Bogen MU 30 PA

[danrulz98]

Hey guys! It's been a while.

Well, I'm not sure where to post such a thing, since it's definitely not a television and I don't think it qualifies as a radio.

It's a public address amplifier and I found it for a pretty cheap price and it's in really nice shape.

But I can't find a darn thing about it. No schematic or anything.
Oh well.. There aren't very many capacitors in it, most of them are ceramics. It has silicon diodes in it and tubes that weren't available until the late 50's.

It's using two 6EU7s for mic/phono preamps. There's 3 mic channels so I'm guessing it's using one 6EU7 for channels 1 and 2 then only using half of the other 6EU7 for channel 3, it might possibly be using the other half for one of the aux (line-level) inputs, but that seems overkill.

Then it has a single 7247, which from what I understand is half of a 12AX7 and half of a 12AU7. I would imagine this is part of the inverting circuit for the output stage?

The output stage is dual 7868 tubes.

If anyone has info on this amp, it'd be much appreciated.

The thing I'm really curious about right now is there's a rheostat between the two output tubes... what on earth could that be for? Biasing?

Photos: https://www.dropbox.com/sh/rvdnlu5jhtvrik0/ktDu6SgLSV
I do have the whole thing, I only took pictures of the chassis.

[N2IXK]

The rheostat appears to be a "hum balance" control. It is connected across the filament supply, with the wiper grounded. It forms a movable center tap for the filament, used to "buck out" any residual hum.

To adjust, disconnect the input, turn the volume up, and adjust for minimum hum from the speaker.

[danrulz98]

That's interesting! But it's going between pin 3 on both output tubes and don't seem to be going to ground.

[wa2ise]

People over at AudioKarma (our sister site) can also help.
http://www.audiokarma.org/forums/forumdisplay.php?f=14