Standby switch thud...

[Bob M.]

Does anyone have a cure/fix for the standby switch thud noise in a mid-60 Fender blackface (Pro Reverb) amp? It thuds both ways: going from standby to play mode and also going from play mode to standby, although the former is worse than the latter. It drives my friend/amp owner up the wall. I've tried a few cures for this over the years but this one in this amp is eluding me. Any help/suggestions are appreciated.
Thanks, Bob M.

[The Dude]

The switch could be bad and arcing. I'd try temporarily subbing in a new switch first to see if that corrects the problem. If not the switch, I'd try putting a resistor or cap across the switch. Here's a good article on standby switches including remedies for this problem.

https://www.valvewizard.co.uk/standby.html

[Chuck H]

Or... Tell your guy to stop using the standby switch. There's been a lot of discussion here debunking the validity and usefulness of standby switches for guitar amps. I have to admit that my own proclivities are contrary because of decades of routine. But the truth actually has been revealed by the higher minds here. And elsewhere too. I've even read a few threads in the last couple of years where home brews are built without a standby switch at all. So I guess it's sinking in? The day may even come when known manufacturers stop using them. Time will tell on that I suppose. At the very least there doesn't seem to be any harm in NOT using the standby switch on a tube guitar amp as far as I've read here.

[pdf64]

Exactly!
Leave the switch closed and let the GZ34 ramp the HT voltage up smoothly.
The standby switch contacts are probably somewhat eroded etc from the inductive back emf flyback voltage spike causing arcing as the switch is opened.
https://el34world.com/charts/Schemat..._schematic.pdf

[Helmholtz]

I did not investigate this, but my impression is that standby popping increases when power tubes are not well balanced.
This would make sense as the B+ jump is a common mode disturbance which will be better suppressed with balanced tubes.

[Bob M.]

Thanks everyone for your replies. Thanks, The Dude, for your on-point answer and the very nice Valve Wizard article, which I think I've read before.
I knew people would have a variety of solutions re: the thud. I know I could tell the guy, 'Don't use the stand-by switch.' But it's his amp and he's entitled to have everything on it working as it came from the manufacturer. If you have an issue with standbys, take it up with Leo. Unfortunately, it would be a one-sided conversation.

I did remove and clean the switch thoroughly and it measured infinity (open) and 0.00 ohms (closed) on my Fluke DMM but I don't believe that's a definitive test as to the condition of the contacts inside the switch. It may still be an arcing switch. As The Dude suggests, I'll replace the switch. This new-ish standby switch already had the extra resistor and capacitor on the standby switch as suggested by the Valve Wizard article, which I replaced with the same but brand new - also to no avail.

As for matched power tubes, the two 6L6 type tubes in this Pro Reverb are within 1mA of each other. Many repair shops I know of consider power tubes within 2 mA as a good match. More research is needed on your theory, Helmholtz.

Few musicians are science-based characters and it's hard to change behaviors even in the face of overwhelming evidence. Many musicians are creatures of habit (if it's not cigarettes or beer, it's the standby habit). I normally don't add standbys to my own builds but I don't editorialize to customers. The other side of the coin is that someday he may sell this very nice, good sounding 1965 Pro Reverb to a collector or vintage guy who will not be impressed with a standby switch that is not in top working condition and will cause him to lose $$$ on a sale. I have to protect my customers by alerting them to such eventualities. Or I could tell him to quit guitar and take up the flute where he will never again have a standby issue. Or he might be in an altogether different standby mode - waiting for a rock band that wants a flute player.

Bob M.

[Helmholtz]

If the new switch doesn't improve things I'd check the choke using an LCR meter.

[g1]

Another thing you might try is pulling tubes one at a time to see if you can isolate an area that may be causing the thud.

[Mick Bailey]

Also, check the lead dress - sometimes a spike can couple into the preamp from the power supply wiring.

[Bob M.]

Thank you, again, everyone.
Bob M.

[Mars Amp Repair]

Was just thinking (dangerous, I know as the mind iS a terrible thing), but I digress...If the bias on the output tubes is not immediately present or at a too low level (not negative enough) given charging of the filter cap there, that might cause the inordinate 'Thiump' coming out of Stby.
I've seen home-brew amps where the designer derived the bias from the voltage that was switched by the standby switch.
Now I know this is not how Fender does it, but you might for grins monitor the bias voltage and see if takes a second to get to it's proper negative potential, (gets less negative enough) coming out of standby.
Just a thought...Glen

[pdf64]

Was just thinking (dangerous, I know as the mind iS a terrible thing), but I digress...
I've seen home-brew amps where the designer derived the bias from the voltage that was switched by the standby switch.
...

Marshall built its JMP era 50 watters like that for nearly a decade!
Based on that classic Marshall FU, lots of kit amp vendors are now publishing layouts with that same ridiculous arrangement.

https://el34world.com/charts/Schemat...d_50w_1987.pdf
https://el34world.com/charts/Schemat...r_vol_2204.pdf
https://www.mojotone.com/Amp_Kits/Br...sh_800_SCH.pdf

[Mars Amp Repair]

Marshall built its JMP era 50 watters like that for nearly a decade!
Based on that classic Marshall FU, lots of kit amp vendors are now publishing layouts with that same ridiculous arrangement.

https://el34world.com/charts/Schemat...d_50w_1987.pdf
https://el34world.com/charts/Schemat...r_vol_2204.pdf
https://www.mojotone.com/Amp_Kits/Br...sh_800_SCH.pdf

Well, there ya go!

[Chuck H]

Ok, but...

The OP indicated that this thump happens in both directions (on and off). Bias voltage is present and (hopefully) stabilized once the amp has been running. This inclines me to look elsewhere than the bias circuit arrangement.

JM2C

[The Dude]

Another thought: The thump may be from the powering up of a preamp stage and have little to do with the output stage. I'd try removing the PI and see if the amp still thumps. This might either confirm or eliminate the power amp as the cause.