Uh, OH.....Looks like the Metropoulos Metroplex MK II is having issues with volume drops........

[harddriver]

And George is going out of business........................................Looks like he is trying to figure things out but how much longer will he support these amps?

https://www.thegearpage.net/board/i...os-metroplex-mk-ii-50w-volume-issues.2733097/

But here's an issue: It sometimes changes volume noticeably without me touching any controls. I've never opened the back panel since receiving it by UPS -- I just plugged it in and played. Does the change in volume mean a vacuum tube is seated poorly or something?

Response from VelvetGeorge:
December 10, 2025
Make that three amps with this strange issue. I have one here on the bench that did it too. But thanks to Murphy's Law, it has not done it since it came back to me. I've been playing it everyday. I've watched the voltage. Yesterday I replaced he opto in the master volume circuit, just to eliminate that as the culprit.

Please give me some data to work with and I'll track this down.

Does it happen every time you play?



Does it happen right away or after the amp has been on for a certain amount of time?

Does it happen on both channels?

Is there something in the FX loop when it happens?

Are you using the footswitch when it happens? If so, does unplugging the footswitch and using the push/pull knob change the issue?

Sorry for this trouble, I'll get it sorted.

VelvetGeorge
December 12, 2025
Thanks for the data and suggestions! I don't think we have a consensus yet on when it happens or why. I really need to see it on the bench where I can monitor the 12V supply and the state of the opto when it happens.
I suspect one of two things are happening:

the opto IS getting voltage but isn't working correctly

the opto is NOT getting voltage

That said, the FX loop and tube suggestions are just as viable. One test that can locate the fault is to send signal into the loop return. This eliminates the entire preamp and the send circuit of the loop. The signal passes through the return circuit, the master volume, phase invertor and power amp. If the issue happens here we can localize it.

 

[ZEN Amps]

Yeah saw that. Must be frustrating for him - multiple users reporting the issue but none are wiling to send an amp back to him. He is diagnosing remotely, and partly via TGP responses. Ouch.

 

[glpg80]

He didn’t have optos in the first metroplex and mine didn’t have any volume drop issues. If anything that amp was loud as absolute fuck. Insane headroom that would easily deafen you.

My personal opinion is that he’s got an issue with the 12V heater supply. I think whatever circuit he’s using can’t supply the current demands, goes into protection, volume drops, current drops, circuit comes out of protection, and volume returns. You have to build in temperature de-regulation into designs so this doesn’t happen.

It’s possible it’s due to the opto isolators but this sounds like a power supply issue as the chances its circuit design related are fairly slim IMO.