600:600 output trx

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Winston OBoogie

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ruffrecords said:
OK, understood. So I drew a load line from plate volts =0 and Ia =30mA through the -5V Vg and Ia = 15mA point. From load line I estimate a 10V swing in Vg gives a 16mA swing in Ia which gives gm=1.6mA/V so Ra = 625 ohms. Have I gone wrong somewhere?

Hey Ian,
I had trouble seeing the datasheet you used while laying a straight edge on it with this tablet so used the JJ one which was a bit easier  to read.
I got a slightly different gm than you but, different manufacturer, slight variance, less graduations on the J.J.  etc.  so...

I got about 15mA for 10v swing which is close enough to your 1.6mA/V for me.  This means that Ra would be even nearer to Jensen's value  @ 667 ohms.  Therefore, you were correct sir. Jensen were correct.  I was wrong.  Sorry.

No idea where my head was 🙄
 

ruffrecords

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Winston O'Boogie said:
I was wrong.  Sorry.

No idea where my head was 🙄

Hey Winston. No problem mate. As I am fond of saying, the only people not making mistakes are those doing nothing.

And just to get back to the OPs original question, that push pull stage has a 15mA quiescent current . That is plenty to provide 400mW of output power into a 600 ohm load (+26dBm) but not via a 1:1 transformer.

400mW into 600 ohms is close to 26mA rms or 36mA peak. The White Follower running at 15mA can only supply a maximum peak current of 30mA.

If you only want +20dBm into 600 then the rms current is only 13mA and peak of just over 18mA so the White should manage this with a 1:1 transformer.

Cheers

ian

Cheers

Ian
 

PRR

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Winston O'Boogie said:
...PRR ...Hopefully he can advise.
John Broskie and Alex Cavalli have thrashed the WCF far better than I could.
http://www.tubecad.com/october99/page4.html
https://www.tubecad.com/2006/10/blog0083.htm

1/gm is good for low-Z loads. High-Z loads should correct for load impedance. This happens when load is above half of plate resistance. So 6SN7/12AU7 driving 600r even 2.4k, correction is small; put a 6080 in there and you should work it out.

Broskie also notes "Plate resistance and transconductance vary."

(For more fun: if you push a WCF to MAXimum output, gm varies from near zero (say 0.5X)  to about 1.4X the idle value...)

My tedious sims only show that a WCF's THD will null, only if *exactly* balanced. In practice this suggests trimming two +/-20% tubes to equal current and 2% equality of Vpk. Since real tubes are not even constant one minute to the next, you get a very low distortion residual which changes minute to minute. I wonder(!!) if that could be more distressing than a higher THD which stays about the same all the time.

Overload of a tube WCF (assuming near-correct Ra) is almost always about not running enough idle current.
 

kambo

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PRR said:
John Broskie and Alex Cavalli have thrashed the WCF far better than I could.
http://www.tubecad.com/october99/page4.html
https://www.tubecad.com/2006/10/blog0083.htm

1/gm is good for low-Z loads. High-Z loads should correct for load impedance. This happens when load is above half of plate resistance. So 6SN7/12AU7 driving 600r even 2.4k, correction is small; put a 6080 in there and you should work it out.

Broskie also notes "Plate resistance and transconductance vary."

(For more fun: if you push a WCF to MAXimum output, gm varies from near zero (say 0.5X)  to about 1.4X the idle value...)

My tedious sims only show that a WCF's THD will null, only if *exactly* balanced. In practice this suggests trimming two +/-20% tubes to equal current and 2% equality of Vpk. Since real tubes are not even constant one minute to the next, you get a very low distortion residual which changes minute to minute. I wonder(!!) if that could be more distressing than a higher THD which stays about the same all the time.

Overload of a tube WCF (assuming near-correct Ra) is almost always about not running enough idle current.

OMG, thank you so much :)
i was wondering all about that.... specially +/-20% tubes, and changes by min to min !


 

Winston OBoogie

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PRR said:
I wonder(!!) if that could be more distressing than a higher THD which stays about the same all the time.

Many thanks for chiming in PRR, I learned a few things there for sure.
I've read Broskie's stuff but it's been a while for sure.

Regarding a more consistent yet higher THD -  as we know, it all depends on the spectral distribution of these distortions.
I don't remember where I first read about degenerating the top cathode to linearize it somewhat  (could have been Broskie?)  but I did find it to be an advantage in some areas, albeit one with a slightly higher overall THD. 
With both top & bottom cathode R's in circuit, higher harmonics came down at the expense of a raised 2nd.  And of course, a raised output impedance
I'm not smart enough to instinctively know the mechanism behind why the 2nd is raised, it could just have been a condition of my particular operating points and not a universal one?

Not much (if anything) new under the vacuum-tubed Sun.  A good read and grasp of the lit.  should enlighten me.

Cheers and thanks again.
 

Winston OBoogie

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ruffrecords said:
Hey Winston. No problem mate. As I am fond of saying, the only people not making mistakes are those doing nothing.

Well, thanks again for taking the time out to check it Ian.

Friends, colleagues etc. from my time in the U.S.  call me John  - my middle name. 
Family and friends back here in the UK call me David - my Christian name.
Some folks on here call me Winston of course.
And any of the above is perfectly fine,
as is 'Hinson'.

Cheers,
D.J.H. (W.O'B.) 🙂

 

PRR

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> the mechanism behind why the 2nd is raised

To a first approximation, both tubes must see the same current and voltage. This is usually not at top-grid exactly equal to half of supply. There's some lesser effects (especially if you do/don't have extra resistors). In sim it is a very fine trim. In real life it may not be practical to pursue a THD null which will drift.
 

kambo

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PRR said:
> the mechanism behind why the 2nd is raised

To a first approximation, both tubes must see the same current and voltage. This is usually not at top-grid exactly equal to half of supply. There's some lesser effects (especially if you do/don't have extra resistors). In sim it is a very fine trim. In real life it may not be practical to pursue a THD null which will drift.

