Please explain resistors at power amp section

[shocki]

I have been looking at several Fender blackface schematics. What I realize ist that there are all very similar.

If you take this schematic.
http://www.thevintagesound.com/ffg/s...b763_schem.gif

You have a 1k and 4700Ohm resistor building some voltage divider that delivers different voltages to different parts of the amp. In other examples (for example Pro Reverb) you have different values. Also depending on the revision (Silverface)

Basicly those to resistors deliver different voltages to the preamp section and PI is this correct? Or what is the purpose and why are exactly those values chosen?

Thanks alot...

[kleuck]

They are here to improve the efficiency of the filter caps (RC Ripple Filter Calculator) and to separate the stages (less sag from the pre when the power stage is pumping on the PT)

[exclamationmark]

This should answer all your questions (only because explaining it myself would be just as long). Pay attention to the "Pre-amp supply" section.

The Valve Wizard

[shocki]

Thanks for the info. I forgot to mention that of course acts as a filter also. But what it also does is drope the voltages in the preamp stages. I will read the rest in detail later
How to choose a resistor is explained here.

The value of resistor depends on the voltage you want to drop for the screen-grids. The current through the resistor will consist of the screen current plus the current drawn by the pre-amp (22m and 23mA making 45mA total in this case).
If we wanted the screen voltage to be 300V we need to drop 350 - 300 = 50V. Use Ohm's law to find the value of resistor:
50 / 0.045 = 1111 ohms.
So we would use a 1k resistor (wire-wound resistors are commonly available only in the E6 range so bare this is mind.) This would actually drop 1000 * 0.045 = 45V. The average power dissipated in it will be:
(0.045 * 0.045) * 1000 = 2W So we would use a 5W resistor or better.

But how do I know how much voltage I should drop? Is this something I should experiment with? I am thinking about building a simple Blackface (one Channel clone) with a Vibrolux Output section (about 35W) I looked at different power sections and found different values.

BTW this looks great.
http://www.valvewizard.co.uk/Book2.html

I already have his preamp book which is awesome.

[MWJB]

"But how do I know how much voltage I should drop? Is this something I should experiment with? I am thinking about building a simple Blackface (one Channel clone) with a Vibrolux Output section (about 35W) I looked at different power sections and found different values."

Will your one channel be "Normal", or will it have Reverb & trem? Your preamp tube will pull a shade over 1mA per plate, twice that for the 12AT7 PI tube plus whatever reverb recovery pulls. I'd stick with 4.7K after the choke and adjust the last resistor in the chain to give around 200vdc on the V1 & reverb recovery tube plates (or 300vdc at the B+ rail feeding the V1 plate resistors)? Anything from 220v to 270vdc might be considered normal for a BF style amp, I'd aim for the lower end seeing as you seem to want to start with lower B+. Perhaps 15K would do it for reverb & trem? 27K for a Normal channel....you don't have to be accurate to the volt +/- 20vdc is plenty acceptable.

[shocki]

Yes just the normal channel without reverb and trem. So 4.6k and 15K? Is this more of a "feel" thing than an actual sound thing?

[MWJB]

Yes just the normal channel without reverb and trem. So 4.6k and 15K? Is this more of a "feel" thing than an actual sound thing?

No, I'd start out at 22K or 27K for just a normal channel.

Fenders don't run to the volt as quoted in schems...in fact pretty much no amps do, until you get to small scale bespoke builds where the builder can tell you the volt & milliamp. Preamps draw very little current compared to power tubes and it is very hard to hear deviations in preamp plate voltage of less than 30vdc (untill you get down to the sub 150v mark). So pick a voltage (200vdc on V1 pins 1 & 6 would look to be ball-park, if you got say 220vdc, go with it & see how you like it) and play with that last dropping resistor until you hit it. Then live with the amp for a while, if you want it a bit brighter & cleaner reduce the value of the ??K. The actual resistor value is incidental, a means to an end in hitting the 12AX7 plate voltage you want.

We use the real things to make actual sounds that give us the feel thing. Some might prefer sub 200vdc for a browner sound, others might like 250vdc or more for more brightness & edge...ear testing is key here.