Music Man 100-RD

[xtian]

It's been a while since I've had a Music Man on the bench. This one has 6L6 power tubes. When powered up, hum slowly grows and one tube starts to red plate.

I'm getting my head around the power section. Looks like the signal is fed to the cathodes.

A non-adjustable, fixed bias is fed to pins 5, so I can check for that.

There is a trimmer, TR1, which is also connected to the cathodes--is that a kind of PI balance control? Right now the trimmer is fully CW--how would one adjust it?

Anything else to watch for....aside from the 720vDC on the plates?

[bepone]

it was few monthis this question.. then how much you charge this repair if you ask for solution on internet?

[martin manning]

Recent Musicman discussion: https://ampgarage.com/forum/viewtopic.php?f=6&t=36752 that trimmer is the bias adjustment. I’d suspect a failed driver transistor.

[Stevem]

The 6L6 based amps have less then 600 volts on the plates if I recall right, but non the less be careful.

Turn the amp off and leave the standby switch in the on position and the power supply will drain out in 30 seconds.

[didit]

Hello --

Martin, as usual, on point accusing one or possibly both of those brittle BJTs. Though their failure/death may well have cascaded from surrounding passives. Proactively replacing the electrolytics C19, C20, C21 eliminates obvious probable suspects. A trio of film capacitors would be an inexpensive upgrade/update. That MM bulletin embedded in the prior thread is interesting reading.

Best .. Ian

[WhopperPlate]

it was few monthis this question.. then how much you charge this repair if you ask for solution on internet?

Totally … if you have to reference an outside source in order to execute a repair then you must lower your prices …because of honor … otherwise harakiri for you !!!

I hope the sarcastic ridiculousness is obvious

IMG_4643.jpeg

[xtian]

Proactively replacing the electrolytics C19, C20, C21 eliminates obvious probable suspects.

I cannot spot C21 on the schematic...

[didit]

Proactively replacing the electrolytics C19, C20, C21 eliminates obvious probable suspects.

I cannot spot C21 on the schematic...

My bad - C22 ..

[TUBEDUDE]

The power tubes are driven at the cathodes, and the suppressor/beam forming plates are grounded, but they are internally connected. So how is the cathode modulating the current flow if it's grounded? What am I missing?

[martin manning]

Yeah, that's an error. Other MM schematics show it correctly with the internal g3-k connection and no ground.