The AM Forum

OK, now I am thinking of a pair of 4-65A's as modulators for the two 813's. Thoughts please. I cant seem to find any schematics of the 4-65A being used as such. On the other hand, didnt really dive into the research for to long.
Regards,
Gary

The Eimac datasheet has some data.

This won't be much help, but I was given (in pieces) a military rig (Air Force) that used a pair of 4-65's to modulate a pair of 4-65's. I still have the mod transformer, which was quite small, and did not have much iron in it, but I suppose that would be expected as it was built for installation in various aircraft. The transmitter looked to be in the 100 - 150 watt power class. When I played around with 4-65's I found that they loved high voltage and low current, and in the end, didn't seem worth the trouble when other more plentiful tubes that could exceed their performance were floating around out there. Just my 2 cents of course!

I think they would be fine if you used them in push pull parallel for a pair of 813's. I would run push pull parallel for just one 813. They will add a nice glow to your rig that are otherwise boring to look at 813's do not do. Well maybe some blue glow in them. Don't get me wrong, I like 813's. There are as versatile a 30.06 hunting round. 

Here's a PDF if I can up load it. Tells you all the specs, has circuit diagram for 500 watt rig using 4-65 in the mod and final. 

A pair will work if you turn the 813's way down.
Figure 250 watts out of a pair and a high impiedance.

Nothing else on hand to use?

Besides the low power output, you have to build a regulated screen supply and bias the tubes, and even so the distortion is higher then some other tube choices.

811a's will do 340 or more watts without any screen or bias supply.

If I was to go to the trouble of building regulated screen supplies and bias supplies, I would opt for a good AB1 tube that has low distortion and lots of power output.
4x150a / 4cx250b type tubes, up to 600 watts out with zero driving power, work from 1000 to 2000 volts.

Brett

How about 813s modulating quad 4-65s. They are a high Z tube so 4 would help. They are a better RF tube above 20 meters.

True, they'll work but 811's while are excellent tube are another boring tube to look at. I think home brew rigs should be built for show and glow is what you would get, from 4-65's. They would run red to orange when loaded up or talked up to max. Why not triode connect them?  That is what I would do,  so forget the screen voltage. I've got bunches of them and  I'm still waiting to do something with them. Oh well, just my 3 cents worth.

Thanks for all the input. I do have 811's kicking around and also a nice pair of 8005's. I was just thinking of something different than the same old, same old. Now what I should do is put together the 3CX15000 driven by a 5CX1500, but think that one would get me into real hot water.
Again, thanks guys for the info.
Regards,
Gary...WZ1M

Just out of curiosity Terry, when triode connecting a tube like the 4-65, does the screen get tied to the grid, or to the plate? My thought is that since they would be grid driven with audio, that the screen would be tied to the plate, but I'm not sure. I remember asking this question a few years ago about triode connecting grounded grid 4-400's in a couple of different forums and there were good arguments for screen to grid and also screen to plate, so possibly it would work either way. I have a feeling one way may be better for the tube than the other but don't have a clue which would be best. Any thoughts?

Good luck with the rig Gary. Stay off the high voltage!

Rob W1AEX

If you are running it as a lo-mu triode, tie the screen to the plate.  As a  high-mu triode, tie it to the control grid. 

Remember there is a maximum voltage rating for the screen grid, so if you tie the screen to the plate, the maximum plate voltage will be limited to the maximum rating for screen voltage.

With the control and screen grids tied together, the tube functions as a high mu triode, usually zero bias.  It works well with 813's.  With 807's, a 20K resistor is required between the two grids, and IIRC, the audio is fed to the screen grids.

Please stand by. More to come. Please stand by.

Update:

I said please stand by, some of you out there are sitting.

 Sorry Jim, I didn' see your pdf.

they are cute little bottles but don't store very well .... about half I had in storage had lost their vacuum ... before you commit to a design, TEST

The military T-217 UHF transmitter making 100W out on 225-430MHz used a pair of 4-65's in AB1 to modulate a pair of 4X150's. Plate voltage was 1200-1500 IIRC, and the plate to plate load was about 9000 Ohms. Mod iron was rated 120 watts. 

The 100 watt output URT-12 Coast Guard transmitter uses a pair of 4-65's to mod a pair.  The mod transformer is rated 14KCT to 12K, 90 watts RMS. I do not know the HV but the power transformer is 3500V center tapped at 0.25A so that means maybe 120mA average current for the modulator. Chokes are rated 1400 volts working in that one.

Thanks for clarifying the triode connection parameters Don. That will be helpful with a little project I am working on.

My 4-65 push pull modulator carries the label "AM 773A/ARC-21" and is stamped with the RCA logo. I have never found out much about the rig it came from, but it sure was a complex chunk of gear with numerous Potter & Brumfield relays throughout. Looks kind of funny with the odd shaped pull out handles bolted to the chassis.

Rob,
      Generally speaking tying the screen and plate together is usually used for lower powered class A operation. Tying both grids together us usually used for class B or similar service where you are pushing the grids to draw some grid current. Tying the grids together will usually give a lower voltage gain, but a higher power gain, requiring more drive to kick them along.
 
                                                               the Slab Bacon

Most of the tubes of that style envelope from that era had the same problem. I needed some 826s for an old gonstipated VHF amplifier. I must have went through a least a dozen of them to find a good pair! !   However for some odd reason 829Bs dont seem to have that problem.

                                                                  The Slab Bacon

Dunno about 4-65s as modulators... high voltage low current = high Z - that makes the match to the primary side of the mod iron difficult. Not impossible but not that easy.

I'd run it the other way... 813s or 811s modulating the 4-65s??

Maybe some 4-125s or 4-250s might be nice, they seem to not be "desireable" and go pretty low in most cases....

Or why not a "JJ" style 813s by 813s rig??

Oh, if you strap the 4-65s as triodes the plate Z will be somewhat lower, but still if you want to make power with them the B+ will need to be rather high making the mod iron match and V rating a bit tricky... also they make less pow-air in triode.

                     _-_-WBear2GCR

Because they are boring to look at. His circuit would be just fine. 813's are the kind of bottles that should be hidden. Now Glow, NO Show ;D ;D ;D ;) ;) ;)

Erry-Tay,

Youze needz to get ur ies chzeched?

Take a look at the pic of KC2FXE's rig that I took, I think it is in my gallery. Plenty of glow.
Ya haz ta know how to build for max gloowww!!  ;D

but whatever... bottom line is uze the toobes ya brung witcha!

                         _-_-