The AM Forum

My HB rig has 811A's mod'ing a pair of 813's.  It seems to me that it would be very easy to convert the modulator to 813's.  I have a filament transformer for the 813's...

Is this as simple as I think, ala the "Maul"?

Rich

Why not.
I have considered thay myself several times. One of the most common setups around is triode connected 813's X 813's. You could even power up the screens & drive the tubes with very low power.
Keith
WA1HZK

Not sure what makes a "Maul" but my question would be, why?

If the 811s provide enough audio for modulation, why change the tubes?

You need to change something else to make any sense out of the project.

I am using both tubes in my rigs.  An 811 modulated by two and two 8000's mod by 813s and treat them both as zero biased triodes.  Nothing could be simpler.

js

Triode connected 813's have more scrote than 811s...If you want to modulate a pair of 813s and have big audio, the 813 modulators are the way to go....I've done
 it both ways and prefer the 813s x813s...

Yeah, I agree with 'ACT pretty much ; if you already have enough moxie, at the same plate voltage your just burning 100 watts for the 813's flilaments (10v at 10 amps)vs. half that for the 811's( 6.3v at 8 amps.)    If you need more audio then 813's are the way to go, but to see it you need 2000 volts to 2500 vs. the 1200 to 1500 you have on the 811's.

IF your buying new, then 572B's might be the way to go, same socket, same filament current but still have to raise the plate voltage to get real benefit.

OTH, Johnson in the 500 ran 811's plates way higher than 1500; I think it was 1800 to 2000.  Good for voice peaks if you didn't put continuous sine wave on them very long.

My dual 813 HB transmitter just completed has 1600vdc (loaded) on both the 813's and 811A's with a 6.8v zener in the cathode c.t. of the 811a's.  Tons of audio for 350 to 400 watt carrier.  Way to easy to flat line (neg. mod.) the sucker if I overmodulate, even with my very assymetrical voice.  

The maul is K1JJ's 813's x 813's rig outlined on this and Huz's site; quite strappescent but you need the 2000 plus, plate voltage.

Roger.  Just changing the tubes does not make it a Maul.

But I am curious as to the purpose of the Zener.

js

Jack
The zener sets the resting plate current in the modulators. How do you do it in yours?
John

A pair of 811A's will modulate a single 813 very nicely, but it would be pushing them close to the edge to use them fully modulate a pair, if  you are running them at anywhere their rated CCS power level.  I like to run with plenty of headroom myself, so I would probably triode connect another pair of 813's and use them in the modulator, running both modulator and final somewhere around 1800 to 2000 volts on the plate.

I have run triode-connected 813's on as high as 2500 volts, but the graphite plates would turn cherry red.  I thought it looked "cool" in those  days, but 813's were cheap and plentiful back then.  Now, I would never abuse good transmitting tubes to that extent.

I never tried any of those piece-of-crap 813's that came out with the metal plate, but they would have probably melted into a puddle the way I was running them.

One advantage to using the triode 813's is that it would take minimal modification.  With the 811-A's, you are already driving a pair of zero-bias or near-zero-bias triodes in the modulator, so the modification will be primarily changing out the tube sockets and filament transformer.

Dentron's 160-10L ran 811As at 1800VDC grounded grid. Mine lasted years and was still in grand shape when I sold it.

the 572B's would do the job. Run 2000-2500 on them zero bias. lotsa audio scrote. Probably make 2.5X- 3X the power of the 811a's. modulate a pair of 813's with plenty to spare. Get Down. Swing it. They just drop right into the 811A sockets, no fuss, no muss.

Hi Rich,

Yes, the triode connected 813's are an excellent choice for modulators, especially when modulating a single one with 2000-2500V or using two in the final.  

As Don said, you don't need any cathode bias at all under maybe 2kv or so, but a higher HV will require some diodes in the CT to idle it down.

A pair of 813's X 813's can make a tremendous big rig. I once ran my pair with 3KV and was able to do 1KW out into a dummy load.... 130% modulated. A more conservative HV might be 2200V running 500 W or so with a pair. A single 813 final is good for an EZ 375W out, using 2KV. There's so many options.

The 813 modulator grid/screens can be fed with a simple backwards connected 8 ohm audio transformer with the CT to ground OR consider the new WA3KLR fet cathode driven board. It runs 5W and drives the 813's beautifully.  I now have a single 813 final using 2500V with the 813 modulators driven by this new fet board - using 7-8db of negative feedback. It is the cleanest running plate modulated rig I've built to date. This is a testimonial to how clean the 813's are in triode service. It's probably just a lucky design that makes the 813 and 4-1000A work so well triode connected, but that's a good thing for us to take advantage of common tetrodes as triodes.

Find attached the schematic for the KLR fet cathode audio driver.  I used PLUS 50 vdc on the 813 modulator grids to insure proper peak power and operation when cathode driven.  Each modulator tube needs it's own fil transformer. I don't have any etched boards - I just wired up a simple PC board.

Also here's the old link to the 813's X 813's for new people who may not have seen it. Simplicity plus.

http://www.amwindow.org/tech/htm/813/813.htm

Best of luck with the 813 modulator switch-over, if you do.

T

Being a zero bias tube, it only draws 30 ma with no grid drive.  I use a 1k grid leak to get it to Class C with 40 ma grid drive.

There is a schematic on my web site http://schmidling.com/radio.htm
but ignore the one shown directly and go to the hi res link.

I did not realize this when I first built the rig and put in a bias supply and a grid leak.  Turns out the bias supply is not necessary so I removed it.

This rig is about as simple as it can get.  I had a nice cross mode QSO on 7290 tonight and the SSBer said my audio puts him to shame.

As soon as I find a bigger fil trans I am going to add another 811 in parallel.

js

Distortion is a child of quiescent bias set point.  Orignial literature¹ for many years recommended zero bias for 811'A at plate voltages of 1200 and a bias of -4.5 for plate voltages of 1500 to keep quies. current at less than 50ma. 

So, singing to the choir here, 'cause you already know but some may not...

When you 'chart' ,stack two tubes in push pull their load lines mirror each other with a neat common load line.  Crossover distortion is reduced at some minimum current. Running a little more current will improve distortion but the whole idea for efficiency is to keep as close to class "B" as possible, just a little "A." So I figured that for Ep of 1600 to 1800 a bias of -6.8 would be fine. 

Besides, that's the 10 watter that was available on the blister card rack at Corning Electronics years ago when I bought it for just such emergencies... ;D

1.) any ol' ARRL handbook tube tables.