Tube Guitar Amp #1 (2012)

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Videos are at the bottom of this page.

As of April 18 2012, my first D.I.Y. tube guitar amp appears to be functioning normally. I made a huge mess and a lot of mistakes with it but it came out fairly well for a first try, and was definitely worth the time and expense because I learned quite a bit.

hta1_roughHere is a schematic that’s more or less like what I made, which is based on the AX84 designs (see ax84.com, a website for tube-amp enthusiasts). This schematic is incomplete and has some mistakes in it – the tonestack is not represented and the filament supply of the Hammond 270FX power transformer should marked 5 Amps, not 3 Amps. The transformer, a Hammond 1650F in this case, is matched better to a pair of 6L6’s than to EL34’s, and the poweramp section isn’t right for a pair of 6V6’s, which start red-plating right away.

See below for why the tonestack is left out. If you want to see tonestack schematics you can probably bring up 500 of them in Google in about 20 seconds.

ta1_interior3   ta1_interior2

The vertically-mounted perfboard is for the the power tubes’ negative bias voltage, which I wound up not using because I opted to cathode-bias them instead.

One of the dumbest things I did was to cram most of the DC filter capacitors onto the turret board. I should have used dual can caps for all the filter-cap stages instead of just the first two, so all the ground lines could be run straight back to the star ground.

ta1_underside   ta1_interior1

Since the poweramp is cathode-biased, I don’t have to adjust a negative supply voltage every time I change tubes. I’ve been able to load EL34’s, 6L6’s, and 6V6’s, all with very distinctive results, although I can’t keep using 6V6’s because they overheat in this circuit.

The EQ is not a standard 3-band stack. The bass control is just a high-pass filter, the mid is a twin-T notch, the treble is a low-pass filter. And they’re wired in series, respectively. I’m not thrilled with the EQ but a standard 3-band Fender-style EQ sounded awful with the overdrive stage, probably because of impedance mismatches and certain sections I didn’t wire up all that well.

The tubes in the photos are 6V6’s but I stopped using them because they red-plate (overheat). I used EL34’s for the videos but I’ve been using a 6L6 pair lately because they give me a better low-end response than the EL34’s. That isn’t surprising since the impedance of the output transormer (about 7.7kOhms) is way out of whack with an EL34 pair would expect (3-4kOhms would be better). I’ve got the amp wired for a single 16-Ohm output because at this point all I’ve got is a single 16-Ohm speaker.

ta1_top1   ta1_top2

ta1_back1   ta1_back2

Crunchity-crunch, with a Gibson SG:

Medium/high gain, with a Gibson SG:

Sort of clean, with a Fender Strat:

Medium/high gain, with a Fender Strat:

One thought on “Tube Guitar Amp #1 (2012)

  1. Mike

    I just bought a Les Paul Special II but I am not sure which amp to go for. Seeing that you built your own, do you maybe have some advice and suggestions. I want to go a little bigger so size do matter.

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