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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

Showing 5 responses by ces4x4

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: 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!!
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!!
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
 
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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