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Old 6th December 2003, 04:08 PM   #1
brsanko is offline brsanko  United States
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Default bifilar output trans in single ended amp

I'm curious as to wheather one could use a bifilar output transformer on a single ended amp and just run DC through the second filiment to cancel magnetic flux. And if so where could I find such a transformer. I'm working on the design of my first tube amp. As of now I'm thinking about going with a 12au7 for input/driver and a single ended parallel 2A3 output stage. Any suggestions of comments welcome. Oh yeah where can I get 2A3s. I'm poor so I would probably go with Chinese ones for my first project.
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Old 7th December 2003, 09:45 AM   #2
Fuling is offline Fuling  Sweden
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Iīve been thinking in the same ways many times, but I have never tried it out. One problem might be the fact that there will be a signal voltage present at the cancellation winding too, so I guess the DC has to be supplied through a CCS or a choke.

By the way, havenīt Andrea Ciuffoli done something like this?
A quite regular PP output stage but only one tube is connected to the driver stage, the other tube is only there to balance the DC.
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Old 7th December 2003, 10:14 AM   #3
Jax is offline Jax  Sweden
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This has been discussed during the years but nobody seems to have had a reliable success. Maybe because the bias current in the output tube varies. My own idea is to use a constant current source feeding a low impedance winding and a servo measuring the bias current in the tube to compensate. I have never tried it though.

One weird idea is to use the secondary winding and AC-couple the speaker. Then use a sand state constant current source to cancel the field in the transformer. Complicated, it would be easier to AC-couple the tube to the transformer and feed the tube through a choke.

Extra windings also cause more leakage inductance, something to think about.
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Old 7th December 2003, 11:06 AM   #4
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Originally posted by Jax
it would be easier to AC-couple the tube to the transformer and feed the tube through a choke.
This is a good solution because the transformer can then have high inductance and tight coupling for good LF and HF response respectively. Because it now doesn't need an airgap in the core (no dc flowing in it) this is much easier to achieve.
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Old 7th December 2003, 11:15 AM   #5
Fuling is offline Fuling  Sweden
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Yes, at least in theory. But as Jax points out there must be some reason why noone does it.

The problem with higher leakage inductance should be lower if the cancellation winding only has a few turns and is fed by a high current instead of being similar to the primary winding.

That high DC current could always be used to heat some tubes
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Old 7th December 2003, 11:58 AM   #6
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Bi-Filar will take up a lot of room. Not good for parasitic stuff. It will also have a voltage on it that matches the tube. Cpaacitive coupling to this winding will be substantial.
Now, you could just put in another winding, of say a third of the plate winding count, not absolutely a bi-filar one, and pass higher current through it. at the 'one third' count, you'll develop a third of the voltage and require 9x the current to off set the plate's field.

Just because nobody has tried it is no measure of its workability or potential.

the Tnuctipun will return
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Old 7th December 2003, 12:12 PM   #7
Fuling is offline Fuling  Sweden
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The people who uses 811A, 805 and other tubes that draws grid current often drives them with inverting interstage transformers.

Since the transformers are inverting the grid current and the driver tubes plate current is out of phase, partially cancelling the DC flux in the core. Very clever, but the transformers has to be very well made to work well in inverting mode.

Instead of using servo circuits to get perfect cancellation we could use a tiny airgap that allows some imbalance. I believe itīs benificial for the linearity, by the way. The primary inductance would still be much higher than a regular SE- tranny with a fullsize airgap.
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