Decreasing headroom...on purpose

[Wilder Amplification]

Best is to mod the amp so that the variac only controls the plate voltage, while everything else gets the normal voltage.

Um...on an amp with a single power transformer just exactly how would you mod the variac to only control the voltage of one winding?

And wouldn't you also want to alter the bias voltage to keep it optimum for the lower plate voltage?

[d95err]

It sounds awkward to mod an amp to use a variac for only the anode voltage, when you can do the same thing with a mosfet and a handful of simle components. Powerscaling is easy. Especially in a cathode biased amp where bias adjusts itself automatically.

[nedhogan]

If those caps your on about are filter caps then switching back to the 5y3 should be fine in my opinion.
Id switch it back if I was you but you could raise the plate resistor from 100k to 220k on V1, that adds more break up but its a different flavour break up to that produced by the PI or the rectifier. Original Vox's had 220k in that position anyway.
Lowering the B+ for the sake of a rectifier seems a little extreme to me.

Just noticed how old this thread is... I must be tired!

[Chuck H]

If the amp sounds better with the upgrades, and you just want breakup at lower volume, do this:

Easy and inexpensive to make. More versitility than just lower headroom. Can be built in a small box or on a plate and bolted onto the inside of the cab. EDIT: Oh... The resistors are the inductive aluminum housed jobbies as made by several MFG's. Mount them on aluminum with heat sink paste.

[Regis]

I agree with Chuck. Build his posted schem or one of my DIY attenuators will work. It will handle the power of an AC15 no problem.

In fact, I built a 16 ohm version for that AC30 I bought from you, Will, it sounds great.

[Chuck H]

Regis,

Have you ever scoped the waveform from the output of an L-pad attenuator? I had one once and it sounded a little odd to me. So I scoped the output and it looked sort of like this when the amp was overdriven:

[Diablo]

Regis,

Have you ever scoped the waveform from the output of an L-pad attenuator? I had one once and it sounded a little odd to me. So I scoped the output and it looked sort of like this when the amp was overdriven:

Wow....I bet that attenuator sounds pretty titty.....