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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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16th Nov 2018, 7:27 pm | #1 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Small valve push-pull output transformer?
I'm starting on work to restore a Hallicrafters S27 receiver. This has a pair of 6V6 as the audio-output stage.
In a previous existence my specimen has had the push-pull output transformer replaced by a much bulkier item - probably because the original was intended to drive a 500-Ohm load and someone wanted to use a low-impedance speaker instead. In making this modification they've shifted the S-Meter-zero pot and replaced the S-meter with a smaller one to gain space for the oversized transformer. To reverse this, I need a really-rather-small push-pull output transformer that will match a couple of 6V6 to a modern speaker. As originally designed the transformer had a confusingly-overcomplex feedback winding linked to the tone-control, something that seems deeply unnecessary for a 'communications' receiver. I'm happy to forego this - remember we're not talking ''hi-fi'' here! Suggestions? The smallest P-P OPT I've seen is the sort of thing fitted to my Barker-88 - which is no bigger than the single-ended OPT in a typical late-1950s radio. I wonder - could a multiply-tapped '100V line' transformer work? Seems they've been used in some P-P amplifiers. |
16th Nov 2018, 8:26 pm | #2 |
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Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
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Re: Small valve push-pull output transformer?
A 6v mains transformer with a 115v tapping on the primary will do the job surprisingly well.
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16th Nov 2018, 10:26 pm | #3 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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Re: Small valve push-pull output transformer?
Hi Tanuki, optimum load for PP 6V6 is 8K, so 50:1 ratio for a 3R speaker.
Ed |
16th Nov 2018, 10:49 pm | #4 |
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: Small valve push-pull output transformer?
I would go for the -125C version of this universal type rated at 8W from AES, 2.3 inches across the mounting hole, since its a pair of 6V6's, but you could probably use the 5W or 10W version too and they have taps to get you Z ratio perfect:
https://www.tubesandmore.com/sites/d...p-t125-a-e.pdf But also if you search their websisite you will find many more, especially for low power push pull guitar amp output stages, some with full covers that look old fashioned and suit a vintage radio perfectly. You didn't say what physical dimensions would be ideal. |
18th Nov 2018, 6:22 pm | #5 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: Small valve push-pull output transformer?
The original was quite small - maybe a 2-inch cube. I haven't yet removed the kludged-in replacement from the chassis to see what the mounting centres are. It's got to basically fit in the same space as a pair of octal valves side-by-side (see snapshot of chassis layout - T7 is the one needing to be replaced).
I'm thinking of first of all trying the smallest 100V-line transformer I can find; they're cheap enough so if it works I'll be happy and if it doesn't I've not wasted much money. Like I said, I'm not after hi-fi (it's a communications-receiver - any audio above about 4KHz is definitely unwanted) so going into the realm of guitar-transformers and the like at this point would be overkill. [The S27 is going to need a _lot_ of work before I even show it power: its threescore-years-and-ten of life have not been kind to the cotton-insulated wiring which is definitely not in a condition to handle mains or HT]. |
18th Nov 2018, 11:49 pm | #6 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Small valve push-pull output transformer?
I have heard of people using speaker line transformers as output transformers in low powered guitar amps before so don't see why it shouldn't work as long as it can handle the current (something like a 10 or 15watt line transformer should be ok for 2 6v6's). They wouldn't work well for a single ended amp as the core is not air-gapped and would saturate but for push-pull they should be fine. As for the frequency response a line transformer is likely to be more "hi-fi" than a typical guitar amp transformer, the line transformer is designed for P.A. audio where it has to reproduce the full audio spectrum whereas a guitar amp only has to reproduce a fairly narrow frequency range.
Boater Sam's idea of using a 6v transformer also sounds interesting, I haven't heard of that before but the turns ratio would certainly put it in the ballpark so no reason why it wouldn't work! |
20th Nov 2018, 1:35 am | #7 |
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
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Re: Small valve push-pull output transformer?
A PT1608A is only 2 x 1.69 " across the base holes, a little bit taller.
https://www.tubesandmore.com/product...sh-pull-10w-8k these are HiFi 30Hz to 30 kHz! However, I can promise you that the original 2 inch cube transformer associated with two 6V6's is nothing close to a HiFi transformer, it would have to be 4 times bigger to have a HiFi bass response. A HiFi transformer is not what you want in a radio which has an audio spectrum narrower than that, exactly like a guitar amplifier transformer in fact which has the perfect qualities & frequency response and size for the application in an AM radio. Last edited by Argus25; 20th Nov 2018 at 1:50 am. |