Care and Feeding of the 811A

From the get-go, let me first say I'm not a techy-kinda-guy.  I never thought I would publish my thoughts on this subject, but I feel compelled to, largely because what I read in text and published reports based upon modeling and practical application regarding this legacy triode is so, well.... opposite of what I actually hear about how folks operate these tubes in real life.
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The 811A is really the only valve I ever ran as a linear amplifier, and having two Johnson Couriers in the course of my ham radio life, they seemed to me to be relatively bullet proof.  The Courier had a pair of 'em, and I drove them with my Hallicrafters HT-37, driving them at the tune of what was probably about 25 watts.  My watt meter read an output of over 100 carrier watts, and if I figure things correctly, I probably realized just shy of 400 watts pep.  That amp ran cool, and those tubes were old when I got either one of those Couriers.  The only thing I ever did was either solid-state the power supply in one, or simply rebuild the other, replacing the 866 rectifiers with some other recommended tube that glowed orange, a 25-something or other, I forget.... but it handled the 1700 volts DC just fine.
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Ok, fast forward about two decades, and I find myself once again with an amp, only this time I have a cluster, or "quad" of four 811As.   And I begin refreshing myself back up on how these amps tune, what to expect, etc., how the relay connects (it's a Collins 30L-1), and most importantly, how my amp really wasn't meant for continuous duty cycle modes (cdc modes).    Since my primary intent was just that, AM, pretty much the ultimate in "CDC"modes, my investigations went another direction.  I did a lot of reading and a lot of asking around, and the reading began to depart in results and findings from the on-air asking.
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On the air, I got advice something like this: "I always do max drive and max out.  I always tune to "full smoke", and then, back my drive down to permit a good amount of 'head room' "
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So, what this means is max input, max output (maximum amplification efficiency), reduce the drive.  Ok, makes sense.  Sounds good. In fact, that's what I always did myself.  My max input drive to those two Courier 811A's used to be about 20 watts max.
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But that's not what the on-air guys were talking about.  They were talking about much higher maximum drive which could be northward of 70 watts carrier, 280 watts pep!   Max that, then pull the drive back, without re-tuning the tank.
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Now, that last piece was something different for me.  I never in my life ever altered the input of a final tube without touching up the output, because any change in input alters the load the tube faces, it changes the characteristic impedance, the tube is operating under a slightly different metric.  In most finals, you always "touch up" the finals, last thing.
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But, I get it.  We are talking about a modulated wave that is varying by 3-8 kc, not a cw signal where we only had to worry about one frequency, which never changed.  In AM, the amp has to handle a range of frequencies varying at an audio frequency rate, as high as 16kc side to side of carrier.
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But I began to also pick up comments of another sort.  Folks who "used to run" amps with 811A's, and after a time, sold them and went to 4-400, 3-500z or at least a couple 572's.  They had common complaints as I indexed them in my mind: "they ran really hot", "I had to replace them after a year", "I had to keep my transmissions short". 
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I never had that issue with my Couriers.  I could lay a brick on the key, that amp would stay cool, and it had no fan.  Just a lot of ambient air circulation.

