Problem with amp hum/buzz

[nogoodnamesleft]

so i am working on an amp that has a JCM800 50w power supply/power amp section with a completely different preamp section and when I finished, all worked well. The amp worked for about 20 min and sounded awesome.
I then went in for a mod on the preamp section (add bypass cap to tube 1) and when I turn on the amp there is this very loud hum/buzz happening.

After much investigation here is what I have found......
- hum/buzz is unaffected by preamp section, volume, tonestack, etc. It still amplifies the preamp signal, though quieter.
- the hum goes away when the power tubes are out
- the B+ is half of what it should be from the first rectifier diode (which doubles as a bias supply....220k resistor on). In other words, power to the B+ is only coming from one of the transformer leads once it hits the rectifier...the diode with the 220k resistor reads (post diode) 2v while the other diode reads normal HT.
- the bias circuit resistors are all functioning fine as is the bias pot
- the EL34's are not bad...tested several sets
- the el34's are reading a super high bias....like over 200....i turn it off very quickly!
- replaced the caps from the PI in case they were bad...nothing changes
- checked every connection i can think of
- heaters are fine
- the B+ to ground resistance is over 2M with all tubes and OT connected........huh?

i'm probably missing something easy but am at a loss and losing my mind. I assumed it was the bias circuit, and will probably switch out the 2 caps there to see if it helps, but the B+ to ground resistance being near infinity is confusing to me.

any thoughts?

-

[nogoodnamesleft]

- switched the bias caps....no change
- unplugged the bias supply and PI from the el34's....no change
- resistance from B+ to ground is infinity ohms....unless i put my negative lead on the B+ and the positive lead to ground....then it is 500K
huh?

[nogoodnamesleft]

could this be an OT pr PT problem? They are both brand new.....

[TheGimp]

I'd start with the B+ and find out why it is off.

Possibly a bad diode?

Schematic would help.

[nogoodnamesleft]

all diodes switched out with no change

[kevinkr]

Guitar amplifier being discussed so I will move this thread to Instruments and Amps per forum policy.

[kevinkr]

Originally Posted by nogoodnamesleft

all diodes switched out with no change

Are you using the recommended power transformer or something close with a center tapped high voltage secondary? If you are the transformer could be defective.

[nogoodnamesleft]

Originally Posted by kevinkr

Are you using the recommended power transformer or something close with a center tapped high voltage secondary? If you are the transformer could be defective.

i am using the very same transformers suggested by the layout....i bought them from the company that posts these layouts.

are you thinking i am indeed dealing with a bum trafo?

[nogoodnamesleft]

so now my B+ is not draining when the power is off...It is hovering around 180v and very slowly dropping (a volt every 30 seconds) when it normally drops to like 4v in seconds.
all tubes are plugged in currently

[nogoodnamesleft]

the whole chassis (PT?) vibrates and makes the same sounding noise that is coming out of the speakers.........