The AM Forum

I've been looking into a pair of 572Bs as mudulators rather than 811As for a single 813 class C amp.  I've read pros and cons with a lot of good feedback on the 572Bs.  If I used the 811As, I'd need separate power supplies but got to thinking.  Since the plate dissipation is around 65 for the 811A and 160 for the 572B and max plate voltages of 1500 and 2500 respectively, with the S-22 iron, could ONE supply delivering 2KV under load be used?  How about 1650?

check the skizmatic for a Johnson 500 .... 2kV on both the 4-400 and 811 modulators ....in all fairness, EFJ bought RCA 811a .... whether they were off-the-shelf or special order, I dunnow

They must have at least used the graphite plate versions.  Regardless, Johnson probably biased the hell out of them at that voltage.  I'd like to stick with triads in the mudulator rather than mess around with a screen supply.  HOWEVER.........

Once before I considered using a couple of 4-65As which would work but that tube is notorious for getting gassy.  I have a couple, but.  Here's a 200 watt modulator from the '49 West Coast Handbook using a pair of 813s driven by a pair of 6SJ7s with a single 2 KV supply and no driver iron.  This is really interesting especially considering it has a provision for an aux input and the circuit is really simple.  It's 200 watts.

My modulator that I am building for my 814 transmitter was originally a pair of 811As, I later swapped a pair of 572Bs in there and noticed that they seemed to put out a little bit more power, and might be slightly easier to drive than the 811s were. Why not go with a pair of triode connected 813s to modulate your single 813? Just hook the grid and screen together and run it like it was a triode.

I've seen diagrams for 813s as triodes.  I've diddled around with various ideas for over 10 years...meaning I'm 10 years older and still changing my mind.  But I'm getting close to a build.  I just sold an ART-13 mod iron after deciding to build around the UTC S-22. I can use it with a couple of 6L6s if I wanted. It won't care.  
  
I'll set it up with a pair of sockets for 811As.  I could plug in a couple of 809s and run them at 500 to 1,000 VDC and get 85 to 145 watts.  The S-22 won't care.  I can run 811As between 700 and 1500 VDC. I can muster 175 to 340 watts output. It means two power supplies...eventually.  No problem; just more work. The S-22 will start complaining when I push it over 250 watts or so.

RF-wise I'm looking at around 350 watts input...under 400 watts for sure.

As for the 813 RF PA, it probably won't care either if I start with 1250 or so and work my way up with one supply.  It just makes sense.  And if I decide to build the 120 watt AM rig with parallel 5933s mentioned in another thread, I'll just plug in the 809s and run the works at 600 volts.

That's how to kill every mudulator bird with 4 tubes.  Ha!  I can plate mudulate my DX-60B with 809s.  Just turn down the gain control. What a hoot!

Gotta a Junkston "500" in the shack......first set of 811A's lasted 11 years. I think they were Cetrons. Either way; 811A's or 572B's are plug & play in the 500.

With graphite plates on the 572B's, they should handle 2KV with ease.

Craig,

My transmitter started out as just going to be a single 814 modulated by a pair of 809s. The modulator I'm using was built with 811As originally but I didn't think I needed that power so I was going to use 809s. Well not being able to find 809s made me get 811As, and then I wasn't satisfied with how they were running so I had a quad of 572Bs from a blown up linear, I picked the best two out of the group and dropped them in the modulator. The RF deck I figure could be used with 814s as it is designed, or possibly 807s if I want to ever drop the power level later on, with just a few changes, since the 814 and 807 both use the same socket and plate cap and I believe are pretty close to each other in design.

That's what I was thinking Craig.  I'll start the build with just the S-22 mod iron, tubes and filament iron on a basic chassis.  I can add a bias supply later.  I'll probably start with 811As and an 813 PA on another chassis from a single 1250 supply and mess around with it from there.  The HV supply will be on a separate chassis.  I can add the 2 KV on a 4th chassis to finish.  I should end up with 2KV PA and 1350 Mod under load.  I see the 809, 811A and 572B can all use the same socket and wiring.  Home free.

Yeah, the socket for the 814 and 807 might be the same but you would have to reconfigure it.  I'm not familiar with the 814 except the 807 is a tetrode while the 814 is a pentode.

Both are actually technically beam power tubes and have the exact same pinout: #1 and #5 are for filaments, #2 is the screen, #3 is the grid, and #4 is the beam forming plates. The only differences between the two (other than physical and electrical characteristics) is the 807 is indirectly heated and ties the cathode and beam forming plates together (some versions did inside of the glass envelope, others did it inside of the base) where the 814 has a filament cathode, and every datasheet I have seen says to ground pin 4 to the center tap of the filament transformer or the negative side of the filament if you use DC. The other difference is the 807 uses a 6.3 volt filament and the 814 is a 10 volt. I'm not sure why some data sheets for the 814 mention grid 3 as a suppressor when the same sheets call that tube a beam power tube, which are not the same thing.

The 813 is also a beam power tube, if you look at one on a schematic it looks like a pentode but it is not. Now the 803 is a true pentode, with three grids.

Back on topic, you shouldn't need a bias supply with the 572s at that voltage, assuming you are running class B. Zero bias should work.

The Alpha 811A is $16.95 each. The price is 1/4th that of a 572B from RF Parts. I use 4 of them Alpha 811A's as a class BC RF linear amplifier for AM. Makes 180 watts carrier with headroom for 140% positive peaks. Running about 1700V on them with 12v bias (just above cutoff), the plates just show a little red. For class B modulator use, I wouldn't go higher in plate voltage, and would bias accordingly, perhaps -6v to -9v @ 1650 or thereabouts.

Jim
Wd5JKO


http://www.rkrdesignsllc.com/products/transmitting-tubes/811a-tube-glass-triode/

Forgot about the "beam" part.  I'm going to stick with plan A; 813/811A combo beginning with about 1000 to 1250 volts into the S-22 and work my way up.

A ham 'friend' borrowed my S-22 for use in his Viking 500. It worked fine except for the size. Unfortunately he decided to keep the thing and never return it. Never loan a modulation transformer... :'(

Jim
Wd5JKO

Jim, there is an old saying:"The quickest way to lose a friend is to loan him money"  It must apply to modulation transformers too.....

Nowadays getting money is easier than getting good iron.

Sometimes its worth lending $50 to get rid of someone with out shooting them .... ..

klc

But never lend them your mudulator iron, hihi!

Teach a loser to borrow money from you, then you're rid of him for a month. Catch him stealing money from you ...

Where's that 50? usually gets rid of them.

As far as the stealing...... https://www.youtube.com/watch?v=8Xjr2hnOHiM

klc

And at last resort,there's always Judge Judy!