Stereo Class A. Can outputs be reconfigured for ct pp xfmr, mono o/p?

[rob_h]

I have a fixer upper stereo SE class A 7695 amp with 12AX7 drivers.  With i/p changed to mono, can the output transformers be replaced with a normal pp/ct xfmr, with same Z, to be a class A Push-Pull output amp?   It would be 4.5w dual/stereo o/p to 10w PP mono o/p?  Is it a feasible idea, or do I not know enough not to ask the question?

[PRR]

Are you SURE it has a power transformer? 7695 is intended for transformerless (live chassis!!) designs.

The simplest thing to do is mono the inputs, strap the outputs. It will be 9 Watts parallel SE. Nothing wrong with that.

You "could" buy a new OT, convert one driver stage to a phase inverter, and run push-pull. If you stay "Class A" the gross power hardly changes, though a lot of distortion cancels. Instead of 11% at 9W you might now claim 5% at 10W. Hardly seems worth the trouble. 'Specially for guitar.

The specified load for push-pull is 1.5K. It will be VERY hard to find a good 10W 1.5K OT. Or even a 20W 3K OT to re-rate.

[rob_h]

Thanks for your input. It seems the devil's in the details.  Schematic attached.
HOT CHASSIS: Yes, it is a "hot chassis" amp with a half wave rectifier to boot.  I am going to add an isolation xfmr, 50va or 75va. 
     The schematic shows a "circuit common-to-chassis" cap, and a cap from the "stereo" o/p xfmr "low side-to-circuit common", both .02mf/1kv.  The "main" o/p xfmr's low side is tied to chassis.
     I am not familiar with this technique, but, with isolation power xfmr sec.: 
1) Connect X1 to the 120v input which goes to the series heaters then to the rectifier.
2) Connect X2 to "schematic common" and connect ground from 3-wire power cord to chassis. 
After that I don't know whether to
1) leave the two .02mf caps, letting the "circuit common" float relative to the "grounded chassis" through the caps, or, 2) tie the ground to the chassis and the circuit common.
     If #2, what happens with the .02mf cap's functions?
STRAPP THE OUTPUTS: Do you mean connecting them in series or parallel?  What about the difference in o/p xfmr low side connections?   
  (silvertone SE stereo phono amp with reverb)

[rob_h]

p.s.
Output transformers are same on low side.  Just realized that the "main" output xfmr low side, tied to chassis is .02mf from circuit common, as is the "stereo" output xfmr secondary, so they are both tied with .02mf caps to circuit common, electrically, dvdn though one is also tied directly to chassis. Check me on that please.
Thanks to anyone who wades through this post!
Sincerely,
rob_h

[PRR]

> The "main" o/p xfmr's low side is tied to chassis.

The whole thing frightens me. Chassis-to-line isolation seems hit-and-miss, ad-hoc as a simple mono recordplayer grew stereo and reverb. Also the caps are surely lowest-bidder, and now 40 years old. Also, I've found a 2268A  schematic showing alternate grounding. And I know someone who worked in a similar factory... mistakes happened.

> I am going to add an isolation xfmr, 50va or 75va.

Essential.

Power seems to be 19 Watts in heaters and 36 Watts worth of DC. The 50VA may work; 75VA seems a much safer bet.

Tie Green wall-wire to chassis, solid, so it CAN'T lose contact.

Then feed the 120V _secondary_ of the isolation tranny to the original line terminals.

Then add chassis-mount _metal_ guitar jack, and bond the sleeve to B- near the small tubes.

VERIFY dead-zero volts from chassis to wall-plate screw, amp on or off.

With this connection, none of the dubious "isolation" caps matter. (They should be shorted-out by the chassis to B- bond.)

You "could" jumper-together the two top-lugs of the Volume pot and feed guitar in here. Then jumper together the speaker leads from the OTs. Since each was 8 ohms, the parallel connection is 4 ohms. (You could clear all the other stuff from the second OT speaker leads, and wire both windings series for 16 ohms.)

Input sensitivity is about 0.5 volts. It will take a hot pickup and a strong arm (or a booster pedal) to coax all 9 Watts out.

That's if it even plays. Junk like this, almost all caps are bad or will go bad soon after awaking from a 30 year sleep. Being very low-bid parts, even resistors may go bad.

How ugly is the construction?

I'm thinking to tie-together both 7695 and OTs, treat that like a Champ's 6V6 and OT, then (reluctantly) torching everything else off and building a Champ-like front-end.

[printer2]

Had a console stereo that used these tubes, was disappointed about the lack of PT. In the end I built a Champ type amp with a 6CW5 with a 16 ohm load rather than 8. Nice big OT as compared to regular guitar transformers given the wattage. I was thinking of using it with a 6L6 given that idle current for the transformer should be in the 100mA range and with the limited bass range for guitar the transformer should be able to do much more than the 7695's 4.5W. Using the 7695's I would parallel up the outputs and drive them as a pair.