3C24s AKA HK24G's as modulators?

[N3DRB The Derb]

anyone ever used a pair of HK/Eimac 24G's as modulator tubes ?  Tiny little things they are, but I'm thinking they might be a good match for the S21 in the MiniStrap modulator at around 1000 volts. They're so common it seems a shame to not try them out.

handbook sez 2 of them will make abt 115 audio watts. I'm gonna try them out in the 100 watt mod deck.

811's in the bigger 250 watt deck I guess. How normal and boring.  

hmmm...at that plate voltage the P-P load resistance is off the scale. maybe not so hot a idea. 

[KE6DF]

The data sheet only shows 70 watts output from a pair of them at 1000 v in class B.

That's probably a CCS rating, but 115 watts would be a bit high even for ICAS service.

Perhaps use two pair (4 tubes). That would also lower the P-P impedance they could work into.

[WQ9E]

Tim,

If you need a full 100 watts out you need to give them some more plate voltage or do as IYH suggested and run 4 of them in push pull parallel.  My Globe Champ 175 uses 4 6L6 tubes this way and I believe the Halli HT-9 also used a 4 tube modulator.

Not that it really makes much difference but my reference sources pair the 24G with the 3C28 while the 3C24 is listed with a group of other tubes.  However, the ratings are very similar.

Rodger WQ9E

[N3DRB The Derb]

that would just halve the given resistance, right? Devide by 2?

that would kinda be cool looking.  Funky.

P-P triode connected 42's or 6F6's @300v probably drive em ok.

809's would be better and less trouble, but I dont think I'll be running into any of them any time soon.

the 100 watt power level is kinda dicey in regards to tube choices. 809 would be my first choice if I could get some.

[The Slab Bacon]

A quad of 6L6s would easily produce around 100w at a leisurely 400-450v. Just wrap a little feedback around it and away you go!!

Howeva a quad of 24G gasmatrons would look pretty kool and perform about the same except for needing a little more drive than the AB1 tetrodze. Running them P-P / parallel would also help tame down that god-awful plate impedance.

809's arent all that hard to find, I picked up a pair at the last How Cow fester for $20. (spares for the glob chump)

[KE6DF]

Assuming you want to run the modulator and RF stage both from the same 1000v supply, then if you shoot for 100 Watts out of the modulator, you would want to run the final at 200W.

So, for a 200W final at 1KV, you would draw 200 ma from the supply and have a 5K Ohm plate impedance.

Assuming you want about a 1.3 to 1 turns ratio on the modulation transformer for 125% modulation capability, then that works out to

1.3 squared time 5000 = 8450 Ohm p-p impedance reflected thru the mod transformer back to the modulator plates.

Now a single pair of 3c24's has a p-p impedance spec of 15.5K Ohms at 1KV.

So using two pairs would be a pretty good match for the 8450 Ohms reflected impedance.

So it seems like the two pairs solution pencils out pretty nicely.

If I've messed up this analysis, I'm sure one of the guru's will correct me.

BTW, there are a couple 809s on ebay right now. But if you already have the 3c24's you might as well use them.

[N3DRB The Derb]

well, either would be good to go. Actually, come to think of it, I have a UTC S-44 (575-525-0-525-575) @ 500ma ready to go. quad 6L6's would enjoy that.  I could run the whole mod deck off of the 1 xfmr/PS. that appeals to me GREATLY.

I think my mind got made up for me.

[K1JJ]

Derb,

The 24-G is probably the best low power old buzzard triode out there to build a rig wid.

Back around 1996? I built up a TPTG single 24-G Hartley class C final modulated by a pair of p-p 24G's.  It was a ONE tube RF transmitter. I put 1800 volts on the whole thing and man did it scream!  It did about 100 watts output and well modulated. Amazing for a 25w tube. VERY high efficiency when driven hard.  I loaded it up until the plate glow was at its limit. There was more to go with that kind of voltage, but the tube wud melt.  It was built on a piece of plywood that was covered with PC copper board as a ground plane.

I was using a dipole with openwire spaced about 18" apart, from the tower to the ground with no spacers. That TPTG rig would go wheeo-wheeeeeo- wheeeo, when the wind blew if you listened with the BFO on. The openwire was slowly moving to and fro, changing the load impedance of the oscillator.  I used a vacuum cap to get the best stability possible, plus with a vernier, it made it easy to get on frequency.  It was pretty stable with no wind. A 3-diode limiter circuit was very important to keep the rig well modulated without cutting it off on negative peaks. The TPTG oscillator wud simply die otherwise when going over 100%...

It took some dicking with to make it work correctly, but in the end it was a really nice transmitter. The tubes all glowed red when running full strap. Don't know why I tore it down, but think I'll build another one just like it.

The problem with 24-G's is they seem to be gassy - at least the batch I found. I had about 15 of them, but when I fired them up for a test, only four were good, even after hours of precautionary filament break-in, etc. Most would flash over with just 300 volts on the plate.  So I have just enuff for another rig.

I could scan and post my (clear) handwritten schematic, with notes, if anyone is interested in building it or one like it.  A TPTG is a real  "SBE" rig that Tron always refers to...


T

[N3DRB The Derb]

I REALLY hope these HK257B's are not gassy. I only paid 10 bucks each for them, but I want to use em. I got 4. If too many are bad, I'll have to slide over to boring old 813's 

At least the sockets and connections are the same. Just change the fil transformer ----  and I already have a 813 fil unit.

[W7TFO]

HK24's and 24G's are essentially the same tube, just the grid comes out the side on one.

You can check those 257's dry (no filament V) by tying all the base pins together and hooking that one side of the tube to a transformer that will go up to at least 2kV.  The other goes to the plate cap.  Crank it up on a variac in subdued light and see if there is a discharge inside.  If so, run it a bit in on-off cycles at enough voltage to get a good glow, and usually it will kill the gas.

I use an old neon sign transformer for this.  Works for any tube big enough to handle the ionization flow, I wouldn't try it on any tube without a plate cap.

This tip is from a 1950's Machlett Labs engineering article.