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Guitar Tube Amp

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introGuitar 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...

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

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)
Guitar Tube Amp
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step 1Danger, 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 p…


step 2How did this project get started?

First, I like the sound of the 50L6 power tube in my vintage Kay amp. Although weaker than many common types (6V6, 6L6, etc.), nearly 80% of the output of a 6V6 can be achi…


step 3The Tubes

AvailabilityIf we still wanted 50L6 tubes, they are fairly plentiful--lots of radios used these tubes. Same with 12AX7's, they're still being manufactured today and are ple…


step 4Components

Choice of components are alway contentious for tube amp builders. Some insist that one part or another is integral to the process. Hmmm. Although there maybe some truth, th…


step 5The Chassis

The chassis was originally a flat sheet of steel, which I reclaimed from a defunct VCR. Check out that snazzy "high voltage" symbol stamped into the metal...The steel was t…


step 6The Power Supply

Traditional tube amp power supplies are old school--relatively high voltage, with big "iron," and generally not regulated. Typically, they supply a range of voltages for di…


step 7The Heater Power Supply(s)

Unfortunately, the filament secondary for my power transformer isn't a separate winding, and doesn't have a center tap. Maybe I could disassemble the trannie and see if the…


step 8The Preamp Stages

It might look like the preamp circuit was lifted directly from an existing design--excluding the tone control (see the next section.) But I really did design it from scratc…


step 9The Preamp Schematic

I intended to include a section here on using loadlines to design a preamp stage. I think I'll hold off, and keep it general.There first preamp stage uses very typical guit…


step 10Controls

The Tone / Volume ControlThis is one of the odd parts of the build. Instead of a more conventional tone circuit, I chose to modify the "Big Muff" filter schematic instead. …


step 11Power Amp Stage

The simplest tube amp type is Class A, single-ended. Without going into too much detail, Class A amps are considered to be the richest, warmest sounding type of audio ampli…


step 12Output Transformer, Part 1

Figuring Load Resistance Mathematically (WARNING: Math Content)We have an educated guess, using the datasheet (see the previous step):1833 ohm for two tubes.The alternative…


step 13Output Transformer, part 2

Ok, so I already had a Hammond 125C, which is an 8 watt, "universal" OT.Much of the testing with this amp was done with the Hammond. But it's not ideal--it's better used in…


step 14Wiring

WiringI used point-to-point wiring for the project.Oh, yeah. So many components were removed and substituted during the testing phase, this became a real mess. The tone con…


step 15The Cabinet, Construction

Most of the plywood for the cabinet was "reclaimed" lumber, scavenged locally. I've collected pieces for a while, mostly to use as boat building stations (forms.) I've got …


step 16The Cabinet, Dowel Joints

The chassis supports and cross-brace were attached with a dowel joint.There might be easier ways to do it, but it's a nicely-hidden joint. It also prevents difficulties whe…


step 17The Cabinet, Outer Covering

The cabinet was skinned with a roll of black vinyl that's been laying around here forever. Contact cement is the best adhesive for this application.Apply the cement to both…


step 18The Cabinet, more construction details

Front Panel Dress and TopThe panel dress and top were cut after the rest of the case was assembled and the chassis was fitted. They were then covered with vinyl.The front p…


step 19Links

The Valve WizardGREAT stuff, especially the "Triode Gain Stage" PDF.http://www.freewebs.com/valvewizard/index.htmlTales From The Tone Lounge-- Mods and Odds!Really good pra…


step 20OH, Man...I wish I had...

No project is without lessons learned. In this case, I wish I had:--Used a heaver, possibly stock, metal chassis.--Substituted a 10 inch speaker.--Used a turret board, inst…


step 21Finding parts for a build (transformers, etc.)

The "Iron"If anyone replicates this build, it's unlikely they'll find a 142V transformer. So how to replace that part?-- One solution is a 230V primary (euro mains), 300-0-…


step 22The End???

More parallel 6DG6GT SE designs (or parallel PP) are churning away in my head. A three tube, 11 or 12 watt amp would be quite impressive. Or two parallel stages of two tube…


step 23Update, V0.2

Here's the entire schematic (V0.2), with a few changes...-- The whole schematic is now a single graphic.-- One of the power amp cathode bypass resistors has been reduced fr…


step 24Update, V0.3

Sept, 2008I continue to refine the amp. It's been several months since the initial build, and it's still going strong. Any concerns I had-- for instance the power tube cath…


step 25Local NFB Option, V0.4

Late Sept., 2008While reading about the weird additions to the Fender Bassman (Ver AB165), I noticed a comment about a local Negative FeedBack loop on one of the gain stage…


242 comments
1-50 of 242
Oct 26, 2009. 7:31 AMDajer93 says:
I found a pre amp in an old speaker. It has a volume, a treble and a bass knob. What do you thing would it be good for the amp?

