HT on cathode? CHB 35

[Tonewood]

On the Bogen CHB35 schematic the 12AX7, V2A & V2B have the cathodes connected to 240 volts via a 220k resistor. Can anyone explain what this is for?
Thanks
bogen_chb35a.pdf

[g1]

Maybe an error? I don't see why they would want to send DC to the speaker via the feedback network.

[Tom Phillips]

On the Bogen CHB35 schematic the 12AX7, V2A & V2B have the cathodes connected to 240 volts via a 220k resistor. Can anyone explain what this is for?

Note the 680 Ohm and 470 Ohm resistors shown in the circuit. Together with the 220k resistors they form voltage dividers and when you do the math it shows that there would be 0.74V on the V2A cathode and 0.51V on the V2B cathode. Therefore, it appears to me that the setup is a rare implementation of a hybrid bias scheme. Since there is also current flowing through the tube the actual voltage would be affected by that current. It's certainly a strange circuit.

[Tonewood]

Note the 680 Ohm and 470 Ohm resistors shown in the circuit. Together with the 220k resistors they form voltage dividers and when you do the math it shows that there would be 0.74V on the V2A cathode and 0.51V on the V2B cathode. Therefore, it appears to me that the setup is a rare implementation of a hybrid bias scheme. Since there is also current flowing through the tube the actual voltage would be affected by that current. It's certainly a strange circuit.

Thanks Tom, that seems to be what is happening. Wonder why this method instead of just the appropriate cathode resistor.

g-one, the schem is the same as the circuit. Weird ckt but the amp does work just fine.
Thanks.

[Tom Phillips]

...Wonder why this method instead of just the appropriate cathode resistor...

I've been looking through my old books but have not found any mention of the circuit Bogen used in the CHB35A. I suppose that if it produced some wonderful result we would see it more commonly used. I'm still curious and will keep looking. Perhaps someone will chime in that knows the design reasons for the circuit.
Tom

[jazbo8]

This appearantly "odd" bias scheme was used by Bogen on several amps for remote control, i.e., control the level of inputs from far away, since these amplifiers were typically installed in racks in locked rooms.

[Enzo]

How would that work? I see the remote volume connected to the two remote terminals on the terminal strip, but they don;t seem connected to that bias system.

Page 7 of this refers to the RVC
http://www.epanorama.net/sff/Audio/P..._Amplifier.pdf

[jazbo8]

The cathode bias from the B+ is just a simple voltage divider, the remote control appears to be just a potentiometer, so I guess it varies the plate voltages of V1A & V1B thus controlling their gain.