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Alexontherocks 9th January 2012 11:05 AM

High voltage driver for AB2 operation GU81m tubes
1 Attachment(s)
Dear All, out of another users's kindness and expertise (lem8r) a design was developed to push a gu81m, full pentode design, into AB2 operation.

the design is attached to this message.

Please feel free to comment....I m just a messenger so hopefully lem8r will jump in to reply to any observations:


bear 9th January 2012 02:11 PM

Seems like the long way around... using a KT88 to provide voltage drive to a Mosfet follower?? No power is required, only voltage swing from the tube driving the Mosfet.

I think I'd consider other means myself, and perhaps decide if a Mosfet is what I want or if I want to drive the output tube from another tube or not.

I'm not too sure about the bias means for the Mosfets either, but perhaps that will be sufficient...


SY 9th January 2012 02:34 PM

Well, pentodes are a good idea when you need lots of swing, but yeah, I'm with bear, KT88 is not an optimum choice. I'd scout through some databooks and find a HV pentode with a more reasonable power rating (a few watts, not 35!), then start experimenting. There are some interesting linearization tricks you can do with the pentodes in a differential configuration.

Alexontherocks 9th January 2012 02:46 PM

thankyou to both of you for the inputs.

Unfortunately custom transformers must be made to suit specific design requirements. The idea is to have a working design to fabricate transformers....with a working basis experimentation can begin...

I personally do not have enough spare material to simulate this design on a breadboard

What would greatly help would be to have a working design with very good or good chances of working adequately.. Modifications would go from there to an optimized design.

iko 9th January 2012 02:59 PM

Have you seen this? Apparently it's a working prototype.

Bandersnatch 9th January 2012 03:15 PM

AB2 on a GU81m? Are you nuts? A pair in full class A is still going to deliver 400W, give or take all without swinging the grid positive. You have an OPT good for 400W? AB2 ought to be capable of nearly 1kW output. Back to the 'Nuts?' question...

iko 9th January 2012 03:18 PM

OT, hey Douglas, can I ask you privately a few questions about your 4E27 amp? I'm planning to build something around the few 4E27 tubes I got.

Alexontherocks 9th January 2012 03:21 PM

A custom transformer can be made for this application.

Bandersnatch 9th January 2012 04:00 PM


Originally Posted by Alexontherocks (
A custom transformer can be made for this application.

I don't believe it. Biggest I have seen is for the Altec 260. 813 amp, power output under 300W and I will bet against going any bigger w/o significant issues. Go ahead though, I'd love to see how you do it.

Alexontherocks 9th January 2012 04:05 PM

will be happy to surprise you.. :)

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