‘81 Princeton Reverb Bias Question

[Helmholtz]

Make sure that:
- the 220k is well soldered
- the red wire connects to point B between the 1k and 4.7k (formerly 18k) resistors.

[Perkinsman]

The red wire doesn’t connect at point B. There are two white wires.

[Helmholtz]

The red wire doesn’t connect at point B. There are two white wires.

The wire that connects to the 220k resistor in your picture looks red. Where does its other end connect?

Your problem is a missing connecting between the upper end of the 220k resistor (middle of board) and its supply at point B.

[g1]

Yes, that red wire has to go somewhere. It could be called either B or A depending which schematic you look at. Either way it's got to go to where that 4K7 meets the 1K.

[Helmholtz]

Yes, that red wire has to go somewhere. It could be called either B or A depending which schematic you look at. Either way it's got to go to where that 4K7 meets the 1K.

THanks for reminding me that the screen supply node is called A in the later schematic. I still tend to stick to the B1270 version as the amp had the two 18K resistors. But it may well be even another version.

[g1]

I'm not sure whether this unit has the boost pedal or not.

[Perkinsman]

The red wire (from 220k resistor) isn't connected to anything as you can see I am pointing to in the pic. Just below is Point A, to the right of the 4.7k & point B to the left. Are you saying that the red wire should be connected to point B?

[The Dude]

Yes, that red wire has to go somewhere. It could be called either B or A depending which schematic you look at. Either way it's got to go to where that 4K7 meets the 1K.

^^^^^^^ That.

[Helmholtz]

The red wire (from the 2-220k resiistors) isn't connected to anything as you can see I am pointing to in the pic. Just below is Point A, to the right of the 4.7k & point B to the left. Are you saying that the red wire should be connected to point B?

It looks as if the red wire is soldered to a separate eyelet. According to the B1270 layout this eyelet should be connected below the board to the eyelet that connects the 1k and 4.7k (formerly 18k) resistors. Somehow your amp must have lost that connection. You may directly solder the red wire to the common eyelet of the 1k and 4.7k resistors.

Notice, I don't use the labels A and B anymore - having to refer to two different schematics makes me dizzy

[Perkinsman]

Success! The red wire was moved to the point between the 18k & 4.7k. V4b voltages are close to spec but V3 plates are 20% low. IÂ’ll live with it & call it done at this point. Special thanks to Helmholtz etal for your patience & guidance.

[Helmholtz]

but V3 plates are 20% low

While a voltage deviation of -20% is not yet critical, it might indicate a leaky cathode cap (25µ/25V).