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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
marakanetz's avatar
marakanetz
5,473 posts
 
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.
lloydelee21
1,202 posts
 
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!
ces4x4 OP
16 posts
 
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!!
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
4,767 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.
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