TheRadioBoard

Hi, here's a small bench amp I built for use with an 8 ohm input.
A 6AQ5 driven by a 12AX7 (both sections parallel). I cheated and used an 8:1200 ohm input transformer so I could get decent gain with only 2 tubes. I used a resistor and a red LED as a pilot light and wired it to the 6.3v heater line.




Here's the schematic.



Here's the underside. I tried to make this project fairly neat.



Here's the rear view.




It gives me 2.5 watts driving 5 speakers in my shop nice and clean. More than enough volume.

Doug

Nice amp hergi. . I'm using a LM386 myself because it is easy to change between different gain settings and so on with that IC..
But doesn't that input transformer put too much load on, let's say a one tube regen? I'm only asking...

Yes. I tried it without an input transformer and it just didn't have enough gain to use on a regen. So I put the transformer back on and just it on lower impedance devices. It works fine with 100mv or more input. A 6AU6 gives much more gain than a 12AX7 so I use that on my regen.

Doug

Regarding red LED and resistor.

If you hooked it directly to 6.3 V rms, that's 9 volts peak reverse on the LED.

LEDs may light nicely, but they aren't good rectifiers. I usually use a 1N4001 in series (or you can put it in anti-parallel) to remove the reverse voltage from the LED.

On AC, you can also hook 2 LEDs in anti-parallel, so that one of them is always conducting and the other only sees about 1.5 V reverse.

Your LED probably will work, at least for a while, with -9V peak, but it's safer to eliminate the reverse voltage. LEDs are usually rated for 4 or 5 V reverse.

Rich

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Homebrew Radio ex-Silicon Valley

Nice build...

Usually in a amp, the 12AX7(or other dual triode) is configured so the first section is a preamp, with a gain control between it and the second section...

Tom

"LEDs may light nicely, but they aren't good rectifiers. I usually use a 1N4001 in series (or you can put it in anti-parallel) to remove the reverse voltage from the LED"

Thanks for the tips. I put a 1N4002 in series with the LED to eliminate that future problem.

"Usually in a amp, the 12AX7(or other dual triode) is configured so the first section is a preamp, with a gain control between it and the second section... "

I tried that and I had way too much gain with the 12AX7. I will try it again with a 12AT7 or 12AU7 wired like you say. I just winged it with this amp by using generic schematics from my tube manual and stuff I had on hand. I don't especially like using input transformers but it's an easy cheat
for a gain stage.
Doug

Possibly you would not need the transformer if it were configured with the two triodes in series??? If that didn't work satisfactorily, I'd think a 12AU7 would be fine in series, it only has about 20% of the gain of a 12AX7... There are also tubes such as the 6BK7, 6BQ7, 6BZ7 etc, that everyone seems to overlook...

Tom