Author Topic: Any advice on building a push-pull amplifier with toroidal OPTs?  (Read 461 times)

Offline Qualityten

Any advice on building a push-pull amplifier with toroidal OPTs?
« on: November 20, 2020, 08:56:16 PM »
A year ago, I bought a huge power transformer from a Leak Stereo 50.  And then I recently got a pair of toroidal push-pull output transformers.  6K6 primary impedance and 49% screen taps. The transformers sound pretty good just hooked up to my Mullard 5-20 monoblocs, but I'm thinking of building a stand-alone stereo power amp with these, using EL34 valves.

Does anybody have any experience of using toroidal output transformers and any suggested circuits.  I have read passing comments about DC imbalance, but don't understand these.

Offline handsome

Re: Any advice on building a push-pull amplifier with toroidal O
« Reply #1 on: November 20, 2020, 10:29:16 PM »
In order for a valve to behave as an amplifier it has to pass some DC current so that it can pass both positive and negative excursions of the signal. Transformers only react to AC current (signals) DC current will saturate a transformer limiting the amount of AC it will be able to pass and in extreme cases collapse its magnetic field stopping it from behaving like a transformer.

In a single-ended output stage there is notionally one valve so the transformer has to pass all of its current. In order to not saturated at that particular DC current the transformer is given an air-gap and has to be quite large - bigger core and more windings to get good frequency response

 Ideally each pair of valves in a PP output stage pass equal amounts of current. Each valve’s current passes through ‘half’ the transformer in the opposite direction to the other valve. So effectively the two currents cancel. The transformer can now be smaller plus it only needs a very small air gap just to deal with the fact that the two valves will never be perfectly matched, will age differently and the two “halves” of the transformer may not be perfectly matched themselves, so the currents wont cancel perfectly leaving some small amount of DC current. Good frequency response is easier to achieve than with a larger air gap and a physically larger core.

Toroids are far more efficient transformers; they use much smaller cores and operate very. Toroidal transformers can have excellent frequency response but they will severely saturate with even the tiniest amount of imbalance current.

So if you are using a toroidal transformer you have to design in extra circuitry to maintain equal currents in the output valves and thus perfect cancellation at all time in the transformer.

Offline Qualityten

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #2 on: November 20, 2020, 11:26:09 PM »
Thanks for the explanation Handsome!  Do you or any other forum member know where I can find an example of the necessary circuitry? I am aware of Menno v d Veen’s books, but haven’t got them.

Ideally, I’d like an example of a P-P circuit to study.

Offline handsome

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #3 on: November 21, 2020, 09:01:25 AM »
its basically an automatic bias circuit - at its simplest: a small resistor in the cathode of each output tube is used to measure the current through each tube, the current through one tube would be used as a reference a comparator (usually an opamp) then compares that reference to the other tube(s) and adjusts the bias of the other tube(s) as necessary.

Offline handsome

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #4 on: November 21, 2020, 09:09:58 AM »
More accurately it is a servo circuit.

Offline charles

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #5 on: November 21, 2020, 11:29:00 AM »
I have Menno van der Veen's book:

"Modern High-End Valve Amplifiers based on toroidal transformers""

Try to get hold of this book.


« Last Edit: November 21, 2020, 11:33:13 AM by charles »

Offline Qualityten

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #6 on: November 21, 2020, 04:27:06 PM »
I've identified a few articles and papers by Menno van der Veen and will look out for the book.  Still interested to hear from anyone who has built a push pull amp with toroidal outputs.  I haven't found any circuit diagrams so far, except for a guitar amp by vand der Veen.

Offline handsome

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #7 on: November 21, 2020, 06:02:29 PM »
You can substitute a toroidal output transformer into any PP amplifier design - as long as you implement some sort of bias servo. The difference between a 6k6 toroidal output transformer and a 6k6 EI transformer is the physical construction. The electrical characteristics will be the same, the performance of the toroid is likely to be better though. Its like substituting a toroid for an EI mains transformer of the same output voltage - the toroid is likely to be smaller and generate less interference but both will produce the same electrical result. So any design requiring a 6k6 OPT will work for you.

Offline Qualityten

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #8 on: November 22, 2020, 01:33:27 AM »
Thanks again Handsome.  Can you possibly point me to where I can find a circuit diagram for a bias servo?  (Sorry if this sounds like I'm too lazy to study for myself.  I've been working today and although I have found some Menno v d Veen articles, I have not read them yet!)  There is the amazing Audi-O-Moron build here https://www.avforums.co.za/index.php/topic,79166.0.html[/url, but that's more than I need!
« Last Edit: November 22, 2020, 01:36:08 AM by Qualityten »

Offline Mars

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Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #9 on: November 23, 2020, 10:46:43 AM »
Hi Qualityten

Have you read about the "Blumlen Garter" circuit?




https://www.tubecad.com/2009/04/blog0163.htm


http://www.tubecad.com/2005/May/blog0046.htm

It works well for cathode bias amps with separate cathode resistors.
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Offline Qualityten

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #10 on: November 23, 2020, 11:51:27 AM »
Wow, thanks Karel.  This is going to take my tube studies to the next level!

Offline El Sid

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #11 on: November 23, 2020, 12:43:34 PM »
Hi Qualityten

Have you read about the "Blumlen Garter" circuit?




https://www.tubecad.com/2009/04/blog0163.htm


http://www.tubecad.com/2005/May/blog0046.htm

It works well for cathode bias amps with separate cathode resistors.

Have you ever tried this, Karel? If so, what were the results?

A year ago, I bought a huge power transformer from a Leak Stereo 50.  And then I recently got a pair of toroidal push-pull output transformers.  6K6 primary impedance and 49% screen taps. The transformers sound pretty good just hooked up to my Mullard 5-20 monoblocs, but I'm thinking of building a stand-alone stereo power amp with these, using EL34 valves.

Does anybody have any experience of using toroidal output transformers and any suggested circuits.  I have read passing comments about DC imbalance, but don't understand these.

There are quite a lot of projects over at diyaudio.com (in the Tubes/Valves section)  using toroidal OPTs

Offline handsome

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #12 on: November 23, 2020, 06:29:19 PM »
Read Broskie’s second article; the Garter circuit is wasteful of HT and doesn’t balance very well - you want proper servos on that output stage

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Offline Qualityten

Re: Any advice on building a push-pull amplifier with toroidal OPTs?
« Reply #14 on: November 24, 2020, 11:11:33 PM »
6k OPT : http://www.dalojan.nl/KT88_PP.html

You could also search here : http://lilienthalengineering.com/100-amplifiers-chapter-1/100-amplifiers-part-4
Thanks for both of these interesting links!  I was not aware of the Dalojan one and the lilienthalengineering one has a CCS explanation as part of the discussion of the Brimar – Thorn VA-12 . EL506 PP, 1967.   :thumbs: