kambo

600:600 output trx
« on: August 08, 2019, 01:31:49 PM »
can i dive 600:600 with White Cathode Follower, if i set it correctly?
or any other topology for 600:600 out trx on tubes?


edit: yup, according to ltspice it works... lets order a 600:600 out trx :)
« Last Edit: August 08, 2019, 01:56:51 PM by kambo »


Re: 600:600 output trx
« Reply #1 on: August 08, 2019, 05:57:32 PM »
Like anything, it depends on how it's implemented. 
There's an optimum value of plate resistor to achieve good balance between the push and the pull.   And if your transformer is loaded on the secondary with 600r then you'll need a bit more grunt in quiescent current than, say, the LA-2a white follower - circa 30mA of drive gets you to typical Pro gear headroom levels into 600z.  You'll also need a bigger cap for your low end response.
A clean supply is advised as the psrr isn't great with the 'white'. 
About the most linear and readily available valve might be an ECC99 from J.J. , they're decent little things.   But then, maybe you don't want linear?
Paralleling a couple of lesser powered valves is OK if you don't have anything beefy enough.

Otherwise...  😋

Re: 600:600 output trx
« Reply #2 on: August 08, 2019, 06:22:56 PM »
Forgot:  In case you weren't aware,  the correct plate resistor value for balance is 1/gm of the valve you're using, give or take some depending on load etc.   But that gets you in the right ballpark to check out things further.
As you were...

john12ax7

Re: 600:600 output trx
« Reply #3 on: August 08, 2019, 07:42:53 PM »
Why not get a step down? Tubes can put out a lot of voltage, but struggle more with current.

kambo

Re: 600:600 output trx
« Reply #4 on: August 08, 2019, 07:52:53 PM »
Forgot:  In case you weren't aware,  the correct plate resistor value for balance is 1/gm of the valve you're using, give or take some depending on load etc.   But that gets you in the right ballpark to check out things further.
As you were...

thank you for this :) and above!


kambo

Re: 600:600 output trx
« Reply #5 on: August 08, 2019, 07:59:43 PM »
Why not get a step down? Tubes can put out a lot of voltage, but struggle more with current.

4:1 working just fine...  this is just experimental, educational... 

PRR

Re: 600:600 output trx
« Reply #6 on: August 09, 2019, 12:26:24 AM »
> 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.

BluegrassDan

Re: 600:600 output trx
« Reply #7 on: August 11, 2019, 10:21:56 AM »
I’ve been using the Jensen 4:1 output tx and have wondered the same: whether a 2:1 or 1:1 would give good results....higher output level.

Using the Jensen cascode follower schematic with 12BH7.

Re: 600:600 output trx
« Reply #8 on: August 11, 2019, 01:37:00 PM »
I’ve been using the Jensen 4:1 output tx and have wondered the same: whether a 2:1 or 1:1 would give good results....higher output level.

Using the Jensen cascode follower schematic with 12BH7.

Relaxing the requirements from 1:1 to 2:1 would keep the dissipation down. 
Looking at the Jensen schematic, I wouldn't call it a "cascode" circuit.  True, a cascoded triode achieves about the same rp and reduction of miller effect as that of a pentode, but this is just a pentode driven circuit.
I think the 12BH7 in this circuit has a too *high* anode resistor, something like 5100rp/16.5u gets about  310 ohm resistor instead of the 680 used there.  A further reduction in levels of higher harmonics (at the expense of a higher output impedance and slightly higher H2) would be to linearize it further by degenerating the top cathode.   Another 330 ohm resistor between top cathode and bottom plate with signal taken from the bottom of the resistor. In that case,  the top anode resistor would be raised from 310 ohms to 310 + 330 = 640 ohms.  Now closer to the 680 shown value.
Otherwise, all other comments above by PRR, John12ax7, and myself still apply.  😊

Original Jensen schematic attached:

Edit: If another 330 ohm resistor is added to the top cathode, then I think R4 should be about 22K1 rather than 30K to keep the output at about half of HT = 125V.   Dissipation is on the edge of the noted 3W then so, two 44K 2W in //  would be better.
« Last Edit: August 11, 2019, 11:54:39 PM by Winston O'Boogie »

ruffrecords

Re: 600:600 output trx
« Reply #9 on: August 11, 2019, 04:17:55 PM »
I’ve been using the Jensen 4:1 output tx and have wondered the same: whether a 2:1 or 1:1 would give good results....higher output level.

Using the Jensen cascode follower schematic with 12BH7.

People often say the thing about the White follower is that it has a very low output impedance. The thing about the White Follower is that it has a very low small signal output impedance. The thing about all output stages is low output impedance does not mean high drive capability. What primarily determines output drive capability (in a class A circuit) is standing current (closely followed by plate resistance)

Suppose you want to drive +20dBu rms into a 600 ohm load. The rms current in the load will be 7.75/600 = 12.9mA. The peak current swing is therefore  root 2 times this or 18.25mA. With a 4:1 transformer the primary current only needs to swing each way by a quarter of this about 4.5mA.  A push pull stage like the White Follower can in theory swing up to twice its standing current in either direction so a 3mA quiescent current ought to be fine. With a 2:1 transformer the current will need to swing 9mA so you will need twice the standing current.

