Universal(ish) "T-Bar" for 6386 alternative

[thecr4ne]

I've been looking into various builds for Fairchild 670 's and I recently came across Manley's "T-bar mod" for their Vari-mu, as an option to use a pair of triode wired pentodes (6BA6s I believe) in place of the rare and expensive 6386. I'm wondering if such a T-bar could be designed for use in place of any 6386, or if it would need to be designed specifically for the circuit it's meant to be used in.

Thoughts?

 

[KrIVIUM2323]

Hi,
it can be designed to be used in any place a 6386 can be used (but keep in mind that max anode voltage and max power will be differents however), but as 6ba6 is a pentode wired as a triode you'll have to take care of the way you wire g3 (supressor) and g2 (screen grid).

As they are not either cathode or anode they can't stand too much current in them so you must use a resistor (grid stopper) to limit it. 

Take a look at post 139 in this thread:

https://groupdiy.com/index.php?topic=31788.120

It is about 6k4p-ev (6ba6 russian equivalent) but it explains the issue clearly.

 

[Tubetec]

I think I heard about a mod for the manley using type 5899 vari mu  pentode tube . It might require a few  component changes but apparently  it can be plugged into the existing valve socket . Its fairly readily available/cheap
Might be worth a look .
https://groupdiy.com/index.php?topic=64253.0

 

[pucho812]

the 6386 is not so rare anymore. JJ is rmaking them. I have yet to compare it to a NOS 6386. The price around 120.00(USD)  which is not bad compared to a 400.00(USD) NOS.

 

[CJ]

note that since the 6386 is a dual triode, you will need twice the tube sockets if going the T Bar,

 

[gyraf]

A friend of mine did a through research on the JJ 6386 recreation - and found that they were  very very different from real ones.

To the degree where they hardly could deserve the same name.

Jakob E.

 

[emrr]

....and I found the JJ 6386 performed better in a Collins 26U than any original 6386 I could find out of a dozen I had.  So.....

 

[gyraf]

Measured-better or subjectively better? What approximate productoon year?

I was rather shocked when he showed me the curves (and comparison to real ones) in question - and yes, he knows how to measure these.

Not sure if he will allow me to post the plots though..

Jakob E.

 

[emrr]

Greater compression range, better balance leading to an absence of thumping (26U's are really picky here), better meter stability.  I have roughly a dozen various old 6386's, none performed as well, not even close.  In short to say they may not be like an original 6386, but the practical reality of finding a truly good real 6386 is poor in 2017, as it was in 2007.    Curves would be interesting.  I would think different specific circuits could bend the practical results in or out of a 'correct' range.   

Now, 6BA6 or 6BJ6, as asked, a whole different question.  Buy a box of 100 for the price of one good 6386, certainly not 50 pairs, but maybe 20? 

 

[bluebird]

....and I found the JJ 6386 performed better in a Collins 26U than any original 6386 I could find out of a dozen I had.  So.....

I found they very to a degree. Some guest engineers in the studio I work at had two 670's. They had them outfitted with new sets of the JJ's. One unit sounded good but the other sounded "weak" just wasn't right. We switched tubes in the units and the "weak" sound followed the tubes. I think I might have played around for and hour or so switching around tubes in the two channels of the weak unit till I got it sounding decent. I have also made my own data for my Weston tube tester for the 6386. Some of the new JJ's I have tested were off from one side to the other. I don't remember the exact details because it was a few years ago but I did decide to use 6BA6's in my projects from then on. I eventually got some curve tracer hardware to match the 6BA6's.

I have roughly a dozen various old 6386's, none performed as well, not even close. 

Its possible you could have had a lot of weak ones.  I do remember a couple of the GE's I had tested a lot stronger that the new JJ's on the tube tester at least.  Some tested the same, some weaker.

Anyhow that my experience.

 

[CJ]

just a note, the 5749 is the military version of the 6BA6, mechanically stronger but same specs,