Dynaco 440 Transformer Cathode/Screen Taps 6P1P 6AQ5 UL transformer - diyAudio
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Old 24th September 2010, 04:59 AM   #1
TheGimp is offline TheGimp  United States
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Default Dynaco 440 Transformer Cathode/Screen Taps 6P1P 6AQ5 UL transformer

Long title but I hope it will provide helpful for future searches.

So what am I asking?

I've been reading a bit on UL and Schade feedback and in particular “An Ultra Linear Amplifier feedback Including UL” by Hafler and Keros,

And “ULTRA-LINEAR DISTRIBUTED LOAD AUDIO AMPLIFIERS”

ultra-linear

I'm also interested in trying transformer cathode feedback

In looking at this transformer, last page bottom right :

http://www.the-planet.org/pdf/transformer.pdf

… it seems to me that the use of the separate winding for Screen taps is great for tubes that have much lower screen voltages wrt plate voltage. However, I’m confused by the alternate use for cathode feedback.

Wouldn’t that provide an awfully lot of cathode feedback (40% of plate voltage) ?

Most of the commercial schematics I’ve seen with cathode feedback use the output windings with the 4 ohm tap tied to ground and one cathode to the common terminal, and the other cathode to the 16 ohm tap?

I’m interested in getting Edcor to make an opt for 6P1P/-EV and 6AQ5. I expect this may also be suitable for 6V6 and other derivatives. The smallest transformer they currently offer is a 25W one and a bit big for these tubes (at least the smaller output ones). One possibility would be to make it compatible with the 6BQ5, but there are more compromises to be made in this case.

If the compromises become to great I’d leave out the 6BQ5, but I think setting it as the top end would make for a nice transformer with good versatility although it would add a slight cost compared to leaving this out.

I figure with the 6BQ5 I would need to specify 17W, and without it 10W, presuming response at rated output from 20Hz to 20KHz, UL 40%taps, 4, 8, 16 ohm output taps.

Another fly in the ointment would be the impedance. 6BQ5 looks like it is best at 8K p-p, and the 6AQ5, 12AB5, 6P1P/-EV are normally specified at 10K and it appears to me that 12K would be even better if one were pushing lower distortion with a slight trade off in lower output power.

I considered a separate UL tap like the A-440 as the 6P1P/-EV is specified for 250V max screen, but expect this would really make the transformer more difficult as it would effectively add an additional 40% secondary winding and it’s associated complexity in interleave and therefore really drive the cost up.

Comments?

Is anyone else interested in 10-15W transformers? Or is this just the “rework an old console” level?

I’ve got Hammond 1608s/1609s but I’m not completely satisfied with them. Too much base extension.

The CXPP10-8-10K transformers are nice and greatly exceeded my expectations, but I’d like to get something at the next level.
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Old 25th September 2010, 05:08 PM   #2
TheGimp is offline TheGimp  United States
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1. a cathode feedback winding is doing for fixed bias what an unbypassed resistor does for cathode bias. That is, it provides 100% negative feedback to the cathode.

2. The optimum feedback percentage for UL is closely related to the plate screen grid and cathode geometry. This is typically around 40%.

3. using the4/8/16 ohm output windings for cathode feedback in fixed bias will only allow balanced (null) current through speaker int eh 16 ohm tap configuration.

Therefore:

Whatever feedback scheme is used, an individual winding should be used for it.

The use of a common winding for either UL or cathode feedback must compromize one of them since UL will require 40% of the plate swing as feedback which can run in the order of several hundred volts, and cathode feedback will only require a small percentage of this (maybe 25% of screen swing for cathode swing in large output tubes). eg, plate at 800V, screen at 400V, fixed bias -70V, screen swing is +/-400V and cathode is only +/-70V so ratio is 17.5% (70/400).

Using the output windings for cathode feedback will result in dc bias of the speaker system if used with 4 ohm (worse) or 8 ohm (Second worse) speakers, and therefore should only be used with 16 ohm speakers. In addition using either the 4 or 8 ohm taps provides asymmetrical loading of the output, and thus the output tubes themselves if in class AB. In class A the reflected load will be symmetrical, but will still produce a dc bias of the speaker system.

My conclusion based on the above understanding is that any feedback should optimally be done with separate windings.

Is this correct?
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Old 27th September 2010, 05:18 AM   #3
TheGimp is offline TheGimp  United States
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Oh com on. 123 views and no comments?

Surely I can't be that right.

Math Error?

nit pick?

Help me here guys (and gals)...

I am to be loving low power out PP amps.

Got a Magnavox SE console yesterday for $20 at the "Salvation Army Store".

Damn if the power transformer wasn't toast.

"If it weren't for bad luck I'd have no luck at all."

Last edited by TheGimp; 27th September 2010 at 05:24 AM.
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Old 27th September 2010, 03:24 PM   #4
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Sounds like a deep dark depression of excessive misery.

What is the DC resistance of the 440 output windings?
And idle current of 6P1P biased for a smooth AB crossing?

Only the woofer will see any DC offset after crossover,
can it handle the worst case offset? If it can, what you
worried about?

