Plate Voltage for 811A

[WB6VHE]

Greetings All!

Is it possible to run 1800 volts on the plates of 811A's in class-b modulator
service, ICAS, without damaging the tubes?  The specs give 1500 as the max
plate voltage.

Regards, Ken
W5KFS
ex-WB6VHE

[w4bfs]

Hi Ken ...  an obscure spec somewhere lists 2kV as max plate Voltage for an 811A but will require an increase in bias from 4.5V to somewhere around 9V or so ... a bigger problem for modulator service will be a likely increase in pp load impedance and peak voltages .. interesting question...73 ...John

[N3DRB The Derb]

I would think you could get away with 1800 V on them, but that would be absolute maximum. Why not just plug in 572 B's and get almost 4X the plate dissipation to boot?

4 572B's = 10 811A's in plate dissipation. Swing the munky up to 2500 volts with zero bias. mo betta.

I think the higher the plate voltage the more unstable they get as well. at 17~1800 vdc things get hairy.

[WA5VGO]

Johnson ran 2000 volts on them without any problems.

Darrell

[WQ9E]

Darrell is correct and supposedly Johnson got the blessing from RCA to run them at this higher voltage.  You just have to adjust the bias so that you don't have excessive plate dissipation.  They don't seem to have any reduced lifespan issues in the Viking 500.

Typical operation in the 500 is 2000 volts on the plate, -9 volts grid bias, and 35 mils idle current.

Rodger WQ9E

[KM1H]

If you are going to run that voltage stick to USA built brands. Also consider a small fan to keep air moving.

There is an obscure reference in the 4/63 data sheet to a Note 3 which says at 2000V with a grid bias of -2V. However there is no Note 3 annotation anywhere in that sheet. It may have been carried over from an earlier release that was cancelled in the 4/63 revision.

 
As far as imports the Svetlana 572B works well as a modulator and the thick carbon plate can take a lot more abuse than the thin sheet metal in the 811A.

Carl
KM1H

[KE6DF]

Attached is a version of the 1963 datasheet with a "note 3" about 2000vdc operation.

Note 3 is referenced about 5 lines above the note on a line giving a plate current spec.

[Opcom]

Even so the lack of a set of data for 2KV operation is a concern for designers. For builders and operators, 2KV is fine. I've run a pair from 2450VAC as a self rectifying tesla coil oscillator and never had a problem. After all, 2KV is the positive modulation peak per 1KV class C modulated amplifier CCS ratings and ICAS is 1250/2500Vpeak. I think the only issue is bias and dissipation.

[KM1H]

Attached is a version of the 1963 datasheet with a "note 3" about 2000vdc operation.

Note 3 is referenced about 5 lines above the note on a line giving a plate current spec.

Ahhh, silly me. It was at a page break and I failed to put brain into gear . Thats the same sheet I have here.

Carl
KM1H

[N3DRB The Derb]

I think the only issue is bias and dissipation.

mainly the last. in the Gonsets, you get 200 more watts input (400 vs. 600 ) on AM by going to 572b's and gain zero bias operation to boot.

in modulator service, much more munky would be swung. ( excuse me for being too technical there ) you'd have to worry a bit about the current. 572B's are little piggy tubes, they like current. yer mod iron may protest by crapping out.

[WB6VHE]

Thanks to all who replied to my question!  I am running the 811A's with 1800 volts on the
plates, a few volts of bias (~10 v) and they work fabulously!  I had previously used
100TH's in this modulator, but discovered that the driver stage did not have enough
oooomph to drive them hard enough to fully modulate the final (an 813).  Now, there is
loads of audio, the driver kicks the pants off the 811's and 100% modulation is a breeze.
Thanks again everyone, and 73!

[KA1ZGC]

I'm obviously late to the party, but for what it's worth:

I'm running 1650V on a pair of 811s in my modulator. I'd have to look again, but I don't think there's any bias on those. They barely show a blush in the center of the anode face when yelling obsceneties at slopbuckets, and modulate the piss out of the 4-250A with only a few watts input audio.

Timtron's got about 1800V on the 811A mod tubes in his BC-610B-as-in-Bastardized. Not sure if/how he's got them biased. Also readily modulate the piss out of his 4-400 with very little input audio.

10 volts seems like a lot of bias at 1800V B+; but if it's working, it's working.

[WB6VHE]

Thanks for the reply, OM!  The bias might be less than 10 volts; I measured it once,
but can't recall the number that well.  The idling current is 30 mA at 1850 volts. 
Modulator works great!

[W2JBL]

some Collins collector is probabaly gonna cry when he reads this but i run 1650 volts on my T47/ART-13 with no trouble at all. the RCA 811A's i have in it don't see to mind and i get a ton of audio. in my Heath Warrior i run 1800 on the 811A's, with 10 volts bias and it gets a bit dicey...i have few with nice 1/4" holes in the plates from that amp (i use it on SSB) but they still work, plus the hole in the plate makes a nifty way to check the tread wear on the grids...

[KM1H]

some Collins collector is probabaly gonna cry when he reads this but i run 1650 volts on my T47/ART-13 with no trouble at all. the RCA 811A's i have in it don't see to mind and i get a ton of audio. in my Heath Warrior i run 1800 on the 811A's, with 10 volts bias and it gets a bit dicey...i have few with nice 1/4" holes in the plates from that amp (i use it on SSB) but they still work, plus the hole in the plate makes a nifty way to check the tread wear on the grids...

I havent pushed my Navy version ATC much beyond stock voltages. I keep hearing that the mod xfmr likes to crap out when pushed which might explain all the bad ones that people get suckered into at hamfests and Epay.

Carl
KM1H

[WU2D]

Speaking of ART-13, I have a variac on mine and run 1500V on AM and 2200V on CW. Don't spare the air - a muffin fan on the rear blowing out keeps the old ART-13 happy.

Mike WU2D

[W9ZSL]

Hi Mike! Fancy meeting you here. Fine bottles those 811As. My good buddy Ed K9FWR just sent me some photos of the modulator he just finished. Beautiful and he's using them to modulate a 4-250A to full PEP. Your bias is absolutely key. But the modulation transformer is another thing. I lucked into a tub of butter for my project. I scored a Hammond 175 watter with 15K ohm plate-to-plate. My PA voltage to my 4-125A will be about 1700 VDC. I need around 1KV on the 811As. Scored a UTC CG-103 5-25 Hy swinging choke and the matching CG-303W plate xfmr. I should end up with about 1075 on the modulator plates. Perfect for da 350 watt input AM beastie LB-1T-350.

Congrats, you got it working!    Da Mad Mix