Ampex 351 hum when dc heaters grounded

[iampoor1]

Hello

I am dealing with a somewhat perplexing issue and am sure I'm missing something silly! I have an ampex 351  on the bench, and whenever I have the v3 tube in circuit (which as you can see in the schematic grounds the dc filament supply), I get an incredibly strong 60Hz hummmm. The hum will usually be readable on the meter at about -18...so it's pretty loud. If I remove the tube, the strong hum goes away, but of course we get increased noise from an ungrounded filament supply. I have tried multiple tubes and the issue stays relativlythe same...minus some tubes appear to hum louder than others (I haven't confirmed this with any measurements yet)

I have checked the ground on pin 9 of the v3 tube and it is connected. I have continuity between the heaters of v1/2/3 as well. There does not appear to be a break in these connections.  Dc voltages between the heaters (pins 4 and 5) are ~12v, interestingly eniugh, when i measure the dc voltages from the positive side of the dc filament supply to the enter tapped ground, I get 11.5v on one side and .5 from the negative side to the center tapped ground. I am assuming this dc filament supply should be balanced in reference to the center tap?

Also, one other interesting  note. In any of the 4 playback\record\bias\erase switch settings, hum stays the same, even with v1 and v2 removed, however if months switch inbetween the record and playback modes....hum disappezrs.

I will be taking more measurements later...just messed around with this for a couple hours trying  to figure out where the hell the hum was coming from! Any ideas and a sanity check are appreciated

Schematic here
https://usermanual.wiki/Document/Ampex351schematics1962.901134551-User-Guide-Page-4.png

 

[radardoug]

You have ' remove the tube' twice with different results. Which is it? Your diagnosis is completely unclear. We cant help you if you cant give accurate faultfinding info.

 

[iampoor1]

You have ' remove the tube' twice with different results. Which is it? Your diagnosis is completely unclear. We cant help you if you cant give accurate faultfinding info.

Sorry, made a few typos. I have corrected them. Thank you.

Also I'd my description is still too hard to follow...ill try and make a video. I have been having a hard time describing the issue, and could be missing some important information heh

 

[radardoug]

Have you got a load across the input?

 

[iampoor1]

Have you got a load across the input?

Yes, terminated with 150 ohms. Hum level is constant whether or not the input gain control is at 0 or max as well.

 

[radardoug]

Well I think you will find the hum has nothing to do with that supply. You either haven't got a ground on somewhere, or your test setup is faulty.

 

[smilan]

Hi, I'm having a similar issue with an Ampex PR-10.
Have you managed to remove this hum?