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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hbohlius7 months ago

Harika!

gmoon (author)  hbohlius7 months ago

Song!

msholden7 months ago

Hi gmoon this looks real good

gmoon (author)  msholden7 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.

Benjamick7 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)  Benjamick7 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 gmoon7 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)  Benjamick7 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 gmoon7 months ago

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

gmoon (author)  Benjamick7 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 gmoon7 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)  Benjamick7 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 gmoon7 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)  Benjamick7 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)  gmoon7 months ago

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

Benjamick gmoon7 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)  Benjamick7 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 gmoon7 months ago

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

gmoon (author)  Benjamick7 months ago

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

Benjamick7 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)  Benjamick7 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 gmoon7 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)  Benjamick7 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 gmoon7 months ago
Thanks.
By the way, your amp souded real nice.
Good job.
Benjamick7 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.
Benjamick7 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
Benjamick7 months ago
Thanks.
I was thinking of using 12AU7s for the preamp stage, and i thought of an EL84 'cause i saw the schematic of the LXH2, which uses two EL84 at low voltages.
I also considered using 20 or 30v to get more power. How about that ?
gmoon (author)  Benjamick7 months ago
I'd never heard of the LXH2, but I eventually found the schematic. If others say it works and sounds good, there must be something to it...

Sure, why not try two 12au7 stages in a valvecaster type setup? Unless "all tube" is important, I'd probably leave the two opamp phase inverter of the LXH2 intact--those two opamps are unity-gain anyways. Just replace the two preamp opamps with the tubes.

Otherwise you'll have to figure out a phase inverter that works at low voltages (again, you could look to the low voltage tubes, schematics of car radios, etc, from that period). Might be easily done with another 12au7 at low voltage, or it might be more difficult (the PI gain needs to be at least unity). You won't know until you try.

Any small push-pull output transformer with a 6K input impedance would work fine here. Should be cheaper than a typical EL84 OT.
Benjamick8 months ago
Hi, gmoon.
I'd like to build a tube amp, but i know little about electronics, and i'm scared of high voltages, so i designed a 12V amp.
Would an EL84 work at 12V ?
And how do i calculate the output power ?
gmoon (author)  Benjamick8 months ago
Hi, didn't see this earlier.

I doubt you'd get much at all from output tubes at 12V. It kinda works with some preamp tubes, but then not too much above unity.

When figuring output power, I look to the tube data sheets--they usually list several configurations (SE, PP) at a handful of voltages. Unfortunately, not for low voltage operation.

You could look into low voltage power tubes--they were used in stuff like car radios.
gmoon, or anyone that knows:
What is the list of materials/estimated cost of building this?
begooder131 year ago
gmoon, or anyone that can help:

I've been working on my version of this amp for some time now. It's so close to being done but I have this issue. See the video link. http://youtu.be/CWk6nhGLEV0 I'm all new to this (I'm actually a drummer, not a guitarist and a mechanical engineer, not electrical) but I have enjoyed learning and creating something really cool. Any help is greatly appreciated!
gmoon (author)  begooder131 year ago
I can't quite hear where you say you're placing the DMM probe, but I suspect you might have some high freq oscillations, parasitic or otherwise...the probe could be drawing off the high frequencies. Maybe a place a 150pF cap to ground at that point??
I was touching the probe between the power supply and the power tube plates. Is it ok to connect the cap from this point to ground?
gmoon (author)  begooder131 year ago
If it's HF oscillation, it could be happening in an earlier stage. I'd first try a small cap to ground somewhere before the power tube grids.

But to tame it at the power tubes like your probe, which could be acting like a conjunctive filter-- a cap (or a cap & resistor in series) parallel with the output transformer primary. That's basically a cap between the power tube plates and the B+ voltage. Google "Dr. Z conjunctive filter".
knexp901 year ago
Hey! I have been looking at this project for about a month now. Seems legit. I want to use your schematic for a science fari project comparing low wattage tupe amplification to solid state. It's going to be hard to keep unbaised, seeing how yours is performing. I just ordered 2 12AX7's some 6v6GT's and a 5Y3GT was thrown in. should be here soon. The one thing I am a bit concerned about finding are the input and output transformers. I don't really want to drop 30 bucks a pop for them, and was wondering if salvaging may be an option in this case. Is there any other transformer i can use for either? I really could use some advice. Everyday i look at this. SO excited about the build. Cheers!
gmoon (author)  knexp901 year ago
Thanks! I think if you're planning to go with 6V6's, then you either want to clone a Fender Deluxe, or if you stick with the parallel SE topology, you should look at the Angela Super single-ended amp (search for it, Angela.com doesn't seem to have a direct link anymore). My amp doesn't use the 6V6GT tubes.

It's probably harder to find transformers for this project than to find typical tube transformers...I built this partly around the power transformer, but the output transformer is definitely an oddball one, too. That's why I documented it so well, to explain the design decisions.

The transformers will always be the most expensive part of a tube amp. If you're planning on using a tube rectifier, you'll need to use the equivalent of the Fender Deluxe power transformer (three secondaries--HV, 6.3V and 5V).
knexp90 gmoon1 year ago
Thanks for the response. Looked around and found the "Magnatone varsity deluxe model 108" from the mid-fifties. uses 5y3 for the rectifier, a 12AX7 preamp, and a 6v6GT for the power tubes. Thanks again!

The hardest part about this project is still going to be the power and output trannies. Going down to the local thrift stores to try to see if i can find a suitable replacement.
gmoon (author)  knexp901 year ago
Sure--if you're not just looking for a parallel single-ended projects, there are TONS of single-ended amps that utilize that tube set.

The most common and obvious one would be a Fender Champ. You'll find all sorts of available transformers for a single 6V6GT...not to mention component layouts, if you check around.

When I answered earlier, I just assumed you were looking for the same topology as this project. You have much more freedom (and commercially available components) when using just one 6V6GT as the output tube...
knexp90 gmoon1 year ago
Hey, thanks for the help you have been giving me, you'll have a special spot in my report. :) I have but a few more for you, however. I tore apart a 1981 modular component systems model 3275, and got a 15 pound power transformer with six outputs. has no markings on it other than a bunch of random numbers. none of them pull up anything. there was no output transformer. What should i be looking for on the market output transformer wise? Also, how would i know what watt resisters i need. it's a low watt amp, so would i need to be ordering online, or do you think a local radio-shack would carry what i need. I thank you once again for helping out a newbie. Cheers!
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