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Old 27th March 2006, 02:24 PM   #11
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EC8010 is right, just put the resistor (with a cap across it) between the cathode and the transformer. Many pentodes are terribly non linear in the region near positive grid bias, so PowerDrive may not be a serious advantage here. I am using it in the current prototype because I wired 6AV5's into my 300B amp. I will do a lot more testing when I return from an upcomming trip (mid April).

If you try CFB with push pull you would ground the 4 ohm tap, and use the 0 and 16 ohm taps for the cathode connections. In a transformer the impedance ratio is the square of the turns ratio, so the 4 ohm tap is the CT of the 0 to 16 ohm winding. Most P-P transformers wind the 4 ohm winding with heavier wire than the 8 and 16 ohm windings, so they have different DC resistances. This is often the case with the 2 halves of the primary as well. Many P-P transformers have different amounts of inductance in each winding. This causes different amounts of feedback to each tube, and the whole effect varies with frequency. The tube to tube balance that you are trying to achieve gets upset with imbalanced feedback.

Audio Research made P-P amplifiers that used CFB successfully. The schematics for their amps can still be found here:

http://www.arcdb.ws/

Look at the schematic for the D76. This idea obviously works well with their transformers. I tried it with several generic transformers, and the results were not pretty.
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Old 27th March 2006, 03:33 PM   #12
rdf is offline rdf  Canada
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Two quick questions. Why put the cathode resistor in series with the speaker? It's standard practice but currently my amp has the speaker in series with the cathode bypass cap and leaves the cathode resistor from cathode to ground. No DC through the speaker and a more direct connection between speaker and cathode. I realize this places the cathode resistor in parallel with the speaker, in the case of this EL84 though it's 270 ohms and I can make an argument it has a benefit in midly taming speakers with wild impedance variations. It measures well, any idea why it's not used?

The other question relates to a Rowe/AMI jukebox amp I bought for iron. The P-P OPTs have 70 volt windings, anyone every try using something similar for cathode feedback? Thx!
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Old 27th March 2006, 03:45 PM   #13
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by rdf
It measures well, any idea why it's not used?
How about, "Because nobody else thought of it"? Good idea.

As Tubelab says, cathode feedback in push-pull circuits does rather assume symmetry in the output transformer, and that's not usually the case. However, it costs nothing to try it, and sometimes the results are well worthwhile. In my experience, you trade HF ringing and stability against distortion. 3-6dB of cfb seems to work well, but more than that can be a problem. Further, the lower mu valves don't seem to respond well to cfb (even if the windings have been adjusted to give 6dB of cfb). If you've got a 70V winding, try it and see...
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Old 27th March 2006, 04:08 PM   #14
jane is offline jane  Norway
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Quote:
Originally posted by rdf
It measures well, any idea why it's not used?
Do you mean something like this?

Click the image to open in full size.
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Old 27th March 2006, 08:49 PM   #15
rdf is offline rdf  Canada
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Thx EC8010, there's just no way I'ld ever believe that in field as throughly plowed as tube circuits I came up with anything combining originality and value. Easier to assume the technique saw use and was abandoned in the distant past for perfomance or economic reasons. You mentioned before low mu tubes caused more problems, did you ever determine the cause? Or were you applying large amounts of cathode feedback via extra windings? It just seems somewhat counterintuitive a circuit with less open loop gain could be more sensitive to the application of moderate feedback. My transformers (Rowe R-4359A amp) were meant for 7868, no ultralinear winding, so eventually I'll try JJ 7591A both pentode and triode mode using the 70 volt taps for cathode feedback to hammer down the output impedance. I'm canvassing as much prior experience as possible to avoid releasing magic $$$moke beforehand though.

Yup Jane, almost exactly like that. Significantly different front end and bias point for the EL84. I see that design uses 470uF for the cahtode cap. It's the one potential 'gotcha' I found. It's possible to achieve decpetively flat response into a load resisitor by tailoring the cap with small values at the expense of seriously reduced damping factor. My amp was dead flat to 20 Hz using 22uF, at the same frequency the DF was under 1. At least now I know what that sounds like. Currently it's 220uF but I've yet to remeasure the Zout.
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Old 27th March 2006, 09:17 PM   #16
EC8010 is offline EC8010  United Kingdom
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Hello rdf, no, I'm afraid I didn't find any explanation as to why low mu valves didn't seem to benefit from cfb as much as higher mu valves. I have direct experience with EL84 and N78 (triode and pentode), 6S4A, 2A3, all in push-pull, and 6528 in single-ended, also anecdotal of triode-strapped 813. The measured (and listening) results of adding cfb to push-pull UL N78 were that it was an absolute winner. 6528 and 813 were well worthwhile. 6S4A and EL84 weren't bad, but not really worth the degradation of 10kHz square wave response. However, 2A3, with a specially wound symmetrical transformer to give 6dB cfb, was a disaster and if it hadn't been for the reduced gain and output resistance, I'd have thought I applied the feedback incorrectly (I even swapped it to make sure). Wherever possible, I used more than one type of output transformer in my tests.
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Old 27th March 2006, 11:19 PM   #17
rdf is offline rdf  Canada
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Thx EC8010. I wasn't familiar with the N78. Wow, what a nice tube! I can understand why it responded so well to cathode feedback. Very linear and the Marconi spec sheet shows 4 watts out (about 5.5 volt 0-peak into 8 ohms) with 5 volts swing at the grid in pentode. That's a pretty good return. Surprised about the EL84 though. I used the technique on the EL84 family in triode with two different transformers. The first a James SE in fixed bias, currently a Hammond 1628se as shown in Jane's schematic. It worked well both times but the James exceptionally so sonically. I really can't say much about square wave response, the Hammond's a defective design and the James donating parts for a KT100 (or 814) SE so no longer available for testing. I recall the James rings pretty good at 60-80 kHz with an EL84 in any case.
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Old 28th March 2006, 12:12 AM   #18
DougL is offline DougL  United States
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I found this link today while looking at an Audio Research D40 from 20 years ago.

