• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Cathode and other types of feedback

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Tubelab,

Thanks for broadening my horizons.

I guess, being fortunate enough to have moderately reasonable transformer winders locally, thus making it economical to design and obtain my own few transformers, I have been suitably shielded from the wicked world out there. With our economy imports are comparatively expensive and local stocks of tube output transformers non-existing, so..... (Btw, I did not question that there are differences in winding resistance, just how that could cause different inductances - but not to worry. With what you mentioned, anything is possible.)

And I have also been looked upon quite strangely for suggesting that THEIR own output transformers are not suitable for hi-fi work, because, see, they work quite well in PA amplifiers!

Sic transit gloria (and us veterans).

Sorry if this was a bit off-thread.
 
Let's use a simple model. The inductance of a solenoid is proportional to the square of the number of turns and the cross-sectional area. With a layered winding, the number of turns in the two halves may be equal, but there will be a difference in the area between the inner and outer layer. So perhaps that's why the two halves show a different inductance?
 
SY, me friend....

Yes, OK! One does forget basics sometimes!

One tends to think that with iron-cored products this does not really matter, most of the flux being concentrated in the core anyway - but there!

Though I have checked in the past and cannot recall finding much of a difference, you will now have me burn midnight oil (locally) and go and recheck on the few output transformers in my possession. I think more than likely my own designs will not show much difference because I sectionalise, interleaving also primary windings, but I have a few of the type described by Tubelab.
 
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Joined 2003
Further, an outer layer will have a different leakage inductance to an inner layer, because leakage inductance is just that - it's flux that was lost on the path from primary to secondary, and that's the real problem. The problems with cfb turn up at HF. Sadly, most transformers are constructed as Tubelab describes - it's most unusual to find transformers that are properly balanced.

If you don't believe me about leakage flux, try waving a search coil near an output transformer. The places where you pick up the most signal are on axis of the coil (as you'd expect) and at the edges of the coil.
 
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Joined 2009
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.

Apologies for resurrecting an old thread.

Hopefully to prevent someone spending lots of money on some exotic Japanese transformers: I found the above to be the case on Hashimoto HWC-30-8 PP that I bought for putting into an Audio Note PP 6V6 amplifier which is a direct clone of the Sansui HF-V6K (1955) / Sansui HF-V60 (1956). Hashimoto made at one stage the OPT for Sansui so thought it would be a satisfying amplifier, unfortunately it was not.
 
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