Voltages on Fender PI

[sbirkenstock]

Hi Everybody,

I wonder why Fender BF Vibroverb has such high voltages on the phase inverter. They are around 116 on the plate and 118 on the cathode.
For the Fender BF Bassman AA 864 I can hardly read the plan, but it looks like 78 volts.
Why are they not at or around ground level like the preamp tubes?

I could not find any Marshall Schematics that mention voltages.
Is it the same there?

Thank you,
Stephan

[dinkotom]

you mean 118 on cathodes and 116 on tail resistor?
http://www.thevintagesound.com/ffg/sche ... _schem.gif

[teemuk]

Why are they not at or around ground level like the preamp tubes?

Because the other half of the LTP is a common grid amp and takes its input via cathode.

If you bias cathode near, or to, zero volts then you limit the voltage swing on it to very low level and the LTP seizes to work as intented.

It's not a generic single-ended common cathode stage, so you can't design it like one...

[sbirkenstock]

Still, it´s mainly about the voltage diffence between grid and cathode.
Usually on a 12ax7 or 12at7 the grid is 2 volts away from the cathode.
Same at the vibroverb. The schematics say 118v at the cathode, 116v at the grid.
The 116 are shown at the tail resistor, but there is no voltage drop on 1 Meg resistor. So 116 as well on the grid.

Since the grid is fixed here, the cathode should change up or down 2 volts.
But it is set to 118 volts, so the cathode can swing from 116 to 120.
I don´t see a reason why it is so high.

How much is this on a JTM 45 or a JMP 50?

[matt h]

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