122 caps

[Matt Green]

I took a peek at the inside of my 122 amp today and found a cap attached to the rear 6550 had come off one of its locations. I've attached a pic. Can anyone tell me where the other side goes?

[Jaim]

Appears to be the 200uf cathode bypass cap - minus end to ground, plus end to either 6550 cathode pin 8. The pin 8's are also common to the 150 ohm 10 watt cathode resistor so it could also attach there.

Jim

[Matt Green]

Here's the schem.

[muckelroy]

Among other things, R17 (which is supposed to be 470 ohms, 1/2 watt resistor) has been replaced with two resistors in parallel. It looks like a 12 K ohm 1 watt, and a 39 Kohm 1/2 watt, but I can't be sure from the photo.

[David Anderson]

No, that 12K resistor is in the plate supply to the motor switching 12AU7. They do stretch in value sometimes, so someone probably added a resistor in parallel to bring it into the right range. It's not the best practice, but it works.

[muckelroy]

Of course.. sorry. I can't blame lack of coffee this time. The 470 ohm is just out of the frame of the photo (if it's there).

[David Anderson]

I'm sure the vast majority of us already have the 122 schematic, and many of us have it memorized. The loose capacitor is the cathode bypass for the 150 Ohm cathode resistor. The loose end is the positive lead that should be attached to the non-grounded terminal of the cathode resistor.

[Matt Green]

This is weird, the positive is supposed to go-to pin 8 and neg to the pin with the two 220 resistors,right? I think it was put in backwards. I just found it like this.

[Matt Green]

BTW, I'm having a hell of a time finding a cap to replace this one. Any ideas?

[Matt Green]

Here's a clearer pic of the socket without the offending cap. Pin 8 is on left. middle pin with green wire. Roughly 8 oclock

[Matt Green]

[David Anderson]

It looks like the cathode bypass capacitor was originally correctly installed. It was not installed backwards. I hate always to be the person saying this, but if you have no idea about how to interpret schematics relative to the components in front of you or how to source the appropriate replacement parts, it would be wiser to hire someone who knows what he's doing to do this kind of work. You are making me scared for the fate of this amp.

But for the sake of saving the amp, pins 1 and 2 of the rear 6550 socket are at ground potential which is where the negative end of the capacitor is supposed to be connected. The positive capacitor lead, in stock form, is soldered to one terminal of the cathode resistor, which is the white ceramic wirewound resistor sticking up through the chassis. It's the resistor terminal with the GREEN wire on it.

Just resolder the free lead of the capacitor to the resistor terminal with the green wire.

[Matt Green]

Thank you Mr Anderson for the simple explanation.