Scratchy balance pot on Ultimate PAS build

I've put together a pre amp using the Regulated Power Supply, PAS Aikido and PH10 boards including new blue volume and balance pots. Also added the filament mod which has reduced the hum problems to a certain extent but not entirely.

My question and concern has to do with an annoying scratchiness that I hear when I pan the balance pot from the center position to the right speaker position.  The sound and accompanying violent woofer movement comes out of the LEFT speaker. If I pan the balance pot to the left there is no scratchiness. I had the same problem using the original PAS balance pot so I switched in a new balance pot from tubes4hifi so I don't think it could be the pot.

Does this sound like it could be tube related? I've tried different 12AU7's from my stash since I have quite a few to play around with.  I would appreciate some advice before I break down and order 4 more 12AU7's. Thanks

DC on the balance pot left channel, across its internal resistive element?  Check it out with a voltmeter.

Thanks for tip GP49. I checked the DC voltage on the left balance pot as you suggested. Testing from the grd to the left output with the pot is centered there is negligible DC, but as I rotate the pot to the right I'm seeing voltages of up to 150 mV. I saw no DC on the right channel.

The pre-amp sounds terrific otherwise, I wonder if there is an easy fix.

150mV...no wonder the crackling is loud and the woofer pumps.  That could be enough to drive the linestage to full output.

I don't have a schematic for your preamp setup, so the following is rather generic:

The varying resistance to ground of the left channel balance control is varying the DC as you rotate the balance control toward the right channel.  Unless there is a wiring error, that DC can only come from the input side of the balance control or the output side.  In a stock PAS, if one turns the volume control all the way to ZERO, that shunts any stray DC that could come from the control grid of a shorted, defective tube.  IF in your preamp the volume control follows the balance control like it does in the stock PAS:  Do you still see the varying DC on the voltmeter when the volume control is at zero?

If it is getting in from the input side:  does this happen on phono, or line level, or both?  Is it on any particular line level source?  The phono stage and the external line level sources should be the only sources of DC to the input of the balance control.

Further testing shows that I'm getting up to 150mV DC and the scratchy sound with pumping woofer even with the volume at zero. I see DC when selector is on Phono and all the other line inputs.  Additionally if I turn up the volume control I begin to see DC even when the balance pot is centered.  

Looks like I may have to throw in a DC blocking cap somewhere ?

Just to see what happens: swap the first tube after the volume control, between channels.  If the problem moves to the right channel, you have a bad tube.

If not, put the tubes back, then insert a shorting plug into a line input, and select that input.  What do you get?

seems you are getting DC on the balance pot from somewhere, but the phono output is capacitively coupled, so shouldn't be any DC there.
Is your balance control grounded on both channels?   Common ground to the line stage?

Roy, my balance control is only grounded to the line stage board through the left channel but I have the short ground jumper linking both (right and left) negative channels on the pot as shown in the wiring diagrams.  Tell me if that's incorrect but I believe that's the correct set up. I did try additionally jumping the spare twisted pair white right ground wire directly to the line stage board but this didn't help.

GP49, still haven't tried the shorting test yet since I don't have any of those and don't have the time or energy after a long day at work to try to make up a couple. I'll see if Radio Shack has any in stock.

I did see some promising results when I switched the first tube from the left channel with the tube on the far right of the board. The scratchy sound has now been reduced significantly and I'm only reading about 20 mV as I pan the pot to the right. The odd thing is the sound still isn't present as I pan to the left. That's not what I expected!

Maybe my tubes are to blame for this after all. These four tubes were the ones that tested strongest of my 12AU7 stash but I have to admit I never really should rely completely on my tester. Seems the best tubes tester is the particular piece of equipment you intend to use the tube in.

I also disconnected all my inputs and learned that the sound is still there regardless of what's connected to the preamp and what position the selector switch is on. 

So after all these latest observations do you think I should just go ahead and buy 4 brand new matched tubes and hope for the best or are there any other ideas to pursue?

yes, that's the preferred connection for the balance control to prevent a ground loop, although in most cases I can't hear any difference.
Not sure at all why you are having this problem.
Also you are correct, the very best tube tester is the equipment you will use that tube in.
My VTA70 amp is more accurate for testing/matching tubes than ANY tube tester I have ever had,
and for preamps, especially phono amps, hearing the tube in the circuit is the best tester.

I just broke down and ordered a quad of new matched Electro Harmonix 12AU7's.  Hope I didn't waste my money. I usually don't like to just buy replacement tubes hoping that will solve the problem but in this case it may be the right choice. I ws working with quite a hodge podge of old tubes pulled from test equipment and old communication receivers.

I'll report back after I the new ones are in place. Thanks for the ideas guys.

Elrick wrote:I just broke down and ordered a quad of new matched Electro Harmonix 12AU7's.  Hope I didn't waste my money. I usually don't like to just buy replacement tubes hoping that will solve the problem but in this case it may be the right choice. I ws working with quite a hodge podge of old tubes pulled from test equipment and old communication receivers.

I'll report back after I the new ones are in place. Thanks for the ideas guys.

According to the schematics there is caps before the balance pot. If you have DC here a cap is leaking OR you have a wiring error. New tubes cannot fix this ( and the pas uses 12AX7 not 12 AU7 ).

yes, the PAS-AIKIDO board uses 4 12AU7 tubes

tubes4hifi wrote:yes, the PAS-AIKIDO board uses 4 12AU7 tubes

Oops, my mistake, i thought it was about dynaco PAS 2/3.  I will take back everything i have said and strongly argument for the opposite ;-)

Peter (and everyone in general) read post 1, then speak !

The new matched quad of replacement tubes arrived last week and I'm happy to report that the problem has impoved. I've move them around a number of times and have settled on the best combination for the 4 of them. Also decided to run the preamp through my variac that the rest of the system employs at about 116V instead of house power at about 123V. This also decreased the problem even more to the point of almost going away entirely.

I guess I'll just have to live with it and just keep my paws off the balance pot.

Glad the new tubes helped. Still, I can't imagine a competent designer putting out a circuit with DC across a user-operated control. Something may still be amiss, especially if the condition does not appear in the other channel.

Thought I would add some final thoughts to this odd problem and not leave it unfinished with a negative spin. The scratchy noise issue has been completely eliminated. Chock it up to an issue with the tubes I was using. I never thought for a moment that it was due to a board design issue. Most likely operator error.

Yesterday for S's & G's I decided to replace the left and right twisted pair output wires to the RCA jacks from the Aikido board using some nice shielded coax salvaged from an old dead pH probe from work. This has reduced any hum to a level where I can't hear it from my listening chair so that's more progress. If I stick my ear right next to the right speaker I can still hear a slight hum but for me it's a non issue.

I also decided to try some O rings on each tube to help reduce microphonics. The final job for this Ultimate PAS transformation is to drill a series of ventilation holes to the top of the cabinet. Inside the case it measured over 140F the other night after about 6 hours of continuous high volume entertainment.

This unit has been a little touchy to put together but I'm no expert builder. My stereo has never sounded better. Thanks for the tips and help.