back-to-back trafos

bluemonk · 08-22-2009, 10:35 AM

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 · 08-22-2009, 02:53 PM

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 · 08-23-2009, 11:13 AM

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 · 08-23-2009, 02:32 PM

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 · 08-23-2009, 02:56 PM

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 · 08-23-2009, 05:10 PM

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 · 08-23-2009, 05:42 PM

Thanks a lot for your help

Giacomo

bluemonk · 11-02-2009, 08:18 AM

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 · 11-02-2009, 03:21 PM

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

bluemonk · 11-02-2009, 05:12 PM

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 · 11-02-2009, 05:23 PM

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 · 11-02-2009, 05:51 PM

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 · 11-02-2009, 05:58 PM

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 · 11-02-2009, 06:41 PM

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

bluemonk · 11-02-2009, 06:51 PM

Many thanks!

Regards

08-22-2009, 10:35 AM	#1
bluemonk Member Join Date: Aug 2006 Posts: 71	back-to-back trafos 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

11-02-2009, 05:23 PM	#11
Steve Conner Supporting Member Join Date: May 2006 Location: Glasgow, Scotland Posts: 2,999	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. __________________ "Ohhhh miracle bulb shines feebly"

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08-22-2009, 02:53 PM	#2
Albert Kreuzer Senior Member Join Date: May 2006 Posts: 287	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

08-23-2009, 11:13 AM	#3
bluemonk Member Join Date: Aug 2006 Posts: 71	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

08-23-2009, 02:32 PM	#4
Albert Kreuzer Senior Member Join Date: May 2006 Posts: 287	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

08-23-2009, 02:56 PM	#5
bluemonk Member Join Date: Aug 2006 Posts: 71	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

08-23-2009, 05:10 PM	#6
Albert Kreuzer Senior Member Join Date: May 2006 Posts: 287	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

08-23-2009, 05:42 PM	#7
bluemonk Member Join Date: Aug 2006 Posts: 71	Thanks a lot for your help Giacomo

11-02-2009, 08:18 AM	#8
bluemonk Member Join Date: Aug 2006 Posts: 71	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

11-02-2009, 03:21 PM	#9
Albert Kreuzer Senior Member Join Date: May 2006 Posts: 287	Can you be more specific? What are the voltages for "normal" use?

11-02-2009, 05:12 PM	#10
bluemonk Member Join Date: Aug 2006 Posts: 71	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

11-02-2009, 05:51 PM	#12
Albert Kreuzer Senior Member Join Date: May 2006 Posts: 287	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

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11-02-2009, 05:58 PM	#13
bluemonk Member Join Date: Aug 2006 Posts: 71	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

11-02-2009, 06:41 PM	#14
Albert Kreuzer Senior Member Join Date: May 2006 Posts: 287	Easy - leave the center tap unused. Connect 6V to 6V: Done.

11-02-2009, 06:51 PM	#15
bluemonk Member Join Date: Aug 2006 Posts: 71	Many thanks! Regards