ADL 1000 distorting on kick/bass.. but otherwise sounds fine?

[Indecline]

I’ve had an ADL 1000 for a couple years and it’s always had this problem.. but it only affects kick/bass.. on all other material it sounds great with zero distortion  ???

If I insert it in a kick, about every third kick or so it distorts. Same on bass, fine for a few seconds then it distorts, then fine for a while longer till it happens again. It works perfectly on any other material (vocals, horns etc), but on kick/bass it craps out intermittently. I’ve read about discrepancies in part values between units (old models vs new, 1000 vs 1500), but not sure if that is true. FWIW, this is a newer unit with a PCB layout if that matters.

I found a post by Jim Williams mentioning upping the coupling caps on the output tubes (12ax7, 12bh7) because it “robs the low end and causes phase shift.” Which kinda makes sense if the caps aren’t holding enough charge for the transients.

I’ve swapped the ax7, but not the bh7. I have access to a vintage la2a I could swap if needed, but I don’t think it’s the tubes since it works fine otherwise..

Any help would be great!

 

[dbelousov]

It makes sense because Jim believes that their capacitance is too small for given impedances. It's easy to check and measure, but he is usually right.

With bigger caps, there will be more lows, hence more distortion.

Check the tubes first.

 

[Indecline]

It makes sense because Jim believes that their capacitance is too small for given impedances. It's easy to check and measure, but he is usually right.

With bigger caps, there will be more lows, hence more distortion.

Check the tubes first.

I’ll grab parts for Jim’s recommendation since I can get those in town, and order tubes if that doesn’t solve the issue first.

Also to note for future ADL owners: I did two other mods recommended by Jim. Changed R34 in the in the LA2A schematic to 15K (was 22k), and changed R7 to 2.7k (was 1K).

A couple others on the web have said the PCB version is missing R30, mine PCB has that on board, so that was fixed by ADL at some point. C14 is missing (variable bypass cap) which I did not change.