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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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Old 30th Aug 2019, 8:00 pm   #41
ChristianFletcher
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Default Re: Interstage Audio coupling transformer

I think Cobolt told me it was an old design even back in the day they built it. I wonder if they based it on an older design that needed the voltage gain offered by the transformer for lower gain valves etc.

Only managed to tune in 3 junk talk rubbish stations on MW and only radio 4 on LW. That’s pretty normal for me. Interesting the radio is almost silent between stations. Still getting used to the reaction control appears it’s main function is to put the radio into Oscillation. But that’s always been me experience of reaction controls LOL. Must be me.

I think I’m going to stick to collecting radios of the 1930 this will be my third. I cannot figure out why I like them as they are simple devices, perhaps that’s me.

Chris
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Old 30th Aug 2019, 10:47 pm   #42
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Default Re: Interstage Audio coupling transformer

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... I have retained the -4.5 volt grid bias to the pentode output via a 500k resistor and also capacitive coupled to the detector output. The radio tunes and itís as loud as I would ever want a radio to be. Sound quality is about as good as any 1930s radio I have owned. Now Iím still going to rewind the transformer, but cannot help thinking why did they bother to use a transformer when capacitor coupling works well and would have been cheaper.
Hmmm... Not only cheaper, but better fidelity.

However, transformer coupling does offer the possibility of extra gain. There's not only the 3x which is possible with a decent transformer (Ferranti even made a 1:5 step-up transformer), but the previous valve can be loaded with the transformer primary and thereby operate at near full HT on its anode, which would give greater signal handling and greater gain from the valve itself (although that won't apply with your shunt-fed circuit). And it will work down to a lower battery voltage. Also, the transformer secondary gives a very much lower DC resistance in the grid circuit of the following valve, which can be important if it is operating into grid-current.

In your case probably none of these apply, and if RC coupling works well enough then the arguments for the transformer are not very strong. All the same, it would be great to read your reports on the rewind, and the performance attained!
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Old 31st Aug 2019, 1:02 pm   #43
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Thanks, Argus! (It's not actually a case of 'more turns is always better' because as turns increases, winding self-capacitance also increases so as low-frequency bass response improves, high-frequency response drops off.
Yes generally true, however there are two things significantly helping in this particular case. The rewind will not have paper between the layers and all other things being equal, the secondary's inter-winding capacitance with its jumble wind will be lower than the original winding's capacitance. Although the original might have been jumble wound, but in any case it is much less of a problem with an auto-transformer than an isolating transformer design with the same ratios as the total turns numbers for the same transformation ratios are less.

Generally the upper audio frequency response only starts to get troublesome due to secondary capacitance for valve radio work with 1:5 isolation inter-stage transformers, which is probably why this was about the highest ratios that most manufacturers pushed it to, or maybe 1:6.

So since this particular transformer also is fairly small in size, it probably would do well with some more turns most likely, within sensible limits, certainly no less than the original.
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Old 31st Aug 2019, 1:14 pm   #44
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Now I’m still going to rewind the transformer, but cannot help thinking why did they bother to use a transformer when capacitor coupling works well and would have been cheaper.

Chris
Of course in these radios gain came at a premium price, every amount was a precious commodity in radios of the 1920's and 30's. So it was a good idea to add passive gain with the transformer, rather than another valve stage. You are probably in an area of strong transmitter fields and all seems well without the transformer. But on that threshold where the radio was in a peripheral area far from the transmitter, the transformer gives some advantage.

This more applies to isolating transformers: Due to to the transformers band-pass response they are helpful in filtering off the RF component from the grid leak detector stage's anode that drive the primary in many 1920/30's TRF radio designs, so these RF signals do not end up at the grid of the output valve, this saves on some capacitors. And they have the other advantages Kalee20 pointed out too.

