Ampeg Reverberocket II Hum

[fuzzman72]

Hello

I'm servicing an Ampeg Reverberocket II guitar amp.

Here's the schematic:




I've already done the following:

Add a three prong cable, replaced all the electrolytics, changed drifted resistors, new tubes in the pre and matched pair in the final stage.

The amp delivers a noticeable hum that changes with volume pot turning.
It's not a buzz, it's more like a bass note.
I've scoped out the noise across the speaker with 5 ms/div and 50 mv/div

Volume: Max
Treble: 0
Bass: Max



Though I'm new to the use of O-scope this appear to me like a 50Hz (I'm in Europe) noise. It's about 70 mV P-P.

The ground scheme is a sort of "buss ground" with the only connection to the chassis at ground lug of the input jack.
The first input has a ground switch (that is working properly) the other one has no switch at all.

I've tried almost everything, tube swapping, checked all ground connections, tried the "Chinese stick trick"...nothing the hum is still there.
I'm running out of ideas now.
Does anyone has a suggestion?

[danFrank]

If you already replaced the filter capacitors, then I would suspect one of the pre-amp tubes having a heater to cathode short. This will give you a hum that gets louder as you raise the volume; especially if it is the first, input tube. Do you have any extra tubes? Start switching them out to find the guilty one.
Daniel

[fuzzman72]

Thank you Daniel, I've already tried swapping the tubes with known good ones and move them in their socket to see if something would happen with no results.
In the previous post I forgot to mention that all the audio connections are made through shielded cables. The shield is grounded at one end only of course.

[Wavebourn]

I have not seen this particular amp, but I silenced some Ampeg SVTs that had ground wire soldered in the wrong place, from power supply to preamp section. As I concluded, they were manufactured with such a wiring error. May be something similar here.

[fuzzman72]

Hello Wavebourn

Grounds connections is what I'm suspecting of.
Here are two pictures of the chassis before the cap job.

[Wavebourn]

Did you solder electrolytics to exactly the same places?

Check if ground goes from transformer/rectifier by the single pair of wires directly to the filter cap, and nothing more goes through this wires. Also, some VR tube is drawn on the schematic diagram. Does it glow?

[fuzzman72]

Excuse me, what VR stands for?

[Frank Berry]

Voltage Regulator

[Frank Berry]

It may be a neon power indicator rather than a VR tube. Hard to tell. The schematic isn't too clear.

[Wavebourn]

Ah, never mind. It is only a pilot signal lamp. I downloaded and resized the picture. The only purpose it has other wire is to short it by power switch.