Bluesmaster PI reprise

[martin manning]

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 (?)

When you are at this extreme limit, are you CW or CCW on the trimmer, and what values do you have in for the fixed resistors and the trimmer?

[Raoul Duke]

Trimmer is 10k, plates are 110k/100k. Trimmer balances at/near the 2 o’clock limit with the wiper being 6 o’clock.

This situation has me wanting to pull and try every pair of 6L6GCs I have here just to see what happens, lol.

[sluckey]

Does this balance trimmer issue cause any sound issues?

[Raoul Duke]

No Sir, no crackle or strange sounds either static or while sweeping. I tried just to see if the pot was at fault, seems fine.

[martin manning]

Trimmer is 10k, plates are 110k/100k. Trimmer balances at/near the 2 o’clock limit with the wiper being 6 o’clock.

I'm trying to get to what the resistances are when it is as near as possible to balanced, i.e. how many ohms on the input side plate vs the other side?

[sluckey]

No Sir, no crackle or strange sounds either static or while sweeping. I tried just to see if the pot was at fault, seems fine.

What I meant was does it affect the quality of the guitar sound?

[Raoul Duke]

Trimmer is 10k, plates are 110k/100k. Trimmer balances at/near the 2 o’clock limit with the wiper being 6 o’clock.

I'm trying to get to what the resistances are when it is as near as possible to balanced, i.e. how many ohms on the input side plate vs the other side?

Martin, balanced at 0 mvdc I measure 120 at the supply side/110k and 99 at the feedback side/100k for both JJ sets. The EH set is apx 1-1.5 mvdc at the limit (which is just a hair past the previous measurements).

[Raoul Duke]

No Sir, no crackle or strange sounds either static or while sweeping. I tried just to see if the pot was at fault, seems fine.

What I meant was does it affect the quality of the guitar sound?

No - everything sounds pretty much the same to me with the exception of the subtle variations between extremes WRT feedback and harmonics. Nothing sounds obviously wrong or odd.

[Raoul Duke]

Just tried a pair of old but strong pair of Mesa STR-415 (Sylvania/Philips “10,000 hour” tubes). Same result as the EHs.

Was it Agent 86 that said “missed by thaaaat much…”?

[martin manning]

balanced at 0 mvdc I measure 120 at the supply side/110k and 99 at the feedback side/100k for both JJ sets. The EH set is apx 1-1.5 mvdc at the limit.

So in theory if you went to 115k and 95k for the resistors a 10k pot would balance in the center with 120k and 100k.

I did some playing with a simulation of the BM power amp using the values shown in the layout, and found that the BM PI needs the ~20k offset in the plate resistor values to balance, with the trimmer set at 55%. Looks like your real-world hardware agrees, just shifted by 5k.

[Raoul Duke]

All right
Thanks Martin - that makes way more sense than my anecdotal extrapolations, lol.

I’ll get those values in and do that swap and report back. Funny how the subtleties make all the difference sometimes.

Thanks again for doing that simulation. My fingers are crossed

And as always - thanks as well for some more education.

[dorrisant]

balanced at 0 mvdc I measure 120 at the supply side/110k and 99 at the feedback side/100k for both JJ sets. The EH set is apx 1-1.5 mvdc at the limit.

So in theory if you went to 115k and 95k for the resistors a 10k pot would balance in the center with 120k and 100k.

I did some playing with a simulation of the BM power amp using the values shown in the layout, and found that the BM PI needs the ~20k offset in the plate resistor values to balance, with the trimmer set at 55%. Looks like your real-world hardware agrees, just shifted by 5k.

Not to get off topic, but how does that compare with others, like #124 or #183?

[martin manning]

Not to get off topic, but how does that compare with others, like #124 or #183?

With the 124 PI (plate loads, tail resistors, and NFB), it balances at 36%. Seems like the process was to get it within range of a 10k trim with the fixed resistors, where the required offset in plate loads is driven by the PI tail resistance.

[Raoul Duke]

Martin,
Meant to ask - is this slight change in the PI plates due to my building this as a 50w, individual fine-tuning, or proof and correction of the layout/schematic over time?

Just curious. Thanks!

[martin manning]

I don't know, maybe the amp that the layout was drawn from had a slightly different situation wrt the tube matching. I don't think the 50W vs. 100W has much to do with it.