Bluesmaster PI trimmer

[greiswig]

I gather from the stock D'Lite layout that the "high" side of the PI plate voltage is supposed to be on the same side that gets feedback, since the lower plate resistor is there.

I also notice that just switching out the tube can reverse this.

So...how important is it that the feedback side be higher in voltage? Important enough that every time you change it you should crack it open and check and adjust the trimmer?

[dogears]

The feedback side gets the bigger resistor. If a layout says something else, it is wrong.

I gather from the stock D'Lite layout that the "high" side of the PI plate voltage is supposed to be on the same side that gets feedback, since the lower plate resistor is there.

I also notice that just switching out the tube can reverse this.

So...how important is it that the feedback side be higher in voltage? Important enough that every time you change it you should crack it open and check and adjust the trimmer?

[dobbhill]

In the PI, I believe it's about balance, not voltage. The trimmer should be adjusted for AC balance every time the tube is changed.
D

[Fischerman]

Been meaning to ask this for a while and this seems like a good place (I hope you don't mind greiswig!): When adjusting the PI trimmer...does or has anybody disconnected the NFB (which tends to 'correct' or balance the PI) to do the adjustment and then reconnect it when done?

I realize that 'whatever sounds best' is the right setting and we play the amp with the NFB connected but still...was curious.

[Structo]

Sorry I'm getting confused between the layout and schematic.
On my eyelet board I put the higher value resistor on the left side of the plate resistor pair of the PI.
Is that correct?

[Fischerman]

The main 'input' side of the PI gets the lower value plate load resistor and the second side gets the higher value plate load resistor. I can't say left/right because it depends which side of the chassis you're on and how it's wired.

EDIT: The grid that gets the input from the preamp (should connect to a .022uF cap and 1M resistor on the board)...that corresponding plate gets the lower value. The grid that is AC grounded through the .1uF cap (and Presence circuit)...that corresponding plate gets the higher value.

[Structo]

Ok, yeah I should have mentioned that I was talking about the rear of the amp facing me, and it is a D'Lite.
But I think I have it right so no worries.

[greiswig]

The feedback side gets the bigger resistor. If a layout says something else, it is wrong.

Okay, I checked my layouts, and I was reading them wrong. You're right: the higher value resistor goes on the feedback side.

So...how important is this, and what is the effect of changing it around? Like I said, a tube change can change this drastically, and on amps without a trimmer there, a user is pretty much incapable of doing anything about it other than trying to find a balanced PI tube.

[Fischerman]

It just makes the PI even more unbalanced. You're likely getting a lot more signal at the PI main input plate than you are at the feedback plate. A BM PI just makes it even worse because the BM PI doesn't balance as easily due to the lower tail resistor (9.1k instead of 24k). Sometimes even with 82k/100k you can't get the 100k side up enough to match the 82k side...when using the BM PI that is...but when using a 24k tail it's much easier to balance.

This is one reason a balanced PI tube is not necessarily better and may actually be worse in some amps. If you don't know how unbalanced your PI is...and in which direction...then it's impossible to say if a balanced PI tube will result in a more balanced PI or not...it may make it more unbalanced.

[Structo]

I have read some pros and cons of using a balanced PI tube.
Some say it is essential while others say having it off balance influences more harmonic content in the tone.
Surely you wouldn't want it way out of balance side to side.
But even with so called balanced tubes there probably is enough difference to negate the perfectly balanced theory.

BTW, how do you adjust a trimmer there, by the plate voltage?

[dogears]

HUGE difference Incredibly noticeable. You are so far out of balance the amp will be dead feeling and not bloom at all. I had an amp that had this very issue once. When I swapped the plates there was an amazing improvement. We are not talking minute trimmer balance here. But, massive total imbalance. Big difference.

So...how important is this, and what is the effect of changing it around? Like I said, a tube change can change this drastically, and on amps without a trimmer there, a user is pretty much incapable of doing anything about it other than trying to find a balanced PI tube.

[Structo]

So do you look at the plate voltage to see how balanced the PI is?

[dogears]

Nope. Not at all. Only a scope.

Just set to a 12-13K difference as a baseline. Then adjust trimmer a little each way while hitting the low E string. Set to taste. I use a balanced PI tube since they will give me repeatable results and are screened better.

So do you look at the plate voltage to see how balanced the PI is?

[greiswig]

Nope. Not at all. Only a scope.

Just set to a 12-13K difference as a baseline. Then adjust trimmer a little each way while hitting the low E string. Set to taste. I use a balanced PI tube since they will give me repeatable results and are screened better.

So do you look at the plate voltage to see how balanced the PI is?

I've heard others describe the "sweet spot" as often being around 5-10V difference, but now I know which side is supposed to be higher. Even with the difference in resistors, on some of my tubes the feedback side goes higher, voltage-wise.

In order to cope with that fact, and partly because I just like redundant parts, I set up my BM inverter circuit to have a 1M trimmer in parallel with a 180k resistor on the non-FB side. That gives me a lot of adjustment room. I know...it's not HAD approved. But I tried it, and it seems to sound good.

[dobbhill]

This is how I do it. Got the method from David Manley of Manley Labs/ Vacuum Tube Logic.
https://tubeamparchive.com/viewtopic.ph ... ht=balance


D