How important is AC balance?

[jgordon]

I have a set of tubes from Jim Mcshane at home. I have my citation II on a fedex truck about to be delivered. I'd like nothing more than to put them both together tonight.

However, after ordering the tubes from Jim, I read his biasing instructions. At the bottom, there was a discussion on AC balancing, using resistors and test signals and whatnot. Had not seen this prior to ordering.

http://pages.prodigy.net/jimmcshane/adjust2.htm

I am reasonably comfortable with the idea of the DC biasing. I can read a meter and turn a screw. No idea what the AC balance is about. How important is it?

His pages also say that you should put the AC balance pots to the middle before doing anything. This is a restored amp (terry dewick) which was running in the system of a fairly experienced tube guy prior to my purchase. Can I assume that his AC balance was ok and leave it that way, or should I set them to the midpoint?

Thanks, appreciate your help.

 

[Westy56]

Assume that everything needs to be adjusted and follow jims steps to the tee.
A good volt meter will set your mind at ease.

Congrats on the delivery today.


Steve

 

[jgordon]

That's what I was afraid of. Any thoughts on where to find the appropriate parts? Resistors, wiring, and "test signal?"

And will I damage the amp or tubes if I just adjust dc bias and listen for a few days while the resistors and other parts arrive?

 

[Maron]

Your UPS guy is gonna hate you be careful when you get done and don't grab the resistors right away. They will be smokin hot!

 

[audiodon]

I think you can make haste slowly. After all, the amp was advertised as working. At least the tubeless one that recently sold on Audiogon was . . . if that's the one you were smart enough to grab.

Consider AC balance n'stuff refinement.

 

[Maron]

Radio shack may have the resistirs. I got mine from Mouser, they are 25 watters housed in aluminum. Looks like Westy56 is using the same. As far as the test signal, on the back right side under the RCA input jack is the test signal RCA jack. You just connect that to the input of the side you are testing via an RCA cable. Once everything is hooked up, turn the selector switch to the proper setting and make any necessary adjustments.

What has been done to the Deuce? Any upgrades?

 

[jgordon]

That is the one. Restored, Terry Dewick, and the pictures and history had me hooked. Terry vouched for the job he did on it in 2005 and its current condition. I was a little starstruck to have spoken with Terry and Jim within a few days. Adds to the excitement of the amp.

Much appreciated on the input and on the congrats. Shipping package was 75 lbs. Ouch. My doormen likely are not my biggest fans right now.

So I'll do the DC bias tonight and consider it acceptable, and then order the parts to make sure I'm good on AC. Unless Steve (who's given me a lot of great advice so far) or Jim (who I emailed last night) strenuously object.

 

[jgordon]

This is the work Terry did in 2005.

 

[Westy56]

Resistors are easy to find.

You can buy cheap white ones that the leads eventually break, or get the metal ones which last.
Don't worry if you can only find 15 ohm, they will work just fine.
You need skinny RCA heads for balancing channel B. Setting the AC balance takes less time than bias adjustment.

Just make sure the amp is warmed up. Recheck all bias adjustments with no incoming signal.


Steve

 

[jgordon]

Resistors are easy to find for those that know what they're looking for. i am in no way conversant in electronics. It's the main reason I dove for this amp - plug and play.

Jim is sending me a resistor and an RCA cable for testing. He's proven a great resource and source of advice. Thanks for your tips. Pictures to come this evening. I'm like a kid on christmas, cannot wait to get home and open my presents.

 

[dshoaf]

If you are using the same tubes in the same sockets as the last time the amp was adjusted, there's little need to redo bias and AC balance.

If, however, you're installing new output tubes, then you need to do the DC bias first. The amp should be warmed up at that time. Then move on to the AC balance.

I note from the repair sheet that the resistors in the cathodes of the amp weren't replaced. Jim's site discusses the change to another value and its implications. This isn't something you'll want to worry about right off the back but is good backup info. Consider it another step in ways to improve the amp a bit further.

When the amp comes in, be sure that the meter on the back has its needle falling to point where it is marked for natural rest. Mine doesn't and I've reverted to using a meter across each of the cathode resistors. Jim's site has a discussion on how to measure bias in this way. If the meter works, use it.

On your original posted question re: what is AC balance: This adjustment adjusts a sine wave passing through the amp for minimum harmonic distortion. The ideal way to adjust is to use a distortion analyzer and set the pot in each channel. HK built an approximation of this tester into the amp which uses the meter on the back and the extra RCA jack, which is a 60 hz sine wave generator. Pretty cool, actually, for something from the early 60s.

Hope that helps.

Cheers,

David