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In Reply to: Re: what's the application? posted by Dave Cigna on April 2, 2007 at 14:01:20:
it is not possible to wound this bifilar, this gives a very high capacitance between both windings, much to high for the high plate resistance of your tubes. And you get an very high coupling capacitance, you can also use an coupling capacitor. The next fact is the stray inductance is proportional to the main inductance, and high stray inductance is reducing the high frequency response.And permalloy will loose its magnetic characteristics if you have any DC magnetisation. The warm up time is critical, because different plate current in both tubes.
Follow Ups:
Gerd, I don't see the problem. I'm thinking of a dual bay bobbin with a bifilar pair in each bay (one of them reverse wound.) Each half of the 1+1:1+1 transformer will have its own bay. No interleaving at all is necessary, so the shunt capacitance will be low. The capacitance between primary and secondary will be very high, but I don't see it being a problem unless the secondary is loaded down, which it won't be in this case.
Hi Gerd:re: bifilar wound transformers. Yep... you get very high capacitance btwn the two windings and very low leakage L.
Above a certain frequency your no longer transformer coupling at all but essentially capacitor coupling.
In other words--- ironically, for the fellas who have suffered from capacitoritis--- bifilar tranneys promote excellent (i.e., large) capacitive coupling btwn the primary and secondary windings.
And of course this primary to secondary "capacitance" is in the signal path---- cause essentially above a certain critical frequency it (i.e., the capacitance) is THE signal path.
I guess it could be proven that above a certain frequency all transformers behave as capacitively coupled circuits whether they are bifilar or not. Unfortunately that doesn't tell us a lot about how any specific implementation behaves.The most recent pair i measured has what appears to be about 4nf of "coupling capacitance" and when loaded down in circuit, the corner frequency of the transition was in the 15-20K range. (anybody care to guess at the source impedance driving this since that matters too)
The very interesting and often ignored difference between the simple view of the bifilar as a cap coupled circuit and reality is that at low frequency the circuit behaves as a magnetically coupled one so the behavior is very stable and gives you the predictable 6dB per octave rolloff with no squirrels jumping in to bite you as frequency goes down.
> I guess it could be proven that above a certain frequency all transformers behave as capacitively coupled circuits whether they are bifilar or not. Unfortunately that doesn't tell us a lot about how any specific implementation behaves.Hello, this is correct for transformers like OPTs, but good input transformers using a static shield between the windings to prevent this kind of coupling
Also ever a squirrel did not bite me, if I used a coupling capacitor ;-)
RC coupling is no problem, the low end roll off is 6dB / octave too.Another fact is, I dont need an IT for 1:1, this gives more problems than it will solve.
If the problem is only the headroom at a given power supply, I will better use a center tapped plate choke. This works symmetrically, with the same coupling between both windings primary, and gives very stable 12db/Octave in combination with a fitting coupling capacitor...The last question is, why one needs at all a complicated interstage transformer with all its problems and the necessary compromises?
After the invention of the radio quite more intelligent concepts were developed. A typical representative is the Studio Amp V69 from Telefunken.
An input transformer provides for galvanic separation and settles also the phase splitting. Afterwards to it the concentrated amplification is made, by local inverse feedback develops a supertube with high power and large amplification.
The inverse feedback of both circles is coupled over the primary windings of the OPT, an extremely stable function results.
Reference to the link: Everything left of the Input transformer can be void, but then 1,5Vrms are needed with 5-7 KOhms source impedance for full power
Regards
Gerd
I personally don't see the situation as direly as you do. As long as the transformer will be driving a high impedance the bifilar will not be an issue and the minimal gap needed to allow for a 1-2ma offset will still allow upwards of 800hy's even with a 49% nickel core. The 800hy number is end to end so each tube will see a load of 400hy's placing the -1dB point at ~10hz.Another octave on the low end could easily be gained if the current through the tubes is kept matched allowing the gap to be further shrunk at the cost of permissible imbalance.
dave
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