Bluesmaster PI reprise

[Raoul Duke]

Update:

I am on a quest to get this PI to balance with some adjustment travel to spare using the Ampeg method. To that end, I’ve tried every 12AX7 that I own in the PI as well as swapping the 10k trimmer for a 20k. What I’ve found is that no matter which tube I use (including three that I’ve recently had tested and verified “matched triodes” by a tech with both a Maximatcher and eTracer) and regardless of which trimmer (10k or 20k), my adjustment range is very small. If looking at the PI trimmer from the back of the chassis where the wiper is at 6 o’clock, my range is from 2 to 3 o’clock (with 2 being the CCW stop) with both trimmers. Additionally, 2 o’clock doesn’t always get me there - especially with the 3 matched tubes - all 3 have a lower limit of around 1.6 mVdc.

I’m about to try the “drop plate load values and install 25k” suggestion, but given the pattern that has emerged here - I’m wondering if only one side’s value needs to change (i.e. 100k/91k)? The consistent “2-3 o’clock sweet spot” seems odd to me and I’m concerned that dropping both plates proportionately will yield the same result. Should I be looking at the tail resistor value as well?

Any and all opinions are welcome.
Thanks!

[martin manning]

Hmm. The adjustment range in mV with a 5k trimmer will only be around +/-5.

If you are at less than half-power, try increasing the signal until you get there. With an 8-ohm load, 25W would be 14.4V RMS. I'm wondering if you are not driving the output tubes fully into cut-off.

[Raoul Duke]

Thanks Martin!
I’ll definitely try that and post results. I did notice that the more signal I fed in, the more the adjustment moved toward center - but I didn’t go up to 14.4. IIRC I was fiddling between 11.5 and 13.5 and noticed the (slight) change.

Been searching around here for clues and have found some older posts that talk about HAD adjusting the plate load values and it seems there’s a little wiggle room around what the layouts show. The only consistent thing is that he seemed to avoid the Fender 100k/80k combo in favor of consistently higher values in the PI. Interesting reading.

[martin manning]

Just to be sure, do you have the fixed resistors installed correctly? 110k on the left, 91k on the right as viewed from the back of the amp.

[Raoul Duke]

Yes, I wondered if I had reversed them as well (that would be the typical mistake I’d make, lol.) I re-verified all the resistor values from the V3a .02uf grid cap going right to left on the main board and everything matches the layout.

Could the unusual OPT have an effect on this?

[martin manning]

Could the unusual OPT have an effect on this?

Not likely.

[Raoul Duke]

Just warmed everything up and tried 14.4V. At 400hz and it made very little difference; but at 100hz (I think I read that Gil uses 100hz?) gained me back apx 1.5mv. I understand this really isn’t much but thought it worth mentioning. The adjustment range is still from the CCW limit at 2 o’clock back to about 3 o’clock.

I’ve tried 3 pairs of well-matched 6L6GCs: the JJ set in my 102, my “trusty” start-up set of JJs, and the new EHs. Both sets of JJs balance at 12.5V between 2 and 3 o’clock and the EHs hit the CCW stop at apx 1.2 mV. Both sets of JJs balance toward the center of the 10k trimmer in my 102 and 2nd gen.

What else is left but adjusting plate resistor value?

[martin manning]

I think that's where you are. When you run out of adjustment are you bringing the plate loads closer together in value? Note your 102 has 120 and 110, 10k apart, and the BM as-drawn has 110 and 92, 18k apart. If they need to come closer together, replace the 92 with a 100.

[martin manning]

Are you saying the JJ's will balance in the BM, but the EH will not?

[Raoul Duke]

Yes, the JJs barely balance - one pair just a hair back from the 2 o’clock stop, the other at the full 2 o’clock limit.

Yes, the EH plate loads are within 1mv at the 2 o’clock from apx. 4 mv at 6 o’clock - so I feel like closing that gap a little should buy me the adjustment I need. I would think it should work fine with the 10k at that point?

I looked at the plate values on both my other amps and noticed they were closer - so that reinforced my hunch even more. Thanks for the guidance Martin! I’ll post results.

[Raoul Duke]

Ok, went ahead and swapped a 100k in place of the 91k and reinstalled the 10k trimmer thinking it would help me dial in better all things (plate resistor values) considered. What has actually happened is the ~1mvdc gap I had at the 2 o’clock stop on the trimmer is now ~7mvdc. The range on the trimmer with these resistor values is now 7mvdc at 2 o’clock limit and 18.5 at the 10 o’clock limit where before (110k/91k) it was 1 and 11.2mvdc respectively.

Also, when feeding the 400hz sine wave into either end with the PI trimmer centered, the pin 8 cathode balance measurement scales up and down pretty closely with the AC load; I.e. 12V across the load resistor equals 12mv at the cathodes. I don’t know if this means anything, but it seems a little too coincidental.

My inexperienced gut feeling is that the 110k might need to be 100k and the 91k should go back in? More confused now - but determined to figure this out

[martin manning]

This is weird. Can you measure the resistance from the plate leads to the B+ lead feeding the trimmer at the extremes of trimmer rotation? Let's make sure that the trimmer's resistance is getting added to the plate loads as it should be.

[Raoul Duke]

On the 110k (V3b):
Cw: 109.4
Ccw: 119.7

On the 100k (V3a):
Cw: 109.1
Ccw: 98.7

Measured from the plate leads at the .02uf coupling cap eyelets on the board to the wiper/B+3 eyelet.

A couple hours ago I lifted the board to see if I knocked the V3a plate lead out while replacing the resistor. Yikes! I don’t want to do that again unless it’s absolutely necessary. The lead wasn’t completely out, but certainly a dubious connection. I pushed it up and back through the eyelet and re-hooked it. Solid now; and while I was down there - checked everything else.

[martin manning]

Well the resistance checks out. You can balance it with the JJ,s, but not with the EH? If so it seems like the EH transconductances are not closely matched.

[Raoul Duke]

I thought about that but couldn’t reconcile why the JJs balance in the same (extreme limit) part of the trimmer travel where the EHs come within ~1 mvdc - which leads me to believe all three pairs are close in quality (?) - and I think the JJs are better than average because my “start-up pair” has biased fine in everything from vintage Fenders to these three D-clones. Granted, this pair has only been put in a PI balance circuit three times - but they balanced fine in my first two builds. The second set of JJs didn’t need much bias or balance adjustment following the start-up pair in the first two; which is what I base my qualitative assumption on.

Of course; I’m just going on limited knowledge and none of my supporting info is scientific, lol.

I got the tubes from Viva Tubes and they appear close based on the labels (2140/15.5, 2340/15.4). I’ve always had great luck and service with them - never a crappy tube (far as I can tell). I’ll email them and see if they have had any QC issues with these.

Back to the three similar sets balancing in roughly the same place - should I try a different value on the supply side with the 91k back to the feedback side or is this likely not a solution?

I know I’m all over the place here and appreciate your patience . Thanks!!!