Sloclone..hums like h...

[thomasdj]

Hi, really struggling with my new build..sloclone..got a loud 50hz hummmmm picked up somewhere.

All voltage looks great..a little lower than stated in the schematic, but okay.

The hum increase with gain and master volume. There is no difference except from volume when switching between Clean and OD channel..the hum is rock steady.

Pulling V1 and V2 doesn't affect the hum but grounding the shielded wire at V3 grid kills the hum!
Going through FX return sounds sweet with no hum

Got 10pcs 12ax7's (5 brand new) and have tried them all as V5, V4, V3, V2, V1..no improvements!

I've attached some pics of the amp..don't mind the extras..all is wired completely like the official layout..I think.
6.3v CT winding are elevated
5v winding is used for the switching circuit - grounded at chassis

The circuit ground looks like the official layout:

Pots are grounded on the buss
Input is grounded on the same buss
Ground buss goes to the preamp board and back to the PS board
Bias pots (I use individual bias pots) are grounded at preamp board
One wire goes from the PS board to the chassis (PT nut)
HV center tap goes to the same PT nut
Power amp cathodes goes to the same PT log

All jacks are isolated from chassis (verified!)

Any help on this one??

Best regards

Thomas

[kg]

instead of pulling v1 and v2, try shorting out the various stage grids to ground. work your way "upstream" from v3. report back!

[thomasdj]

instead of pulling v1 and v2, try shorting out the various stage grids to ground. work your way "upstream" from v3. report back!

Thank you for reply!

Grounding v3 grid kills the hum..v2 and v1 doesn't..i've been thinking all day and suspect the cap at the elevated heaters

[tedmich]

on the bright side you can take off 10 days of work and replace every single component 4 times and STILL not equal the price Mike S. charges for one of these

[thomasdj]

on the bright side you can take off 10 days of work and replace every single component 4 times and STILL not equal the price Mike S. charges for one of these

Hi, liked that one!

Update...with the scope.i can see the 50hz on the cathode cap on v3 an back to v1...

[kg]

Thank you for reply!

Grounding v3 grid kills the hum..v2 and v1 doesn't..i've been thinking all day and suspect the cap at the elevated heaters

disconnect point C from the b+ rail and return it directly to ground, see if that changes anything.

you can also use a relatively high value coupling cap (ie, 0.1uf/400v) and shunt the two plates feeding the grid of v3 (v1 pin 1 and v2 pin 6) and see what happens.

that stopper on v3 is pretty big, and the node it's fed from isn't exactly low impedance either...

[thomasdj]

disconnect point C from the b+ rail and return it directly to ground, see if that changes anything.

you can also use a relatively high value coupling cap (ie, 0.1uf/400v) and shunt the two plates feeding the grid of v3 (v1 pin 1 and v2 pin 6) and see what happens.

that stopper on v3 is pretty big, and the node it's fed from isn't exactly low impedance either...

Hi kg...thank you for your reply!

I've used some tin with solder flux which has shorted my tube sockets..measured 12M ohms from pin 4 to pin 7...i changed all the sockets and the amp is dead silent! The tin is in the trash can now...

[kg]

awesome news!

[Mick Bailey]

I've used some tin with solder flux which has shorted my tube sockets..measured 12M ohms from pin 4 to pin 7...i changed all the sockets and the amp is dead silent! The tin is in the trash can now...

What brand was your flux? Resin flux won't usually short, but acid flux will.

[52 Bill]

What brand was your flux? Resin flux won't usually short, but acid flux will.

And did you use the same flux for the rest of the build?