Heathkit Volume problem

[Madman1]

Hey guys, so I have a aa-151 that’s in super nice original condition other than the few coupling caps that a tech changed as a precaution a few months back. I’m having a volume difference between the two channels. At first one channel would barely have any sound until half volume so I deoxit and then lubed it and it seemed to be good. Put it away for a couple weeks and tried it again the other day and noticed that one channel had to be at 3/4 to have the same volume as the other at 1/2 volume. Possibly not as clear also. I just put a set of barely broken in el84s in so that shouldn’t be the problem. So is this an obvious volume pot that’s dirty or maybe worn out or could there be something else internal that’s causing the issue? I guess it’ll go back to my tech but, wanted to see if anybody has prior experience with this issue in these units.
Thanks,
Mark

 

[Big Harry]

I own two AA151's and I think your issue is the volume pots and their tracking. One of mine has a slight imbalance between channels that can be compensated for by adjusting the individual volume pots for the correct balance as the pots have a friction coupling which is what I do with mine. The other alternative is to replace both pots with the hope that the new pots will track correctly.

 

[dcgillespie]

Mad -- You can prove out if the culprit is the volume control or not by using either a high level stereo source of known balance, or a high level mono source that is connected to both both channels -- BUT -- the source must have its own level control.

Operate the amplifier in the configuration as described, with the level control on the source full down. Turn the volume control on the Heath up full. Regulate the volume with the level control on the source. If the the sound is balanced in this scenario, then the problem is in fact the Heath's volume control. If it is still unbalanced however, then there are other issues to address in the Heath.

I hope this helps!

Dave

 

[Big Harry]

I found with both of my AA151's and on an AA151 that a friend owns, even after a complete rebuild which included all new caps, most of the resistors and any marginal tubes being replaced, that I still had a channel balance problem. I finally ohmed out both sections of the volume controls and found that there is a tracking problem with all three units, but one that appears to have a bunch more time on it is the one that has a noticeable balance issue. I used Dave's method by turning up the volume and feeding a signal from my audio generator into each channel while looking at the trace from the amp's output on my scope which led me to the volume controls.

 

[Madman1]

Thank you both for your advice. Now could I feed my advent 300 receiver/preamp into the aux input for the source? If that’s the case then I should just use a cdplayer or iPhone running into the advent?
Now BIG HARRY, have you replaced the volume pots in any of the 3 or do you just live with it?
Thanks again!

 

[Big Harry]

I would use your CD player connected to the aux input of your AA151 for a signal source. The amp with the marginal pot is used in my garage system so I'm not worried about a bit of channel imbalance. I have both pots adjusted to a slightly different level which works fine in my case. I also have a Dynaco PAS2 preamp with the same problem. The volume pots are worn which caused me to have to set the balance control off to one side. On that one, I put a resistor in series with the input leg of the lower resistance pot to get both channels to track correctly. Like Dave says, be sure that you do some testing before condemning the volume pots as the problem. Fortunately Mark Oppat at oldradioparts.net should be able to supply you with a new control if you decide go the replacement route.

 

[Madman1]

Thanks again Big Harry, you know I purchased a volume pot for my eico Hf-85 preamp from Mark. Honestly the newer one tracks only slightly better than my old one did. Now my CD player has no volume control of its own so I wasn’t sure if that would work. Oh well, I’ll probably just sell the Heathkit but, I want to be upfront with anyone that is interested.

 

[Big Harry]

If your cd player doesn't have a volume control than you can't use it for an input signal. You would have to use your phone if it has a level control in it. The problem with the older tube equipment is its age and the amount of use it has seen. The quality of the pots used in most equipment built many years ago to me is suspect at best as far as the tracking between pots which causes unequal volume levels between the channels. Even the new carbon pots can be suspect as far as tracking goes. The AA151 is a decent sounding amp even in stock form which is what both of mine are. Both of mine are good for about 14 watts per channel before they start to clip which to me is acceptable for use with efficient speakers. I'm using a pair of Bozak speakers on the garage system AA151 which sounds really good to me.

 

[triode17]

Don't forget, a vintage volume control is made of carbon composition so it will change value over time like a CC resistor, worse even because its an open (to the air) carbon comp. resistor.