I have a vexing issue made more so by that I solved the same issue earlier and now it's back, this time without a findable solution as of this time.
A 5E3-type circuit I'm working on is producing a low level 60 cycle hum on standby. Live the hum is louder, although in each instance it is not obtrusive. I would say that, on standby, the hum is what I would expect from the circuit when live, and the live hum is a little more than that.
I have checked the voltages on the V1-V2-V3-V4 plates on standby and they all seem normal - a trivial number of millivolts. Live, the values are all pretty high - I have bigger transformers in there. But they seem reasonable - in the low 200's on the preamp tubes and low 400's on the EL34's.
For the filter caps - on standby they're all running about 445mV. Is this normal?
The transformer/rectifier connections all seem legit - moreover the amp was quiet on standby earlier so this seems like an unlikely source.
One thing I'm unsure of, however, is that the rectifier (I'm using a plugin diode) pins 2 and 8 are at 327V on standby, and 423 when powered up. I frankly don't know what the values should be here.
Other ground connections:
Cathode bias 250Ω resistor - grounded
Input jacks -> potentiometers - grounded
PT high voltage center tap - grounded
PT shield wire (Allen TP40D) - grounded
PT 50V bias tap - grounded (should this be grounded or heat shrinked? It didn't cause an issue earlier)
Here's my standby switch wiring:
1. Rectifier pin 8 to Standby
2. Standby to positive end of first filter cap.
3. OT center tap to positive end of first filter cap.
One thing I just noticed is that the turret lug at the base of the + end of the first filter cap is a little loose in its footing. I don't imagine that makes a difference but I don't know exactly.
I seem to be running dry on solutions. What else can I check?
Topic: Standby Grounding Issue (Read 3685 times)