Instructables
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)
 
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JeffC424 days ago

I like your style. I read alot more than I understood, but nice job. And nice sound too. I've been working on alot of ideas of my own the past few weeks. I have noted the trend on the lower watt tube amps, and the high dollars they expect. bleh... gimme a junk pile and a few weeks on the internet. ;) well done. cab reminds me of an old gibson blended with a marshall cutout for the controls. tastey.

gmoon (author)  JeffC422 days ago

Thanks man :-)

I'd like to rebuild this with a different power transformer; clean it up, update it. Most of these low-wattage amps nowadays don't have enough headroom for me...

JeffC4 gmoon9 days ago
I am on a mission myself to try to put together something truly affordable and somewhat easy to assemble, yet usable as a guitar amp.
The cost of tube amp kits and projects are just too insane for my taste right now, plus, Many of us have seen this before. They come in vogue, price goes thru the roof and then the bottom drops out when players start to see the cost and limitations of tube amps. Just bring a nice hot one out of a warm building into the freezing winter air up north once.
Anyhoo. solid state it is. But there really isnt anything out there put into a package a person could one stop shop and solder together, at least not at a realistic price. like you said, low wattage is pretty common among what is out there.
at this point, I am looking at a 60 watt post amp board preassembled and coupling it with some tone pre-amps of my own making or of the effects pedal variety reboxed into a classic style cabinet. alot to learn here for me, I'm no engineer, but i do have a concept and the interweb.
So far I have soldered up a tonemender board, using scavenged parts from old power supplies, modems, whatever was around, and had a few 4558 ic's in my stash along with a pack of new resistors. yet to get my pots and hook it up and to be honest, the layout was just too tight for my liking, and suspect i may have overheated and probably have some solder bleeds. so wish me luck there.
So jump in this and get us 100 watts of highly affordable amp project with pcbs for sale and a bag o parts.(less transformers and chassis and cabinets, or make those extras)
or is it just impossible to source out enough parts to build a good amp for less than a good used amp?
gmoon (author)  JeffC48 days ago

That's a good question...might be equally applicable to tube amps as well as SS. At this point, I've fixed several multi-channel tube amps; amps that were much cheaper to buy busted (and fix) than it would cost to purchase the parts alone.

For SS amps, I just gave my young cousin an old Harmony B3500 amp. That amp (working) with the cabinet (not working) only cost me $22 USD. And I recently bought an working PA (100 A) with reverb and EQ that sounds pretty good with guitar and stompboxes. For $50.

I can't turn my back on DIY, but unless some of the parts can be found used, there isn't much financial incentive. And modular amp building seems pretty inexpensive--finding manufactured power amps, etcs., then slapping a preamp module to it.

With the "reuse" ethos, it's still possible to make cool projects cheaply. But some of those are a little hard to duplicate...(that's why I'm always looking for junked amps, transformers, etc).

Interesting project, I've built several hi-fi tube amps in the past, but now only work in silicon.

Anyway, you might want to look int the 6W6GT as output tubes.

Same basic specs as 50L6 or 6DG6, but with a plate voltage rating of 300V.

You might be able to use a higher plate supply voltage with a more common transformer voltage - and get a bit more power output to boot.

Just some food for thought.

how do you drain the electricity from the amp

use a light bulb on the caps

It is always best to drain the charge off of a capacitor through a properly rated power resistor than to dump it all of a sudden, it could cause unintended consequences.

gmoon (author)  titsanonymous6 years ago
The current is stored in the "filter caps," which are connected after the power supply rectifier. Essentially, you need to drain those capacitors; shorting them by connecting the leads together, or by connecting the positive (+) cap terminal to the GND.

There's more info here, on my amp rebuild project.
n0ukf gmoon6 years ago
If you short across the caps with a wire (or tool) you'll get a high current snap as it arcs. If you use a resistor (perhaps 1K, at least 1W) it'll take a little longer to drain but it'll drop the arc intensity.
gmoon (author)  n0ukf6 years ago
Thanks! (the link above also mentions using a jumper / resistor combo:)

-- OR jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here...

It is the Amps (current). What I mean is it is the amount of current that you push through your body. If your body resistance is low enough and the voltage you come into contact with is high enough to cause your body to conduct or pass as low as 100mA, it could possibly kill you, or at least do damage. In other words it ends up being the amount of power your body dissipates. generally you can get a pretty good sensation out of as low as 5mA. The Ohms Law relationships should be understood.

gmoon (author)  rene.stover.91 month ago

It's both amperes and voltage--which is pretty much as I read your comment.

High-voltage static electricity won't kill you, not enough current. Low-voltage 1.5V batteries can supply well over 1 amp in short circuit, but not enough voltage to overcome skin resistance.

Correct, Amps times Volts= Watts. It is the power dissipated that does the work in any type of body!
hbohlius8 months ago

Harika!

gmoon (author)  hbohlius8 months ago

Song!

msholden8 months ago

Hi gmoon this looks real good

gmoon (author)  msholden8 months ago

Thanks, mate. It's a real rat's nest from all the changes, but still sounds pretty good. Wanna make some more "original" designs someday, but there's always an old project amp here to fix.

