Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JLS on November 13, 2017, 11:45:27 am
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Not that I am considering attempting this but curiosity had me wondering while I sit in an airport. Wondering if supplementing lower wattage transformers (PT & OT) is safe or even viable for inserting into an original schematic. For example, instead of building a 100 watt head for two reasons, cost and loudness, and reduce the output to say 15 watts. Would swapping the transformers be ok or is there other mods that would need to be considered? I've read where the more tubes involved equals the more output. I'm unsure if reducing the amount of output tubes would be required as well.
Thanks,
JLS
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Sure you can. You would go with a different reflected primary impedance on the OT,
and not require the large PT a bigger amp would want.
The power tube feed (bias/grid) resistors would change...
And the bias circuit would have different values.
Totally doable.
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Lower voltage equals lower power output if all other things are equal, but you can't just put a power transformer meant for a 15 watt amp into a 100 watt design.
This would reduce power output compared to the original 100 watts, but it would still consume (and output) more power than the 15 watt design. This would fry the power transformer.
Output transformers with different characteristics (higher impedance) could also reduce power, but you wouldn't just install a smaller OPT which was otherwise similar-- you'd be pushing 100 watts through a 15 watt part and you'd fry that too.
The best way to do this is take a poweramp and matching power supply, like the Marshall 18 watt, and pair it with the preamp from an amp that gives your desired sound, whether that's an SLO100 or a Fender Twin or whatever.
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My statement considers that he is building a... for example, a 15 watt type amp.
Figure out what tube sound you like for power tubes, and put the preamp type you like in front.
You would want to provide the good choice of plate voltage, for your power tubes that you want.
The rest of the power supply is adjusted to what you need, in the rest of the amp.
You can even cathode bias to bring the output down some more.
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reduce the output to say 15 watts.
it would work, till Ohms law and thermodynamics are proven to be true :icon_biggrin:
any pre can drive any PA with a little tweaking n math, so if you like a 100w pre but only want 15W, find 15W amp, cut n paste, tweak and do math