Quote from: argenta on January 14, 2025, 10:06:11 PMwhen testing for shorts between the transistors and the heat sink I observe the following:
The meter reads short circuit initially, then over a few seconds resistance increases to open. Why is that?

Those transistor collectors are connected to the supply rails.  The heatsink is grounded to the chassis.  When you measure, you are measuring from supply rail to ground. 
What you are seeing is the meter charging up the supply rail filter caps.

Yes, maybe it would be better if it was called the 'break-time' switch.   

I understand what Merlin says about cathode stripping being a myth, but I prefer to use the switches as prescribed by the manufacturer for turn-on and turn-off.
He gets very adamant about his anti-standby stance and it ends up putting me in the "methinks he doth protest too much" zone.

I have suspicions about tube warm up possibly being involved with tubes going microphonic, but have absolutely no hard data to back that up.

Your meter should have a spec for the AC volts range as far as frequency response.
My true RMS meter says 45Hz to 2Khz.  A lot of them are just for measuring AC volts in the 60 to 100Hz area.
Yes, scope is best bet for higher freqs.

As a general rule, if it's a cap issue the hum should be 120Hz.  60Hz hum would be more likely ground issues, as outlined by phatt above.

Quote from: saturated on January 05, 2025, 05:56:58 AMAlso it is toob rectified if it matters

Yes it matters.  With solid-state rectifier you do not want to hit the tube's plates with instant DC voltage before the heaters warm up, so standby is a way to deal with this.
With tube-rectifier, the DC builds up slowly while the heaters are warming up, so you don't really need a standby switch.

The T has the metal tab, the TF is fully plastic encapsulated, so no insulator required.
Not sure if the metal style transfers heat better to the heatsink.

Quote from: Paul Marossy on December 28, 2024, 07:18:26 PMSo despite the notion that any old FET should work, this circuit is very picky about what FET it will accept.

Not sure where that was said.  'Any old' would still require it be n-channel jfet with same pinout.
2N5952 has a different pinout than the PN4093 or J113, so it will not work if installed with the same orientation.

I guess it does not have a 3-prong AC plug with chassis connected to ground pin?

Quote from: saturated on December 24, 2024, 02:00:42 PMAfter a little research maybe it's possibly a TDA2030 or LM1875 ?

TDA2030 was what I was thinking as well.  Here is their G10XL schematic which may be similar.

Sounds like phantom power for condenser mics.  Modern stuff uses 48V for phantom.

One thing I noticed on the layout, the fets have the middle leg bent forward, whereas the small bipolars have the middle leg bent to the back.
Since the fets had all been changed out by someone else previously, make sure they are not installed 'backwards'.

Agree with JM that there may be other issues rather than the fets which are just being used as switches.

A jfet is low resistance (or 'on') in a non-powered state (or when no voltage applied at gate).  These are N-channel so they require a negative voltage at their gate pin to turn them 'off'.
Source to drain resistance should measure less than 100 ohms with amp un-powered.
Removing the fet from the circuit will simulate the fet in it's 'off' state (which is high resistance from source to drain).

Quote from: saturated on December 17, 2024, 04:20:47 PMI'm trying to decipher terminals that show up in the pictures 🖼� of many of them

I have attempted to identify and lable appropriately....

What the heck is S- and S+ ?

I can't guarantee that this is correct, but seems to make 'sense'. 

https://forum.allaboutcircuits.com/threads/open-chassis-power-supply-connections-s-and-s.69047/

Quote from: argenta on December 17, 2024, 10:08:16 PMSo here's an update. On replacing all 4 power transistors, I eliminated the loud switch on pop.

No you did not eliminate the turn on pop. 
As soon as you turned on the amp it blew.  There is probably DC on the output, which is why you hear the hum.  If you keep running it with DC at the output, it will burn out the speaker as well.

Please post the schematic you are using.