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Old 30th October 2012, 01:06 AM   #1
john65b is online now john65b  United States
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Default Tube amp - modified output tranny for direct driving ESL

Hey all, I am going to be starting a tube amp project - a 6H30 (gain stage) / 12BH7 (phase splitter) / KT88 (output) tube amp (kinda like a Quicksilver 8417 or Bogen MO100), and I have all the components. The output tranny is a edcor 60W and 3500 ohm primary impedance, and 4 and 8 ohm tap output.

Now what I really wanted to do was drive a pair of either ML CLS, or a setup consisting of a series of acoustat panels, using a modified acoustat MK-121 interface. This setup would be for frequencies over 200hz....

But I was wondering if there was a tranny I could use instead of the edcor that would have the 3500 ohm primary impedance on the KT88 plates (just like the edcors) , but secondary be a 7- 1 step up to the 3500v - 5000v direct to the ESL panels. The B+ of the tube amp should be around 475v.

Not exactly direct drive (as I am not driving directly from the plates), but sorta...

Anyway, would this work, and does such a tranny exist? 7:1 to 10:1 step-up and 3500 ohm impedance on plate side? What would the impedance need to be on the esl side?

Go easy on me if it is a ridiculous idea...I'm thenthitive....
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Last edited by john65b; 30th October 2012 at 01:15 AM.
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Old 30th October 2012, 12:03 PM   #2
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The idea is not bad at all!
Actually its a very good idea!
I have that ide myself :-)

I will use a 300B drivin an interstage Lundal 1677 transformer drivin two GM70 tubes on the output.
From there i have custom built output transformer 1:3.5 from SOWTER.
The GM70s is fed with 1350volts and is loaded with coils 50H on each GM70.
That way I will get the double swing output 2*1100V=2200Volts and multiplied with the transformer turn ratio 3,5 it will give me a total sving on the plates of 7000 volts.

Good enough i think.
4 coils and 4 transformer was hand made by Mr Brian Sowter himself...
I guess he like challanges!
He could meet my spec with wery low capacitance which is important also the isolation capability.

So a short answer to your question is YES! This is the way to go!
Good luck!
Bear in mind... everything above 48Volts is really dangerous.
300volts requires lot of skills.
1200volts is for experts only or really really stupid guys.
Im the later one i guess...
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Old 30th October 2012, 12:51 PM   #3
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This has been discussed before and is not practical.
The more voltage you have on the primary the more turns it takes to keep the core from saturating at a given frequency and core area.
This means that many more turns on the secondary and this will raise the transformers parasitic capacitance dramatically.
Doing this can lower it's resonate frequency well below 20khz and will severely limit the transformers bandwidth.
Yes, it seems logical but it doesn't work like that and is not practical unless you have some really big iron to hold all of the extra turns while still providing enough room for some thick insulation in between the turn layers in order to keep the transformer capacitance at a minimum.
The more turns you have the more copper losses you have as well!
I am not saying it can't be done but it is not cost effective nor will you gain any performance.


FWIW !!

jer

Last edited by geraldfryjr; 30th October 2012 at 01:19 PM.
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Old 30th October 2012, 01:49 PM   #4
tyu is offline tyu  United States
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Well i am all about Diy but ......Jim Did this with the Acoustat tube sevos..... just copy this....Go for it....Less you need to Make a new mouse trap??
I had the Xs Thay were a pr of M3s type on the floor....
The Tube Sevos Sounded Great....These Amps Will drive ANY ESL.....ML ,SL,Quad,......What was the Down side an why did i move on to the 121 interfaces......The THD was so low that you just wounted more an more Output....an it Not going to happen.....
To me it like a 500 Watt pr ch Mcintosh SS amp.....Sounds Sweet.....but has no headroom becase of the autio-fourmers on it outputs.....you can use all it output with Low THD....leves mOST wonting More...
An this is not just me.... I sold Mc an saw a lot of people who lov the sound move away from the Mc SS Amps....just did not have the drive thay wounted for ribbons are Even Mc own speakers...
With mods the 121 can sound as good an can be driven with my OTLs M60s ....60watt tube amps....Goodluck
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Last edited by tyu; 30th October 2012 at 01:53 PM.
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Old 30th October 2012, 02:22 PM   #5
john65b is online now john65b  United States
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Tyu, the servos did not have output trannies, but I was enquiring about having a reverse output tranny on my tube amp - a stepup instead of conventional step down - to drive the esls
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Old 30th October 2012, 02:31 PM   #6
tyu is offline tyu  United States
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Did mean go too far off the topic......
I have used.... edcor tranfourmers.... thay will wind any tranfourmer you can come up with......if it can be made thay well do it... An there $$ is as good as any i have found.....
But i see the sevos type tube amps as a ezer way to drive a ESL if this is someing anyone wonts to do.........Goodluck...
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Old 30th October 2012, 03:37 PM   #7
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As i mentioned... write down your requirement specification and send it out to transformer suppliers and let dem send you a quote.
Sowter fullfilled my specifications at a reasonable price i have to say, and YES, there is many layers of insulation here and there.
And a massive piece of iron i can tell.

Remember that the secondary on those transformers is probably much better than the QUAD ESL63 original transformers....
The weight only is more than double, on each transformers.
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Old 30th October 2012, 05:55 PM   #8
john65b is online now john65b  United States
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Dumb question - what would the output impedance need to be on such a tranny connecting to the ESL?
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Old 30th October 2012, 06:35 PM   #9
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As we all know ESLs already have matching transformers. I expect the allure of eliminating one transformer in the signal train is powerful.

I would certainly give it a shot despite admonitions to the contrary. The only thing that would concern me is that bias voltage on the secondary side. Just as an output transformer can saturate when there is too much unbalanced DC current through the primary, so to can it saturate on the secondary side.

So the actual device might have to be of the cut-core variety. The other issue is that there is likely a learning curve for anyone fabricating such a part. Be prepared to built between 5 and 20 units getting it right.

If there is an argument for going direct-drive, that latter issue may well be it.
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Old 30th October 2012, 09:16 PM   #10
john65b is online now john65b  United States
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Quote:
The other issue is that there is likely a learning curve for anyone fabricating such a part. Be prepared to built between 5 and 20 units getting it right.
Yikes!!

The secondary CT of this step up tranny would be grounded, and ESL bias placed on the diaphragm, just like a MK-121.

Hate to start this and have to quit as it may be over my head, especially since I have all the components for the tube amp AND all the interface already.
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All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian

Last edited by john65b; 30th October 2012 at 09:19 PM.
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