Power Supply Headache

[david.rogers]

I have just purchased a valve PS for a valve pre-amp from a Hong Kong based supplier on eBay. The PS looks well specified, using a 6Z4, 12B4A and a 12AT7. I would post the schematic but the file size is 250KB so I cant. I can email it to anyone interested though. There is one design complexity which is very annoying. It requires a mains transformer with no less than 4 windings! 300-0-300 @ 80mA for the HT, 15v @ 1.6A for the regulated heater supply to the Preamp, 6.3V @ 1.6A for the 6Z4 heater and 12.6V @ 0.7A for the 12B4A and 12AT7 fed in parallel. I am assuming - I hope I have this right - that a power transformer with a suitable HT winding (300V) and 6.3V @ 2.3A would at least allow me to feed all three PS valves. Is there likely to be any special reason why the 6Z4 heater is fed separately from the other two valves? Of course it would be nice to supply ALL the valve heaters (the PS valves plus four 12AX7s in the preamp frpm a single transformer, but this is probably beyond my capabilities! Any ideas?

Thanks
David

[NickG]

The rectifier fillament is at B+ voltage, so you would want to keep that seperate from the rest of the heaters in the amp.

[Paul Barker]

There are a few companies that could bespoke wind that for you, isn't there one on the Isle of White? I don't get mine bespoke wound but I'm sure someone could supply you with the information.

otoh it's ever so easy to hand wind the low voltage requirements on a torroid. Buy a torroid with a pair of 6v windings, use on for the rectifier and the other for the 12b4a and 12at7 in parallel, pins configured for 6.3 volts instead of 12.6 volts (this will require minor adjustment to the circuit connections to those two valves' bases, which we can help you with on production of diagram and pictures).

Now you only need 600v centre tapped and 15v. You might find that on one transformer but the 15v is unlikely, so I suggest you wind your 15v on the above torroid. Wind 30 turns, turn on the transformer and measure the voltage on your turns, devide by 30 and you have turns per volt. Multiply by 16.5 and you have the correct number of turns for a transformer with 10% regulation (typical of a small torroid). Your secondaries should then be about right under load. You can use one of the insulated wires out of 1mm twin and earth to wind it with, doesn't matter how uggly. Make sure the original torroid you purchase is 50 watts minimum. Don't get 7 volt windings that would end up over voltage on the 12at7, use 6v and it will come up within 5%, maybe a little over on that winding, just put a resistor in series if so. Other windings should come up right, 15v winding you adjust no of turns on test.

Now you are only shoping for 600vct, easier to find on ePay.

Go to RS or similar for the torroid and obviously make sure it has a hole in the middle isn't encapsulated or potted.

[david.rogers]

Thanks for your helpful replies. It seems slightly odd that the heaters to the PS valves are all fed AC. Would it be sensible to feed the 12B4A and 12AT7 valves DC? If this makes sense - and if I could track down a transformer that has windings at 600V, 6.3V and between (say) 12-30V - I wouldn't need the 15V feed as heater DC is regulated by a LM317, and could of course be adjusted.

[Dave the bass]

Hello, I've bought the same PSU and Phono Pre-amp I beleive, the chap I bought it from in Hong Kong was selling the transformer you speak of .
It was listed as...

"RCore Transformer for Tube Pre-Amplifier,Tube Regulator Item number: 5833127258"

Don't know if that helps you at all, I can't find anymore on his website. It has all the windings you describe, the primary is 2x115 Volt coils, when wired in series on our UK 240 (ish) AC mains I get a slightly higher o/p (unsurprisingly) form the all windings even under load.

All the best

Dave.