I completely restored my Plush 10560 bass amp, replacing all the electrolytic caps and most carbon comp resistors. Also replaced the death cap and installed a properly grounded 3 prong power cord. I'm the original owner. The amp always had excessive gain. Back in the day, they designed them so zero to 4 was the maximum usable gain setting. Made people think their amps were more powerful then they actually were. Anyway, cutting to the chase... Because this gain from the 12ax7 preamp tubes is so high, I get too much microphonics. Anyone see a problem if I replace the (2) 12ax7 preamp tubes with 12at7's to make the volume control more linear and reduce microphonics?
Why switch tubes at all?
Unnecessary expense and will bias wrong, since you are not touching cathode resistors.
12AU7 is VERY different from 12AX7, just compare datasheet suggested values. Day and night.
To tame gain simply remove the 25uFx25V at first preamp triode cathode.
Should do, but if not enough, repeat with next triode cathode cap.
Unnecessary expense and will bias wrong, since you are not touching cathode resistors.
12AU7 is VERY different from 12AX7, just compare datasheet suggested values. Day and night.
To tame gain simply remove the 25uFx25V at first preamp triode cathode.
Should do, but if not enough, repeat with next triode cathode cap.
Agree with JMF that a 12AT7 or a 12AU7 would require a different cathode resistor for proper bias (i.e. same plate current as 12AX7). You might get away with a 12AY7, though.
There's one caveat:
Sometimes removing the cathode cap from the input triode gives rise to heater buzz.
If so and if it's not sufficient to remove the cathode cap from the second triode, I would insert a voltage divider after the input stage.
There's one caveat:
Sometimes removing the cathode cap from the input triode gives rise to heater buzz.
If so and if it's not sufficient to remove the cathode cap from the second triode, I would insert a voltage divider after the input stage.
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