Oct 26, 2009. 11:32 AMgmoon (author) says:
Kinda hard to know without more information..

Is it tube, solid-state, or just controls? A preamp module? An amp itself?

Oct 27, 2009. 2:38 AMDajer93 says:
I think its a preamp module, but I dont know exactly. How can I make sure, what type is it?

Oct 27, 2009. 4:27 AMgmoon (author) says:
Post a couple photos here?

Oct 27, 2009. 6:53 AMDajer93 says:
Here you are, thanks for your helps :)

DSCF7223.JPGDSCF7224.JPG
Oct 27, 2009. 8:42 AMgmoon (author) says:
I'd say that's an entire self-contained amplifier.

Why?

-- There's a power rectifying (4 diodes on the left) and filtering circuit (two large caps on the left.) Looks a little over-kill for just a preamp.

-- And there are two power transistors attached to a large metal heat-sink. It could be two channels, or it could be a single-channel push-pull amp.

A preamp wouldn't need the big transistors and the heat sink--that's something you only see in power amps. Or the two "transistors" may be discrete power amplifiers in a TO-220 package.

There may be other transistors (or ICs) I can't see behind the POTs, and they'd serve as the preamp. Somewhere off camera is a transformer or a wall power supply.

So this could work on it's own as an amp...


Oct 27, 2009. 9:58 AMDajer93 says:
So what do you think, could I join on it a bigger speaker? Or how could I achieve a higher effiniency? Would it work well with a guitar?

Anyway, sorry for my stupid questions, but I'm a beginner in electronics, and I just don't know anything (but I want to learn it, it's interesting). :)

Oct 27, 2009. 5:17 PMgmoon (author) says:
It could work with guitar. No idea about how it might sound, however.

Higher efficiency speakers are real--look at speaker specs, and there's usually a chart for SPL (Sound Pressure Level), measured in decibels. Look at the sensitivity column on this page . A 3dB increase in SPL is the equivalent of doubling the amps wattage.

Double the wattage doesn't really equal twice as loud (that's log function.) But it is noticeably louder.

Rather than spending lots of cash on new guitar speakers, you should try it with other speakers you have on hand. There should be an ideal "load" for the amp (speaker ohms rating), and normally it's loudest at that point. So try to match the load, if possible.

Oct 28, 2009. 1:23 AMDajer93 says:
Okay, and thanks for all of your help :)

Aug 10, 2009. 10:12 PMeric m says:
Way too complex for a beginner.
Aug 11, 2009. 6:56 PMgmoon (author) says:
That's likely true for most...
Aug 4, 2009. 7:07 PMisaacnseals says:
NOS tubes are not always plentiful....trying to find specific less common or popular tubes can become fairly hard. In your design you should use what you have available.
Aug 5, 2009. 8:21 PMgmoon (author) says:
???

In this case, 6DG6's are VERY plentiful, and cheaper than many classic amp power tubes-- 6V6, 6L6, etc.
Jul 6, 2009. 2:51 AMglen1990 says:
hahaha......yer im done that a fue times ...left the power in ...gives a nice belt thow..wehn your wokring with power amp..that use high voltage :P
Jun 13, 2009. 11:35 PMplanetmuncher says:
whats funny, when i was taking apart an old amp i got for free, i left it plugged into the wall, i touched that amplifier....F***! i was right next to my dad he was laughing at me yeah always unplug it!
Jun 11, 2009. 6:27 AMRetroTechno says:
You should enter this in the "Art of Sound" contest!
Jun 11, 2009. 7:20 PMgmoon (author) says:
Thanks, but I think the contest is just for newer 'ibles. I hope to have time to throw together a project or two before it's over...

I love Robot (Danger, Will Robinson, Danger!)
Jun 1, 2009. 11:57 AMSycan says:
I loved this build, right down to the burlap. Instructables such as yours are an inspiration and really, why this site is great.

BTW, you're a damn good guitar player too.

Sycan
Jun 1, 2009. 6:19 PMgmoon (author) says:
Thanks, man. The chops are rusty, but I appreciate it.

If you're serious about building an amp (diy, on the cheap), I'd be glad to point you in the right direction. I'm planning on putting together a 6L6 pushpull project anyway.

