Music Man 112-RD-50 has fizzy clean sound

[Helmholtz]

If the grid voltage is lower than 30V, crossover distortion will be increased.
Higher than 30V would lower crossover distortion.
In fact the transistor can add a little class A power a low output - if properly biased.

[g1]

Screen voltage for power tubes should be checked as well.

[Helmholtz]

..and make sure the global NFB is functional.

[Coop]

I already changed the 30v 1 amp zener diode. All voltages are good. I’ve just accepted it for what it is. Thanks to you all for your time, help, guidance, and cool stories. You guys are the best!

[Helmholtz]

. I’ve just accepted it for what it is.

How this now?

There are ways to increase idle current and lower crossover distortion - if necessary.
But there are other open questions.

[Leo_Gnardo]

I already changed the 30v 1 amp zener diode. All voltages are good. I've just accepted it for what it is. Thanks to you all for your time, help, guidance, and cool stories. You guys are the best!

Believe me you have made the best choice. Enjoy the rad OD channel!

[Leo_Gnardo]

There are ways to increase idle current and lower crossover distortion - if necessary.

Certainly! Rip out that silly transistor bias/drive system, drill a big hole to install a 12AT7 & copy the classic Fender/Musicman invert/drive circuit, also cobble a bias circuit that provides appropriate voltage. Most of a day's work plus a handful of parts. I should have mentioned it days ago, apologies to all. Probably get a bit more power overall besides with output tubes' cathodes at/near ground potential.

Trying to improve the existing bias/drive circuit is clearly an exercise in turd polishing, I have proven to myself conclusively on this amp.

I've offered my solution to RD50 owners several times but not one of them wanted to pay appropriately for the improvement, say $400 mostly labor. Working within budget is part of dealing with customers. I have yet to hear "cost is no object" even from the rock stars.

[Helmholtz]

Certainly! ...

I meant things like increasing Zener voltage or lowering the 75k resistors (R68, R69).

8mA cathode current might not be enough.

[glebert]

I've offered my solution to RD50 owners several times but not one of them wanted to pay appropriately for the improvement, say $400 mostly labor. Working within budget is part of dealing with customers. I have yet to hear "cost is no object" even from the rock stars.

You have to figure they bought the amp with that sound so they were OK with it at some point.

[Leo_Gnardo]

I meant things like increasing Zener voltage or lowering the 75k resistors (R68, R69).

8mA cathode current might not be enough.

Believe me I tried everything. 8 mA wasn't enough. I could see the notch reduce only a tiny bit, & guessed more like 12-15 mA would be the charm. After many hours spent faffing to little success the opening song on David Gilmour's first solo album rang in my ears "There's no way out of here, when you come in, you're in for good." Time to button it up & leave well enough alone.