Modulation transformer for a driver transformer?

ironradio · 28th December 2008, 08:41 AM

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!

Circlotron · 28th December 2008, 01:00 PM

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

SY · 28th December 2008, 01:38 PM

Quote:

Ideas? Recommendations?

Yes, rethink the design. What topology are you using for the output stage?

Tweeker · 28th December 2008, 02:34 PM

Not sure about the Collins, but, many mod xfrms are spec. for bandwidth of voice only, FWIW (for example 300hz-6K).

ironradio · 28th December 2008, 05:00 PM

Quote:

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....

SY · 28th December 2008, 05:10 PM

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.

ironradio · 28th December 2008, 06:05 PM

Well, I have already ordered the Sowter so perhaps I should stick with the driver transformer approach...

28th December 2008, 08:41 AM	#1
ironradio diyAudio Member Join Date: Feb 2005 Location: Oregon	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!

28th December 2008, 01:00 PM	#2
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	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 __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/attachment.php?s=&postid=353269&stamp=1079781945

28th December 2008, 02:34 PM	#4
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	Not sure about the Collins, but, many mod xfrms are spec. for bandwidth of voice only, FWIW (for example 300hz-6K). __________________ Be sure your foil hat has a good low impedance ground.

28th December 2008, 05:10 PM	#6
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX	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. __________________ "It is not seemly, after wiping your nose, to spread out your handkerchief and peer into it as if pearls and rubies might have fallen out of your head." - Erasmus

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28th December 2008, 06:05 PM	#7
ironradio diyAudio Member Join Date: Feb 2005 Location: Oregon	Well, I have already ordered the Sowter so perhaps I should stick with the driver transformer approach...

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