Step one: find a cast off or cheap tube organ from the 50s or early 60's... that'll get you the iron (power and output transformers) and if you're lucky some decent tubes.

Jun 3, 2009. 2:05 PMSycan says:
Thanks for the tip on old tube organs. A quick check with Craigslist turned up quite a few prospects including an old Baldwin for $25 in a Christian thrift store. The ad wasn't specific as to model, but it's old and huge. I'm on my way to check it out now. I'm not taking the truck just yet , but at least I'll have a basis for research.

Thanks
May 30, 2009. 10:29 AMThe Jamalam says:
Is that a yamaha pacifica model you're playing? If it is, I have the 012
May 30, 2009. 1:14 PMgmoon (author) says:
The guitar in the first vid is an Ibanez (RX-something, I forget.) So-so pickups...but great action and real fun to play. Probably very similar to your Yamaha.

The second vid features my "Trash-o-caster," which was my very first instructable project...
Nov 30, 2008. 6:55 PMElJefeUno says:
Anything above 40 volts is generally considered "high voltage", as it's enough volatage to break down your skin's inherent insulation and allow current to flow across your heart high enough to kill you. I've built tube amps, and I recommend to anyone aspiring to build on that they use extreme caution while working with them. Discharge all caps before working on it (as stated above), and take great care when using a multimeter to troubleshoot. I personally use "gator clamps" when checking high voltages, so that I can do it hands off and not risk my life.
Apr 13, 2009. 4:33 PMsonofkikkoman says:
but there is a big difference in 40vac and 40vdc
Nov 30, 2008. 8:47 PMgmoon (author) says:
Thanks, good comment.
Apr 9, 2009. 1:07 PMNebraska G says:
Hey man B.A. amp thinking about building one, but u wouldn't have a list of what u used as far as materials would you? plz ty!
Apr 9, 2009. 6:26 PMgmoon (author) says:
Naw, sorry. The closest thing is the schematic...
Apr 11, 2009. 9:47 PMNebraska G says:
KK thnx
Apr 2, 2009. 5:39 PMThe-X says:
Very nice instructable! You should make a higher wattage simple amp, as I'm hopeless in building my own design tube amps haha. I'm looking forward to building a head version of this with my audiophile father.
Apr 3, 2009. 5:27 AMgmoon (author) says:
Thanks! I've been collecting parts to build a 6L6 cathode-biased, push-pull amp. Probably about 18-20 watts...
Mar 8, 2009. 12:44 AMnoobamp says:
hey! Its been a while! after about 90 bux, the amp is great and all is well, maybe a little bassy for my liking, but thats fine! If you wanna hear it, check out this vid, the tube amp is nice and driven by my buddy on the SG..........http://www.youtube.com/watch?v=yWS_HVYN0ys

I have a query........If I were to buy one of those cheapy 110v to 220v 50W transformers (like for travel), run it through a volt doubler, and use it to power a 12ax7 and a 6v6, would it **theoretically** work as a simple champ amp? If I can get 440v outta the doubler and math out the required resisitors for the plates, shouldn't I be able to pull it off? I know I am not that smart, so somebody out there has actively refrained from doing it for a reason.....

Mar 8, 2009. 6:12 AMgmoon (author) says:
Nice groove. Since we were never sure the amp was designed for guitar, I'm not surprised it's bassy. You might look at the coupling caps (between each stage), and change them out. Typical values for a guitar amp are 0.02 to 0.005uF. If they are larger (0.04+), they'll boost the bass.

If you have room for two input jacks, maybe change the input circuit to a standard Fender "input pad." The input pad on my build is very similar--three resistors to form a simple "high" and "low" input schema. They work with the native capacitance of the tube itself, so there's no coupling cap before the first tube.

"Not that smart" is not having an original idea (or plowing ahead when someone tells you it's a bad idea ;-), so I'd have to say you're pretty intelligent.

But yeah, there is a problem with those euro power adapters--they are autotransformers , which don't isolate the incoming mains from the boosted voltage. For cheapness sake, they forgo a second coil. Non-isolated amps are considered unsafe.

But you can try a cheap 1:1 isolation transformer and a voltage doubler (or maybe even use it backwards: US output -> euro input for 1:2 ratio if there isn't too much current draw.) With a SS rectifier there's a 1.4 multiplier converting RMS AC to DC, so 240 AC = ~330 DC, which is enough voltage for a 6V6 output tube...

