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Old 7th May 2011, 04:32 PM   #1
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Default on the toroids again

Hi everyone!
A quick question,
Will this 30VA power transformer, present a reflected load somewhere near 7K/10K, hooking 8 ohm on the secondary?
Rapid Electronics - Electrical & Power
I mean with the primary series connected and parallel secondary., just as this guy does:
http://www.triodeguy.com/Triodeguy%2...0AMP%20SCH.pdf
push pull use, of course, and CCSed (sink)
Thanks
J.
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Old 7th May 2011, 04:51 PM   #2
DF96 is offline DF96  England
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240V to 6V is 40:1, so the impedance ratio is 1600:1. 8R becomes 12.8k. Whether it will make a decent audio transformer is a separate question. My guess is OK for a guitar, no good for hi-fi. You will need to ensure DC balance.
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Old 7th May 2011, 05:26 PM   #3
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Thanks!, so I was(quasi) on the ball park, In fact, a 230V/7V unit will be better , but hard to find..
It seems that the measures implemented by Triodeguy, solve the DC unbalance issue.
Forumer Shoog had a lot of experience with toroids, and apparently managed to solve the problem too.
I was thinking on a cheap self splitting circuit, not a true high quality unit.
Thanks again
J.
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Old 7th May 2011, 05:42 PM   #4
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It's plain easy to add some windings on a Toroid. Just add a few windings, and connect it in series with the 6V Winding till you reach 7V.
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Old 7th May 2011, 06:01 PM   #5
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Quote:
Originally Posted by the_manta View Post
Just add a few windings, and connect it in series with the 6V Winding till you reach 7V.
Good idea, the units are cheap enough, so I think I can risk this mod...
Cheers
J.
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Old 7th May 2011, 06:13 PM   #6
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Mosquito, you didn't ask, but I don't think a 50 Hz power transformer will make a very good audio output transformer.
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Old 7th May 2011, 07:39 PM   #7
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Just a tip better than a 60Hz one.
Out of joke, it's a matter of perspective, just suppose you're going to a place where there is no chace to found real OPT's no EDCOR, no Hammond, ...and you still want to have fun with tubes...
Besides some forumers report excellent results with those toroids (at least the Talema units) So, for a cheap 8W self split circuit, let give it a try...

Last edited by Mosquito; 7th May 2011 at 07:50 PM.
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Old 7th May 2011, 09:28 PM   #8
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OPTs have always been a problem for me too in the UK. I wish Edcor was here, but they are not and home grown stuff is expensive.

First I would say that you need a much bigger toroid than the power suggests, because the frequency you want to get down to is < 50Hz. So you need more iron!

Secondly you are not bound by the output impedance of the tubes because you can apply feedback to lower it. Not global feedback (to avoid any stability issues), but local between the anode of the output tube to the cathode of its driver tube. Paralleling up tubes is also an option.

So if you have a 12k output impedance from your tube and want to drive a 8R load you need sqrt(12000/8) = 39:1, but if you add enough feedback to reduce the impedance by a factor of 4 you'll need sqrt(3000/8) = 19:1 instead - into 12V transformer territory.
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Old 7th May 2011, 10:09 PM   #9
DF96 is offline DF96  England
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You may be confusing output impedance with optimum load. Feedback changes the former, but not the latter.
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Old 7th May 2011, 10:24 PM   #10
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Quote:
Originally Posted by DF96 View Post
You may be confusing output impedance with optimum load. Feedback changes the former, but not the latter.
Yes I think I am, how can I find out a little more about how to distinguish the two? TIA!
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