B+ and filament transformer

A lot of people here doing low power tube stuff seem to be looking for a cheap source of these. I'm wondering if using a simple 6 or 12V secondary transformer with dual primaries would be the easy way to go, at least in 120V land. Power one 120V winding, and use the other for HV with a doubler.

Consider the Altec 436C supply, schematic attached, they do similar and get a happy B+. The only caveats being to double the current rating because you are only using one primary, and possible issues with the unexpected voltage deferential between the two 120V windings.

Gene

Apparently published 2003. Not considered novel then.

An objection is that the two primaries are not expected to be "very" far apart, being connected to known voltages on one power line. It is possible a huge line-circuit voltage difference could break-down the insulation. However basic insulation is good for 500V, and we hope not to be working 500V away from line reference ground. However some cheap windings are really cheap and primary breakdown is not unknown. As-designed, that's just a puff of smoke. Used for isolation, the breakdown could give shock.

Eons ago I saw a DIY (?) project that used back-to-back transformers which stepped down to 12 VAC (?) then stepped it back up to 240 VAC.

Bri

  There are some transformers with totally separate windings, with a plastic shield in between, awfully if you want to get away with signal as some do but great for what you are doing. I have done it with no problems at all, 220V here, I even used 220V:12V feeding a 9V:220V so I get a bit higher than 220V at the output, pushing a little the specs but I got away with that for a mic pre. I don't remember, if from the same 12V I took the filaments or I use a third one to be able to fit more smaller transformers in a small box, as height was a constrain.

JS

Let me take another shot at this, I don't write clearly very too these days often much.

PRR said:

Apparently published 2003. Not considered novel then.

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Not sure what you mean by "novel", as I referenced a 1950's schematic. The intention is a money saving trick for the tube guys here. A dual voltage primary and a single filament supply is going to be a lot easier and cheaper to come by, than a filament supply plus a specific HV secondary. 120 doubled to ~300VDC B+ seems to be in the ballpark of what the tube preamp guys are clamoring for. Works for that Altec.

An objection is that the two primaries are not expected to be "very" far apart, being connected to known voltages on one power line.

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I think I said exactly that, it is my only real caveat with doing this, but I believe it to be only academic, as wired for 120V, there will be 120V between closest windings. More in other possible, but expected configurations.

It is possible a huge line-circuit voltage difference could break-down the insulation.

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Exactly, but how much voltage is there really, with a typical doubler, wired and phased worse-case?. Use a fuse on the line cord.

This is a simple 120/240V input, 12.6V output transformer.

I) Input wired for 240V.

II) Another way to wire in series for 240V, assuming the winding are wound in order of A, then B on top, this puts 240V between the physically closest turns where any arc-over might occur.

III) Same transformer wired for 120V in, both windings in parallel. 120V between closest turns.

IV) Same transformer, 120V into one of the primary windings, and the other one repurposed as a secondary out for B+. Windings don't know from primary or secondary, they will happily push amps in or out.

Gene

> I referenced a 1950's schematic.

The Altec used a then-common transformer, aimed at redesigning transformerless gear safer. That part lingered in the Allied catalog to quite recently, may still be around?

  I missread the first time I answer, I was thinking in the back to back transformer config.

  Besides the insulation issue already under the scope and related safety concerns, with this approach you'll end with twice as much iron as you need, as you only can use half the primary as primary, so the primary current should be around half the rated current unless you expect to abuse the primary, not so recommended as the insulation is already close to the edge and warming it up shouldn't help a lot.
  Note that you also end with twice the iron in a back to back config but insulation and safety are well treated, availability is much wider and flexibility is greater as you can use it on 220V networks, you have many more voltages available as you can use 110V:12V back to a 12V:220V or even a 9V:220V as I did with no problems (YMMV on this last one)

JS

80hinhiding said:

Hey Gene,
I might try out that Altec compressor sometime. Thanks for the reference.
Cheers
Adam

ps. What is PL1 and M1?
Edit: pilot light, and VU meter?

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I got it right here, in the tech docs section. When I joined a couple years ago, I didn't even know what "variable mu" meant. The only mu I knew was spelled "mew", and when you have mews in the back shed, it means a feral cat dropped a litter. Haven't had mews in years, I think the rats around here ate all the feral cats.

So I DL'd this to study, it seemed like a good basic example to learn from.

Yes, pilot light, and a meter with 34 ohm shunt to measure cathode current and indicate the amount of compression.

Gene

Ive made a few ht supplies from shaver panel isolating transformers ,a 20watt core easily will supply 50ma at 330dc .
I can get these at my local electrical supplier for about 10 euros . An off the shelf 12va 12 volt does my fillaments (say a max current of about 800ma)they usually cost about 5 euros . Its a fairly cheap way to go and despite the bother of having to mount two seperate units it all works well ,no safety issues ,no voltage doublers,no back to back transformer compromises.Connecting the transformer in reverse on 230 volts here usually yeilds a slightly lower ht voltage too. 

I did have a look on ebay.com to see if an equivalent 110 volt to 110and 220 was available in the US but I didnt see any ,you have to make sure your not buying an autotransformer  though as that wont provide any mains isolation.

What's wrong with recycling/reusing old stuff?

Got it......I thought we were talking about power transformers with heater taps.... I'll shut up now....lol

they build houses too close together nowadays to be messin with  indoor bar b q action, might take out the whole block, so be careful with the mains stuff,

never constructed the 436 iron , only destruct,  plus Bluebird just bought out all the nickel from mag met, 

right now working on the Peerless 600:600 15036 hybrid on L lams, 

what about the UA174, is that compressor any good?

CJ was typing too fast again    UA made a 175, 176, and 177 but no 174.

80hinhiding said:

Which is more raw and imperfect, the Altec 436c or the UA 175b?

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In it's stock configuration the Altec is more "imperfect", but there are tons of mods to make it into something useful in a modern studio.  The UA 175/6/7 start off great and can be tweaked to be even better. 

436 and 438  (various iterations, A - C for each I think) were famously modified by the BBC and used at EMI to record some of the Beatles stuff.  Stock units were intended for PA applications, but are often used as is in more than a few studios for drums, etc. EMI mods are documented in a number of places.

http://www.preservationsound.com/?p=257

While indulging the thread hijacking here, i’ll say that as for your question earlier about using two 12v transformers back to back, i’ve done that and it works fine. Two 6.3v center-tapped transformers, higher VA unit (if one is indeed higher) connected to mains. Mains->12v#1 (Primary)->12v#2 (secondary)->120v out to voltage doubler for 240v.  Heater supply from 12v#1 in parallel to transformer #2’s connection to the same.

BT

80hinhiding said:

Thanks, I'll keep these in mind as possible builds for the future.

Gene, I didn't mean to hijack your thread topic. 

A

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Hijack all you want, I'm enjoying and learning from where this thread went.

Besides, it seems like the original transformer topic is tapped out, might be time for winding it up, since we've already gotten to the core of it..  ;D

Gene