Bluesmaster PI reprise

[Raoul Duke]

Hi All,
Just put a brand new set (from a reputable dealer) of matched EH 6L6GC in my BM and went through the static bias process. Everything is good; they biased close to where the JJs I started with were.

Then, I went to re-balance the PI and found that I couldn’t get anywhere close. The trim pot centered is ~5.5. Maximum CW drops me to just over 3 and max CCW gets me to around 11. I verified the resistor values, tried a couple of other closely balanced PI tubes and always have the sticky thread up on my I-pad and follow the Gil/Martin process to the letter (tried the signal going into power amp in and normal input - same results either way).

After a bunch of searching here - my conclusion is either put the other tubes back or start fiddling with PI plate resistor values. Is there another choice I’m not aware of?

Any advice is greatly appreciated!
Thanks!

[bepone]

This is measured AC voltage n cathodes. ie current on 1ohm resistor in cathode?

Change place of output tubes in sockets from 1-2 to 3-4

[martin manning]

Just put a brand new set (from a reputable dealer) of matched EH 6L6GC in my BM and went through the static bias process. Everything is good; they biased close to where the JJs I started with were.

What are the individual mV readings? I assume you had the pairs of test jacks on each side shorted together when you were adjusting the trimmer?

[Raoul Duke]

Yes, measured mV DC across cathodes of power tubes with 12v AC measured across load resistor.

Mine is a 50(ish) watt build and I tried swapping the two power tubes around and got similar results - so no improvement with that.

The JJ/Ruby HG 12AX7 I originally was using had 290/272/40.7 (plates a, b, and cathode) compared to the Sovtek WXT I have in there now - which reads 301/275/36.5.

Should I be searching for a tube with closer triode voltages?

[martin manning]

Sorry forgot you are a 50W, so swapping power tubes won't matter.

How close are the power tubes in idle current? Might have to rethink the process.

[Raoul Duke]

Well you generously help so many people it would be difficult to remember everyone’s specific builds Martin, lol.

The tubes are within 2.5 mvdc of each other at idle - which gets closer to 2 mvdc once it’s warm.

[martin manning]

Well that's pretty close. Were the power tubes you had in it before further apart? Of course the transconductance doesn't necessarily go with the idle current...

[Raoul Duke]

The JJs (which I put back in) were about the same variance and within a sixteenth of an inch in bias pot wiper travel compared to the EHs. When I observed all that I figured the PI would be close too. Nope

I don’t understand why, but the bottom line for me is the JJs sound decent and everything adjusts as it should. Can’t really argue with that I guess?

Makes me wonder what I’d have done if these were the tubes I did the build with. I imagine I’d be in the same situation as Dan with his 2nd gen and adjusting plate resistors?

[dbharris]

The JJs (which I put back in) were about the same variance and within a sixteenth of an inch in bias pot wiper travel compared to the EHs. When I observed all that I figured the PI would be close too. Nope

I don’t understand why, but the bottom line for me is the JJs sound decent and everything adjusts as it should. Can’t really argue with that I guess?

Makes me wonder what I’d have done if these were the tubes I did the build with. I imagine I’d be in the same situation as Dan with his 2nd gen and adjusting plate resistors?

Hey Marc,

In my #102 I had a similar issue and solved it by swapping in a 20K trimmer. Might be worth a shot for you.

-Dan

[Raoul Duke]

Hey that’s a simple and effective solution that I didn’t even think of Dan!

Thanks, I just might try that

[martin manning]

Makes me wonder what I’d have done if these were the tubes I did the build with. I imagine I’d be in the same situation as Dan with his 2nd gen and adjusting plate resistors?

That would be one option. Closely related to that would be to drop the values of both plate resistors by 7k or so and install a 25k trimmer.
Have you considered getting an o'scope? There are some decent tablet scopes around that would let you observe the current pulses.
It would help if you could determine just how well matched the PI tubes you have are.

[Raoul Duke]

I think about it a lot - I just get overwhelmed with the choices and which specifications are important and which brands are reliable.

Two channels and a built in function generator are about all I think I need as far as features go. Any suggestions are certainly welcome.

Regarding your suggestion Martin; is changing the plate resistors and the 25k preferable? I think I follow the logic from a proportional perspective - but I imagine there’s some math behind it? Would any changes around the PI plates and trimmer have significant impact on the “whole” circuit or sound at all?

Thanks again!

[martin manning]

With a balanced PI tube, a 10k trimmer should be sufficient. I believe some builders go with a 25k as standard to avoid having to use selected PI tubes. The math is just the relationship between plate load and gain, and how much offset in the loads is needed to equalize the gains in the LTP triodes for a given mismatch in amplification factor mu.

Most of the little tablet scopes don't include function generators, and the few that do seem to get low ratings. The scope-meter seems to be trending, and they are getting to be pretty inexpensive. The good news is you don't need a very high-performance scope for audio work.

[Raoul Duke]

So it is likely that the PI tubes I’ve been using in this build aren’t that well balanced then?

I didn’t really put a lot of thought into selecting the PI tube out of my box of tubes. I’ll dig a little more and see what might be hiding on me and give it a whirl.

Is higher plate voltage desirable in a PI tube? I ask because I know I have a couple of ‘60s GE 5751s around that I’ve tried in the other amps where the plate voltage on “A” and “B” were within 2-3 volts, but they seemed a little on the low side - I.e. where a new JJ would average around 190-200 on each plate, the GEs would average around 170 on each plate. Better balance, but lower plate voltages.

[bepone]

2mV really is very small negligible, so it is ok.. if is wanted zero (but really no needed), change - swap output tubes socket1-socket2 and retry