st-70 rebuild hum

Hi Folks
I rebuilt my st-70 about six weeks ago and have had a fairly loud hum since then that starts when the tubes warm up and doesn't vary with the volume.
The rebuild consisted of:
VTA board with k-40's and the cap upgrade, parts all installed inside the chassis
new 80-40-30-20 quad cap w/80 oriented to the back of the amp
removed front octals and mono switch
put new bias tests near the OT's on top
moved the power switch to the front, shielded the cable and drained it to the chassis grd.
moved input jacks to the rear, shielded and drained to the rca grd

After about two weeks the rectifier tube started to arc so I replaced it.
voltage tests are good
I have:
gone through it and checked all the solder joints
swapped tubes
pulled the power switch and inputs out of the chassis.

The only thing so far that stops the hum is pulling the center tube only vta board.
Any help would be greatly appreciated.
Thanks, Bob

Bob,
Did you use replace the 2 wire line cord with a 3 wire? Sometimes you will get a ground loop hum if connected to something with a ground wire. If you short the inputs do you still hear a hum?

Take some clear photos of the top and bottom and post here, maybe one of us can figure out your problem.

Sal

Hi Sal
When I short the inputs the hum stops!
I still have a 2 wire line cord.
I'm not able to post pictures at this point. I'll try to get that going later today.
Thanks for your reply,
Bob

Hello
I rearranged the cables and power cords and that helped alot.
Then I remove the passive crossover for the Vandersteen subwoofer from between the preamp and the amp and that also helped quite a bit.
Now all that's left is a minor hum that I can easily live with.
Any ideas about the subwoofer crossover?
I really like having it in the system.
Thanks,
Bob

Hey Folks
I just blew my rectifier tube.
Turned the amp off, then turned it back on before it had totally powered down.
It arced then blew a fuse.
Now it arcs every time I try to power it up.
This is the second one in a month that I did this to.
I'm learning some hard lessons here.
Bob

This mod should help too -

https://dynacotubeaudio.forumotion.com/t512-rectifier-modification-for-the-gz34

Thanks for the input.
I've got a new gz34 and quad cap on the way and I'll put in a couple of in 4007's while I'm at it
Bob

You should install either diodes or the tube. But not both.

sailor wrote:You should install either diodes or the tube. But not both.

Normally that's true, but in this application the diodes are intended to be used in conjunction with the rectifier tube.

Best Regards,
Charlie

This Rectifier Mod is all over other forums. I actually got the lead on it from the guy at http://www.dynakitparts.com/. The main reason for this mod is the shorter life span of current production rectifiers (GZ34 in particular)

You could run just the diodes... You want to run the rectifier tube as well I think to get the slow warm up. So the Tube just gets the pre-rectified volts. I'm running the mod and the amp sound great and is silent when idle.

PS - Your issue with the hum. Did you try to simplify things? Just hook up a Right and Left speaker to the ST70. Then hook up a CD player to your amp. Don't turn the CD Player on or you will be blasting music! Just see with the speakers connected... And the CD Player connected direct to the amp but with the power OFF... Hum YES/NO?