Pentode driver and IT?

WE91 · 25th May 2007, 06:40 AM

Hello to all

I have a question about Interstage Transformers with pentode drivers in a SE amp. Why havent I seen any such amps? What's the reason for this? Are there any reasons for why not?

Looking forward to more info on this.

Best Regards

AndreasS · 25th May 2007, 06:47 AM

In the Bouyer ST150 amplifier (1966 made) two 6L6 in pentode configuration drives 4 6L6 in class-B2 using an interstage transformer... (with NFB form secondary of the interstage transformer)

Andreas

BudP · 25th May 2007, 06:56 AM

We91,

The lack of SE IT's in general and pentode SE IT's in particular has to do with how difficult it is to get them to work. There is no such thing as a generic SE IT for pentodes. Each would have to be made for a specific driver tube, run under a specific set of conditions. Then would come all of the cut and try and measuring because the math just falls apart when you get to this level of complexity. If you are willing to spend the money, and it will run to several hundred dollars, you might interest a custom house in attempting it. But, there would be no guarantee you would get what you thought you paid for and that it would work as you wanted. Spend your money on a really good coupling cap.

Bud

Miles Prower · 25th May 2007, 09:14 AM

Quote:

Originally posted by WE91
I have a question about Interstage Transformers with pentode drivers in a SE amp. Why havent I seen any such amps? What's the reason for this? Are there any reasons for why not?

It's not done due to the enormous plate resistance of pentodes. That would require likewise enormous inductances. You also can't use high u triodes for the same reason. with xfmr coupling, you can't use any triode with a u in excess of 20, or so to keep the plate resistance down.

There are some topologies that add lots of local NFB in order to reduce the otherwise unacceptable pentode plate resistance. However, I haven't seen that done in anything other than AM modulators where fidelity isn't a concern.

WE91 · 25th May 2007, 09:30 AM

Thank you very much, guys!
In almost no time at all, my question was answered.

Have a nice weekend!

Best Regards
Aril

25th May 2007, 06:40 AM	#1
WE91 diyAudio Member Join Date: Jan 2005 Location: Norway	Pentode driver and IT? Hello to all I have a question about Interstage Transformers with pentode drivers in a SE amp. Why havent I seen any such amps? What's the reason for this? Are there any reasons for why not? Looking forward to more info on this. Best Regards __________________ Yoda: "May the Force be with You" Acoustic design: www.fmdesign.com Check out the Griffin loudspeakers!

25th May 2007, 09:30 AM	#5
WE91 diyAudio Member Join Date: Jan 2005 Location: Norway	Thank you very much, guys! In almost no time at all, my question was answered. Have a nice weekend! Best Regards Aril __________________ Yoda: "May the Force be with You" Acoustic design: www.fmdesign.com Check out the Griffin loudspeakers!

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25th May 2007, 06:47 AM	#2
AndreasS diyAudio Member Join Date: May 2005 Location: near Duesseldorf	In the Bouyer ST150 amplifier (1966 made) two 6L6 in pentode configuration drives 4 6L6 in class-B2 using an interstage transformer... (with NFB form secondary of the interstage transformer) Andreas

25th May 2007, 06:56 AM	#3
BudP diyAudio Member Join Date: Feb 2007 Location: upper left crust, united snakes	We91, The lack of SE IT's in general and pentode SE IT's in particular has to do with how difficult it is to get them to work. There is no such thing as a generic SE IT for pentodes. Each would have to be made for a specific driver tube, run under a specific set of conditions. Then would come all of the cut and try and measuring because the math just falls apart when you get to this level of complexity. If you are willing to spend the money, and it will run to several hundred dollars, you might interest a custom house in attempting it. But, there would be no guarantee you would get what you thought you paid for and that it would work as you wanted. Spend your money on a really good coupling cap. Bud

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