Gibson Sabre

[lowell]

Schematic - http://www.gibson.com/Files/schematics/Sabre.jpg

This amp has a real flabby bad sound to it. I'm gonna go through it, recap it, possibly replace the speakers etc.

Reason for this post is to ask if someone can explain how the power amp works.

It seems to me that Q10 is in emitter-follower arrangement and that Q11 is in common-emitter arrangement.

I'm curious: Does Q10 provide any gain? Or maybe the secondary on Q10 provides the gain whereas the secondary on Q11 provides less to none?
Are these in push-pull arrangement?
What class is this?

[lowell]

Also there's no voltages listed on the schematic. What does one do in this case if one wants to be sure voltages are good?

[g1]

Yes it is push-pull. The driver xfrmr gives the output devices same signal, opposite phase. Sort of like a Garnet (or other) that uses a phase splitter xfrmr rather than tube phase splitter. I think there may be similar circuits covered in the Darr book. Back before pnp's were readily available and cheap, push-pull had to be done with NPN's only (quasi-complimentary?). With a single rail supply (rather than +/-) you would have the output at 1/2 of Vcc so the speaker would have to be capacitively coupled. A problem with C27 will have quite an impact on the output, make sure it is good.

[lowell]

Ok right on. I found a bad filter cap on the power amp supply, so I replaced all of the filter caps, as they're all very very old. Amp is working far better now. I'd like to be sure the bias is good on the power amp. Am I right in thinking that the Vbe on the power transistors should be .7v?

[Rhodesplyr]

Also, if you haven't done so, target the 35uF crossover caps for replacement. Old electrolytics can really muck up the sound of just about any crossover, often making it sound like there's a blanket over the speaker.

If you can spring for them, try replacing those 35uF caps with metallized polypropylene motor-run (not motor-start!) capacitors. Especially in series with a horn, they will wake things up rather dramatically. I did this on a Lowrey T2 organ's companion amp, which is basically the same thing as the Sabre, with excellent results. Get what power you have to the speakers rather than using it to heat the crossover caps.

Motor run caps are usually considerably cheaper than capacitors marketed specifically for audio crossovers, but they work just fine.

And unlike electrolytics, in this application, they will be a permanent fix.

ASC Capacitors - X386S-35-10-330 - Passive Components - Capacitors - Allied Electronics

[lowell]

Anyone know bout biasing this amp? Vbe on the power transistors should be .7v right?

[rf7]

Vbe on the power transistors should be .7v right?

Yes, that's typical for silicon transistors.

Usually you set the bias by measuring the voltage across the emitter resistors.