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Old 28th December 2008, 07:41 AM   #1
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Default Modulation transformer for a driver transformer?

I have the need to run eight KT88s in parallel push pull and I need a driver transformer that'll do the job. I have a nice 6000 / 6000ct 1:1 20W Collins modulation transformer, would it do the job? I don't know how modulation transformers differ from driver transformers.

I have also been looking at the Sowter 8423. Oh yes, I'm powering the transformer with a 6L6.

Ideas? Recommendations?

Thanks!
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Old 28th December 2008, 12:00 PM   #2
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My guess is that a mod tranny secondary =may= be built to tolerate the full DC current of the final RF stage it is modulating, seeing that the secondary often goes in series with the DC supply to the final. If this is the case the tranny will probably have a gapped core like a normal single ended class A tranny does.

http://en.wikipedia.org/wiki/File:Ammodstage.png
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Old 28th December 2008, 12:38 PM   #3
SY is offline SY  United States
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Quote:
Ideas? Recommendations?
Yes, rethink the design. What topology are you using for the output stage?
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Old 28th December 2008, 01:34 PM   #4
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Not sure about the Collins, but, many mod xfrms are spec. for bandwidth of voice only, FWIW (for example 300hz-6K).
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Old 28th December 2008, 04:00 PM   #5
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Originally posted by SY


Yes, rethink the design. What topology are you using for the output stage?
I am using the Fender 400ps as a guide and I just adding two more KT88s and attempting to come up with about 450WRMS on a single output through a Plitron 4141:

http://ampwares.com/schematics/400ps.pdf

Re: the Collins's frequency response, it's rated 200-5Khz. But I tested it and its flat from 20-20Khz. But perhaps seeing as how the Sowter is a real driver transformer, I should use it....
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Old 28th December 2008, 04:10 PM   #6
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Piece of cake. It's a pentode input, so the total capacitance will be pretty low. If it's being run in AB2, you can use power MOSFETs as source followers to drive the grids, then do the voltage amplification with a plain vanilla diff amp (6SN7 is classic for that function). This will work better than a driver transformer and cost you a fraction of what the Sowter does. If it's AB1, you probably don't even need the MOSFETs, though they'll speed the overload recovery and extend the bandwidth a bit.

In this circuit, they're using the transformer to go from single-ended to balanced, which is much better done at lower levels.
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Old 28th December 2008, 05:05 PM   #7
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Well, I have already ordered the Sowter so perhaps I should stick with the driver transformer approach...
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