active screen grid elements

[Andy Le Blanc]

some time back I found an example of how to dress a triode as a diode.

Fun stuff, you can use it to replace a crystal as a detector for small radio projects, a "flemming Valve" out of radio history.

And, you can use what you might have laying around the bench to noodle around with, its a diode on the cheap, any 8 pin octal or 9-pin mini dual triode can be used.

[Andy Le Blanc]

At some point I wired a couple of these onto the screens of a rig I was noodling with

You get a voltage drop across the diode and you provide de-coupling for what follows the diode.

[Andy Le Blanc]

with slight alteration you get a stupid simple series regulator.

Theres a voltage drop, but thats fine for the screens.

[Andy Le Blanc]

With a little more alteration you get an adjustable supply for the screens.

This can be dressed to the center tap of the output transformer, of further down the power supply.

You can also tie both side of a dual triode together to handle more watts

you also can tie both the screen grids together, or utilize separate screen supplies for each side of the P-P pair.

[Colossal]

Excellent post and PM sent Andy.

[David Root]

Agreed, most interesting! Thanx Andy. Possibility to stabilize screen voltage with respect to varying line voltage, using feedback??

[JMFahey]

No need for feedback, you can use the triode as a regulated screen supply.

Only (slight) problem is getting a, say, 400V reference voltage , you won't find a single Zener that high (that I know of), but you can stack a few, no big deal.
Mouser shows a few in the 120V to 330V range.

STOP THE PRESS:
I forgot (you too) : there is a definite and impassable limit about how much voltage you can apply between cathode and heater (which is grounded) so it will be very hard to get, say, much more than 100/120V at that cathode ..... which of course kills its ability to regulate screens.

[roberto]

The heater of that triode needs a dedicated supply, elevated by 300V or more (depends on the maximum cathode to heater allowed voltage, for 12ax7 is around 200V but depends on the brand).

[Andy Le Blanc]

thats up to you of course... you need to float your heaters for srpp and follower, cascode etc any way.

the rating says you need to keep it with in 200v +/-, so it'll depend upon where your screen V ends up riding.

you only need .15A 12v, so a small 12v ct fil. tranny is cheap and easy.

[JazzGuitarGimp]

Super interesting thread! So, is there a way to use the plate curves of the triode / diode to determine how much voltage drop it will impart?

[matt h]

(deleted)

[JazzGuitarGimp]

12AU7 would be a better choice at 22mA, either run in parallel or one half for each power tube in a duet PP config.

[EtherealWidow]

With having a pot to adjust the cathode bias, wouldn't you run the risk of scratchy noises even not turning the pot because of the DC on it? Maybe a rotary switch instead?