I think I've commented on your stuff before. It's good stuff.

Benjamick8 months ago

Here's another question: does the speaker power rating have to be the same as the amp's, like, could i use a 15W speaker even though my amp probably won't output 15W ?

gmoon (author)  Benjamick8 months ago

So long as the speaker has a higher power capacity rating than the amp it's OK.

You shouldn't use a 15W speaker with a 30W amp, that would blowup the speaker (if the amp were turned up). The standard approach is that the speaker have a wattage capacity as large as the peak output power of the amp...for tube amps that's usually twice their normal power rating (so a 15W amp requires at least a 30W speaker).

For amps with much lower output power than the speaker--no problem. People are always plugging in their little micro "Smokey" amps into 200W Marshall cabs, and that works just fine....

Benjamick gmoon8 months ago

Ok, now i've got a problem: the max plate dissipation watts of the EL84 is 12W, so if i do Va/(Pa/Va) running my amp t 20V, the OT primary impedance should be about 33 ohms. I don't think i can can find anything like that, but i found a 1500 to 600 omhs OT, so do you think i could use that with a 12 ohm speaker, or like a 8 ohm speaker and a 4 ohm resistor in series, or something like that ? (1500:600=30:12)

gmoon (author)  Benjamick8 months ago

Yeah--not totally certain if that formula will work at these low voltages, but every time I run the numbers for an amp it's darn close to the spec'ed impedance. So it's certainly worth using as a starting point.

It's a little crazy but I'm not certain you need an OT--two 16 ohm speakers in series will yield a 32 ohm load resistance. Might work just like that, wired as the load...

Benjamick gmoon8 months ago

Yeah, but i wouldn't get as much volume, right ? And it's certainly cheaper ($2.25)

gmoon (author)  Benjamick8 months ago

The purpose of an output transformer is just to match the high output impedance of the tubes to the low impedance speaker. For devices with low output impedance (such as transistors), no OT is needed. If the impedance of an EL84 @ 20V really is ~33 ohms, there isn't much point in using one.

Certainly the best load would probably be found by experimenting with a various speakers (8,16, 32, 64 ohms) or even w/ the transformer you describe. The formula could be off at these voltages...

Benjamick gmoon8 months ago

I heard that the lower the speaker impedance the more volume you get, but i really don't know (do you?). If it doesn't make volume difference i'll do what you suggested, but speakers are kinda expensive, so if i use, say, an 8 ohm speaker and 24 ohm resistor or something like that, would it work (an sound) as well ?

And if the formula goes off at low voltage, could it go off enough to damage the amp or other negative effects ?

gmoon (author)  Benjamick8 months ago

"I heard that the lower the speaker impedance the more volume you get"

This is true for solid state amplifiers, not for tubes. Solid state amps deliver more and more power with lower load impedance until they short out/melt down (or the internal protection, if any, kicks in and silences them until they cool).

But there's a "sweet spot" for the load resistance of a tube--anything higher or lower still works (to some extent), but doesn't yield as much power. There's a load R vs. power output graph in this very instructable (see it here). The graph shows the max power/min distortion at ~3500 ohms, and the power drops off above and below that value.

Of course you should never run a tube amp without a load (or as a dead short), but for a low wattage amp like yours, anything in the general neighborhood would probably work... Stay away from the extremes and you probably won't harm the tube.

If you place a 24 ohm resistor in series with the speaker it might approach the "ideal" impedance, but it will consume current that you'd want to reach the speaker and act as a voltage divider (speaker will get 1/4 the output voltage), so you'll loose LOTS of power/volume. It would in fact be acting as an attenuator.

Just dig up some cheap speakers at a thrift store or tossed radios, etc., to test it. You might get lucky and like how they sound.

Benjamick gmoon8 months ago

OK, i'll use speakers to match the impedance. But this brought up to another problem: i wanted to make sure i don't use speakers with a lower wattage then the amps' wattage. So i looked for some of those EL84 loadline things and i found some, but they were all completely different.

Is there any way to know which one to consider ?

gmoon (author)  Benjamick8 months ago

I don't think an EL34 @ 20V is going to produce much more than a watt. And speakers in series share the current, so don't worry about the individual wattage per speaker, look at series resistance.

I'm not sure you can draw a loadline with the existing graphs, 0 to 20V would only occupy a small sliver of the graph.

The Load imped. vs. wattage graph I linked to is for a specific voltage (see the little inset--plate V = 200V, screen = 110V). Even at low voltage I'd expect the output graph to have a similar shape. But you'll never find a similar graph for 20V, that's way out of the designed operating voltage for almost all "normal" vacuum tubes.

Most datasheets have examples of operation in typical topologies. If you look at the datasheet for the above tube (6dg6gt), you'll see SE operation at 110V and at 200V. As expected (and you've formulated), the load resistance for those two operational points is 2000 ohms and 4000 ohms.

