Audible low level humm from Vincent st-70

I'm experiencing a low level hum from my speakers when connected to my Will Vincent st-70 that is audible when not playing music. It is constant and does not increase or decrease with volume setting. Replacing the amp with a SS amp eliminates the humm. I have tested all the tubes and replaced the ones that showed weak. All current tubes show strong with two of the Kt77's showing minor grid leakage but still well within the good range. Humm is in both channels. The amp sounds great but I find the humm annoying between discs. Even though my GZ34 rectifier tube tested good could it be the problem?
Update: I swapped out the JJ KT77's for some Electro Harmonix EL34's and the humm dropped by at least half. I like the sound of the KT77's better so I have ordered some fresh Gold Lion Kt77's. So we shall see!!
EL34 has lower gain than KT77, but I doubt about tubes since your humm isn't volume depended. Try swapping AC plug terminals using cheater plug.
Good luck...sounds like you have got 'at least half' the problem solved...and if it really is tubes, then you've got the problem entirely solved!
Marakanetz: Tried swapping AC terminals, no change. Have been listening to the el34's and am going to put the kt77's back in regardless of humm. The el34's just don't sound as good as the 77's IMHO. Not sure what it is but with the 34's it sounds like my SS amp! I've been switching between the two. With the 77's it has more of that tube sound! Maybe the new rectifier tube on the way will help. Heck maybe this says alot about my SS amp!!
Is it 120 or 60 hz hum? If it's 120 hz it would suggest something power supply related since the power supply almost certainly uses full wave rectification which results in 120 hz ripple.

60 hz hum could be pick up from power lines, ground loop problems, heater to cathode leakage in tubes (unlikely since you've tested them and tried substitutes), or any of a number of other things.
Re the hum - it certainly could be the power tubes, although that has never been my experience if they all hum at the same level. If the hum is at the same level in both channels it could be a speaker sensitivity issue (tube or amp noise threshold) or it could be a small tube problem. I've had small tubes (in a different unit) cause a hum but usually only in one channel, not both.

It could be a ground loop problem. Disconnect all of your sources. Then reconnect them one at a time using a cheater plug on the source PC and see if that helps. It did for me.
Haven't tried swapping out the driver tubes. Though I did audition a set of 6gh8a versus the 7199's and wrote in my notes that it seemed background humm increased. I will try a second pair of 7199's. I have a pair of black plate RCA 7199's I haven't tried yet.
Ghostrider45: To the best of my knowledge or I should say my best guess is its 120 hz.
Update: Tried three sets of 7199 driver tubes. All tested excellent on a sencore tube tester. Original RCA set is gray plate, humm is minimul ie livable. Second set RCA black plate, humm increased over gray plated set. Third set is GE 7199's, right channel scratchy/crackling sound, pulled them immediately. I find this interesting as all tested excellent on the tube tester, this only proves to me that testers do not tell all. I'm new at the power amp tube game but I am quickly getting the idea that tubes must be graded by ear. Your experiences in this area are greatly appreciated, thanks.
If it's 120 hz hum, look to the power supply. If you're into mods larger value filter capacitors might be the ticket. I have no idea of the circuit topology of this amp, so it's hard to say much more....
Did a quick search on your amp and came up with this thread:
I just had a similar problem on new Conrad Johnson Amp/Preamp. Here is the response I got from Spearit Sound eliminated the problem. Hope this helps you.

"I’m 99% positive you have a ground loop between the preamp and the power amp. Thank the Underwriting Laboratories for requiring equipment to have the third ground plug. This isn’t a problem with less expensive gear because they use all sorts of filters so you don’t have any hum. The problem is they tend to kill the sound of the components. cj prefers to do a clean design and solve the problem at the source, the wall outlet. The usual fix is cheater plugs. Try one on either the amp or the preamp and see if the hum goes away. If not, try a cheater on both plugs.

It’s possible that the hum is being induced. i.e. a strong emi field near the amp or preamp. It that is the case, and this is rare, it may be necessary to move the components slightly to reduce their susceptibility to emi and rf. Again, I do think the problem is a ground loop."
Yikes! The third prong (ground) on the power cord should never be a problem. If it is, this indicates that the manufacturer has not done their homework and figured out how to ground their equipment properly.

Using a cheater to look for ground loops is OK, but it should not be used on an on-going basis. There are certain types of failures that could result in fire or shock hazard. The equipment should be fixed! So if you have an amp or preamp that hums when the ground prong is plugged into the wall (and this will usually only manifest when connected to other gear) it needs to be returned to the factory and reworked.

If they tell you that is not possible (for example: "thank the UL Laboratory" or some nonsense, like that 'hum' switch on the other thread) sell it and buy something else that is properly engineered! Seriously, there is no excuse for this.
Dimmer switch?