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Simple tube circuit transforming push-pull output transformer into single ended one
Simple tube circuit transforming push-pull output transformer into single ended one
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Old 6th August 2019, 01:27 PM   #1
KeesB is offline KeesB  Netherlands
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Default Simple tube circuit transforming push-pull output transformer into single ended one

Having a 40w 6k6 push pull output transformer I wanted to use it as a single ended one for a PSE VT4C ( the output impedance seemed about right). I therefore needed a tube circuit that would cancel the output tubes anode currents to avoid magnitization of the airgapless core and one that would be both simple and cheap. The current drawn by the tube had to be equal to and opposed to the anode currents of the VT4C's. The schematic shows the VT4C's and the 1000v powersupply and the added floating PL519 circuit whose anode current runs in the opposite direction through the output transformer. Both are drawing 100ma and thereby cancel each other.

The 519 is used as a penthode which has a high output impedance even made higher because of the nondecoupled cathode resistor so as not to ac load the output tubes. With the 10k pot we can vary the anode current from 40- 150ma. With a dvm across the outputs primary we adjust the pot until we see zero volts (within a few tens of a volt).

As the indirectly heated 519 takes some time to settle we must readjust the pot after a few minutes. As the floating counter current circuit carries a 1000v I built it on a piece of wood and a cage around it to prevent an accidental touch. The pot has a plastic knob for the same reason. The 40v, 1A transformer feeds the filament and by tripling the dc output also the anode and screen voltages of the 519. Four 220 mu\ 200v electrolytics take care of the smoothing. ESRC tubes sells the 40KG6A (=PL519) for $12 so the total cost will be about $35 for one channel.

I'm quite happy with the outcome and feel no need to buy a big and costly OT. I know many audio amateurs do have an unused PP OT lying around so I suggest they try the above for a change. The anode current of the 519 can be adjusted over a wide range so it can be used with all kinds of tubes (845, GM 70, 811-3, 811-10, KT88 etc.). You don't have to worry about dc overloading a (possibly) small OT as there is no dc. To prevent ac overloading don't use the amp at full power. Any other tube can be used instead of the 519 but sweep tubes are cheap and strong and will last a life time in this application.
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Old 6th August 2019, 04:06 PM   #2
DF96 is offline DF96  England
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I suspect that what you have drawn is not what you have built. A truly floating circuit cannot take current from anything.

If you are going to draw current from one side of an OPT then you might as well use it for signal too. Less waste, less distortion.
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Old 6th August 2019, 05:42 PM   #3
KeesB is offline KeesB  Netherlands
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There is nothing wrong with the schematic. The 519 circuit is floating because there is no connection with ground, it has it's own powersupply because the 40v. from the transformer is used for the filament and and through tripling dc provides 90v under load.
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Old 6th August 2019, 06:13 PM   #4
HollowState is offline HollowState  United States
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A quite interesting current bucking arrangement. Most do it with one half the primary which sacrifices transformer coupling to a degree. But aren't the primary windings anti phased so coupling is compromised as well? Anyway I seem to remember something about this full primary bucking system years ago in the famous Japanese "MJ Stereo Technology" (Mu Sen To Zi Ken). I had planned to experiment with it but just never got around to it. Thanks for the reminder.
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Old 6th August 2019, 09:28 PM   #5
KeesB is offline KeesB  Netherlands
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Thanks for your reaction.If the windings were antiphase and you would connect half the primary to say 30v ac you would measure zero volts. Instead you'll measure 60v.
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Old 6th August 2019, 11:00 PM   #6
PRR is online now PRR  United States
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It should work. I re-drew it to see the cancellation.

Drawbacks:
Eats double the DC power
PL509 probably absorbs half the power to the load.

Side objections: PL509 is worked slightly above its plate voltage rating, apparently far beyond its dissipation rating, and far beyond its screen rating. These issues can be solved with more bottles and a tap on the 910V multiplier. The lowish rp can be increased with substantial cathode resistor and bias.
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Old 7th August 2019, 01:25 AM   #7
smoking-amp is offline smoking-amp  United States
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Looks like an asymmetric Circlotron.

1000V on the screen grid is not going to set well with that PL509.

Maybe use something like an IXYS IXTP01N100D depletion mode HV Mosfet. Could use just half of the primary (500V) for the current compensation.

You can also put the compensating CCS on the LV OT secondary. A small DC offset will appear on the speaker then however (from secondary winding resistance), unless cap coupled or DC offset compensated.

I have taken the end bells off one OT and just wound 30 turns of Teflon insulated wire thru the available window space. I just hooked that little winding up to a current limiting power supply to neutralize DC. (hardly any AC on it)

Last edited by smoking-amp; 7th August 2019 at 01:39 AM.
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Old 7th August 2019, 03:44 AM   #8
6A3sUMMER is offline 6A3sUMMER  United States
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It seems like a nice idea.

If you want to use your favorite output transformer, just remember to watch out for Transient voltages, not just during large signals, but also at start-up and power-down.

There is another solution, Parafeed

Requires a High Voltage high current choke
Requires a High Voltage Capacitor
No feedback needed
Simplicity (If you can find suitable parts for those output tubes current and voltage requirments).

A choke is very efficient as a current source, lower burden voltage and less B+ required. A solid state CCS device's voltage can only collapse to zero, it can't go the other way beyond zero (but a choke can).

Oh . . . and just for safety sake, you may want to consider grounding the secondary of the output transformer.

Last edited by 6A3sUMMER; 7th August 2019 at 03:52 AM.
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Old 7th August 2019, 06:22 AM   #9
Chris Hornbeck is offline Chris Hornbeck  United States
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One really difficult problem is that there's no 910VDC there. Of course the OP did warn against using the amp at full power.

All good fortune,
Chris
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Old 7th August 2019, 06:33 AM   #10
Lampie519 is offline Lampie519  Netherlands
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It looks like this will only work an a static environment not dynamic. The voltage over the transformer will change dramatically (even opposite voltages can be expected). I would build a conventional push pull version using the PL519 as the "other half" without signal. In this case your transformer will have a different resistance but depending on the secondary load it could still work fine. Additionally the hum will be balanced out as well.

The plate rating is no issue with a PL519 as long as the power rating is not exceeded.
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