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Now, my reading took another direction, as I mentioned.  While some material did seem to indicate the common tuning consensus, other material reminded me of this thing called "Maximum Anode Dissipation".  Wow.  I haven't dealt with plate dissipation in years, since my General Exam, actually.  The MPD (or MAD) is simply the maximum power which the plate of any tube can safely dissipate.  In the case of the 811A, that would be about 65 watts.
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Now, in my past rigs, all built for cw, we routinely went beyond those ratings because of the nature of morse code.  The tube isn't "on" long enough to heat up, so we would drive those finals well beyond their MPD ratings.  In the case of the "R-7" mopa rig described in the 1947 E&E handbook, we could drive the 811A to a grand output of 100 watts, with over 140 watts plate input.  That's a full 75 watts beyond it's ability to safely dissipate the heat off it's plate!! ....but again, morse code is a rapid, on-off keying.   So "max smoke" is maybe ok for this mode.
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Single Sideband, where there is no carrier, only the stored power of the sidebands presents a similar load to any power final.  Only more rapidly so, but there is no continuous carrier to deal with as long as the mic is keyed.  Again, this permits the final to go beyond it's CDC plate dissipation rating.  Actually, well beyond it.  And tuning for "max smoke" can work here too.
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The picture is this: you can get a lot more power out of any tube than its plate can safely handle over time.  Just as we could get 100 watts out of a single 811A driving the snot out of it, if we put a brick on that key, that tube would last about 5 minutes.... maybe.  That plate would give off a very nice cherry red, then go to white, then "boom boom, out goes the lights."
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Anyone who has built and operated an original QSL-40 knows the principle, here.  It's call "pushing".
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Computer models show that as the resistance facing the 811A increases, and as the drive power decreases, it's ability to handle power at the plate also increases.  As this resistance  lowers, and drive increases, ohms law takes place, more current flows, P=IxE happens, and the power which the plate has to handle exponentially increases. Even though more power is expressed at the output, the actual amplification factor decreases. This means that even though you may be getting the power you want out of the final, that plate is working overtime to achieve it.  It's heating up really fast.  There are some diminishing returns happening.
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The handbook tells me that efficient operation of a valve mathematically drives it well beyond that MPD, sometimes to the tune of 10x.  It also says that as this "efficiency" increases, the amplification factor begins to decrease after a point on the curve and increasingly non-linearly so.  As such, we need to define what we mean by efficiency.  
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Efficient electrical operation - power output - or efficient safe plate dissipation with somewhat less electrical efficiency. 
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When you set your drive to max, 70 watts and higher, the output tank is adjusted to dip, or  resonance at a relatively lower Z, lower resistance at maximum output.  This actually increases the power transfer efficiency of the tube!  The model shows that the plate (anode) is being forced to dissipate over twice it's rating.  At this setting, if one tunes to "max smoke" and simply cuts back the drive without changing the Z of that pi output tank, keeping the plate facing that very same impedance, the drive would have to be lowered such that the maximum output of a quad of 811A's would be reduced to about 100 watts, or as Owen Duffy indicates, 70 watts to put those plates in the safe range.  The remaining 260 watts is being dissipated as heat from the plates.  That's 70 watts of amplifier output.  That's what it would take to operate that amp with a brick on the key, and keep those plates happy and safely below it's MPD rating if tuning with max drive for max output, and leaving the output tank tuned right where it is when cutting back the drive.  This is probably why I hear the overheating and shorter valve longevity stories.
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AMers tend to hold that key for almost broadcast-length times.  One fella up in Indiana I actually clocked at about fourteen minutes on a single transmission.  My hat is off to his 3-500Zs.  Maximum plate dissipation, thy name is Amplitude Modulation! 
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Now, I have also heard of a lot of ops replacing their 811A's with 572B's, a direct replacement.  Now, we have a valve that has a MPD of 165 watts, rather than only 65 watts per tube.  Whole different story.  And the "Max Smoke" method probably gets by ok.  But that's 2.5 times the MPD of the 811A.  What does that tell ya?
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Computer modelling shows that when the 811A is driven by minimum drive, then the pi output tank is adjusted to a higher Z, a substantially higher resistance, and the plate can handle higher output power before that plate starts heating up into the danger zone.  The suggested drive was about 7-10 watts!  This falls in line with what I read earlier about the relationship of the output impedance to the input drive. 
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So, this was why many of these amps, such as the 30L-1 have actually told their owners in print to stick with SSB or CW, and that their amps were not built for AM or RTTY.  And indeed, using the "full smoke" method, they are not.  
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But it doesn't mean they cannot. 
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This morning I talked to a fella who told me about his Ameritron 811 amp he used to have, how hot it got, and how he had to keep his transmissions short.  He eventually sold it and got his (probably current) amp, I think it's a Heath SB220.  Good for him.  He can safely tune to "full smoke" with all the headroom in the world to PEP that 1600 watts!
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As he was talking, I noticed my own amp.... four 811A's.... which I have installed two thermometers, one digital - sensing both the ambient air and the evacuated air.... idling at 75 degrees Fahrenheit.   When it was my turn in the round table, I keyed down for about two minutes, maybe three.  My max temp at any time this morning was 85f.    The ambient temp in the shack was 72f.   When my shack temp goes up to 80f as it can do in the afternoon, that amp maxes at 95f after about a two to three minute key down.
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I attribute that to tuning according to what the plate dissipation will handle.  Taking the advice of the data supplied, I drive the amp with about 10 watts from my FT-101EX.  I tune to a dip at 225 mA.  I am realizing 125 watts carrier, about the same if I used the "full smoke" method, and have equal head-room.   Apart from a slight hum (I need to eventually replace the filter caps in the Collins) my signal sounds great from all reports.  One fella even scoped me at his receive.  Alles ist in Ordnung.
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This might also explain why I had such a good experience with my Couriers, and why they ran rather cool without forced air.  Tuned to max with minimum drive.  My old HT-37 could produce maybe 20 watts of carrier. 
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Likewise, the chief thing that I notice is how cool this amp runs compared to the on-air descriptions that I have heard over the past half year of asking around and simply picking up conversation on the subject.
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This all made sense to me and reminded me of my earlier years building cw rigs, mindful that I could get away with "full smoke" with my Class C cw amps because heck, I was doing 20 wpm!  They wouldn't stay keyed long enough to heat up.  But oh yes, during tune-up, I saw many a plate glow with fierce cherry-red ness unless I unkeyed.  I was reminded that I was skating by with my over-drive.
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So, am I over obsessive?  Maybe.  But I am also on a budget.  I need these valves to last a long time, longer than the year or so I have heard so many describe as their replacement cycle.  I can't help but think that "max smoke" is the main culprit.
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Another phenomena I find: when I do get into conversations regarding linear amplifier tuning and the 811A's, MPD is usually never mentioned or even considered.  When I describe all I have mentioned above in this post, I usually receive an annoyed tone, being told that I need to do maximum smoke, that that's all there is too it!!
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Well, as one op from KY told me (the one who described how hot is Ameritron 811 got and how he had to replace the valves about once a year!): "well, Gary, you do what works for you.  And if you're happy, that's all that matters."
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I agree.  But I also want my tubes to be happy as well.
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Well.... a year from now we'll see how things go.  BTW, KA4USN, who was the prior owner of this 30L-1 indicates the valves to be several years old.  Not ancient, but not new.  Ron is an old CW op who used to build his own equipment, we got our licenses together.  Methinks he also tunes with MPD in mind.   Just my suspicion.

