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VSi60 Bias Adjustment

Today I checked the bias settings on my VSi60 and I get a 0 millivolt reading from one of the tubes.   The amp was warmed up for 30 minutes before the test but was not connected to my speakers while testing.  The other 3 tubes show a 60mv reading.

To try to see if it was a bad tube I did try another tube set (KT-120) in the amplifier and I got the same result.   So it's probably not a bad tube.

I'm fairly new to tube amplifiers but I have had the VSi60 for a couple of years and I do like it.

My question is what do I do?   I cannot fit the plastic screwdriver that came with the amp into the slot of the bias adjustment of the tube that needs adjusting.   Should I unscrew the trim plate on top and make a bias adjustment?   Or is the amp broken and in need of service?    The amp sounds fine but I would like to know what's going on.





johndyson
5 responses Add your response
lostbears's avatar
lostbears
537 posts
 

 The tube with 0 bias is bad. When it went bad it took out the protection resistor.  The resistor needs to be replaced, It is a simple fix. Bring the amp to your local ARC dealer and he can replace the resistor. He will charge you very little if anything. 

 How old are the tubes and how many hours are on them? You need to replace the bad tube and depending on the hours maybe all the KT120 tubes. Also you should check the 6H30 driver tube. If a 6H30 tube goes bad, it can take out one of the KT120 tubes. But it is far more likely that the KT120 tube just went bad on it's own.


imhififan's avatar
imhififan
2,119 posts
 
Today I checked the bias settings on my VSi60 and I get a 0 millivolt reading from one of the tubes.   The amp was warmed up for 30 minutes before the test but was not connected to my speakers while testing. The other 3 tubes show a 60mv reading
I think the speakers should be connected to the amp outputs.
johndyson OP
15 posts
 
Thanks for the response.   It's good to figure out what is going on.    I bought the amp used on Audiogon and it came with 6550 and KT-120 tubes.   I don't know how many hours are on them.
lostbears's avatar
lostbears
537 posts
 

 I agree, you should never run the amp without speakers attached.

 I would replace all the tubes since you don't know how old they are. Could well be that the tube was at the end of it's lifespan and that is why it failed. Email Jim McShane, you can google his name with the word tubes. He burns the tubes in a bit before he matches them just like ARC. But he is much less expensive than ARC. A real stand up guy. I have been buying my tubes from him for years.

 I would still bring the amp to the local ARC dealer and ask him to replace the resistor. Most dealers will help you out hoping that in the future you may become a customer. It is a real quick and easy fix. I would bring the new tubes so that he can test the amp.



johndyson OP
15 posts
 
John Rutan at Audio Connection replaced the resistor and as one of the 6550 tubes was bad he switched the tubes to the KT120's that I had.   John also said that I should  to replace the 6H30 tubes because they were tired.

The repair was fine but I prefer the 6550 tubes to the KT120's in this amp.   I prefer the sound stage and the tone of the 6550's.  I ordered a new tubes. 
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