quality of plate supply chokes; differences?

[emrr]

Haven't much tried to get my head around it yet, but wondering if anyone can comment on practial quality differences between various types of plate feed chokes for preamp tubes. Immediately coming to mind are the various UTC units; O-13 (50 H @ 3 mA, 6K ohms), A-30 (250 H @ 5 mA, 6K ohms), HC-115 (400 H @ 2.5 mA, 6K ohms).

Right off, I'm thinking inductance is inductance. Is shielding the only real comparative difference, given identical Henry/mA ratings? Clearly the UTC units have different characteristics, so it's apples to oranges there. Is there more that will impact potential sound?

Thanks,

 

[NewYorkDave]

Distributed capacitance is a huge factor.

 

[emrr]

makes sense. If I'd dig around more first:

http://www.electronics-tutorials.com/basics/chokes.htm says:

A choke exhibits some degree of self-capacitance or "distributed capacitance". This capacitance in conjunction with the design inductance are resonant at some particular frequency.

At low frequencies this capacitance has virtually no effect. The resistance is the inherent resistance in the choke both at ac and dc. When the operating frequency is raised the "distributed capacitance" starts to become significant at a point where L and C form a parallel resonant circuit. Increasing the operating frequency further again we find the choke's reactance is dominated by the capacitance to a point where it is now a series resonant circuit. At this point the chokes performance becomes seriously impaired.


Not quite the same, but an informative read:

http://www.kk5dr.com/chokes.htm
http://www.kk5dr.com/TestAchoke.htm



Back to some books.....

 

[CJ]

A parafeed choke needs to be constructed for the full audio bandwidth,

The power supply choke needs to be designed to smooth out 120 cps hum, but what about audio?

I guess it should pass audio as well, since there is going to be audio currents on the B+ line, but do people know this>

Some do, so don't, some don't care either way, a choke is a choke , right?

V72/76 uses three chamber plate choke for 1/9 th the capacitance.

(Divide C by Ns^2, where Ns = number of sections in choke.

No more three chamber bobbins, so you roll your own.
'
I have a small UTC choke here, time to hack it.

Pwr Supply Choke:

"The induction value of Bartolucci supply and plate chokes does not change when a DC current is applied. With several brands the measured inductance can read correct, but with current applied the inductance can decrease as far as 10% of the actual value. Bartolucci has rigorously designed all their chokes to eliminate this problem. Core size and winding techniques have been adapted to offer the best supply and plate chokes available."

Plate Chokes:
" A plate choke needs to meet the same standards as that of a single ended output transformer due to the wide range of audio frequencies that it has to let pass. Therefore each plate choke is in fact an output transformer but without a secondary

To find out which value suits your application fill in the following formula:

fo = desired frequency
Rp = plate resistance of the tube"

http://www.audiokit.it/ITAENG/Trasformatori/Bartolucci/Bartolucci.htm

 

[emrr]

I'd assume the UTC's I mentioned, and any other types sold as parallel feed plate chokes would meet acceptable audio standards for frequency response, but it's not addressed in supporting lit. Honing in on the assumption, one might think they at least meet the standards of the line in question (HA, A, or O); maybe or maybe not. Could all exceed for all I know.

Maybe more or less interchangeable so long as shielding accceptable.

I have some 8000 ohm RCA's for single 6J7 that are the size of the outputs (small coffee cans).

Reasonable as well to assume shielding differences transpose exactly to transformer parameters; an O series choke is as well shielded as an O series transformer, etc etc.

Lots of assumptions; no real testing done here yet. Which one do you have to hack, CJ?

 

[emrr]

Looking over here:

http://www.groupdiy.com/index.php?topic=24961&postdays=0&postorder=asc&start=45

I see the following concerning PP parafeed usage:

[quote author="edanderson"]the edcor 1:1 transformers are wound bifilar, with both windings taken out the center tap at half the total turns count. it is easy and has good performance for 1:1 (cinemag and jensen do it also, among others). so if you connect primary and secondary together in series, you would have a choke with a DCR balanced center tap (since the two windings are identical). something like the xsm 15k:15k (EI625).

ed[/quote]


Edcor are full bandwidth transformers that don't like DC operation in SE mode, but people are getting away with PP applications, so seems like a good option in a PP instance. Worth trying.

More relevant Edcor stuff found here:

http://www.groupdiy.com/index.php?topic=10180&postdays=0&postorder=asc&highlight=expect+dcr+inductance+scale+nearly+rated+impedance+close&start=30

 

[lassoharp]

Was going to start a new thread under the heading OP-6 choke but I opted to start here since there's lots of good discussion and relevant links.

It would seem as emrr has stated elsewhere that the OP-6 is an odd bird for RCA being that it uses cap coupling on the output. Was wondering if anyone had any thoughts as to why RCA chose this particular set up given the number of variables to be dealt with.  Specific advantages on this particular circuit?  I'm guessing perhaps gain .

Some interesting remarks from PRR here regarding LC resonance effects and cost:


http://www.groupdiy.com/index.php?topic=11798.msg137062#msg137062


Also,  does anyone know the specs of this particular choke section of the OT?

 

[emrr]

Off the top of my head, OP-6 should work fine with anything equivalent to the other standard RCA SE outputs (25K:600, or whatever), and a typical 3 mA choke (UTC O-13, etc etc etc).  I'm remembering 6Kish.  Of course if you had an actual standard RCA, you might sub it without the choke and see what happened.    My guess is they got it smaller and lighter incorporating the choke and output in the same can, versus equivalent performance from a plate coupled output on it's own. 

The problem with building an OP-6 is the 4 tap attenuator incorporating variable feedback; no specs mentioned. 

 

[lassoharp]

Thanks Doug,

There didn't look to be any major problems with just  using a good DC+ OT as is.  Gonna try that in a few hours.  It's still an interesting specimen.  The only other voltage amp I've come across that uses the load choke is the V-72.

Have you done much listening to a stock unit?  Anything stand out as different from other RCA 6j7 types other than gain?


Interstage atten -  Noticed that on the schematic.  Ever tried to measure grid to grid resistance?

 

[emrr]

There are lots of pre-WWII voltage amps with choke loads.  Virtually none post-war. 

I think the stock unit I had access to had problems, as it sounded pretty eh and gave a fairly dismal response chart, at any and all attenuator settings.  I've heard reports of very fine response charts, and plenty of people (thanks to the politicking of Stephen Sank) think it's the f'in holy grail. 

I have attenuator measurements, but haven't added them up into anything useful yet.  Not sure I have all data. 

 

[emrr]

OP-6 output choke I measured was 4030 ohms, the output primary was 1414 ohms, and the voltage loss through the transformer (suggesting ratio) was 20 dB.  

 

[lassoharp]

Thanks again Doug!

I had a feeling it would have at least 1 M.  Should be fun rigging up a sub.

I may have something close to those relative values for the load.

Off to crunch some numbers.