Metropoulos Forum

Hi friends, since few years I got a custom build that I've made with the metropoulos pdf. it got some basic adjustment for EU and got Marstran transformers. During few years it was on 2204 specs but since 1 year I changed it for a 1987 (I got a 2203 since).
2 months ago an horrible sound came and sometimes the amps froze on this bad buzz/sound and I needed to standby it and put on again and it disappear for 2 min and come back...
I put some new preamp tubes and did some new solder inside and it seemed that it was better, at house volume with an attenuator.
But yesterday, in band practice situation, after one hour, it come back!.The sound really happen when I strum the low e string on certain frets (I'm tuned to C). Just to say I use fuzzboxes too (bat pharaoh) but it's more

https://youtu.be/wiP5WFroOLg

Someone have an idea of the problem? Need to change my power tubes (they're a bit old)? Something in the preamp?
Thanks a lot for your help

I would definitely replace the power tubes and check the bias current and bias voltage for stable readings first. It almost sounds as if the standby switch has gone bad though. Switching it off and on helps to remake the contacts temporarily until vibration from a low note causes it to act up again. You could easily clip a lead across the standby or solder a jumper across to check this theory. Just turn the amp on and off with the power switch and play it to see if the problem disappears.

So I changed the power tubes and do some new solders and on the standby switch, also I don't know about how the bias was, but it was set on 16mA and my new tubes needs 40mA (442V on the fuse). Apparently I don't have issues now but I tested it on a bedroom level, the amps seems to be less louder too, that is a great thing!

After 3 weeks, the problem came back yesterday... It's so bad, I don't find what is the problem inside.

You could have parasitic oscillation, inaudible, you need a scope.

Are you able to make a recording of the problem when it happen?

Since you can "reset" the amp by switching off the b+ voltage, I would look towards the power supply for the problem. Try a new standby switch if you have one available. The switches used in these amps are not really rated for the DC voltage that we use them for, so it's a chance yours may be bad. I would also look for a bad electrolytic capacitor in the b+ rail or perhaps a bad solder joint.

danman wrote: ↑

Sun Jun 02, 2019 6:33 pm

Since you can "reset" the amp by switching off the b+ voltage, I would look towards the power supply for the problem. Try a new standby switch if you have one available. The switches used in these amps are not really rated for the DC voltage that we use them for, so it's a chance yours may be bad. I would also look for a bad electrolytic capacitor in the b+ rail or perhaps a bad solder joint.

My thought, it sounds mechanically induced arcing, ie a poor contact induced by vibration. I guess it does not do it if the amp is sat on a cushion?

Did you try the croc clip across the standbt switch? Check your filter caps, fuse holders, in fact all the power supply solder joints as you may have a mecahnical joint that is dry some where, also presume your output selector is good and you speaker cables?