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GordonW 5th May 2008 03:26 PM

Toroidal output transformers with screen drive- why not?
 
Just had an interesting thought.

Toroidal output transformers seem to have one drawback... the problem with very quick magnetic saturation due to primary DC current mismatch between the two sides, using push-pull output tubes.

Well... seems that most screen-drive amps can get away with VERY LOW bias current... to the tune of 3ma to 5ma DC on each tube, for a push-pull pair!

This seems to be an ideal situation for toroidal outputs, it would seem to me. With that little bias current, even large-percentage shifts in bias current would make for very small actual changes in DC current differential between the two sides...

Anyone tried this, or have any ideas as to why this might or might not work?

Thanks!

Regards,
Gordon.

JoshK 5th May 2008 04:53 PM

Neat observation. However, who makes reasonable priced toroid OPTs? Plitron's are very expensive IIRC.

alexg 5th May 2008 05:58 PM

Re: Toroidal output transformers with screen drive- why not?
 
Quote:

Originally posted by GordonW
Just had an interesting thought.

Toroidal output transformers seem to have one drawback... the problem with very quick magnetic saturation due to primary DC current mismatch between the two sides, using push-pull output tubes.

I am using a 120VA mains toroidal transformer as a PP OPT for my KT88 amp and I am measuring DC imbalance of 6ma and I am not getting any magnetic saturation on them.

The KT88 are cathode biased and triode strapped, producing around 20 watts per channel. Each power tube has its own cathode resistor and bypass, I previously configured it to have a shared resistor and when I installed individual cathode resistors it seems to be more balanced.

GordonW 5th May 2008 09:21 PM

Re: Re: Toroidal output transformers with screen drive- why not?
 
Quote:

Originally posted by alexg


I am using a 120VA mains toroidal transformer as a PP OPT for my KT88 amp and I am measuring DC imbalance of 6ma and I am not getting any magnetic saturation on them.


That's kind of the direction I was going with my thinking, when starting this discussion. :scratch2:

The lower impedance ratios of power transformers could be just what is needed for a PPP screen-drive amp...

Regards,
Gordon.

woodturner-fran 5th May 2008 10:48 PM

A user here Shoog has used power toroidals in a few amps and I can tell you they sound great. I think he goes for 4-5 times over the wattage.

Fran

SY 5th May 2008 11:01 PM

I would think that you'd have to have the transconductances matched pretty tightly for this to perform well.

alexg 6th May 2008 01:30 AM

Quote:

Originally posted by woodturner-fran
A user here Shoog has used power toroidals in a few amps and I can tell you they sound great. I think he goes for 4-5 times over the wattage.

Fran

I got the idea of using toroidal OPT from Shoog, and have been closely following his work.

I did a 6as7/6080 parafeed using a 40VA mains transformer and it sounded excellent!

Shoog 6th May 2008 09:29 AM

I am encountering the first serious issues with the Tabor clone I am currently building. I get big overshoot on all square wave leading edges. I needed to install a Zobel on the primary to tame this. I haven't got the same issue with other amps using the same transformers. I can only presume that the very low output impedance of the plate to plate design is interacting with the interwinding capacitance to over excite the transformer.
Certainly an issue to watch for and a good scope is needed to diagnose. Aparently there are no free lunches.

I also find that wiring up a transformer in one direction will induce ringing and then reversing the wiring will make it go away.

Shoog


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