ARC D40 Schematic

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Old 28th March 2006, 06:23 PM   #19
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Quote:
Originally posted by tubelab.com
Most P-P transformers wind the 4 ohm winding with heavier wire than the 8 and 16 ohm windings, so they have different DC resistances. This is often the case with the 2 halves of the primary as well. Many P-P transformers have different amounts of inductance in each winding. This causes different amounts of feedback to each tube, and the whole effect varies with frequency. The tube to tube balance that you are trying to achieve gets upset with imbalanced feedback.
Tubelab,
I have some difficulty here. I do not see how different dc resistances in windings will influence the degree of feedback, which is dependant on winding ratio (apart from secondary issues like capacitance, but I do not think you bring that in here). Likewise it is unclear how P-P transformers can have differing inductances in each winding, if the turns ratio is equal (as surely it is supposed to be). I have never found that, although I have probably measured fewer transformers than you. Lastly, will unbalanced feedback not tend to minimise the influence of tube differences rather than upset it? (I take it you are talking generally here).

As an aside, are there output transformers having a physical tap on a 16 ohm winding for 4 ohms, as compared to bringing out the 2 equal windings and wiring either in serie or in parallel? What a waste and unbalanced arrangement!

Then a remark about using tertiary windings for feedback purposes (especially global): If these are unloaded, leakage reactance plays a very diminished role, making higher degrees of NFB possible without stability problems (depending of course on transformer design). Several such circuits existed long ago, making a pentode output stage almost equivalent to a triode one. A 70V winding might serve this purpose.

The drop in frequency response at the loudspeaker winding itself will still be there as this is not included in the feedback loop, but the total effect appeared to have had an advantage over feedback including the (loaded) loudpseaker winding. Excellent square wave response was displayed.

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Old 28th March 2006, 07:10 PM   #20
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The different DC resistances are just another symptom of the underlying problem. Low cost output transformers are often wound in this fashion. One half of the primary goes on first, then the secondary, then the second half of the primary. Better quality transformers will interleave the secondary into the two primaries, but often the two primary halves are wound consecutively. This leads to differences in inductance and distributed capacitance between the two halves. I have taken several transformers appart and this seems to be a common practice. Since the winding that goes on last has a longer winding path, there is more wire in that section. The DC resistance is one clue that this is the case, however I have seen many transformers where this (longer) winding is wound with thicker wire.

In the low cost transformers that I have examined the secondary is one continuous winding with taps for 4 and 8 ohms. Often each section is wound with progressively smaller wire. Since the 0 to 4 ohm section carries the most current is is wound with the thickest wire. The wire is spliced to a thinner wire, and the winding continues. The splice is brought out to form the 4 ohm tap. The winding is spliced again to an even smaller wire and this splice forms the 8 ohm tap. The (thinner) winding continues until the end of the secondary which is the 16 ohm tap. Yes this is unbalanced, but it is common practice.

Transformers wound in this fashion do not work well for CFB using a circuit similar to the Audio research designs mentioned earlier. I have a large quantity of P-P transformers that are wound as described above. They work reasonable well in standard P-P circuits with no feedback (my 300B P-P amp for example) or in circuits that use a single feedback loop from the amp output to the driver or input stage. Any attempts at balanced feedback results in lumpy frequency response above 5KHz and they sound bad.

I don't know how the unbalanced transformer reacts with the unbalance caused by the tubes, but I tried several different transformers in this amp (P-P 6L6) , and found that each one behaves differently. This experiment wes done several years ago, so I don't remember all of the little details. I remember that the only transformers that I could get to work were some old UTC's and some mains toroids. I don't have many P-P transformers to try though.

There are good transformers with bifilar primaries, and multiple secondaries that can be interconnected to get the desired impedance ratio. These tend to cost more than I am willing to spend though.
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