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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?
ces4x4
ghostrider45
449 posts
 
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.
newbee
4,751 posts
 
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.
ces4x4 OP
16 posts
 
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.
ces4x4 OP
16 posts
 
Ghostrider45: To the best of my knowledge or I should say my best guess is its 120 hz.
ces4x4 OP
16 posts
 
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.
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