Voltage on plate of preamp tubes

[alastair_in_oz]

I was wondering if increasing the voltage on the plates of the preamp tubes would increase the clean headroom. Is that right?

I feel that my amp is breaking up too early: it gets too overdriven before it gets to the sweet spot. The amp is a scratch built DR type amp with clean channel only.

Also what effect does changing the cathode resistor and capacitor have.

A general guide/rule of thumb would be great. Anyone point me in the right direction?

[loudthud]

Generally, there is an optimal plate voltage for maximum headroom with a given plate resistor and B+. Increasing B+ will increase headroom as long as plate voltage tracks within reason. The Fender usual values of Rp=100K Rk=1.5K tracks pretty well. Adjusting plate voltage by itself (by changing the cathode resistor) usually reduces headroom.

[tubeswell]

Try halving the supply resistor for the pre-amp plates (you need a dramatic shift in the supply resistors to get any noticeable lift), or better still trying halving the screen node resistor (which will have more effect). Just keep a weather eye on your screen voltages (you may need to add some 3W 470R screen grid resistors - or you might not)

[Enzo]

May I suggest this: find out WHERE in the amp the breakup occurs. I doubt it is in the first stage or two. FOr example, the PV C30 starts to greak up on the clean after about 4-5 on the volume control. This is clipping in the phase inverter. The preamp stages can handle 10 times the signal without bumping into the sky.

When you ask about changing THE resistor, it makes me thing you are focused on the input stage.