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Old 8th August 2012, 05:47 PM   #11
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Join Date: May 2007
Hey Ilimzn,

All your negative expectations will probably be fullfilled about the grid drive. You will first of all need to run the tubes at so high Ua that there is no room for anything else than almost Class-B. It might have to do with the bias(as you describe) that distortion will not act as in conventional A and AB designs. Ie the, a little to, high distortion will not lineary be lower at lower signal levels.

So as we agreed before a very good solution is to go boosted triode. 4xEL509 will give far better results than 2x6C33C. With EL509 trioded at 170V they will be on par with the 6C33C. 70/100V Ua/+Us works OK for EL509 without risk for the to high Is. This will give a lot lower THD before feedback applied if we compare with the, to hard to handle, grid drive(or partly to any other configuration or tube). Also the low Ua gives us the opportunity to run at higher Iq giving us Class-A higher.

About adding two stages(must be differential) between the concertina and the O/P-stage to keep the phase right, this might be an idea. Unfortunately not in reality even if we also bootstrap the following stages. Only with the Concertina directly to the O/P-stage we get the low Zout expected from the inverted Futterman. If doing like the sandboys we could just add more GNFB to butter it up. But Zout isnīt all.

Instead of doing like they did in the early days of sand we can do like Otala and use local NFB together with lower GNFB.
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Old 11th August 2012, 08:36 PM   #12
ilimzn is offline ilimzn  Croatia
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Location: Zagreb
Quote:
Originally Posted by revintage View Post
Hey Ilimzn,
All your negative expectations will probably be fullfilled about the grid drive. You will first of all need to run the tubes at so high Ua that there is no room for anything else than almost Class-B. It might have to do with the bias(as you describe) that distortion will not act as in conventional A and AB designs. Ie the, a little to, high distortion will not lineary be lower at lower signal levels.
Actually it's not so much Ua, that could be low, but the drive voltage on G2 will have to be higher! In other words - lots of problems with constructing the driver and no payback regarding performance

Quote:
So as we agreed before a very good solution is to go boosted triode. 4xEL509 will give far better results than 2x6C33C. With EL509 trioded at 170V they will be on par with the 6C33C. 70/100V Ua/+Us works OK for EL509 without risk for the to high Is. This will give a lot lower THD before feedback applied if we compare with the, to hard to handle, grid drive(or partly to any other configuration or tube). Also the low Ua gives us the opportunity to run at higher Iq giving us Class-A higher.
One of the biggest disadvantages of the 6S33S is the amount of heat it makes by only running it's heaters :|
To be clear I never intended to try G2 drive for an OTL, for the reasons I stated, I don't see any advantage and a lot of problems and disadvantages.
I will, however, look into the enhanced triode mode. There is still a problem with lowering Uak to near zero if there is no load, I will look into screen currents. I am more specifically interested in PL504s but the same principles apply.

Quote:
About adding two stages(must be differential) between the concertina and the O/P-stage to keep the phase right, this might be an idea. Unfortunately not in reality even if we also bootstrap the following stages. Only with the Concertina directly to the O/P-stage we get the low Zout expected from the inverted Futterman. If doing like the sandboys we could just add more GNFB to butter it up. But Zout isnīt all.
Actually the bootstrapping is the local feedback that makes the inverted Futterman output stage behave as a follower. The intermediate driver stages that provide extra gain to the control grids chosen (G2 in allistairE's case) restore the attocious 'follower' performance of G2 drive, The local feedback thus becomes less local, resulting on potential instability - so, one more drawback. But in my comment I was just exploring the idea, not that I would attempt it, honestly.

Quote:
Instead of doing like they did in the early days of sand we can do like Otala and use local NFB together with lower GNFB.
No disagreement there!
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