BadCat Judah and PPIMV

[The Dude]

Got a BadCat Judah on the bench. I have it all repaired and "working" with one question. It has a PPIMV 1M pot in front of and across the output tube grid resistors. It is factory built this way- not a mod. It's very hard to control on the low end and it reaches max at about 11 o'clock. The pot is good and I even subbed it with a new one to be sure. It seems and acts like a backhoe attempting to do the job of a hand shovel. I subbed in a 250K pot temporarily and it works much better- easier to control on the low end and reaches max output just before the end of travel instead of midway up.

My question is twofold. 1) Are there any potential negatives to just permanently installing the lower value pot? 2) This is a new amp brought to me by a store client. It was sold and returned partially because of this problem. What are your thoughts on "modifying" a brand new amp?

[Enzo]

Perhaps a call to the company? Do we know it is the proper value pot? Like a mistake may have been made at assembly?

[The Dude]

I've had ZERO luck getting schematics or much else out of them, but you're right. Maybe they'll at least tell me what that potentiometer value is supposed to be. I'll give them a shout tomorrow.

[drewl]

That seems right as if you look at the Matchless schems which are similiar, they usually use a 1Meg or 500K pot in the cross line master volume.

Does it have a tone cut pot in parallel also?
Is the pot linear or log?

Changing to a lower value for a better taper "may" lower the output some, but maybe not enough to be noticeable.

[The Dude]

It does have a tone cut in parallel also- a 250k pot and a .0047 cap. The Master pot is a "D" taper log pot. I scoped the output to just before clipping with both the 1M and the 250K. No noticeable loss of output with the 250K. The only difference was that full output was reached with the 1M at about 11 o'clock and the rest of the pot travel made no difference. With the 250K, max output was reached at about 3 o'clock.

How I arrived at 250K was this. I slowly turned up the 1M pot until there was no discernible difference or raise in output as seen on the scope. I then measured the value of the pot at this point. It was approximately 200-220K.

[Chuck H]

It has a PPIMV 1M pot in front of and across the output tube screen resistors.

And then:

It does have a tone cut in parallel also- a 250k pot and a .0047 cap.

Should I assume were talking about the grid circuit and not the screen grid circuit? Not being a pecker, just want clarity. drewl mentioned the earlier Matchless amps using the cross line master that was on the G1 circuits. I'm not familiar with these being referred to as "screens" so I'm just asking.

And as far as I'm concerned anything about the control grid circuit that doesn't alter bias control is still a relative voicing issue. If the 250k pot works better then it IS better. (see the period) If you've tested and found that full output is achieved then that's all that matters. 1M is simply too large and is just a throw back to the previous designs. I suppose there may be some EQ affect from the different cross line impedance, but I wouldn't expect it to be anything detrimental. Possibly not even audible.

This guy say's go for it.

[Jazz P Bass]

I had A Vox AD100VT come through that used the master volume as grid resistors.

Guess what happened when the pot got broken.

Actually it wasn't broken.
The stop detent wasn't stopping the full off rotation.
The wiper ended up on 'nothing' on the wafer.

[The Dude]

Yes, Chuck...I meant to say grid resistors. My bad. Brain fart. Apologies for the confusion. I am going to edit the original post so as not to confuse others who might read this thread. Thanks for pointing out my error.

[Zouto]

I used 500K A pots in different Black Cat Amps and an old Matchless DC-30 for a much better control of the cross line MV. It works fine without any negative side effects. I also tried 250K A pots before, but liked the 500K A's travel much better. A 1M pot for cross line MV acts like an on-off switch and is quite impractical. Old Matchless DC-30s use a 1M pull pot to switch off the cross line MV. All the Bad Cat amps I've seen had that useless 1M pot as MV, so it's a good idea to put in a pot of a more usuable value/range.
Good luck
Zouto

[The Dude]

Thanks for your insights Zouto! I think I'll give the A500K a try. Whether or not it's supposed to be a D1M part or not from the factory, it doesn't work right. As you say, it acts more like a switch than a variable pot. It seems quite unusable as it is.

[The Dude]

I had to order some decent quality pots with the correct physical parameters and I'm still waiting, but in the mean time, I've heard back from BadCat. The factory part is indeed 1M. However, the tech consulted at BadCat also recommends and prefers a 250K. I've got some of each on order. I'll report my findings when they arrive.

[The Dude]

In case anyone's playing along, I got the pots yesterday and tried them both. I ended up going with the 250k and the MV works much better and smoother. It still reaches max out without any problems.