Egnater Rebel 20 hum

[xtian]

Egnater Rebel on the bench, with bad hum. Clues:

Hums with all preamp tubes pulled.

Hum is the same with either pair (6V6 or EL84) of power tubes pulled, but of course NOT with both sets pulled.

Heater voltage is OK: 3.4vac on power tubes, 12.3vdc on preamp sockets (preamp tubes are still yanked).

Hum changes amplitude with varying the bias on power tubes, of course. Note that I can only raise the current to 34mV, but it should be set at 45mV.

HV looks OK at 350vdc.

What next? Bad ground? Bad cap?

[xtian]

Bonus clue: When I was play testing, I held a chord and flipped the power switch off. Hum stopped IMMEDIATELY, while chord slowly died away as caps drained. Doesn't this point to hum in the heater circuit?

[martin manning]

I'm thinking it's in the bias supply, possibly a bad filter cap. How much AC ripple does it have on it?

[xtian]

Ah, but you see, I am not an O-scope user. Simple monkey tools here.

While twisting the bias adjustment pots, I'm seeing -15.6 to -22.5vdc on pins 2 of the EL84 sockets (no tubes installed) and -28.7 to -44.2vdc on pins 5 of the 6V6 sockets.

[martin manning]

Use your multimeter and measure AC voltage at the grid pins.

[xtian]

First look was with Standby switch in Standby position: 0.46vac on all grid pins. Then with Standby switched to Run, 0.75vac.

Significant voltage?

[martin manning]

Yes I think so. It should be closer to 10 mV than 1000.

[dorrisant]

Does the filament chain have a center tap? If not, could it be a bad balance pot or ct resistor? Have never found a schem...

Tony

[Colossal]

That amp has a regulated DC heater section for the preamp section:

17VDC input
0.1uF Ceramic cap
4700uF 25V electrolytic
78121A 12V regulator (with heat sink)
0.1uF Ceramic cap
22uF 25V electrolytic
12VDC output

[dorrisant]

That amp has a regulated DC heater section for the preamp section

Ah... thanks for the correction, Dave.

I have a friend who is "saving pop bottles" until he can afford the parts cost on a new amp. When he gets his new amp he is giving me his Rebel. I am going to make up a schematic once and for all.

Tony

[xtian]

When he gets his new amp he is giving me his Rebel. I am going to make up a schematic once and for all.

Good luck, mate! You'll have to remove all the boards from the chassis. All the traces are hidden from view.

[Colossal]

Yeah, that amp is a bummer to work on. I had an Egnater Tourmaster come across my bench that was DOA and stayed that way. Really engineered to be throw-away electronics. Tiny, tiny parts, all wave soldered...

[Reeltarded]

Teeny tiny

[Colossal]

Teeny tiny

EXACTLY

[dorrisant]

The one Troy has is supposed to be hand wired by Bruce himself. I won't know for sure till I open it up but let's just say I'm not holding my breath.

Tony