Ampeg b25 help

[angelothewolf]

Hi there!,

Im struggling with this Ampeg B25. bad 120hz hum I cannot find the culprit. It came with original caps, some bias trimpot mod. I thought that vintage caps must be the culprit. It wasnt.

I change all the caps., drifted resistors etc. I made what I suspect was the original ground scheme: Ground bus with caps negatives, output tubes cathodes, speaker gnd, hum balance pot gnd, and then to the ground "G" point on the PCB, from this single point to the chassis.

All tubes are new.

Only PI tube and output tubes I get hum. if I pull out the 7199 I get no hum (tested several 7199s). I cannot masure any noticeable ripple in the "E".

"C" node is 428V Im mesuring around 20mV ripple there with the scope with a 1uf/630V cap in series on the prove, (im not sure if that is the right way to do it).
On the grid of the 7199 triode theres a huge 2V 120Hz.

Voltages on 7199 stage:

7199 TRIODE:

1: 296V
9: 109V
8: 128V

7199 PENTODE:

2: 175V
3: 83V
6: 1.9V

The rest on the amp is original, already tried another rectifier tube with no luck

I hope you can give me a clue wih this one, thanks!

[Jon Snell]

120HZ hum is a main smoothing issue. Check the capacitor values and connections. C25/6/7
Check the 7027s are balanced.

[Helmholtz]

Still hum, if you short point Z to ground?
If it's actually 60Hz hum, check the bias filter cap (C22).

[Enzo]

You have no ripple on E, but it likely is not from ripple per se. More likely your grounds. If the E filter cap shares ANY copper with A and B node caps on the way to ground, then the charging ripple currents in those caps can be impressed upon the E cap.

I recall an AMpeg once (GU10 maybe?) that had obviously hummed all its life, And teh cure was to move one ground connection to the other end of a 4 inch wire. Honest.