AB763 Single Channel Question

[Shawnobi]

Hi There,
Been wiring a single channel of a Bandmaster AB763 into an old Hot Rod Deluxe chassis. (Tremolo, No reverb - Normal channel removed).

I've realised I've overlooked something I have to correct.

The Normal And Vibrato channels feed the PI each by a 220K resistor which I believe forms a voltage divider. By removing the normal channel I need to adjust the value of the 220K resistor on the Vibrato channel, yes?

For reference I think the Vibrato is feeding ~275V and the Normal is feeding ~265V according to Fender schematic. So PI is seeing about 135V? SO voltage this is what I need to maintain?

[Enzo]

Wha???

275v is the DC voltage on the plate of the tube. The 0.1uf cap blocks that. Same deal on the other channel, the 0.047uf cap blocks the DC there. They are coupling capacitors, they couple the signal into the next stage without passing the DC. Not sure how you could then expect any 135v at the junction of the resistors. Should be about zero volts DC there.

The two 220k resistors are mixing resistors, not really there as a voltage divider. Just remove the unwanted channel.

[Shawnobi]

Wha???

275v is the DC voltage on the plate of the tube. The 0.1uf cap blocks that. Same deal on the other channel, the 0.047uf cap blocks the DC there. They are coupling capacitors, they couple the signal into the next stage without passing the DC. Not sure how you could then expect any 135v at the junction of the resistors. Should be about zero volts DC there.

The two 220k resistors are mixing resistors, not really there as a voltage divider. Just remove the unwanted channel.

Thanks Enzo. That makes sense. 135V was based on my imaginary Voltage divider. Had missed the DC blocking as I was overlooking the coupling caps.

Thank you very much.

[Gaz]

AB763 Normal Channel only.pdf

[tubeswell]

Here's one of mine along similar lines (but with a James tone stack). The Heyboer OT I used is available from the Weber site. Driving a couple of Weber 12" alnico magnet speakers. Very loud and clean (The 6P3Se has a plate dissipation rating of 20.5W, and is a mil-spec tube - very rugged)