There's plenty of SE amps that abuse this trick for
cathode winding, and no DC cancellation whatsoever.
My old Voice of Music reel to reel comes to mind...

And suppose the 420 transforer with 40% cathode?
Just takes your Schaded Mu down to 2.5. No great
tragedy, just takes a larger voltage swing to drive.
Low Mu linearity might be worth it.

I once ordered a 10W CXPP. Not listed on the menu
in bold print, nor at all... But still was an Edcor design
already on-file, there was no custom charge for it. So
does not hurt to ask.

Last edited by kenpeter; 27th September 2010 at 03:31 PM.
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Old 28th September 2010, 11:56 AM   #5
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hey-Hey!!!,
If I understand correctly, you've got a dynaco A440 output? that will have a 17% tertiary winding( same as the A441 ). It is a bit more Iron than you need for a low power amp; consider it to run KT88 in class A for ~35 Watts. Chicago also offered a few OPT with CFB windings, comparable to A440 is their BO-14. That one has a 10% CFB winding, and 37% U-L taps.

U-L tap location is dependant on operating point and tube type. Some like ~40% and tubes like 6V6 in AB like 20%. Using a fixed g2 supply with CFB delivers U-L performance as well as the CFB; instead of moving g2 to the cathode, it moves cathode towards g2 and the conbination works better than either alone.
cheers,
Douglas
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Old 28th September 2010, 01:24 PM   #6
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Interesting way of looking at it. Let me consider that viewpoint a moment?

So, plate current still travels both windings, screen current travels only the
40% cathode winding. Cathode to screen voltage knows no different. Maybe
there is no difference between this and UL after all??? Waitaminute, then is
total plate current path now 140%, or was this plate winding only a 60%?
G2 may not know any diff, but G1 to 40% cathode surely means something.
Cathode feedback to G1 should have far stronger influence than mere UL.

Now cathodes riding the output taps, might be a slightly different story:
They are seeing the true load as it appears on the far side of the iron.
This is certainly no 40%... Is it enough feedback alone to be significant?

Wether or not a DC offset bothers 4 and 8 taps? I don't know. Certainly
across the 0 to 16 is no problem. Driving 8 from here would alter what is
reflected to the primary, but doesn't preclude one from doing so.

Last edited by kenpeter; 28th September 2010 at 01:39 PM.
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Old 28th September 2010, 01:38 PM   #7
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Yes, adding the cathode winding to the primary changes the reflected load. For the 10% chicago, it yeilds a 21% increase ( 110% squared times the plate winding load ). Haven't seen 40% tertiary coils; either 100% as in the McIntosh or 10-17% as in Chicago, Dynaco and Acrosound ). The McIntosh implementation just cuts the plate coil in half and does account for the total load.
cheers,
Douglas
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Old 28th September 2010, 04:34 PM   #8
TheGimp is offline TheGimp  United States
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Unfortunatly I don't have any A-440s, but was looking at the literature for the A-440 and A-441. I was curious how the make a compromize between Cathode Feedback and Screen Feedback ratios.

So the A-441 is 17% and not 40% feedback winding? I would think that would work for cathode, but would be less than optimum for screen feedback.

No further specifications were given in the Dyanco catalog, and no example of cathode feedback was given, just a note that the A-441 was designed for cathode or screen feedback.

The ARC ST-70 C3 is one design where I saw the cathode feedback from the OPT secondary with the 4 ohm tap to ground.

Some simple math on cathode feedback from output of OPT:

6BQ5/EL84 PP:
Vg-g 17.4V rms
P out 15W
Voltage Swing across 4 Ohms = 7.75Vrms
Ratio of grid drive to feedback = 7.75/(17.4/2) = 88%

ARC ST-70 47W 6L6-GC amp
Vg-g 72V (Peak)
P out 47W
Voltage swing across 4 ohms = 13.74 Vrms
Ratio of grid drive to feedback = 13.74/(72/(2*1.414))=53.96%

A-441 6550 amp
P-out 100W
Vg-g 34Vrms
Voltage swing on feedback tap at 17% with 600V supply presuming a 80% peak = ((600*.8)./1.414) *0.17 = 57.7Vrms
Ratio of grid drive to feedback =34/57.7 = 58.9%

Do these numbers look reasonable?
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Old 28th September 2010, 06:55 PM   #9
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No, they don't look reasonable for the A440/1 example, unless you're talking about CT-end voltage and it looks like you were end-to-end on the other examples. You got about the same FB from a 3-4% coil( the speaker secondary ) as you got from a 17% coil.
cheers,
Douglas
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Old 28th September 2010, 10:00 PM   #10
TheGimp is offline TheGimp  United States
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So I need ot consider the complete p-p swing when comparing it to the feedback?

A-441 6550 amp
P-out 100W
Vg-g 34Vrms
Voltage swing on feedback tap at 17% with 600V supply presuming a 80% peak = ((600*2)*.8)./1.414) *0.17 = 115Vrms
Ratio of grid drive to feedback =34/115 = 29.6%%

Seems like I'm going in the wrong direction.
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