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RE: Stereo to mono conversion advice needed.

OK, assuming you have a stereo amplifier with two channels of pentode, single-ended output...

1) Disconnect one of the output tube grids from its driver and wire every pin of the tube in parallel with the other output tube. Leave both output transformers as-is, so the primaries of the two output transformers are also now in parallel.

2) Wire the secondaries of the two output transformers in series.

3) Attach a speaker with an impedance equal to twice that of the original (or connect both original speakers in series across the two transformers, or leave each original speaker attached to its respective transformer).

4) Disconnect global NFB.

5) Convert the now-unused preamp/driver sockets for tremolo, reverb, additional gain stages, etc.

Note that if there's anything unusual about the circuitry, such as cathode windings on the output transformers, additional steps might need to be taken.

For safety, you'll need to add an isolation transformer (or build a transformer-operated power supply) if the amplifier is AC/DC. Also, don't forget to use a three-wire AC power cord.

Edits: 05/14/21

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