Peavey Classic 20 MH - HUM Problem

[nothingMaker]

Hi, I've a problem with a Peavey Classic 20 MH Head.

The amp have a loud 50Hz Hum on output, evident even with an oscilloscope.
No Hum on the Send Output.

It has been completely recapped but the noise persists.
I've tried different tubes in the Power Amp and in the Preamp, but the same.

Anyone experienced this problem?

[Helmholtz]

The amp have a loud 50Hz Hum on output, evident even with an oscilloscope.

Can you post a scope pic with scope settings info?

Please post amp schematic.

[g1]

There is a classic 20 schematic around, but doesn't seem to be anything for the MH (mini head), which is different (3 preamp tubes).

[Jon Snell]

50HZ hum is a bad ground connection or a leaky heater to cathode in one of the valves.
Not anything to do with HT smoothing as that would be 100HZ.

Why recap it as it is only 3 years old at the most and probably was under warranty.

[nothingMaker]

@g1 exactly! The standard Classic 20 Schematic is different, the Classic 20 MH have a different design.
Unfortunatelly I don't have the correct schematic.

Jon Snell two capacitors appeared to be leaking, so due to difficult to disassembly the amp I've made a complete recap job with better capacitors.

[Helmholtz]

I've often seen OPs being wrong about hum frequency (especially when using a phone app).
As fundamental hum frequency can be key to the root issue I like to verify by analyzing a scope pic.
Also the waveshape can give a clue to the origin.
E.g. heater hum has a distinctive pattern..

[nothingMaker]

Helmholtz this is the scope pic, independent from volume and eq. Thanks a lot

[Helmholtz]

Does the hum persist with the PI tube pulled?

Are power tubes cathode or fixed biased?

[nothingMaker]

Helmholtz the bias is fixed.
Without the PI tube the hum goes away, but the amp has DC power supply for the preamp tubes heather, with filaments wiring in series. Measuring the heather voltage is about 5.7V per tube.

The hum persist also with volume and eq at zero.
There's no hum present at the Send.

In this video in possibile to see part of the schematic:
https://www.youtube.com/watch?app=de...kl3DTbA

[Helmholtz]

[USER="45774"]
Without the PI tube the hum goes away, but the amp has DC power supply for the preamp tubes heather, with filaments wiring in series.

Assuming that the power tubes are AC heated, your result shows that the hum is not caused by the power tubes including the bias supply.

In the video you linked the guy states that the Classic 20MH and the 6505 Microhead share the same output stage and he actually used the schematic of the latter
So if that schematic is available, please find and post it.

[g1]

Here is the 6505 MH schematic

[glebert]

Looking at pics of the Classic 20 MH it looks like it has an effects loop. If that is on a daughter board my bet would be some grounding issue on the board-to-board cables or something like that.

[g1]

Removing any one preamp tube disables them all. So the hum going away when you pull one seems to indicate it is a preamp problem. But then it makes no sense to me that the hum is not present at the send jack.
Also, it seems the PI circuit is solid state, not tube.

[Chuck H]

Yes, it seems an unfortunate byproduct of the series filament arrangement that pulling tubes can't be part of the trouble shooting process in these amps. I think an alligator clip lead for grounding AC and having the scope for signal tracing could work. It's cumbersome but it could work.