Mar 8, 2009. 9:57 PMnoobamp says:
You are as usual, right on it! good call on the autotranny situation ( my as of yet not burned down apartment thanx you!) I will definitely try out your idea......and I am sure that if it works, we may be looking at a sub 50$ build!
Jan 10, 2009. 9:24 PMchristophbruhn says:
What is estimated total cost for this build?
Jan 11, 2009. 8:06 AMgmoon (author) says:
Difficult to estimate, since I scrounged so much (power transformer, 12AX7, chassis, some components, wood, etc.)

The Edcor OT was $40 ($30 + shipping), the power tubes were about $3.50 per, maybe $20 in component cost, and then figure the cost of POTs, switches, pilot lamp, etc. (some of which I already had.)

It was under $100 USD...
Dec 23, 2008. 12:31 PMchipperr says:
How did you hook up “internal ground” to “chassis or earth ground”?

Sometimes these are connected with a large resistor and capacitor in parallel.
Dec 23, 2008. 5:49 PMgmoon (author) says:
They usually are connected, but only at one place (like a "star ground") to prevent any ground loops. How they connect depends on one's grounding scheme.

Aiken Amplification has a nice article on grounding.

Dec 12, 2008. 12:51 AMnoobamp says:
aahhhh! the hummmmm is gone! the motorboating is gone! ITS ALIVE!!!.....the sound is bad qual and muddy/bassy as hell, but the tubes seem to be fried, and the sound just proves it...

I was researching some stuff about class A push pulls (it seems like my amp is one) And it says that distortion is canceled out by the two tubes push/pulling..... I added a switch with 25uf/10uf cathode bypass caps between the two tubes before I read this....are the caps completely useless?
Dec 12, 2008. 5:49 AMgmoon (author) says:
Nice! Yeah, I suspect you need new tubes. That's not surprising.

Yes, push-pull amps inherently cancel some noise. But cathode bypass caps serve a different purpose than power filtering caps. They allow the AC component to bypass the bias resistor, which helps to keep the bias voltage constant.

A stage with cathode bypass caps is almost always louder than one without. And changing the capacitance value has an effect on shaping the sound. A larger cap increases the bass response of the stage.

Dec 15, 2008. 12:22 AMnoobamp says:
how much does the heater current matter in tubes? I am (maybe) going to ebay some cheapy 6bf5 tubes (an apparent substitute for the 6aq5, differing mainly in the fact that the heater current draw is something like 3x higher than the 6aq5's). the svetlana replacement guide says that they are ok replacements aside from the fact that the curr. is diff. It goes on to say that this wont matter unless you go too far (switch out too many 6aq5's to 6bf5's) and fry your power tranny. I am switching 2, so the draw is going from 0.9mA to 2.4mA....This is just me being cheap (4x tubes for a buck thus far), so I am fine nixing the deal and saving a bit of dough for some decent tubes (patience sucks)...Also is the heat going to melt my sockets?

Dec 15, 2008. 5:24 AMgmoon (author) says:
That's .9A to to 2.4A, right? (not mA.) Yes, it really does matter. It's doubtful that the power trannie can supply that much more current. If it was only .2 or .3A difference, I'd say go for it... It isn't worth the gamble.

Sockets themselves wouldn't be a problem.
Nov 25, 2008. 2:13 PMnoobamp says:
Ok...sorry about this again!....i purchased an old amp on ebay for 20 bux...It arrived, and after tracing out the complete circuit (so much squinting!) it turns out that it is a small push/pull with 2 12ax7's on the preamp, 2 6aq5's in the power, and some weird diode rectifier system...point being, if I were to take that apart, build up a simple or parallel 2x6v6 Princeton/angela (since the 6aq5's are so similar to 6v6's) will it work? I guess the question is: are pp trannies and output trannies interchangeable? or am I just going to have to build a pp princeton/deluxe?? thank you so much!!!
Nov 25, 2008. 7:32 PMgmoon (author) says:
A quick look at the datasheet confirms that a 6aq5 is very similar to the 6V6. The load resistance looks to be the same too (so the output transformer, PP or SE, would work with either type of tube.)

It's probably a push-pull OT--one of the 12AX7's is likely the phase inverter. Nothing wrong with PP, it's the norm for most guitar amps.

6AQ5's shouldn't be very rare...why not just use it as designed?
Nov 25, 2008. 9:34 PMnoobamp says:
as usual, you bring up a good point...in such case, Ill plug it in to see if it works to begin with, and worse comes to worse, give it a new cap job. I tried to test the continuity in the output tranny, and it seems that the B+ is completely continuous with one of the primaries, but not so with the other...should I worry about this or is it some weird tranny idiosyncrasy? Say that it is indeed shot on one side (possible?) could I still use the working side as a SE tranny? lol, you must think I'm a goof...
Nov 26, 2008. 8:41 AMgmoon (author) says:
Too bad, if the OT is shorted on one side. Yes, that certainly can and does happen.

