Vox AC30TB gain problem

[rf7]

Somehow, in replacing some parts, I lost most of the output level in the Vibrato/Tremolo channel. Does anyone know how the 3 channels compare in terms of output level? This is the new "handwired" version.

Shouldn't the Vibrato channel be as loud as the other two channels? Right now, it's very attenuated compared to the other 2 channels. I get a good amount of gain out of the first stage, the second stage does not change the level that much, and then when it gets to the 12AU7 stage, the output level at the plate is fine, but by the time it gets to the phase splitter, it's very small compared to the output of the other channels, for the same input level. Any ideas?

[Robert M. Martinelli]

Hi,
the vib/trem channel in an AC30 is indeed the stage with the lesser gain, so I would expect it to be somehow less loud ( The 12AU7 is not there as a gain stage, it's the vib/trem modulator ).

You stated you lost some gain on the vib/trem channel after changing some parts....no disregard intended, but....have you double checked the values of the new parts you threw in? Are they all on spec? I'm asking this because ( it's happened to me more than once ) it's easy to miss a dot on a cap or to misread a resistor's value. Time ago I had what I thought to be 10 K resistors, but when I measured them I found them to be 1K ones....I'm not colorblind, but the red they used for the third stripe was so faint and pale to the point it seemed orange.

Hope this helps

Best regards

Bob

[rf7]

The values on the parts seem to be OK, but I've found about 60 Volts dc on a section of ground plane on the front of the pcb, which was shorted to a capacitor in the circuit. I've got the capacitor short fixed, but need to find where the ground plane is getting the high voltage.

Thanks for your suggestions.

[Robert M. Martinelli]

The values on the parts seem to be OK, but I've found about 60 Volts dc on a section of ground plane on the front of the pcb, which was shorted to a capacitor in the circuit. I've got the capacitor short fixed, but need to find where the ground plane is getting the high voltage.

Thanks for your suggestions.

Again, I'd check for leaky caps, especially in the PS filter section, I don't know the amp's age, but maybe it's time for a recap job, electrolytic cap's lifespan's not infinite.

Hope this helps

Best regards

Bob