I.E., as you've come to suspect, the load resistance drops considerably with the plate V.

You gotta remember you're in uncharted territory here. No guarantees; the next step probably is experimentation...

gmoon (author)  gmoon8 months ago

Sorry, meant to write EL84, not EL34...

Benjamick gmoon8 months ago

So, is your point that i should just try out speakers without worrying too much about their wattage ?

Oh, and about the loadline differences, i figured out the problem was that i didn't consider the screen grid voltage (sounded weird that EL84s could output 2 watts at 20v!)

gmoon (author)  Benjamick8 months ago

Yep.

Well, pentodes do have different characteristics than triodes, and might yield more wattage that I originally thought. But wattage is V x A, so don't expect to get too much due to the lower voltage.

Benjamick gmoon8 months ago

Ok. The speaker should be connected from one side to the plate and from the other to the ground, right ?

gmoon (author)  Benjamick8 months ago

To the B+, like the way a SE OT functions.

Benjamick9 months ago
I tried posting the schematic, but it was really small and you couldn't see anything, so i'll tell you:
it's 4 12AU7 preamp stages (yeah, a valvecaster type setup), all with 100K plate resistor. The third stage also has a 250K plate POT. There's also an EQ and a gain control after the second preamp stage.
For the biases, the two cathodes of each valve have a 25K POT and a 2.2uF (should i double the values since it's 2 cathodes ?).
The signal caps are all 0.02uF, except for the EQ, of couse.
For the output i thoght an SE configuration using a EL84 at 20V or a 12AL8 at 12V, with a 1K resistor for the screen grid.
I put a 1M pot between the output valve and the OT as a Master control.
gmoon (author)  Benjamick9 months ago
Four gain stages?  ;-)

Not sure if you'd want the cathodes of each 12au7 to share the same cathode bias resistor (pot) or not. A pot on each cathode might give you more control. But the shared resistor could work well too.

An additional gain/volume control could be inserted after the first stage. Lotsa high-gain amps do that, call it Gain, OD, Volume1, whatever.

A possible alternative preamp might be three gain stages, then a cathode follower driving the tone controls... Or two gain stages, cathode follower / tone, one recovery gain stage. A CF Isn't necessary, but many high-gain amps that use lossy tone stacks like the extra current a CF can supply for the tone.

You couldn't tie the cathodes together with a single cathode bias resistor if one is a follower. Anyway, just throwing the CF out there...

I'm not certain what value screen resistor would be needed. Those are there primarily to limit screen current. But how much current will an el84 screen grid draw at 20V? Not much (the LXH2 doesn't have one). Since this is a low-power output stage, it might be fun to use a 1K rheostat (1W to 5W) here, as limiting screen current is one way to reduce volume while keeping some hair on the output. Don't know how effective it will be; might only be usable in a small range of adjustment...
Benjamick gmoon9 months ago
I didn't want to do four preamp stages to get LOTS of gain, but to get a decent amount of gain (about as much as yours) AND 2nd order harmonics and all that nice typical tube sound. I don't think i want to add any other pot. There are seven already, and the 250K pot on the third triode plate should be kinda like a gain control, right ?
Anyway, i'll take the screen grid resistor away.
gmoon (author)  Benjamick9 months ago
Whatever works... One can always use trim pots for the cathode bias, if you want to tweak the design but not clutter the panel with POTs that are only set once.

Seems I recall the second triode in the valvecaster has the cathode grounded. I've always assumed it's done like that so that the two stages clip differently, as high-gain amps often alternate the cathode R and plate R values. Or maybe the first triode stage has the largest effect on the sound. With four stages, maybe a panel bias pot for the first tube stage, and trim pots for the next three? Or just ground the next three? Play with it...

You could also add a DPDT to switch in / out a stage or two, for a low / high gain channel effect. In that case, you would probably need a gain pot within the high stage as well, since the high gain channel volume would need to be  tamed relative to the clean.

Anyway--Good Luck!
Benjamick gmoon8 months ago
Thanks.
By the way, your amp souded real nice.
Good job.
Benjamick9 months ago


Benjamick says: 1 second agoReply

I tried posting the schematic, but it was really small and you couldn't see anything, so i'll tell you:
it's 4 12AU7 preamp stages (yeah, a valvecaster type setup), all with 100K plate resistor. The third stage also has a 250K plate POT. There's also an EQ and a gain control after the second preamp stage.
For the biases, the two cathodes of each valve have a 25K POT and a 2.2uF (should i double the values since it's 2 cathodes ?).
The signal caps are all 0.02uF, except for the EQ, of couse.
For the output i thoght an SE configuration using a EL84 at 20V (is 20V still too low ?)or a 12AL8 at 12V, with a 1K resistor for the screen grid.
I put a 1M pot between the output valve and the OT as a Master control.
Benjamick9 months ago
I was thinking of doing an SE configuration, so i think i'll just do an all tube amp using one of the tubes from that site you told me about. Could i show you the schematic i worked out so you can give me your opinion?
Thanks
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