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Setting bias on a vintage ARC

I have some new matched  KT 90 tubes ARC V35 amp.

im setting the bias, but I’m a bit confused. One channel can be set to the 37.5 mVDC, But when I check the other channel, I get 34.00 mVDC.

Where I get confused is there is only one pot, so does turning the pot clockwise or anti clockwise shift the balance to get the same number between the two sides OR does turning it one way or the other raise or lower both sides equally, and if so how should I adjust the the bias setting?

btw the recommended setting is 37.5 mVDC. 
last_lemming

Showing 1 response by lostbears

lostbears's avatar
lostbears
538 posts
 

You need to use matched pairs of tubes. Let the amp warm up and run idle for about 15 minutes or so. The amp needs to be at operating temperature with no signal. I always check bias after listening. Mute the preamp and wait a few minutes. The manual says to set the bias at 75ma per pair. ARC likes to run tubes hot. I would set the bias a little low, maybe 72ma. I ran my 6550 tubes at 62. It does not hurt to run the bias a little low as long as it doesn’t affect the sound.

I would call ARC with the serial number of the amp and see if it has ever been serviced. If the large caps are original, they need to be replaced. If the caps leak, they can damage the circuit board. It doesn’t not hurt to let ARC go through the amp and bring it to spec.
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