Voltage clarification

[Tdale]

The plates in the preamp and OD section, are about 200V. Does that mean that the voltage measured across the plate resistors are about the same? Or are those voltages much lower?

Tommy (don't have any amp to measure on right now)

[Darkbluemurder]

The plates in the preamp and OD section, are about 200V. Does that mean that the voltage measured across the plate resistors are about the same? Or are those voltages much lower?

Tommy (don't have any amp to measure on right now)

The voltages on the plates depend

a) on the B+ at the node to which the plate resistor is connected (higher B+ = higher plate voltage) and

b) on the value of the plate resistor (higher resistor value = lower plate voltage as the voltage drop across the plate resistor increases).

So the voltage drop across the plate resistor is the same ONLY if

a) the B+ is the same AND

b) the plate resistors are of the same value.

Everything else would be pure coincidence.

Cheers
Stephan

[Tdale]

Not sure if I got that... but let me explain what I mean..

If I have a 220K plate resistor on one of my preamp tubes, and measure the votage between ground and plate to be 200V.

Is the voltage across the plate resistor roughly about the same, or are we talking totally different voltages?

I'm asking because I was about to buy some resistors, but then noticed that they had a 250V rating, and I started wondering..

Tommy

[Elcabong]

The voltage across the plate resistor is B+ minus the plate voltage. So if your B+ is at 275V and you have 200V for your plate voltage, 75V is left across the plate resistor.

These values will change as the signal is applied.

[Tdale]

OK.

so 250V resistors will usually be more than enough for preamp use...

Tommy

[tonelab2]

Tommy have a look inside a fender reissue bassman and you'll see 1/8watt and 1/4watt CF resistors in the preamp section. I've just completed a repair on one for a customer and was astounded at the size, so tiny. Of coarse the plate resistors were toast (but that's Fender) I always like to use 1watt on the plates (and everywhere else for that matter) because of the lower noise floor (hiss ect) and draw the line at nothing lower than 1/2watt for that reason. My 70s D clone has mostly CCs and is has less noise than that bassman, I wouldn't recommend CCs
I only used them because I had 'em and they look good but are not really suited to a D amp as you know.