Hybrid Dual Rail Power Supply

[priesty]

Hello all,
Just want to put this idea out there for implementing a dual rail power supply for running valves such as 6550's at design maximum ratings. I found a promising transformer at the following website with a non-centretapped two tapping secondary with seperate Bias Winding and provision for tube rectifier:

Valve Transformer VT1042V

It got my grey matter moving somewhat! I figured that the 450V tapping would be rectified to 600V, but could the 375V tapping be used in conjuntion with a hybrid 5Y3 to give a lower HT for screens and pre-amp? See my schematic for arrangements:



The big question is what is the voltage at node "X"? Morgan Jones and Merlin "Valve Wizard" make reference to this arrangement, but I don't know if you treat the winding voltage as half of the given value (i.e. 162.5Volts) seeing there is no centre tap on the 375V winding. If this is the case, the voltage at "X" would be around 172V and not the hoped for 415V.

Your thoughts?

[Steve Conner]

That circuit looks wrong. Even if you got it right, the only voltage these tapped-winding arrangements can create is half of the main voltage, using a centre-tapped transformer. (Essentially a FWCT and bridge rectifier running together.)

If you use a transformer with the tap anywhere else but the middle, some of the diodes never conduct, the transformer buzzes more, and you get worse ripple, because the rectifiers are running as half-wave. I think the auxiliary rail still ends up half of the main one, too, but maybe Merlin or someone will correct me.

You can't generally just connect up two bridge rectifiers to the same winding like that.

[Merlinb]

I don't think that will work, for the simple reason that the lower half of the secondary is effectively shorted out by that hybrid bridge (at least for current travelling in one direction through it)

[priesty]

Gents, Thanks for your comments, they prove the value of this forum as a sence check of ideas!

[anson]

I would definately not rely solely on my advice regarding this, but it might be possible for you to configure your 450V and 375V taps in the same way that is done with a voltage doubling configuration discussed on these threads:

http://music-electronics-forum.com/t19855/

http://music-electronics-forum.com/t21659/

I am suscpicious that your 375V tap can be placed between two filter caps as discussed in the above threads, so that the junction that is made between the 375V tap and where the two filter caps join can be used as a lower voltage supply than what your B+ would be.

In the past, I have used the voltage doubling configuration, using a bridge and two filter caps in series from B+ to ground, with the center tap placed at the junction of the two caps. I had a B+ that was used for the plates, and the junction was about 250 V, which I used for the screens and the preamp stages. I ended up changing it because i wasn't happy with the low-voltage preamp sound. Other wise it worked great.

Like I said, don't rely solely on me! Hopefully somebody else with more experience will comment on this.