New build nasty 100hz buzzing

[Guitarxtc]

Amp is 3 stage gain 60w el34 fixed bias,
with no guitar plug in, 100hz (tested with tuner)buzz/hum is very loud with volume up from 1 along with loud hiss,with volume to zero there is no hiss or buzz/hum,if i connect only first stage direct to tone stack buzz and hiss is low on max volume.
I changed all filter and signal caps
DC or AC heaters =no change in buzz/hum
i have artificial 100 Ohm center tap on heaters from el34 pin 2-7 to pin 1-8 ground
all resistors are metal film,
voltages on all tubes are normal,
grounding form rectifier to first filter cap,
all power amp grounds to one point near PT
all preamp grounds to input
changed all tubes, tube sockets and diodes,
re-soldered many times all connections to ground
temporary lifted chassis ground
connecting direct ground from other wall socked to chassis with no difference

to me sounds like ground loop somewhere but i have no idea what next to try

on the photo fourth stage and fx loop is disconnected,ignore that

image0 (1).jpeg

[romberg]

Amp is 3 stage gain 60w el34 fixed bias,
with no guitar plug in, 100hz (tested with tuner)buzz/hum is very loud with volume up from 1 along with loud hiss,with volume to zero there is no hiss

Try and narrow down which part of the circuit the noise is entering. Start by pulling V1 and see if it goes away. Then work down the line by pulling V2 and then the PI. If the noise goes away after removing one of these tubes then you can start to look at the supporting components and connections. A schematic might also be helpful for others to see what is going on.

Mike

[Stevem]

100 hz hum can only be coming from the sections of the amp that run on DC, so forget about anything that has to do with the filaments which run on 50 cycle.

You stated you have replaced the filters in search of solving this issue, did you also replace or at least jump out the bias filter(s) ?

Any preamp stage plate decoupling caps leaking a small amount of DC down stream into the next gain stage?

It only takes a few MV to create a hum. Plus throw the bias way off on that next down stream gain stage!

It’s a easy thing to just lift the down stream end of a cap and see if the issue stops.

[Guitarxtc]

100 hz hum can only be coming from the sections of the amp that run on DC, so forget about anything that has to do with the filaments which run on 50 cycle.

You stated you have replaced the filters in search of solving this issue, did you also replace or at least jump out the bias filter(s) ?

Any preamp stage plate decoupling caps leaking a small amount of DC down stream into the next gain stage?

It only takes a few MV to create a hum. Plus throw the bias way off on that next down stream gain stage!

It’s a easy thing to just lift the down stream end of a cap and see if the issue stops.

yes i replaced bias caps,
on first preamp stage after coupling cap about 5mV,
on second from 30 to 50mV

readings on scope

[Guitarxtc]

here is video with no guitar and input cable

IMG_2670 (video-converter.com).mov

[Stevem]

You need to go thru process of elimination to find out where that is coming from.
I am working on a Fender BF Bassman right at this moment,and I just injected 30mv @ 100 hz tone ( -22 dB) into the phase splitter, no less where you have this issue up up in a preamp stage and I could clearly hear it out of the speaker.