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Old 18th Sep 2019, 8:19 pm   #1
G6Tanuki
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Default Transformer loading.

I'm getting together the bits to make a classic 80-metre "table-top" AM transmitter - 807 PA, pair of 6L6 modulator.

For supplying HT/LT I've sourced a nice mains-transformer which has 350-0-350V@120mA, 6.3V@4A and 5V@3A windings.

My plan is to bridge-rectify the 350-0-350V winding to get 700V@70mA HT for the 807 and use the centre-tap on the transformer to get something like 70mA at 360V for the modulator and the low-level audio/RF stages.

This approach is sometimes known as the "Economy" PSU, and is documented in plenty of 1960s RSGB/ARL handbooks.

Playing spreadsheet-games, I need to be able to pull 70mA from the full winding and the same from the centre-tap. Which is somewhat more than the transformer's HT secondary is really rated-for.

On the debit-side since I'll be using solid-state rectifiers I won't be applying any load to the 5V@3A winding (which was clearly intended to power a valve rectifier), so that's 15 Watts of headroom. And during receive-periods the transformer will only be supplying the heaters, so giving it a 'cooling-off' period'.

I'm hoping that these mitigating factors will let me get away with what is at first sight a significant overload of the transformer's HT secondary.

Has anyone else done this sort of thing and got away with it??
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Old 18th Sep 2019, 10:09 pm   #2
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Default Re: Transformer loading.

If you're using it for ICAS operation, you may well get away with it. Continuous broadcasting, forget it.


Might be worth adding the 807 and 6L6 HT to the TX/RX changeover switch or relay.
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Old 18th Sep 2019, 10:15 pm   #3
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Default Re: Transformer loading.

Give the 5V winding a purpose in life running a wee 5V or 12V (doubler) at a fraction of an amp surplus DC fan playing gently over the transformer....
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Old 18th Sep 2019, 10:35 pm   #4
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Default Re: Transformer loading.

Quote:
Originally Posted by turretslug View Post
Give the 5V winding a purpose in life running a wee 5V or 12V (doubler) at a fraction of an amp surplus DC fan playing gently over the transformer....
Definitely.

The transformer could be rated at 70mA because nay more current it will overheat. Or it could be thus rated because any more current and the voltage will drop below spec. Or it could be thus rated because any more current and it will fail its efficiency spec.

We just don't know - but it's most likely the thermal limitation. In which case, playing a fan over it will allow you to uprate to what you need!

Just put a trial load on, with fan, before drilling chassis and making a pukka job.
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Old 19th Sep 2019, 7:07 am   #5
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Default Re: Transformer loading.

err hhmmm,
Rectifying 700 volts ( 350 - 0 - 350 ) will fetch something well in excess of 700 volts, I think about 990 volts. Thats a might high for even a lowly ( and highly prized) 807. 6L6's will become very distressed with more than about 450 applied.
Current wise I think you are OK, as you dont run a modulator while receiving, and unless you are constantly "on the phone" the transformer will have lots of cooldown time.

AS far as your HT voltage goes, you can cut down the excess with a SMALL choke input, then capacitor, then a large choke with capacitor. Regulation isnt so good however.
Something along the lines of 1H, 50 uF, 10H, 50 uF. ( thats off the top of my head).

"MOST" transformers will tolerate some short term overloads without too much stress, although long term they will suffer. If your transformer is local English made, and perhaps from late 1950 or early 1960, I dont see too much of a problem. Modern "replacements" or Asian derived I would be more conservative.

Thats just my quick take of your question, and is offered as an idea only.

Joe
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Old 19th Sep 2019, 5:47 pm   #6
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Default Re: Transformer loading.

Yes I'll be using a choke-input filter for the "HT" so will probably be 650V on-load, which is quite within my happiness-zone for an otherwise lightly-worked 807 (I've got plenty of spares) and the 6L6s will be fed from the centre-tap so only seeing half the end-to-end output of the HT winding. Again, experience with public-address amps tells me that 6L6s are quite happy with 350V on the anodes and 275V on the screens when run in class-A push-pull to give about 15-20 Watts output. Remember that the 'maximum' voltages listed in valve spec-sheets are for the voltage across the valve electrodes directly - when you're using a resistor for cathode-bias you subtract the resultant cathode-bias voltage from the HT to get the voltage the valve actually "sees".

From seeing what others have done, I'm thinking my proposed transformer-usage will be OK: a 1:9 ratio of transmitting to receiving is apparently the 'standard' duty-cycle for rating such things and to me that gives adequate rest-time when the transformer's secondary will not be delivering _any_ HT current at all. It's cettainly less-stressful than having to deliver its full rated current for hours/days/weeks at a time.

A fan is definitely a possibility - though it'll be mains-powered and initially aimed at the 807.

I've got a bunch of the nifty little stick-on temperature-indicating labels with which I can monitor transformer temperature, so I'm going to suck it and see: if the transformer survives, OK - but if it has a hissy-and-smoky-fit I'll just have to go to Mr. Hammond and part with around £150 for a replacement.
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Old 19th Sep 2019, 6:07 pm   #7
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Default Re: Transformer loading.

Wouldn't more of a problem be that one end of the secondary will be at about 1000V? Would it be insulated for this?

I might still have a transformer from the 8690 sweeper, this used BWO's with a 1000V secondary, I think. Failing that I have a 6521 PSU, 1000V @ 200mA, bought to soak test power valves, doesn't work, not going to investigate at those voltages.
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Old 19th Sep 2019, 6:16 pm   #8
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Default Re: Transformer loading.

I don't see why the secondary-end voltages will be questionably-high. I've run the transformer with no load, and experimentally earthed the secondary-ends one at a time - which must surely be the 'worst-case' insulation test for the non-earthed secondary-end; so far it's survived a few days of this with no problems.

Like I said, if it works it works and I'll be happy; if it works-for-a-bit-and-then-goes-bang I'll put it down to experience and buy a Hammond.
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Old 19th Sep 2019, 11:55 pm   #9
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Default Re: Transformer loading.

This approach is sometimes known as the "Economy" PSU, and is documented in plenty of 1960's RSGB/ARRL handbooks.

I don't think this is an "economy" power supply at all. In fact it's quite elegant. It supplies all your necessary voltages, and both these voltages will "sag" together so the actual ratio remains the same. In other words no massive screen volts with minimum plate volts etc.

You can also "lift" the ground end of the rectifier with a small resistor and derive a small negative supply if you want to hard bias. I use this type of power supply quite often.

As far as insulation of the windings goes, I wouldn't trust anything from Asia with that voltage. Most of the transformers I have seen from there have no interleaving insulation, just a whole bunch of windings insulated by the enamel/polyurethane. They are of course insulated between primary and secondary, and between low volts (heaters) and the HT winding.

My experience with English traffos is pretty similar to Australian manufacture, and incorporates paper insulation between layers, so more trustworthy.

Joe
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Old 20th Sep 2019, 10:51 am   #10
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Default Re: Transformer loading.

Isn't the need for interleaved paper (or not) dependent on the wire insulation grade? Whether the right grade has been used is the problem.....
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