2. amps-preamps
  3. 1343744611

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
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.
ghostrider45
449 posts
 
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....
ghostrider45
449 posts
 
Did a quick search on your amp and came up with this thread:

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1341061762&openfrom&1&4#1
endersagain
1 posts
 
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."
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