[marc7]

I finally finished my C2 and I'm trying to track down a hum.  I used my own top plate and wood case.  I 've grounded the gray wire in the trans to a mounting screw and inside a wire from the mounting screw to the earth connection .  With out the ground I measure 75v ac on the top plate. My input and output tubes are microphonic(new or old ones). When I touch the top plate I can greatly reduce the hum.
What am I missing.

[hagtech]

You could start by grounding the top plate to earth prong on power cord.  Post photo.

jh

[marc7]

I did ground the top plate to the earth connection(which includes the plug ground) and it did help the hum and negated the stray voltage.  I still have some hum and the last 12ax7 is still microphonic and when I touch it the hum increases. It doesn't change when I sub a known good tube.  I will post pictures later on this afternoon.

[marc7]

The thick green/yellow wire ties the top plate/trans to earth

[hagtech]

I'd say one of the problems is that you routed the sensitive input wires right across the power transformer leads.

jh

[marc7]

I re-routed the wires so they come in right above the input connection.  Nothing changed.  I still have the hum, the hum is reduced when I touch the toplate, increases when i touch the tube and the tube is microphonic .

[WGH]

I didn't have your exact hum problem, but I had a very sensitive tube and noise whenever I touched a tube or top plate. The solution was to re-solder the ground connections from the first 12AX7 to R203. The second 12AX7's resistor going to ground is R210.

[rotcoddam]

On my C2 I made an isolating enclosure with thick copper foil aroung those output wires, sort of like a small box, and grounded it.  I found the copper at ACE hardware. This made a good difference with hum.
Marty.

[marc7]

I checked my joints and re-soldered all the ground connections and anything that looked questionable front and back.  I think because the tubes are so microphonic that any hum or buzz is being picked up and amplified.  What would induce a known good tube to become microphonic in this circuit ?