Saturday, December 24, 2011

Here's a Lear Siegler era Bogen M60A tubed PA head.

Compared with the Bogen logo from earlier days, I'm going to set my expectations a touch lower on this one. Granted, it's probably twice as powerful, but power isn't everything.

Classy design & good looks still count for something. Back to this amp, survey says: 1967.

A note! This isn't going to be the last note published here, nor is it going to be the most interesting, but - this one is highly informative. Let me run down my analysis of this note (spanning the next couple pictures as well).

1st: The unit was shelved due to low power output, not because of some more serious problem.

I'm going to go ahead and quote the note here, because I'm going to wind up coming back to it in bits and pieces and it'll simply be easier if I'm dealing in text.

With 12AX7 in input 2
it works but is nowhere
near 60w @ 8 Ω.
Check out if worth fixing
(maybe tube refit: eg:
6L6's & 12AX7's)

Here's the top side of the chassis. By the late 1960s sound reinforcement was fast turning into a more sophisticated endeavor, as evidenced by the input/output block at the rear of this unit. At first I thought that the low output power may have been the result of improper configuration of input selector, but wait, let's re-read part of that note:

With 12AX7 in input 2

"input 2"

Well, I do see a 9 pin Noval socket down there labeled "INPUT 2", and seeing how 12AX7 are also Noval base the parts will physically fit, but, 9 pin miniature does not automatically equate to a tube, much less a 12AX7. Note, there is no tube designation call out in the proximity of those sockets. That is because those sockets are wired for input transformers.

Like this. TM-200 input transformer can, on a Noval 9 pin base. Clearly an unpowered tube is going to pose a hindrance to signal. But I digress.

(maybe tube refit: eg:
6L6's & 12AX7's)

I'd also like to point out that there isn't a 12AX7 or a 6L6 in this entire amp. Thus concludes my note decomposition, there's hope for this amp yet.

Guts! The thicker wires are shielded, build quality is pretty robust. Busy, but not overly convoluted. This also pretty much looks exactly like the other LSI Bogen guts I've seen.

I found the schematic on this page on, ton of other Bogen stuff there, in case you wound up here looking for a schematic I didn't post.

This grill hides a spot for a ~3 inch speaker. I can laugh at the notion of a 60 watt powered mixer pushing a front panel mounted 3" speaker, can't I?

Yes, yes I can.

1 comment:

Olli Niemitalo said...

Thanks, I was wondering about the 9-pin "tube" sockets in a Paso Transistor Amplifier T-67-A, one for each microphone input. There is a dummy component in each socket that shorts pin 2 to pin 8 and pin 5 to pin 9. A good place to put an microphone transformer like those manufactured by Bogen.