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Old 2nd Sep 2019, 7:38 pm   #45
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Default Re: Interstage Audio coupling transformer

After a hard day at work itís nice to get home and unwind for the evening! I keep hitting a white spot in the coil where the wire breaks so Iím guessing that was the problem.
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Old 2nd Sep 2019, 8:21 pm   #46
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Default Re: Interstage Audio coupling transformer

5173 turns in a we have hit the tapping.
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Old 2nd Sep 2019, 9:44 pm   #47
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Default Re: Interstage Audio coupling transformer

I'd expect the remainder to be about double the turns so far. Slightly better HF response ought to arise by putting the grid winding on FIRST, despite my earlier post - the shorter turn length means less winding capacitance, on the winding with the higher signal voltage (where capacitance hurts more).

Will be interesting to see if this is the case!
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Old 6th Sep 2019, 5:31 pm   #48
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Default Re: Interstage Audio coupling transformer

I will be having ago at winding the coil this weekend. I was wonderingly if I should apply anything to secure the wire as I wind it. The old transformer had something as I needed to keep wetting it with acetone to unwind it. I have some shellac designed for wood working and also some PCB dope for circuit boards. I was thinking the shellac would give me a bit more working time if I needed it.

I was going to use some single core wire wrapping wire for the lead in wires soldered to the coil wire as it’s the thinnest wire I have.

Any last minute tips or tricks you can give me before I get to wound up ?

Regards Chris
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Old 6th Sep 2019, 6:03 pm   #49
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Default Re: Interstage Audio coupling transformer

I wouldn't use anything to 'stick' the new windings together.

As to lead-outs, yes - I'd suggest attaching the start/tap/end to *flexible, stranded* wire - suitable stuff being salvageable from scrap cat-5 Ethernet patch-leads. Stranded being good here because its flexibility minimises stress on the soldered joints between the winding and leadouts. A bit of really-thin heatshrink sleeve will be fine to cover the soldered transitions.
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Old 6th Sep 2019, 8:05 pm   #50
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Default Re: Interstage Audio coupling transformer

Ok Stranded thanks for the heads up I think I have an old mouse cable that is thin and flexible. Ok I won’t glue the windings then, I just read about using shellac in an article I read on winding transformers and assumed it was necessary

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Old 10th Sep 2019, 12:37 pm   #51
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Default Re: Interstage Audio coupling transformer

Given the number of turns to put on I have had ago at constructing a coil winder and made a quick video showing construction

Thanks Regards Chris


https://youtu.be/8zjvTq2wJnM
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Old 25th Sep 2019, 8:06 pm   #52
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Default Re: Interstage Audio coupling transformer

Just an update I ended up building a simple winding machine from an old battery drill to wind the coil. I thought I would have problems with the wire breaking as it was very fine only 0.08mm but didn’t have any problems with the wire breaking. The hardest part was starting and stopping the winding machine. I also ended up buying the wrong wire size twice ! as I did not realise that the wire diameter did not include the varnish but the varnish depth wasn’t specified so a bit of trial and error.

I fitted the rewound transformer and had a big increase in volume and also sensitivity for weak stations. Much better than linking out the transformer with the capacitor. All in all a good project and I’m really happy with the end result. It’s sounds good to me but would like to know if anyone agrees as not heard many 1930s TRFs to make a comparison with.

Unfortunately I don’t have any pictures just the video so if anyone is interested the coil winding is 15 minutes in.

https://youtu.be/v1eoND0OyhA

Thanks for all the help and advice I couldn’t have done it with all your comments.

Best Regards Chris
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Old 25th Sep 2019, 10:32 pm   #53
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Default Re: Interstage Audio coupling transformer

Very impressive work. The use of a tapped coil is interesting as it uses extra parts but avoids the DC current in the primary of a conventional transformer. Seems like that didn't stop this one from failing though! The 'primary' also appears to be wound over the 'secondary'.
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Old 26th Sep 2019, 6:24 pm   #54
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Default Re: Interstage Audio coupling transformer

Thanks PJL for the nice comments. It was a lot of work but really worthwhile while and I was quite daunted at the idea of winding the coil but it was much easier that I had expected and good fun in the end. It is an auto transformer arrangement but as you say the secondary is wound on first.

Thanks Regards Chris
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Old 26th Sep 2019, 6:40 pm   #55
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Congratulations on a good job - and justified by the performance! Williamson output transformer next project

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It is an auto transformer arrangement but as you say the secondary is wound on first.
See post 47.
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