AA864 PI question

[Randall]

https://cdn.shopify.com/s/files/1/23...97210715640943

I am going thru a AA864 Vibrolux Reverb that I have never found a proper schematic for, so I am working off the AA964 above. I have replaced lots of leaky caps and out of tolerance resistors, which brought the hum and noise down quite a bit. I ran into the PI and I find a couple of wildly different resistor values. The 100, 1M and 470 ohms match, but there is a 82K in place of the 22K, and a 100K in place of the 820 ohm feedback resistor. I'm wondering what to do here. They look original, so I am inclined to leave them. But I wonder what effects these different values would have?

And for the life of me, why does the cathode have 77v on it, and where does it come from? Is it because it is cathode biased? If so why so much? I actually have 120v there, the last time I checked.

[Chuck H]

82k in place of the 22k is almost certainly wrong. With the other values you say are correct to the schem I would expect a 22k. The reason there is 77+ volts on the cathode is that it's elevated above 0V at the bias point. Just like there is a small voltage on the cathode bias of a preamp or power tube. Except that the LTP needs to be elevated higher than those circuits and so there is more voltage. The mistaken 82k where a 22k should be is even more elevation above 0V than there should be and that is why you are reading 120V there.

The 100k feedback series resistor is also wrong. The feedback circuit is a voltage divider. The 820 ohm is the series resistor and the 100 ohm is the shunt. So you can see that by these numbers a 100k feedback resistor means there is no effective feedback in the circuit as it's division would be about 1000/1. Since you have the 100 ohm resistor in place I would say that the correct feedback series resistor value should be 820 ohms.

Please be certain you are identifying the resistor locations correctly because the incorrect values you've indicated just happen to be the standard plate load resistor values for the PI. Wouldn't want to make that mistake. So locate and confirm the plate resistor values for the PI just to be certain those aren't the resistors you're looking at.

[Randall]

I am sure we are talking about the correct resistors. The plate load resistors I already changed out another time before when I did them all.

So, if these two are incorrect, it would seem it came from the factory this way. What to do, change them to values we would expect and possible change the behavior of the amp he just paid a king's ransom for, or leave it as is, because it's "original"?

[Chuck H]

I am sure we are talking about the correct resistors. The plate load resistors I already changed out another time before when I did them all.

So, if these two are incorrect, it would seem it came from the factory this way. What to do, change them to values we would expect and possible change the behavior of the amp he just paid a king's ransom for, or leave it as is, because it's "original"?

Neither of those resistors as they are are a threat to safe operation. If he bought the amp because he loved the tone and had to have it, then leave it as it is. Otherwise... Since you have already changed out resistors then changing two more amounts to a squirt of piss on a forest fire. You can always put it back the way it was if he hates it with the correct values. I'd change it to the correct values and see what he thinks.

[Randall]

Actually, he bought it without even playing it because it is so clean, and was thrilled with it once he got it. I think you are right, I am going to put it right and see how that goes.

Thank you Chuck!