National Westwood model 6422 hum

[cjlectronics]

I am working on a National Westwood 6422 amp (circuit is the same as this Gretsch) that has a hum on the output. I have replaced the power supply caps and the 1k ohm resistor between the 20uf and 10uf in the power supply filter capacitor section. The 1k ohm resistor read 200 ohms and it crumbled when I touched it. One thing to note is that when I scope the 360v B1 voltage (AC coupled) I see a 15 Vp-p ripple but I replaced the 20uf cap and still get the ripple. I have swapped the 6V6 too, no difference. Any ideas?

[52 Bill]

There are two 20uF caps paralleled at B1. Is the hum constant regardless of signal or control settings? How about the rectifier tube? Pull the driver tube, hum still there?

[cjlectronics]

This amp had one 20uf at B1 and one 20uf at B2, along with the 10uf at B2 as well. I know, I checked it multiple times to make sure I wasn't seeing things wrong but the second 20uf was soldered to B2.
I removed this 20uf and have not installed it in B1, YET.
With the input signal grounded, there is a hum. I replaced all of the 12AX7's and no difference. I put another rectifier tube and no difference, hum is still present.
I will pull all 12AX7 and see what happens.

[Enzo]

A certain amount of hum in a single ended amp is normal. Your power tube draws off the first node of the B+, there will always be ripple there, and it will be added to the signal. SIngle ended amps that want to lose this hum will have to add another filter stage before that node. There is only so much that brute force filtering can do in this.