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73, de wd4nka. 
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Just as an addendum, here is probably the best published explanation on this topic, specific to the 811A triode:
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Collins 30L-1 and AM

The Collins 30L-1 is a heritage linear RF amplifier using four 811A triodes.

The manual gives no guidance on the use of the 30L-1 on AM, and that leads some users to think that it isn’t suited. This article explores the 30L-1 using the model described at RF Power Amplifier Tube Performance Computer.

The 30L-1 is specified for 1000W peak DC input in SSB telephony. This imposes one constraint on its use for AM, the other constraint is anode dissipation.

Tuned / loaded for rated SSB telephony

If the PA is tuned and loaded for 1000W peak DC input in SSB telephony, one would expect an output of about 680W PEP.

The carrier power for normal 100% modulated DSB AM is 25% of the PEP, so the carrier power for 680W PEP is 160W. However, at this level, the anode dissipation would be 294W, in excess of the valve ICAS ratings of 260W for four.

If drive was reduce until carrier power (amplifier output) was just 70W, anode dissipation would be just under 260W, and PEP would reach 280W. This clearly does not exploit the 30L-1 very well.

Tuned / loaded for AM

Reloading the 30L-1 for a higher resonant load impedance at the anodes will allow operation at reduced power for AM (due to the dissipation constraint) at higher efficiency and within the dissipation limit.

The RF Power Amplifier Tube Performance Computer gives us a tool for exploring different operating conditions without risking damage to the valves.

If the PA is tuned / loaded for a little over 500W PEP (say 510W) with the least drive, the PI coupler is adjusted for the optimal resonant load for AM within the dissipation limit. AM drive is then adjusted for 122W carrier output, and modulation will take the signal to 488W PEP.

Output power is 74% higher than under the “Tuned / loaded for rated SSB telephony” heading above.

Under these conditions, anode current should be about 228mA DC, and assuming the DC input voltage is 1700V+30V drive, DC input power should be about 1730*0.228=394W, carrier out should be about 136W at the anodes, 122W at the output terminals, leaving anode dissipation at about 394-136=258W.

Summary

The above is a procedure to adjusting the 30-L1 for optimal AM telephony operation within the dissipation limits of four 811As. Under those conditions, it is capable of 490W PEP output, which is about 72% of the PEP capability in SSB telephony.

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© Copyright: Owen Duffy 1995, 2019. All rights reserved.