeping PURE amplification Alive.This is a keeper!! Thank you.

[rode]

Very well explained - maybe too well, as those with no experience might be tempted. Nice job overall though - very thorough.

There are a lot of comments about this amp, so I may have missed it ... Using the 7V tap off the main HT winding is a VERY BAD idea, because you can very easily exceed the maximum heater/cathode voltage (which will cause catastrophic failure), and at the very minimum you get hum if there is any leakage.

Just upgrade the wallwart used for the 12AX7 so it can handle all the heaters. All the circuitry operates at hazardous voltages (including the output valve heaters !!!). This is a potential killer - please amend the circuit to use the small auxiliary heater transformer for all valves - both for safety and to protect the heater-cathode insulation.

The position of the standby switch means that as you switch it off, a HUGE voltage spike is generated in the choke because you've just interrupted the current flow. The spike can (and has) damaged the insulation in the choke.

This can be fixed by using diodes (you'll have to work out where they go, it's too hard to describe in a short message).

Single-ended Class-A output stages always have asymmetrical distortion which many guitarists dislike. A disconnected speaker will usually cause the demise of the output transformer (same mechanism as interrupting the current through the filter choke).

Hope this is helpful.

[gmoon]

Thanks for your comments...

The filament winding is indeed bad for hum, but not much of an issue with the power tubes.

I'm not too concerned about safety. The PT is fully isolated and the chassis grounded to earth. Any catastrophic short between the PT secondaries should just fry the heaters (and / or blow a fuse). Bad for the tubes, but not much of a safety issue. A separate filament transformer would indeed be a better solution from the hum perspective, though.

The standby switch was never added to the circuit (and indeed is overkill in a smaller amp), but I agree with your assessment.

(much of this amp has been largely redesigned / rebuilt in the last two years; maybe someday I'll post an updated schematic...)

Single-ended Class-A output stages always have asymmetrical distortion which many guitarists dislike. A disconnected speaker will usually cause the demise of the output transformer (same mechanism as interrupting the current through the filter choke).

Not sure why you added this. I don't disagree (actually I do disagree about Class-A, at least compared to other topologies), but the dangers of running a tube amp without a load are well known...

[rode]

gmoon,
If you measure the peak and peak+DC between 6DG6GT cathode and heater, you'll find that it is either right at the very limit of the allowable maximum, or may exceed it. This is generally not recommended & that's what I was concerned about.

The safety issue was mentioned because people expect the heater supply to be "safe" low voltage.  Your arrangement causes peak voltages of over 200V on the heater supply.

A properly designed push-pull valve amp will have almost perfectly symmetrical clipping. It's not the Class-A part, but the fact that it's single-ended.  Symmetrical clipping is almost impossible other than within a very narrow range.

There are countless ways to make a valve amp safe (well, safe-ish) with no load, but single-ended output stages are close to impossible to protect.

I rarely make comments on the Instructibles site, but this is an area that I know very well indeed, having designed and worked with valve amps for over 40 years.   Just trying to help ;-)

Cheers, Rod

(see http://sound.westhost.com/valves/ for the info I have available).

[gmoon]

Great stuff on your link, Rod....

[gmoon]

Rod, why would there be 200V on the heaters? I'm not using the 50L6 tubes (seriously--I'd like to know if that's the case.) Also, you're aware that two Si diodes have been place in series with the AC heater voltage to drop it below 7V? It's mentioned in the text...

We can argue about the different "sound" of tube amp topologies ad infinitum, but it's preference, not textbook math or science, You have yours, I have mine. I enjoy both Class A SE (pushed to clipping beyond it's technical Class A definition), AND I enjoy push-pull AB amps too.

For guitar amps, of course. Not for all audio. And I hardly invented the Class A SE guitar amp...

I still don't know why we're discussing "no load" for this amp. It has a load ;-) Do we want to start a separate theory discussion? That would be OK...

Thanks for the link!