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In Reply to: RE: See text... posted by andy evans on September 10, 2016 at 11:00:01
Try contacting the maker of the transformer. Sometimes you can connect the two secondary windings in series, and do what you want, but at half the current rating.
A center tap transformer is designed to only use one half of its winding at a time, letting the other half cool down each half cycle. The volt-amp rating of a transformer is fixed by wire size. You can't double the voltage out without cutting the current rating by half.
A voltage doubler is likely your best option for what you have....assuming you can get by with 1/2 the amps.
Follow Ups:
Ah right - so it's a question of wire thickness. So at 250v the current rating would be half of 50mA, so 25mA? That would change things. Is tthat the convention - if there's a centre tap it's assumed you will use it for a full wave bridge?
So in that case a hybrid bridge with choke input would give 225v at 23mA. If that's correct, that's not impossible for an 01A preamp.
The transformer is a very old RS one - Midget Mains. UK sourced.
Clip lead it together and conduct a soak test. JH
Well - for now using full wave rectification with cap input of 40uF. Mains transformer runs cool - no problems. After two chokes the PSU outputs 155v. May or may not be enough for Ale's Gen 2 Gyrator design. Work in progress..
This has been an informative thread.
Hi Andy,
155V is too low for gyrator loaded 01.
I tried (and use) Ale's gyrator with several DHTs (01, 26, 10, 801).
In 01 preamp 105...110V anode voltage requires at least 200-220V B+ voltage for proper working.
I used 220V shunt regulated (Salas SSHV2) power supply.
"Iccs= 4.98mA (cCCS)
B+= 220V (SSHV2)
Ug= -5.73V (22R W22)
R.C. V4 regulator (R1:4R7, Vin:20.5V)"
Thanks for that. So clearly HT needs to be 200v or more, though Ale was of the opinion that the gyrator itself filters well enough not to need another shunt reg. or glow tubes. I'll have to check all this with him.
I don't think anything is overkill in a DHT preamp - I'd use a mesh AZ1 out of choice, and chokes in the filament supply. It's possible to simplify as long as the difference isn't too audible.
Andy, IIRC the transformer secondary was 125-0-125. As I understand things you have center tap isolated and are using the full secondary as one winding into a cap input filter. Don't know what you are using for a rectifier (please elaborate).
I'd like to know what the secondary voltage is (end to end) under load. It should be around 250 volts (AC). I'm guessing it might be a good deal less and that, rather than heat, might be a better indicator of over load. It would be nice if you could input a signal (play real music say) into the preamp when measuring this voltage.
I wired it as full wave with the centre tap to zero volts and CLCLC filter. The boringly obvious. After all the filtering it went down from 178v to 155v. I didn't use the full 250v winding, ignoring the centre tap.
I think I need the whole of the current rating, so what I've done in the past when I'm short of volts is to add another smaller transformer. I would in this instance take a small 24+24v transformer and add 24v on the end of each side of the HT windings. That should take me up to something like 185v final voltage if I needed a bit more. I've only previously done this with a bridge rectifier - I imagine it works with a centre tap as long as you get the phase right. Can anyone confirm?
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