No, it's not a dumb question. But you shouldn't try to use 1/2 of a PP as a SE OT, unfortunately. You might look into the Hammond 125 series (125E is 15 watt) output transformers. They aren't expensive, and you got the amp really cheap anyhoo...
Nov 27, 2008. 12:07 AMnoobamp says:
So I fired it up with a computer wire, and grounded the chassis...flipped the switch, and about 7 seconds later, I got this humming (with or without a speaker)....I got to see that the power tranny is kicking out 400v on the high volt primaries, and then killed it because the humming wigged me out a bit.....I did this a few times and then...nothing...no caps were charging, my multimeter went to zero.... it turns out the fuse had blown.....question is: I am assuming that one of the main caps blew a long while ago, and whoever chucked the amp did so because it kept blowing fuses... would a cap job fix this or am I way off? it seems like the power tranny is working just fine, 400v and all (I may have some questions about that if you care to listen), and the output tranny is at about 10k ohms on the side that was seemingly blown before...Where may the eerie humming come from? (sorry for the book I just wrote) thanks so much for your help!!
Nov 27, 2008. 6:37 AMgmoon (author) says:
Yeah, you've got a problem. Nothing you can't track down, however. Could be several things. Might be as simple as bad tubes. So working upstream from the mains:

All the usual safety precautions should be observed. Drain the caps, etc. Poking around in an amp is very dangerous. Remember, I'm not telling you to do this... (DON'T do it unless you're competent..)

-- Shorted PT. Replace the fuse, remove all tubes from the amp, power up and check the AC voltage at the primary. If it reads the same as your mains, that's good.

Then check the voltage at the secondaries, and be sure it's symmetrical... on each side of the center tap.

Obvious, blowing a fuse now means a bad PT.

-- Bad rectifier. Replace the fuse, put all the tubes back in except for the 6X5 and power up. If the fuse blows, that's not the problem.

If it doesn't blow, it could be the rectifier or a shorted filter cap.

-- Shorted power tube(s). Replace the fuse, reinsert the 6X5, remove the power tubes from the amp and power up. If the fuse blows, that's not the problem.

If fuse doesn't blow:

Before inserting a new power tube, check the bias resistors (I'm assuming it's cathode bias since it's a cheaper, relatively low powered PP amp.) If the bias resistor(s) are bad, one or more of the power tubes is probably shot, too. But check the bias before inserting a new tube, or you'll lose the tube...

-- Shorted OT. Pull the power and rectifier tubes and check resistance of each side of the primary (with power off.) Check each lead (prim and sec) to the chassis, in case the wiring is shorted to the trannie core.

-- Misc. shorts. The usual suspects--bad sockets, shotty repairs, etc.

Nov 27, 2008. 4:36 PMnoobamp says:
I will make sure to try that out. It seems like ill be able to nail it with no problem!

I am assuming that since my power tranny HV sec. are diode rectified as shown in the picture, the problem can't be a bad rectifier tube, so the issue is a bad filter cap or a bad set of tubes (all of which look clean, but ancient). Thank you for all your help! I am learning an insane amount of stuff from these chats!
power tranny rectification.bmp
Nov 27, 2008. 5:46 PMgmoon (author) says:
Sorry about the 6X5 tube rectifier reference (I've been corresponding about another project, and got the two mixed up.)

Your OT is still a suspect, but I hope that's not the problem. Cap, tube or bias resistor is more likely...

That looks a lot like a voltage doubler...which is a bit unusual, but not unheard of in commercial amps. But it's missing a cap between the PT and the diodes, so I guess the first diode is just spike protection.
Nov 30, 2008. 1:00 AMnoobamp says:
yeah, the rectifier is odd...Ive looked at (literally) dozens of amp schems, and have only seen 1 with a similar 2diode rectifier. I was thinking of maybe gutting the cap (60mfd 150vdc) shown directly after the diode (shown in the pic above), my reasoning being that the filter caps are already included after the second tap...diodeless....is that stupid/safe? I dont know if the cap is doing anything other providing an extra filtering stage aside from the "normal" few seen on most normal amps (which are one to three)....will the power going to the plates work normally (seemingly unrectified)? Or does this cap actually do something purposeful in the rectifying business? I was actually thinking of doing a complete cap overhaul (11 in all) and if you think that would be the foolproof way to go about things without too much tinkering, I'll take that route...

Thanks for your help!
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