• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Show me your screen drive circuits

This has not been built so just understand that. Its just a concept at this point.
 

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Here is something I used to drive a PL508/ 17KW6 SE

Real tough tube. Takes 30 watt idle no problems. whilst spec only says 12 watts.

Here's the bare-bone schematic, I've deliberately left out any specific component values. as its only an experimental circuit.

the pot sets the bias point. between a given range selected by zener voltage, Ive had this run for awhile and observed no runaway phenomenons.

An externally hosted image should be here but it was not working when we last tested it.


Its likely advisable to fuse either the screen drive circuit or the cathode of the tube. As damage to the mosfet e.g. shorted source drain will put your tube at maximum dissipation levels.

Kind regards.
V4lve
 
This one has been built.....about 6 years ago so the memory is fuzzy. 80 watts from a pair of 6AV5's on 550 volts into 3300 ohms. There is a new one on the bench but progress is limited by lack of time.

Is that a 2SK2760 MOSFET?

What are all of your negative source voltages? That's the biggest obstacle for me. Finding another power transformer that delivers all the various voltages for the negative supplies.
 
Yeah,

That's my biggest hesitation with screen drive minus never really using any type of sand for anything.

I have a few transformers that will work for ~ -225V my positive B+ transformer can either deliver ~ 350V or 700V using a doubler.

I'm a bit torn due to space as to whether or not this is the project to try screen drive on.
 
Tubelab got a sick amount of power out of a pair of 13GB5. That tube is so common here in the form of PL500.....I got 150 of them so expect to see more testing soon.

I made that quick post from my phone in wood shop class earlier tonight. I did squeeze 150 watts from a pair of 13GB5's, but that was in conventional G1 drive mode. I sold a lot of tubes to ESRC for store credit, so I got a mixed batch of US branded 13GB5's. Regardless of whose name is painted on the glass, all of them are Mullard XL500's. There were a few oddballs that were made in Korea. They look like they will melt easy (thin plates, thin glass).

Is that a 2SK2760 MOSFET?

All of my old experiments used the Toshiba 2SK2700 which went extinct when ROHS happened. Any N channel mosfet with suitable voltage ratings and a low and constant Crss in the 50+ volt region will work.

What are all of your negative source voltages?

I used bench power supplies for this and all my amp design experiments. Once I figure out what voltages I need, then I figure out the power supply.

The amp I am working on now is very space constrained, so the power supply bits are already chosen. I have a BIG Ebay toroid with a 425 volt CT 3/4 Amp winding. It will be used for a 300 and 600 volt supply. A mosfet regulator will make the screen supply. Heater power comes from a SMPS. I usually use a Triad N-68X to make +/- 150 volts for bias supplies.
 
I used bench power supplies for this and all my amp design experiments. Once I figure out what voltages I need, then I figure out the power supply.

The amp I am working on now is very space constrained, so the power supply bits are already chosen. I have a BIG Ebay toroid with a 425 volt CT 3/4 Amp winding. It will be used for a 300 and 600 volt supply. A mosfet regulator will make the screen supply. Heater power comes from a SMPS. I usually use a Triad N-68X to make +/- 150 volts for bias supplies.



I used bench power supplies for this and all my amp design experiments. Once I figure out what voltages I need, then I figure out the power supply.

The amp I am working on now is very space constrained, so the power supply bits are already chosen. I have a BIG Ebay toroid with a 425 volt CT 3/4 Amp winding. It will be used for a 300 and 600 volt supply. A mosfet regulator will make the screen supply. Heater power comes from a SMPS. I usually use a Triad N-68X to make +/- 150 volts for bias supplies.[/QUOTE]

Hi George,

That's a similar option for me also. I can use a doubler for 650-700V and tap the center of the doubler and feed a screen regulation circuit. I am still thinking about just going with plain Pentode for this amp. I have a load of IRF9610s. Are they of any use?

96B1626F-294E-40EA-9AB5-E19B842A013F-18387-00000D7E67F82B04.jpg


Here is the chassis. I'm debating cutting off the rest of the wall and making more space for PCB.
 
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This one has been built.....about 6 years ago so the memory is fuzzy. 80 watts from a pair of 6AV5's on 550 volts into 3300 ohms. There is a new one on the bench but progress is limited by lack of time.

Hi George,

Digging around, I have some IRF610. Maybe 10 of them. Will they work in the circuit you have here?

Also, I keep reading about the advantages of screen drive being lower idle current. What kind of current does a PS need to be capable of delivering to achieve full output from something like four of the 6CB5A without sag?

Thanks!

Blair