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Air Tight ATM-2 Owners - Help with Bias setting

I recently bought a used Air Tight ATM-2 amp. I'm having some difficulty understanding Bias setting procedure. I was hoping other ATM-2 owner might help shed some light on this. I'm not sure if I'm reading the instructions correctly.

Also, after completing the procedure I turn the bias test switch from V1 through V4 to see if they show where each was set, but for V1 and V3 the needle is way off to the left but while v2 and V4 appear correctly on bias meter.

Here's how I'm doing this:

1. Placed the Auttenators to "Min".
2. Turn the Bias Test Switch to V1
3. Turn the left-channel screw labeled as "Bias" - the 2nd screw from left. Turn it until the needle on meter is within lower end of the sausage shaped thing.
4. Turn the left-channel screw labeled as "Balance" - the 1st screw from left. Turn it until the needle is in lower end of sausage. FYI, this did not budge the needle on left channel at all.
5. Turn the Bias Test switch to V2 then repeat 3 and 4.

Then do the same for right channel (v3 & v4 and assoc screws).

For some reason on my left channel the screw labeled as "Balance" does not seem to affect the needle on bias-meter regardless of if the switch is set on V1 or V2. But
on right channel both screws (Bias and Balance) seem to affect the needle movement.

My reading of the instructions is as follows:

For each of the four switch positions ("v1 & v2 - left channel" and "v3 & v4 right channel") I must first adjust the screw labeled "bias" then the screw labeled "balance" and for each screw adjustment the needle on meter should be somewhere within the sausage shaped bias marker.

I hope you can help clarify the correct procedure, also why the the "balance" adjustment screw for v1 and v2 does not seem to move the needle.
jewel_hasan

Showing 1 response by audiofile9

audiofile9
45 posts
 
Not sure about the exact procedure, but two things I might mention:

There are a lot of "poor" quality KT-88's out there, some were especially prone to jostling such as during shipping.

The Balance is probably meant to adjust for differences between the two tubes of the same channel. Thus, as purely a GUESS, I would think "Bias" should be set until the needle is within the sausage while monitoring V1 - AND - V2, back and forth. I would then expect the "Balance" to adjust until both the V1 and V2 switch positions are showing the same meter reading.

I could be way off, but hopefully that is something to diddle until someone more knowledgeable jumps in.
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