using a 6F6 instead of 6V6 to lower power

[PRNDL]

I'm wondering if it's OK to put in 6F6 (4.5 watt) tubes into an amp that normally uses 6V6 (12 watt) tubes, such as a Deluxe or Deluxe Reverb.

Both have similar plate resistances.

[booj]

bias a pair up and see.

[tubeswell]

You might have to lower the B+.

[Dave H]

I don't think you'd get less power with 6F6s. The 6F6 is 4.5W in single ended operation. It would probably have similar output to 6V6s in push pull.

[Bruce / Mission Amps]

I've used lots of 6K6gts instead of 6V6s in the tweed Deluxe and the amp still makes about 6-8 watts output... and loudly.
The 6K6s are looking for about 10K-12K primary load and a correctly built cathode biased 6V6s amp is usually around 6K5 to 8K5 so the error factor is not crazy enough to hurt the 6K6gts.
It is the higher plate voltage and subsequent plate current that does them in prematurely.
Use the softest rectifier you can, like an old 5Y3GT rectifier or use some 5w 10v zener diodes in the power supply rail to lower the B+ to the power tubes.
What I've done is actually use 3 or 4 five watt, reverse biased zener diodes in series with the OT's center tap lead and screen node to lower the B+ only to the power tubes.
Then just connect the preamp tube's B+ node right to the main B+ supply through a new, 5K to 22K, 1w resistor to compensate for the additional 10vdc-20vdc you would have seen dropping across the screen node.
(Well, that was assuming the screen node was not a choke, but a fixed resistor.)
If it is choke then there was only a couple three volt drop so it isn't worth compensating.
If you choose that zener diode option, make sure the cathodes of the zener diodes are facing away from the output transformer center tap lead.