Tetrodes as triodes

[K9ACT]

If you don't wish to know why, my question is at the end of this.

I have sort of developed a love affair with triodes.  One 8000 then two in parallel; ditto for 811's.
Most recently, I decided to swap my spare 8000 for the two 811's and within a few hours, I had it running like a clock.  Talked to Brent and a few others last night with good reports and concluded it was a good idea.

One legal limit 8000 rig for serious 80 meter stuff and a smaller one to putz with where ever I get the urge.

This morning was clean up time.... get rid of the clip leads, mount the bias supply on the chassis and a little more work on the power supply.

The bias supply is a voltage doubler using a backwards transformer off the 8000 filament.  To avoid abusing the filament while testing, it seemed a good idea to remove the tube.  The twist was followed by a crunch and to my horror, I had forgotten about the grid cap on the side of the tube and the crunch was it's parting with the tube.

It now can only go back on the bookshelf as a bookend where it has spent the past 50 years or so.

During this transition, I had been talking to the Noon Time Forum gang about a new project which was leaning toward a 4-250 but I ended up opting for the 8000 instead.

However, one of the guys (John K9KEU) brought a little present for the host of Oktoberfest yesterday in the form of a 4-400.  I thanked him profusely but confessed that he was about 24 hours late but the next project is just around the corner.  Little did I think it would be less that 24 hours later.

So, having said all that, I can not find another 8000 anywhere so it seems time to dig into the tetrode project.

Two aspects of tetrodes bother me.  The first of course is the screen supply and modulating it but the less obvious is sensitivity and need for serious shielding between grid and plate.  I have done well on the project with only plywood and masonite and no shielding at all.  Just making the grid tank perpendicular to the plate tank.

With this in mind plus the fact that the big rig uses 813's as triodes in the modulator, it occurs to me that one might be able to do this with the 4-400 in the RF deck.

So, is this a nutty idea?  If not, what do I do with the screen?  Tie it to the plate, the grid or leave it alone?

Thoughts please,

js



 

[k7yoo]

Jack--I have a good supply of 810's if you want to go that route. Still a Triode with the grid cap on the side!
Skip

[N3DRB The Derb]

8000's these days are made of unobtanium. 810's make damn good modulators. I used it's baby bro a pair of 805's to modulate a pair of 812 H's (carbon plate 812's) and they swing a mad munky.

[Opcom]

4-400's can be used as zero bias triodes by tying the grid to screen through a 50K 10 watt resistor and driving the screens. The reason for that is the control grid is rated 10W dissipation max. The screen is rated 35W. The drive requirement is 500-750V peak positive voltage on the screen depending on the plate voltage and desired power output and the grid resistor also is adjusted depending on those values. I don't know anyone who has done this for RF. I have seen it for a modulator where a backwards-connected 50 watt tube output transformer was driven by a solid state amp.