Screen Drive Push Pull ideas

[whitelabrat]

Scrap the IRF840B thing. They appear to be intended for power supply use.

[tubelab.com]

My "universal driver board" is now somewhat alive. I used it last night to extract 80 watts from a pair of Chinese 6L6GC's in conventional control grid drive. I was pushing them well into AB2 but far short of the glow region. Pictures of that experiment with the schematic and parts values for the current version are in this thread:

6L6GC AB2 Amp

Comming soon, glowing screen grids. I have to make a 2500 mile road trip next week so I don't know if I will get to crank this up before I leave or not. If so I will report the results here. I should be able to stop at the Dayton hamfest for some fresh specimens to "test".

[whitelabrat]

I'm going to go off the ranch a bit. What is the likelihood that the Universal Driver could be used for Parallel Push Pull screen drive? I pulled the trigger on a pair of Antek AT-1008's that have a low primary resistance at 2022 ohm. Maybe too low. The price was right and I've been wanting to play with toroidal transformers. Would the mosfets be able to pull it off?

The TEK power transformer has plenty of 6.3v juice for all eight power tubes and the four pre and driver tubes. No worries there. Even more off topic I just noticed I've got a good number of 6CL6's while parting out the old scope.

[tubelab.com]

What is the likelihood that the Universal Driver could be used for Parallel Push Pull screen drive?.....Would the mosfets be able to pull it off?

The mosfets are limited by their current capablity (I am using 2 amp fets) and their dissipation (mine are rated at 35 watts and I have a BIG heat sink). The setup that I have has succesfully melted the grid in a single tube, so driving two or three in a non glowing situation should not be a problem.

I usually recommend seperate mosfets for each tube when individual bias adjustment is needed. In a multi-tube screen drive setup you can tie the screen grids together and use the control grid for individual bias adjustments.

I am finally headed towards home in what will have been a 3000 mile road trip (currently using hotel WiFi). Sherri drove most of today, so I laid out a new version of the driver board. I eliminated the "what if" parts that didn't make the cut allowing a smaller board, and added jumpers so that 6 or 12 volt tubes can be used. It will likely be 2 or 3 weeks before I have time to make one.

I have a large collection of tubes to try in push pull with the board, from 45's to big transmitting tubes. I also have a pair of rather LARGE Plitron toroidal OPT's that are rated at 400 watts 1250 ohms. I will find several ways to feed them and pick the best. I have already found that a single pair of BIG sweep tubes (6LW6) can drive these things to 170 watts in conventional control grid drive. The Plitrons, and toroidal OPT's in general are quite sensitive to bias current imbalance. Time for experiments has been limited lately though.

[whitelabrat]

Mouser has a few 2SK2700 so I've jumped on a bunch before they're gone. Crss 10 instead of 6 can't be too bad?

I've also been wondering how I'll be able to deal with a power supply at or over 500v if my caps are only good for 500v. I'm thinking a pair of the big ASC Widow Makers in serial should be safe for 700v while providing about 300uF of smoothing. Not that 300uF at 700v is safe at all.

I've got my workbench set up in the garage finally so I hope to do some testing on the power transformer. I estimate about 26A in 6.3v.

[Tony]

I have a large collection of tubes to try in push pull with the board, from 45's to big transmitting tubes. I also have a pair of rather LARGE Plitron toroidal OPT's that are rated at 400 watts 1250 ohms. I will find several ways to feed them and pick the best. I have already found that a single pair of BIG sweep tubes (6LW6) can drive these things to 170 watts in conventional control grid drive. The Plitrons, and toroidal OPT's in general are quite sensitive to bias current imbalance. Time for experiments has been limited lately though.

hi tubelab, do you have any 4D32's in your stash? if so any plans to try them out? i wonder how they will fare compared to the 6LW6's.....

[tubelab.com]

hi tubelab, do you have any 4D32's in your stash?

I don't have any of these. I have seen them in Stans (ESRC) warehouse, but he didn't sem too interesed in selling them to me.

The universal driver board that I was building for these experiments is on hold right now. I have been working on an octal driver board that is very similar to the tag board that I used for some of my previous screen drive experiments. Right now it has been used for conventional control grid drive, and it works very well. Screen drive experiments will come soon.

[Tony]

Thanks tubelab, i purchased 8 of these from surpluss sales at about 22bucks a pop....the data sheets says 125watt classAB2 at a plate b+ of 650 volts, was intending to use as pentode amp. screen is low at 250 volts so i wonder these tubes could be good candidates for your screen drives...

[Merlinb]

This isn't an area I've ventured into, and I was wondering, what is the point of screen drive?
The only advantage I can think of is that it would be easier to DC couple to the driver stage, but is there something else I'm missing about this topology?

[SY]

Efficiency and linearity. My amp idles at 3mA per output tube and coughs up 150 watts for a pair of sweep tubes. No crossover distortion, unlike similar near-class-B conditions with conventional grid drive.

If you're RF-savvy, think of this as analogous to zero-bias triodes in a final.