kambo

LA2a / jensen : measuring WCF
« on: August 14, 2019, 06:13:36 PM »
so couple of 12BH7 arrived...

how do i measure/observe top and bottom half of 12BH7 WCF !
ac current on rk and ra ????



Re: LA2a / jensen : measuring WCF
« Reply #1 on: August 14, 2019, 07:01:49 PM »
so couple of 12BH7 arrived...

how do i measure/observe top and bottom half of 12BH7 WCF !
ac current on rk and ra ????

Same as any other iteration, standing current can be measured across the cathode resistor or anode resistor.    AC balance of the top and bottom push-pull is done by calculation. 
The LA-2a white follower output is way off being efficient and AC balanced with that 10K load resistor though.  For sure. 

The LA-2a was designed by a Mr Jim Laurence, not by Universal.  Bill Putnam Snr bought the rights and simply manufactured it as was.  Sure, it met spec of the day but, we can do better.

I did, while at UA as analogue design engineer, try to get it secretly corrected for the reissues  but it wouldn't then have been historically correct so...

If it were me, I'd take that 10K Ra resistor and make it about a few K ohms resistor (start at 6 or 7K)  and feed another smoothing cap with it - 10uF or so.    See below for more on this resistor value. 
Now, use a properly calculated load Ra from that cap/resistor  junction to the top anode.    Again, use 1/gm at the specific operating point for this calculation.  Adding a bit to this resistance is OK, maybe up to 1.5 times, but not more.
If the new Ra and the now additional smoothing cap add up to about the old value of 10k using the same 1K cathode bias operating current, the DC voltages will be about the same as the original, or as near as need be.   The additional filter for B+ is no bad thing in a white follower as they have poorer psrr than a straight cathode follower.

That's my opinion.  Been there done it on this 👍

Re: LA2a / jensen : measuring WCF
« Reply #2 on: August 14, 2019, 07:38:20 PM »
It's a pretty cold "White" at 4mA.  Not an issue if feeding high impedance like the UTC A24.
I get about a 2K as a better Ra in the LA-2a.  So old 10K is 8K feeding a cap.  New Ra from there = 2K.
Again, 1.5 times this would be OK so, 7K and 3K respectively.

I'm assuming you're cap coupling to this as in the LA-2a.

I think I might have had my fill of white followers for now.   Read the original patent a few times and about 30 pages of various infos today.

Ciao...

P.S.  if I'm off on the gm it's because the datasheet I'm able to see isn't very detailed.   I advise checking  the gm for yourself before you proceed. 

Also edited for clarity.
« Last Edit: August 14, 2019, 08:14:24 PM by Winston O'Boogie »

kambo

Re: LA2a / jensen : measuring WCF
« Reply #3 on: August 14, 2019, 08:00:29 PM »
thanks for the infos, yea read all that while back!
no cap from preamp stage to WCF, if that what u asked...

so,the big question is :
whats happening if i am doing push push on wcf... is it hurting the tubes feeling (joke )?
does it even care ?


Re: LA2a / jensen : measuring WCF
« Reply #4 on: August 14, 2019, 08:09:51 PM »
thanks for the infos, yea read all that while back!
no cap from preamp stage to WCF, if that what u asked...

so,the big question is :
whats happening if i am doing push push on wcf... is it hurting the tubes feeling (joke )?
does it even care ?

The bottom tube gets a bigger voltage swing due to the larger Ra,  it's pushing up against the top cathode.
No real point in a white follower then.  Might as well use a cathode follower with a current source underneath.  This won't have the ability, like the White, to drive a theoretical twice its standing current into the load.  Does that matter?  Depends on the load you're driving. 
Do folks complain about old LA-2a's not being push-pull balanced?  No.  So maybe none of this ultimately freakin' matters at the end of the day! 

Forgot to address this part : I don't know what your DC voltage is coming off the proceeding plate so that's why I said I was assuming a cap couple. I also don't know your HT voltage. 
If DC coupling, you won't need the 470K bootstrapped grid resistor of the LA-2a.   Assuming a similar HT supply as UA, you want an output voltage at the bottom of the upper 1K cathode resistor of about half this HT.  And there's a 4 volt difference between output and the input grid, so if your proceeding plate DC is around half of this HT less a few volts then, close enough.  Sounds like you aren't requiring maximum voltage swing into low loads from what you've said in other threads so, just build it and use it.   


« Last Edit: August 14, 2019, 08:40:35 PM by Winston O'Boogie »

kambo

Re: LA2a / jensen : measuring WCF
« Reply #5 on: August 14, 2019, 08:31:43 PM »
The bottom tube gets a bigger voltage swing due to the larger Ra,  it's pushing up against the top cathode.
No real point in a white follower then.  Might as well use a cathode follower with a current source underneath.  This won't have the ability, like the White, to drive a theoretical twice its standing current into the load.  Does that matter?  Depends on the load you're driving. 
Do folks complain about old LA-2a's not being push-pull balanced?  No.  So maybe none of this ultimately freakin' matters at the end of the day!   😄

technically matters! real life not so sure as u say...
end of the day, i dont care if i am using wcf, 12ax7 or bc107,
all i care is my client happy with my sound designs or not (also my self too)!
but i hate using technically perfect, but no character equipment! just waste of time and money!

kambo

Re: LA2a / jensen : measuring WCF
« Reply #6 on: August 14, 2019, 08:35:51 PM »
btw, manley uses 2k and 305r

Re: LA2a / jensen : measuring WCF
« Reply #7 on: August 14, 2019, 08:53:22 PM »
btw, manley uses 2k and 305r

I know the Manley circuits quite well, knew and dealt with David before his departure (bought his Neve 24 channel 1081 filled desk  for one), and was friends with Eve Anna, staying at her place a few times.  They make good stuff.

I think the circuit you're talking about is an older Manley design, might still be currently used, but it's possibly an older David one.
The whole issue of optimising White followers was not something widely known about or utilised until, I think, the 1990's when there was an article in Glass Audio magazine. 
As I've said elsewhere, most of them out there in commercial products seem to have a too high Ra.   I'd do it by the book myself since it costs nothing, but I'm not losing sleep over what others do.   

kambo

Re: LA2a / jensen : measuring WCF
« Reply #8 on: August 14, 2019, 09:02:05 PM »
I know the Manley circuits quite well, knew and dealt with David before his departure (bought his Neve 24 channel 1081 filled desk  for one), and was friends with Eve Anna, staying at her place a few times.  They make good stuff.

I think the circuit you're talking about is an older Manley design, might still be currently used, but it's possibly an older David one.
The whole issue of optimising White followers was not something widely known about or utilised until, I think, the 1990's when there was an article in Glass Audio magazine. 
As I've said elsewhere, most of them out there in commercial products seem to have a too high Ra.   I'd do it by the book myself since it costs nothing, but I'm not losing sleep over what others do.

everyone has their reasons to use different Ra...
you think David didnt know how to calculate perfectly balanced WCF  ;D

Re: LA2a / jensen : measuring WCF
« Reply #9 on: August 14, 2019, 09:17:20 PM »
everyone has their reasons to use different Ra...
you think David didnt know how to calculate perfectly balanced WCF  ;D

I can't say what David did or didn't know.   All I can comment on is what I might have known about in the past, and what I might have known since then, or know now. 
The answer is always the same regardless, not enough.



kambo

Re: LA2a / jensen : measuring WCF
« Reply #10 on: August 14, 2019, 10:02:10 PM »
as Ian said, its about standing current u r picking... and as PRR said, u have ton of tolerance!
and also, PRR said probably 20 zillion times, that u have to look at the whole schematic,
not just the one section of it!


 

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