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In Reply to: what's the application? posted by Dave Cigna on April 1, 2007 at 14:45:51:
Hello Dave,The driver stage is 12AT7 diff amp running at 9mA per tube / 250V plate. I am just juggling with the idea now, and although seductive in many respects i realize that at mu 65 and rp 13K it is a difficult tube to load properly.
Besides the price, what would be the disadvantages/trade-offs you see? IIRC, it is difficult to build high impedance trans with good freq response, but since there are practically no current flow maybe the use of permalloy would circonvent the problem... But at a hefty price tag for sure...
Follow Ups:
Well, I suppose you really do need 400H or 500H in your case. The difficulty is that it takes a lot of wire which will make it more difficult to get high frequency response. But I think it could probably be done, especially with 1:1 which can be wound bifilar. Without actually trying I can't say for sure how well it would work...Permalloy might be an option, but you would really need to get the DC current balanced without a gap.
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.
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
Of course I am of the frame of mind that an IT transformer "has" to be high nickel. A high Rp tube only makes matters worse when it comes to bass. Now if more IT makers offered a "feedback winding" we would have ways around the problem. But WTF, feedback is a nasty word round these parts so I reckon it don't matter:)
Hello Russ,The 12AT7 was my choice because i wanted a two-stage amp and its "low" rp wrt its high mu allowed me two get high input sensitivity.
If i drop this requirement, another option would be chaining two stages of low-mu low-rp tubes, like the 5687 (mu 16, rp 2K). The gain before the power stage would no longer be a problem and the low driving rp would ease things for coupling; however it is a supplemental stage with all its colorations. On the plus side, since my amp is fully differential (and i want it to stay that way), two driver stages with identical op points would cancel their odd harmonics a bit - the evens being canceled by the diff setup. Worth a bit of thinking about...In your opinion/experience, which is best - more or less stages?
Joris:If you want to keep the 12AT7 in an LTP arrangement but get lower output impedance and enhanced drive capability--- would an SRPP arrangement be helpful or a consideration?
What made me think of it is that one of my all time fave sounding amps... the Baby O designed by Gordon Rankin used a 12AT7 SRPP as the front end tube to drive a 2A3 single ended tube.
couldn't you keep the LTP arrangement and SRPP each side of the long tail paired tubes? And then, perhaps, tie down the cathodes with a stiff CCS? Or am I missing something?
Mike,Yes, great idea. Another solution to keep drive low would be a CF (probably mosfet) DC coupled to the input stage, but that's a whole different can of worms. The SRPP LTP keeps me at two stages and provides the low drive Z and full mu, then coupling using a regular gapped 1:1 IT of around 20K Z is easy. No more messing with plate CCS or chokes, but a higher B+ will have to be provided.
Thanks to you and all who responded.
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