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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 14th Feb 2019, 4:03 am   #1
Terry_VK5TM
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Default Transformer winding article.

While archiving a load of paperwork, I came across an article on winding replacement vintage interstage coupling transformers.

This will probably not be new info to most here, but I have posted it to my Drive account for download if anybody wants it (it is far too large to post directly here - 10Mb+).

It also references a previous article on winding output transformers, also included in pdf (beginning part of the pdf).

Note that there is also a reference to an article in the April issue - this is a misprint and is the above mentioned other article.

https://drive.google.com/file/d/1mcD...ew?usp=sharing

Any problems accessing it, let me know and I can make alternative arrangements to access it.

(Mods - please move this if I have put it in the wrong place).
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Old 14th Feb 2019, 4:40 am   #2
Argus25
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Default Re: Transformer winding article.

Quote:
Originally Posted by Terry_VK5TM View Post
I came across an article on winding replacement vintage interstage coupling transformers.
That is a helpful article.

There is another way to deal with faulty interstage transformers in vintage radios. Use a part AES supply the PT431:

https://www.tubesandmore.com/product...ssembly-p-t156

So the original lams can often be kept and the faulty winding just replaced by this 1/2 x 1/2 bobbin. For a bigger size the bobbin from their PT124E (3/4 x 3/4) can be used. This avoids the rewind with a lot of fine wire.

One other thing , interesting about interstage audio transformers, the equation that defines their frequency response was practically unobtainium. The response is very dependent on the plate resistance of the driving valve. The equation is in this article on page 15, about my Grebe radio. I derived it from some equations presented by Terman. It does a very good job of predicting the transformer's frequency response shown on pg 16:

http://worldphaco.com/uploads/THE_GREBE_MU-1.pdf
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Old 14th Feb 2019, 11:23 am   #3
kalee20
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Default Re: Transformer winding article.

Interesting article!

For interstage transformers, winding self-capacitance is a significant limiting factor for high frequency response (together with leakage inductance). Even with just a 3:1 step-up ratio, 1pF extra secondary capacitance hurts as much as 9pF of primary capacitance.

I therefore - and I have only wound a couple of intervalve transformers - wind the secondary FIRST. Being nearer the core, the average turn length is less than for the outer winding. So, the wire length for the same number of turns is less, and as the capacitance between one turn and an adjacent turn is proportional to the length of each turn, the shorter turn length reduces winding self-capacitance.

The primary (wound last) of course then has more self-capacitance than if wound first, but with fewer turns, as above, this is much less significant.

From a design point of view, an intervalve coupling transformer is a really unhappy thing to design. The valve driving it is likely to have a fairly high ra, which causes a HF roll-off with shunt capacitance, and an LF roll-off with the shunt inductance. So, for good LF response you need a high inductance. But this means lots of turns hence high capacitance. Using a valve with lower ra helps (LF roll-off goes down and HF roll-off goes up) but that is rarely an option! Using an exotic core material helps get high inductance with fewer turns - but if you have DC flowing through the primary (as you would, if the transformer carries anode current) you can't really take advantage of this as you have to add a gap to prevent saturation. So you just have to be super-clever and use a low-capacitance winding layout.
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Old 14th Feb 2019, 5:09 pm   #4
Argus25
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Default Re: Transformer winding article.

Quote:
Originally Posted by kalee20 View Post
Interesting article!

For interstage transformers, winding self-capacitance is a significant limiting factor for high frequency response (together with leakage inductance). Even with just a 3:1 step-up ratio, 1pF extra secondary capacitance hurts as much as 9pF of primary capacitance.

I therefore - and I have only wound a couple of intervalve transformers - wind the secondary FIRST.
Yes the capacitance in each winding is effectively transformed into the other by the square of the turns ratio and the main issue is the secondary capacitance. So it is especially bad with a 1:5 ratio type.

Despite the expense and weight, they do provide "voltage magnification" which really helps in many radios with only a few valves. They also provide a BPF function in the audio range eliminating the RF carrier and DC isolation between stages. I have seen the transformer replaced (repairs) in some sets by R-C coupling and its much more problematic and less effective. However interstage transformers are adroit at picking up magnetic radiation (almost as good as a hearing aid's telecoil) and will sense a switchmode psu nearby!

One other version of them, which is less problematic from the design perspective, is the type made for class B operation. These are usually step down rather than step up as they have to provide grid driver power.
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Old 14th Feb 2019, 6:19 pm   #5
kalee20
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Default Re: Transformer winding article.

Yes, coupling by transformer is elegant, and has big benefits.

You can go single-ended to push-pull.

You have just one component (the transformer) in place of three (load resistor, capacitor, grid-leak resistor).

Problems with 'that' capacitor all go away.

A slightly gassy valve is not an issue, because the DC grid resistance is much lower (kΩ rather than approaching MΩ).

You can break ground loops because the secondary output is fully floating.

If it wasn't a matter of having to design and make them, they'd be brilliant!
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Old 14th Feb 2019, 10:29 pm   #6
Argus25
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Default Re: Transformer winding article.

Quote:
Originally Posted by kalee20 View Post

A slightly gassy valve is not an issue, because the DC grid resistance is much lower (kΩ rather than approaching MΩ).
I hadn't thought of that one. And it also helps prove the point about the best way to test a valve, in the actual circuit it is used in.
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Old 14th Feb 2019, 11:19 pm   #7
Argus25
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Default Re: Transformer winding article.

Kalee20,

Have a look at the Samson brand driver transformer in this amplifier it is a 1 to (1.5 + 1.5). I was amazed at the frequency response of this one is basically close to flat from 50Hz to 12kHz, well in excess of any speakers at the time it was made in 1923. It actually required 560pF capacitance added to the secondary to get a flat response and eliminate a rise at 8kHz, when used with a UX112A driver valve. I never expected it would be that good. Because they were not "audiophile types" nobody seemed interested in these transformers and they sat on ebay for ages at a low price.

I can only hear to about 9kHz now, so 12Khz is plenty for me.The -3dB point for the whole amplifier is 34Hz & 13.5kHz. Samson's output choke is about 72H inductance plate to plate.

(This amplifier has a speaker impedance conversion transformer I wound on a small lathe with a primary over 70H, 4000T of 37awg wire and an input transformer I wound so I could drive it from an iPod, or a 1920's style radio)

http://worldphaco.com/uploads/UX-171...amplifier..pdf

Last edited by Argus25; 14th Feb 2019 at 11:24 pm.
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