Metropoulos Forum

Hey Folks-
I am finishing up my new 50 watt build and am seeing some pretty unhappy looking voltages. The heaters all look correct, and I am getting normal looking readings for negative bias voltage. But I am seeing a full 450 volts at pins 1 and 6 of V1 and V3, and pins 1, 6, & 7 of V2. This is with tubes not installed. Something seems greatly amiss, and while I step back from the amp for a couple of hours to get my head screwed back on, I thought I would see if anyone had any thoughts on an obvious screw up that I might not be thinking of. Any help is greatly appreciated!

You'll have to put the preamp tubes in to read anything close to proper voltages through the amp. Right now with no tubes at all, you're not drawing any current, so the voltages don't drop down in line. Put in the 12AX7's and you should see the voltages come close to the charts.

Thanks for the reply!

I was aware that voltages would change with tubes- I just didn't expect to see them THIS high. I'll put the tubes in when I get back and see what happens.

Thanks!

a friends comet build had 519v in the preamp even with an ecc83 installed

Ok!
I put the tubes in, and all of the voltages were spot on. I continued testing from there, and found that my MV (the Metro kit version) is causing an oscillation. If the MV is around 2, I can turn the channel volumes all the way up with no oscillation. But any time I turn the MV up past two, whether a guitar is plugged in or not, and regardless of the channel volume settings, the squealing begins. I have built more than 10 amps, but this is the first ever with a MV, so I am suspecting the problem might be there. Also, the amp seems really gain-y, far more so than the last Metro 50 watter I built.

Any thoughts?

I should mention that rather than using the terminal strip sent in the Metro MV kit, I moved the two 220k resistors onto the MV pot and ran shielded wire from there to the junction of the two .022uf caps and the jumper from the bias supply. I then ran shielded wire to the 5.6k resistors on the tubes. I grounded one end of each shielded wire. Does this sound like a kosher way to accomplish the MV?

Thanks for your help!

The squealing sounds like positve feedback. Did the phases get reversed when you install the MV? Or perhaps the OT primaries are reversed?