replacing transformers with lower rated units

[PositiveNegativeMan]

This question is purely hypothetical, and excuse me please my ignorance, but I am still learning...

Suppose a 50 watt amplifier with two EL34 push pull power section needs a new output transformer. Funding being limited and time being short, ordering a direct replacement is not an option. Suppose a 30 watt amplifier in good condition is available, with a 2 6L6 push pull power section.

If I use the output transformer from the lower rated amp, but keep the original power transformer, I know that I run the risk of frying the OT. After all, that power transformer was intended to supply way more. But suppose I swap both transformers.

Ok, I would never get the same power from the amp, because the replacement power transformer was never built to supply that much. Obviously the character of the amp would be different but would I actually be running the risk of destroying anything? What I mean is, wouldn't the transformers be matched to each other? So could the power transformer deliver enough to destroy the output transformer? I don't see why [told you, I'm learning] because it will only transform the power coming to it - which depends on the power transformer - which was matched to it in the first place...

Please remember this is perely hypothetical!!!

[Steve Conner]

Well first of all, it probably wouldn't burn the 30 watt OT if you put it in the 50 watt amp. But if you were worried, you could increase the EL34s' screen resistors to throttle their power down towards 30 watts. The PT won't supply it if the tubes don't ask for it.

Or just put the 6L6s in it, they usually give less than 50W in an amp that would give 50W with EL34s.

[MWJB]

Further to Steve's suggestion, you could also go easy on the plate current.

Going back to your earlier point, power & output transformers aren't really "matched" to each other, they don't really know each other exists. Essentially an amp's power is related to output tube voltage, but if you put a low voltage, high current 80W PT in a 30W amp, it would still be a 30W amp...just with an overbuilt power transformer. Likewise, a 100W OT in a 30W amp will give better headroom, but it won't greatly increase actual output.

Problems start when you pump too much plate current into a too small OT, or draw too much B+ & more importantly heater current from a too small PT. EL34 draw more heater current than 6L6, so it's safer to go the other way. Check the data sheets re heater current, add up all the tubes, add a margin for safety (typically 25-30% for production amps, up to 100% for boutique). If you try and draw 3.6A from a 3A PT all your secondary voltages will be low and you may burn it up! More sabotage than "modification"!