I have never had much joy with White Followers, they seems to clip long before they ought to. I much prefer an SRPP running at about 6mA which will easily poke +20dBu into 600 ohms via a 2:1 transformer.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


Re: 600:600 output trx
« Reply #10 on: August 11, 2019, 05:12:17 PM »
I have never had much joy with White Followers, they seems to clip long before they ought to. I much prefer an SRPP running at about 6mA which will easily poke +20dBu into 600 ohms via a 2:1 transformer.

Cheers

Ian

I have myself seen some very strange distortion characteristics when a 'White' follower isn't balanced properly.  And I would say that most of the circuits I see out there are not balanced, most having too high a value anode resistor.  Maybe this is in the mistaken belief it's then more of a CCS, maybe the value is pulled out of thin air or from some other topology seen elsewhere?
Regardless, In theory, providing there is enough quiescent current and voltage swing for the load and swing required, distortion can be nulled to a very negligible level.  Once the correct value top load R is established, this will be the optimum for that valve regardless of load. 
I can't say I've played much with SRPP, maybe I'd prefer them too 🙂

Edit:  I guess I'm saying that, if Bluegrass Dan is using the Jensen circuit, even if he changes nothing else, the 680 ohm top resistor should be closer to 310 ohms, regardless of whether he uses a 4:1,  or 2:1, or goes straight out unbalanced.
« Last Edit: August 11, 2019, 05:47:39 PM by Winston O'Boogie »

Re: 600:600 output trx
« Reply #11 on: August 11, 2019, 05:40:45 PM »
While at Universal Audio, there were two minor 'modifications' I wanted to implement in their historic units.  One was an incorrect value resistor that had snuck into the BOM for the 1176-LN which was off the charts wrong but had been used since being first re-introduced.  It was felt best to leave it as is so all new units would match, even if historically incorrect.   This was a wrong call in my view. 

The second modification was to lower that 10K load on the 12BH7 white follower and, maybe DC couple to it from the prior stage.  It was demonstrably better in all regards on the A.P. and software analyzer with these changes but it was felt better to leave it be.  I understand this rational a little better, although I doubt anyone would have noticed a different coloured carbon comp resistor on the turret board. 

Whatever gives you the result you're after I suppose...

kambo

Re: 600:600 output trx
« Reply #12 on: August 11, 2019, 08:44:36 PM »

I have never had much joy with White Followers, they seems to clip long before they ought to.

Cheers

Ian

according to Jensen, you have 26dbu in to 600 ohm!
at which part its clipping i wonder ?



Re: 600:600 output trx
« Reply #13 on: August 11, 2019, 09:26:29 PM »
according to Jensen, you have 26dbu in to 600 ohm!
at which part its clipping i wonder ?

I suspect the transformer would crap out above that.   However, again, the Jensen schematic isn't optimised for equal push-pull.
We need a better value for Ra which is 680 ohms in the Jensen schematic.

VgRa/rpmu/rp = Vg/rp

Solved for Ra equals:

muVgRa/rp² = Vg/rp
muRa/rp = 1
muRa = rp

So:

Ra = rp/mu.

Ra for a 12BH7 is 5100.  Mu is 16.5.

So Ra = 310 ohms.



BluegrassDan

Re: 600:600 output trx
« Reply #14 on: August 11, 2019, 09:35:20 PM »
I’m gonna try what you suggested and report back.

Re: 600:600 output trx
« Reply #15 on: August 11, 2019, 09:56:56 PM »
I’m gonna try what you suggested and report back.
👍
With or without the extra cathode resistor?
If with, add the extra 330R to the calculated Ra 310R

Either way, it'll be a happier little pusher-n-puller.
« Last Edit: August 11, 2019, 11:35:13 PM by Winston O'Boogie »

kambo

Re: 600:600 output trx
« Reply #16 on: August 11, 2019, 11:04:43 PM »
I suspect the transformer would crap out above that.   However, again, the Jensen schematic isn't optimised for equal push-pull.
We need a better value for Ra which is 680 ohms in the Jensen schematic.

VgRa/rpmu/rp = Vg/rp

Solved for Ra equals:

muVgRa/rp² = Vg/rp
muRa/rp = 1
muRa = rp

So:

Ra = rp/mu.

Ra for a 12BH7 is 5100.  Mu is 16.5.

So Ra = 310 ohms.

jensen schematic is not bypassed!

Re: 600:600 output trx
« Reply #17 on: August 11, 2019, 11:32:57 PM »
jensen schematic is not bypassed!

Correct.  It shouldn't be.

kambo

Re: 600:600 output trx
« Reply #18 on: August 11, 2019, 11:53:59 PM »
just tuned 6sn7 wcf  2:1 out...  3.4 mA  8)
my SM57 never sounded this good :)


edit: THD 0.045%

Re: 600:600 output trx
« Reply #19 on: August 11, 2019, 11:56:30 PM »
just tuned 6sn7 wcf  2:1 out...  3.4 mA  8)
my SM57 never sounded this good :)

👍  What are the circuit values?  Ra etc. 

I'd take a nice octal like a 6SN7 over most 9 pin miniatures most any day of the week. 

Edit:. A 6SN7 can take a fair amount of current at normal working voltages, you could push that 3.4mA up if need be.
« Last Edit: August 12, 2019, 12:01:12 AM by Winston O'Boogie »


 

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