The Amp Garage

So, what reverb tank have you got, what's its code?

Where has that circuit design come from? If it's your's, it would be beneficial for you to explain your design choices. https://www.valvewizard.co.uk/reverbdriver.html Why the potential divider on the tank output? Why such a heavy load of 32k on the tank output? Exactly what tank have you got? They'...

You need to provide details, really a schematic of what you're referring to, for useful relevant suggestions to be made.
Whatever, the tank delay is largely a set characteristic of the tank, they're available with short, medium and long delays.

for the PI input, I prefer 1nf on bassy amps with nfb and 2n2 otherwise on the normal ch. 3-5nf is fine. You’re using some remarkably low values ​​there. I’ve never seen such low values ​​used between the phase inverter and the power amp before. Doesn’t that end up altering the preamp’s tone a bit ...

So 54W class A output from a pair of 6550 is possible, and cathode bias has no downside for class A, it's just that each 6550 would need to idle at nearly 60W

There's no trem so it's not a match for any likely Fender guitar amp of that era.
It seems closest to the AA864 Bassman to me https://el34world.com/charts/Schematics ... _schem.pdf

My friend has that amp model, it's closer to schematic though, with a 22k trimmer acting as a variable load on the tank output. Where your's has a 10k resistor. Yes the reverb was horribly noisy / hummy. The simplest most effective fix I found was to disconnect the footswitch cable from the tagstrip...

If only someone had already written this up
www.amparchives.com/2019/10/12/the-evol ... uperleads/

Expecting the amp to output 60W because the OT is rated at 60W seems wrongheaded?
Whatever, drawing the loadlines would help answer your queries. These days that's much easier https://www.vtadiy.com/loadline-calcula ... alculator/

How about if you temporarily rig up a 1M pot as a variable resistor master volume, connected between TP23 and a local ground eg P3B-1 (or the grounded leg of R68), by the LTP phase splitter socket (ie at its input)?

the master volume doesnt help in the fx loop. Just to be clear, have you already tried an additional master volume patched into the fx loop? Certainly removing C11 will allow more high harmonics, probably making the issue worse. My suggestion, if the above doesn't help, is to increase the value of ...

... with the master lower than 5 the overdrive gets worse as you turn down the master with more unwanted gritty/harsher overdrive. not sure why. It might be parasitic capacitance between the clipping stage and later stages, after the master volume, allowing high frequencies to bleed / leak through....

The HRDIII schematic is in the public domain, I can find that. Is it the same as the IV in regards of your queries?
If not, maybe post it here, a user needs to be logged in to see attachments.

Schematic?

The trem oscillator is immediately adjacent to the LTP input / channel mixer; I wonder whether the circuit board is conductive?