Well....I wasn't gonna mention it but since you brought it up 

I was reading this the day before yesterday and of course I instantly remembered you talking about your car radio

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Diagram and text Art Margolis

My first tube "anything" did not have a standby switch, but it took about 5 minutes before any noise came out of my 1957 VW Beetles all tube radio.
It freaked some people out how the radio came on all by itself, it just needed time to warm up.

Quote from: g1 on January 14, 2025, 01:41:37 PMYes, 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.

Well for me personally I'll only live for another 20 or so years so whether I use the standby or not the Valves will still out last me.

It's interesting to note that nearly All those old mantle radios and Gramophones did not have a standby.

I once inquired about an old Mantle Valve radio in an Antique shop which had been owned by the same Family for 3 generations.
The owner who would have been in his 60's told me that it belonged to his grandad. It had been running every working day since he could remember.

I think this falls under one those unsolvable mystery options like;
Some people leave the toilet seat down while some people get nasty if you leave it up.  
My wife lost here wedding ring down the loo because she used to leave it up.
and I said,, "well that is why they come with a lid dear"  

YMMV, cheers, Phil.

Nice fix.

Happy New Year all. I did indeed replace the power transistors and the driver ones as well. With the power transistors, I made sure they are insulated from the chassis, however, when 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?

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.

Yep..agree.

I removed Q4 and the amp is 100% now!

So, I will replace that FET and all is well.

Thanks for the assistance.

It appears to be a mute circuit.

If you think about it there is a clue in the word ""Standby"".
Band takes a short break, flick it on standby so when you come back flick the standby and play. There is no waiting for the amp to warm up again.

To me that would have been the obvious logical reason for the Standby switch.
No doubt there will be some monster circuits that need warmup time before HT is applied but for most small to medium wattage amps don't loose sleep if you forget to switch to standby before you power up.

I recall reading long ago that there are some standby designs that cause more harm than good.
Phil.