'64 BF Twin Reverb Rebuild Questions

[martin manning]

Please correct me if I am off base here , but would it not be safe to conclude that since every step up in secondary winding impeadance is about 1/3rd more windings that your getting greater coupling to the primary winding , and this greater coupling makes for better responce and in turn tone?

Each doubling of impedance is 41% more active secondary turns. Where did the 7% better sound come from, and what does that mean? I have previously heard people say that using the entire secondary is somehow better, but I've never heard any explanation of how or why. Anybody?

[Stevem]

I am just going on the basis of what takes place thru the action of interleave winding a OT and its 7 layers of such.
That produces better primary to secondary coupling for better tone / fidelity , this is a fact , so then on a non interleaved wound OT would not using its 16 ohm tap sound better than the 4 ohm tap as the full secondary winding is mating now with the full primary winding, it must make for some level of better, no?

If nothing else it sure as heck eases the current load on the OTs secondary side since you have more wire carrying less current, no?

I thought it was 33% , as 41% make for over 100 windings?

[strelok]

I'm no transformer expert. However I think that when it comes to guitar amps everything is so non-linear and subjective that its really hard to say what's better or not. I'm just throwing this out there as an idea, but perhaps the not so great coupling could be a good thing. It could potentially introduce a distortion or frequency response that makes the amp sound better (to someones ears, worse to another's, maybe).

All I'm really getting at is that it might be wise to not confuse good tone with fidelity. Certainly the improved coupling from an interleaved transformer is a good thing in a hifi amp where the most accurate reproduction of the original signal as very clear and well defined goal. To a guitar amp though fidelity is the enemy (in the sense of fidelity being defined as the amplifiers ability to cleanly and accurately reproduce the input signal at the output).

I just wanted to point that out as it can lead you down a bit of a rabbit hole once you start trying to get everything performing at its theoretical optimum. I've seen happen before. Guitar amps designed with meticulous optimization. Every staged engineered to give the best performance it can possibly get, using the best components available, every cap and resistor selected to give the best possible frequency response. A high voltage power supply with heavy regulation and filtering etc etc. Then you plug it in and it has a nice clean sound, you start to turn the gain up and all of a sudden your slammed into a brick wall of some of the worst tube distortion you've ever heard. This is just an extreme example, but I felt it might be worthwhile to point out that tone and fidelity shouldn't be confused for the same thing IMO.

[martin manning]

With no interleaving, the secondary windings closest to the primary coil could always be used regardless of which impedance tap is in use. My thinking is that with interleaving, using the lower output impedance taps would leave part of the secondary coil unused, and reduce the degree of and symmetry of the interleaving. Since interleaving is done to improve HF response, using the lower output impedance taps would be likely to result in some HF roll-off. It's possible that the winding pattern and connections could be arranged to minimize that effect.

Doubling the output impedance requires decreasing the turns ratio by the square root of two. Primary turns are fixed, so that means adding 41% more active turns to the secondary.

[Stevem]

Ok got it, thanks much!

[pdf64]

By coincidence, I've been pestering Hammond for freq response info for their OTs with tapped secondaries, using an example of the 1650G, as I've got one and it's got a lot of taps (3.5, 8, 16, 250, 500ohms).
They've got around to doing the tests, not at full power unfortunately, see links for the charts
https://drive.google.com/file/d/0BzZK4L ... sp=sharing
https://drive.google.com/file/d/0BzZK4L ... sp=sharing
https://drive.google.com/file/d/0BzZK4L ... sp=sharing
https://drive.google.com/file/d/0BzZK4L ... sp=sharing
https://drive.google.com/file/d/0BzZK4L ... sp=sharing

They seem to confirm martin's hypothesis, but a surprise for me was the slightly increased low end roll off with the 3.5 tap.
I got that OT as I wanted a 6k6 primary for a JTM45 type amp, and I also wanted to use it as a test bed for varying the load using the 500 ohm output - I happen to have a bunch of power resistors that suited that purpose.