Using 100V line transformers as p-p o/p transformers - UK Vintage Radio Repair and Restoration Discussion Forum

Steve_Bell · 12th Nov 2022, 10:45 pm

Post moved from this thread:

https://www.vintage-radio.net/forum/...d.php?t=195920

How do you use a 100v line transformer as a push-pull?

Electronpusher0 · 13th Nov 2022, 9:08 am

Quote:

Originally Posted by Steve_Bell

How do you use a 100v line transformer as a push-pull?

To take the 15W as an example
Connect as follows

Anode 1 - Com
Screen 1 - 15W
HT (centre) - 5W
Screen 2 - 2.5W
Anode 2 - 1.25W

See the attached spread sheet for the calcs.

The 15W is the only one that gives precisely matching screen taps but I have tested the 30W version and despite the slight mismatch I could not measure any detrimental effects. My main amplifier used in the house is a Mullard 5-10 using the 30W ones.

Peter

Electronpusher0 · 13th Nov 2022, 9:24 am

I forgot to add that the Silicon chip (and Everyday Practical Electronics) Currawong valve amplifier used 100V line transformers

https://www.radiomuseum.org/r/silico...ifier_kit.html

Peter

G6Tanuki · 13th Nov 2022, 3:22 pm

I have used the same approach in a transmitter modulator, like the Codar AT5 but with an 807 PA and 6L6 as modulator in Class AB1. The anode currents balance out more or less so that you can use quite a small transformer and still get 100% modulation without the core saturation being an issue.

Cruisin Marine · 13th Nov 2022, 5:15 pm

I have used the same approach in a transmitter modulator, like the Codar AT5 but with an 807 PA and 6L6 as modulator in Class AB1. The anode currents balance out more or less so that you can use quite a small transformer and still get 100% modulation without the core saturation being an issue.

How does that work then?
I just looked at a very small ctt diag. for and AT5, and it looks like a centre tapped transformer with one side feeding the anode of the mod valve and the other feeding the RF PA anode via the usual RFC, The HT goes to the centre tap it seems.
Would I be correct in assuming that the currents are 180 degrees out of phase and cancel (well partially in reality) and not add?

G6Tanuki · 13th Nov 2022, 5:22 pm

Yes, essentially it is a kind of 1:1 autotransformer with equal and opposite current flowing in the two halves so that the magnetic fields sum to zero under static conditions. Applying audio drive to the modulator unbalances the field contribution from the modulator, which results in the audio output being applied to the PA stage. The only downside is that you have to use the same HT on both the PA and modulator valves, but since an 807 is really a 6L6 with a top cap they are reasonably well matched.

retailer · 14th Nov 2022, 11:34 am

The forum members at australian guitar gear forum (https://www.aggh.net/discussion/index.php?board=8.0) have made use of 100V line Tx's in guitar amps, I haven't tried one my self but by all accounts they work well.

daviddeakin · 14th Nov 2022, 3:35 pm

Remember that they're only designed for 100Vac across the primary, above which they're likely to saturate (at 50Hz? 100Hz? Who knows). At typical valve primary impedances that means you'll never get more than 2-3 watts out at the low audio end, before saturation.

12th Nov 2022, 10:45 pm	#1
Steve_Bell Hexode Join Date: Sep 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 475	Using 100V line transformers as p-p o/p transformers Post moved from this thread: https://www.vintage-radio.net/forum/...d.php?t=195920 How do you use a 100v line transformer as a push-pull? Last edited by Cobaltblue; 13th Nov 2022 at 1:37 pm. Reason: New thread created

13th Nov 2022, 9:24 am	#3
Electronpusher0 Nonode Join Date: Jun 2016 Location: Bognor Regis, West Sussex, UK. Posts: 2,300	Re: 100v line transformers I forgot to add that the Silicon chip (and Everyday Practical Electronics) Currawong valve amplifier used 100V line transformers https://www.radiomuseum.org/r/silico...ifier_kit.html Peter

13th Nov 2022, 3:22 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: 100v line transformers I have used the same approach in a transmitter modulator, like the Codar AT5 but with an 807 PA and 6L6 as modulator in Class AB1. The anode currents balance out more or less so that you can use quite a small transformer and still get 100% modulation without the core saturation being an issue. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

13th Nov 2022, 5:15 pm	#5
Cruisin Marine Octode Join Date: Jul 2018 Location: Worthing, West Sussex, UK. Posts: 1,002	Re: Using 100V line transformers as p-p o/p transformers I have used the same approach in a transmitter modulator, like the Codar AT5 but with an 807 PA and 6L6 as modulator in Class AB1. The anode currents balance out more or less so that you can use quite a small transformer and still get 100% modulation without the core saturation being an issue. How does that work then? I just looked at a very small ctt diag. for and AT5, and it looks like a centre tapped transformer with one side feeding the anode of the mod valve and the other feeding the RF PA anode via the usual RFC, The HT goes to the centre tap it seems. Would I be correct in assuming that the currents are 180 degrees out of phase and cancel (well partially in reality) and not add? __________________ "Behind every crowd, there's a silver Moonshine"

13th Nov 2022, 5:22 pm	#6
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: Using 100V line transformers as p-p o/p transformers Yes, essentially it is a kind of 1:1 autotransformer with equal and opposite current flowing in the two halves so that the magnetic fields sum to zero under static conditions. Applying audio drive to the modulator unbalances the field contribution from the modulator, which results in the audio output being applied to the PA stage. The only downside is that you have to use the same HT on both the PA and modulator valves, but since an 807 is really a 6L6 with a top cap they are reasonably well matched. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

14th Nov 2022, 11:34 am	#7
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: Using 100V line transformers as p-p o/p transformers The forum members at australian guitar gear forum (https://www.aggh.net/discussion/index.php?board=8.0) have made use of 100V line Tx's in guitar amps, I haven't tried one my self but by all accounts they work well.

14th Nov 2022, 3:35 pm	#8
daviddeakin Hexode Join Date: Apr 2007 Location: W Yorks, UK. Posts: 407	Re: Using 100V line transformers as p-p o/p transformers Remember that they're only designed for 100Vac across the primary, above which they're likely to saturate (at 50Hz? 100Hz? Who knows). At typical valve primary impedances that means you'll never get more than 2-3 watts out at the low audio end, before saturation.