Metropoulos Forum

Good day everyone - anyone....
I built, some years ago; a Metro JTM45
it as always been a great amp;
love it!
I don't fiddle as much as i used too on the electronics.
but I was installing new power tubes and I noticed, though I never noticed before, that the B+ voltage is almost 20 v different from one tube to the other.
one is 400v & the other is +/- 419v.
is this normal?
then of course the bias is different by about 9 mv or so
should I try to correct this?
this amp is very responsive to tweaks and money spent on quality tubes is always rewarded.
fabulous amp all the way round.
any thought would be appreciated.

This is not normal if you have a traditional bias circuit in the amp. The plate voltage on the output tubes should read the same and depending on how unmatched the output tubes are will show a difference in current draw at the cathodes.

any thoughts on the possible cause?

CP1961 wrote: ↑

Fri Nov 24, 2017 2:50 pm

any thoughts on the possible cause?

Assuming the solder joints from the transformer to the anodes are good and both screen resistors are of the correct value and in spec. then the most likely cause would be badly mismatched (or a defective output tube) the grid leak resistors (220k) should also be checked and the PI couplers also of the correct value and not leaking.

As the current draw of the output tube goes up then the plate voltage goes down proportionally, so if one of your tubes is biased very hot then the plate voltage will be lower than expected.

Ok thanks, I know that it is not the tubes I have lots of 6l6's & a set of KT66's but changing the tubes dosen't effect the condition or the the position.
I will check those others things
appreciate the help!