back-to-back trafos

[bluemonk]

Hello. I need a PS for my HB tube preamp and I think that a couple of back-to-back little trafos will do the job.
Heaters are wired for 6,3V. Primary should be 240V and secondary 320V (to get about 420V B+).
I "calculated" with my imagination this solution:

T1 - 240V - 6V (heaters wired here, after DC rectification)
T2 - 240V - 4,5V

Should it work? Should I get actually about 320V output?
Thanks in advance

[Albert Kreuzer]

In theory, your calculation is right.
In the real world, the second transformer will go into saturation if you operate it above it's rated voltage.
The smoke will come out - no good.

Better use a 9V->240V transformer and a voltage doubler.

Cheers,
Albert

[bluemonk]

Ok. Thanks for taking me back in the real world
Using a voltage doubler I believe it needs higher current trafos. The preamp (Nachbaur design) is based on two tubes (12ax7-12at7)... 1A trafos should be enough?

TIA

[Albert Kreuzer]

Yes, 1A should be fine for the second trafo. The first one better be 1.5 or 2A because of the tube heaters. You could get away with 1A, but it will run hot.

But for a preamp you don't really need 420V.
With a 6V->240V transformer you'll get something around 300VDC under load, should be plenty for a preamp.

Cheers,
Albert

[bluemonk]

Yes I know it seems too much, running a preamp over 400V.
But this design requires such a high voltage, more hi-fi style than guitar gear, I think:
A Studio Preamplifier
I use it for acoustic guitar with piezo pickup. Works nice (at moment it's powered by a big on-the-fly power-amp supply)

Cheers
Giacomo

[Albert Kreuzer]

I have looked at the circuit and I can't see a reason why it wouldn't work with 300V.
But if you want to stay with the design, build a voltage doubler as previously discussed.

Cheers,
Albert

[bluemonk]

Thanks a lot for your help

Giacomo

[bluemonk]

I got a pair of suitable transformers for back-to-back trick.
I am confused by the center tap.
The first PT has no CT; the second has CT.
How can they be correctly wired together?

Thanks in adavance

[Albert Kreuzer]

Can you be more specific? What are the voltages for "normal" use?

[bluemonk]

Here are the specs:

First PT:
Primary 220V - Secondary 0-6V (no center tap), 2A

Second PT:
Primary 220V - Secondary: 6V -0- 6V (center tapped), 0,8A

I wonder about the correct wiring of back-to-back secondaries since only one PT is centertapped

[Steve Conner]

You'll need to use one half of the second transformer's winding. 6-0-6V is a 12V winding with a tap in the middle.

[Albert Kreuzer]

What Steve said.

But this still won't give you 420V. After rectifying you'll have either 300V (normal) or 600V (voltage doubler).
IMO the preamp would work fine with 300.

Cheers,
Albert

[bluemonk]

I apologize... 2nd trafo is 3V-0-3V indeed.
My mistake... sorry
So how should they be wired?

Yes I'm going with about 300V HT+. It's the easy way to go just now.

Thanks

[Albert Kreuzer]

Easy - leave the center tap unused.
Connect 6V to 6V: Done.

[bluemonk]

Many thanks!

Regards