DC current on top and bottom is almost exact, 3.45 - 3.44 on lo - 10.45-10.42 on high current setting!
THD is freaking drifting  ;D ;D ;D that i dont like! its drifting with any Ra Rk anyway! just how/when drifting is changing!
so, slapped a 600 ohm on out transformer permanently, 
set the distortion to 0.28%, current to 9.5 mA... the suckker cant drift anymoe :)))) stuck on 0.28%THD  :)


 

Winston OBoogie

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PRR said:
> the mechanism behind why the 2nd is raised

To a first approximation, both tubes must see the same current and voltage. This is usually not at top-grid exactly equal to half of supply.
So that's where the assymetry is,  hence mo' 2nd.  Ah-Ha!  Thank you sir.
 

Winston OBoogie

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kambo said:
DC current on top and bottom is almost exact, 3.45 - 3.44 on lo - 10.45-10.42 on high current setting

Of course it is, it always will be the same with this circuit.
We are talking about a push-pull balance which is aiming to supply an equal, but inverted "signal" to the bottom tube.  None of this changes the standing current. 

 

kambo

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Winston O'Boogie said:
Of course it is, it always will be the same with this circuit.
We are talking about a push-pull balance which is aiming to supply an equal, but inverted "signal" to the bottom tube.  None of this changes the standing current.

nope, DC current is not the same, is there is a perfect tube out there that i dont know of it... at least with the tubes i have tried, u have to trim it!
at least my triode matched (as the sticker on tube says) tubes needed some trim!
 

Winston OBoogie

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kambo said:
nope, DC current is not the same, is there is a perfect tube out there that i dont know of it... at least with the tubes i have tried, u have to trim it!
at least my triode matched (as the sticker on tube says) tubes needed some trim!

Earlier you said: "3.45 - 3.44 on lo - 10.45-10.42 on high". 

That's the same. 
Unless you're planning on cloning a few Tektronix tube 'scopes.

In tube audio however, it's a miniscule difference between 3.45mA and 3.44mA.  I bet it changes more than that if you happen to look at it in the wrong manner.  I bet Uri Geller could change it just as much with his mind.  it isn't going to make any difference to your THD, or anything that matters.
If you don't like what you're getting, you need to look elsewhere in the topology.  Or a different topology.


 

kambo

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Winston O'Boogie said:
Earlier you said: "3.45 - 3.44 on lo - 10.45-10.42 on high". 

That's the same. 
Unless you're planning on cloning a few Tektronix tube 'scopes.

In tube audio however, it's a miniscule difference between 3.45mA and 3.44mA.  I bet it changes more than that if you happen to look at it in the wrong manner.  I bet Uri Geller could change it just as much with his mind.  it isn't going to make any difference to your THD, or anything that matters.
If you don't like what you're getting, you need to look elsewhere in the topology.  Or a different topology.


those values are after the trimming  8)
 

Winston OBoogie

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kambo said:
those values are after the trimming  8)

I don't know what or where you're trimming but, the curves of two triodes, albeit ones  inside the same bottle and built up by the same little old lady on Tuesday afternoon between 2:33 and 2:47pm, will not be matched across the whole range.  They may match at one point, but be off a bit elsewhere. 
It shouldn't matter in the grand scheme.
If it does matter to you, then buy several and plot the curves on a curve tracer or by hand.  Pick the best few and parallel them to distribute the spread.

Plenty of vari-mu compressors out there with dual triodes that are being asked to swing all over the place, the poor buggers are subjected to much more abuse than a White cathode Follower.  They aren't perfectly matched at every point during all this turmoil.
And yet folks still make records using them.





 

Winston OBoogie

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More food for thought regarding supplying top and bottom halves of a follower with equal but opposite polarity signals can be found in Fred Forssell's neat little circuit (pdf attached). 

The 12au7 output could just as easily be a 12bh7 or an ECC99 etc.  The input pair a 6SL7 (or 6SN7 for lower gain).  The 2SK170 current source can be any npn type of your choosing since 2SK170's are almost as much per ounce as gold these days.
The anode load resistors should be sized appropriately for your input valve choice - taking note of the bootstrap connection which is 2/3rds up (or 1/3rd down for our friends in the opposite hemisphere). 

The topology was  published by Fred  in Glass Audio circa early 1990's.  Credit is also given to Bascom King in the article.





 

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kambo

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PRR said:
> can i dive 600:600 with White Cathode Follower

My car "can" climb Goose Hill in 4th gear.

But it sure isn't happy.

My little 4-banger is the 12AU7 of engines. If I replaced it with a 455 V-8, that would be like a 6080, and I could climb Goose Hill in 4th or drive 600r with authority.

The difference is cost/weight. Throwing cast-iron or empty vacuum at a problem is poor use of potential leverage. As long as you use a transformer, may as well use some leverage and slip the WCF into 2nd gear where it will be happier.

those 6080s pulling 2.5Amp on heater :eek:
power trx pretty large for 7.5amp ....two aps and a power supply aint gonna fit the same case!
 

kambo

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PRR said:
My little 4-banger is the 12AU7 of engines. If I replaced it with a 455 V-8, that would be like a 6080, and I could climb Goose Hill in 4th or drive 600r with authority.

might as well use the power!
going for the  6AS7,  is 50-60mA idle current enough to drive 600:600 ohm trx with authority!

damn, this thing gonna be so heavy!

edit: finally decided.... i am gonna build this as a separate unit, and hook it up to what mic pre needs it! 
Ian's mu's sounds great, i had them back home, and couple of my 6sn7s..i am good to go!



 

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