Why don't my 811A tubes hold up in the AL-811/H ?

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Roy Lincoln:
 The 3 and 4 tube Ameritron amps that use 811A tubes are very popular. A large number of users experience short tube life and wonder why? Lets examine that question.
 RCA made the 811A in the late '30's, about '38 or '39. RCA made the originals and made them under high quality production standards. Anyone else who manufactured them did so under license and were making copies. RCA wrote the book on the 811A and no one presumably knew more about the tube than they did.
 There are 2 ratings applied to power amplifier tubes, CCS and ICAS. CCS stands for Continuous Commercial Service and ICAS stands for Intermittant Commercial/Amateur Service. The RCA rating of the 811A was 45 watts plate dissipation in CCS and 65 watts plate dissipation in ICAS.
 Examining what this means, an 811A in CCS can run about 128 watts input and at 65% efficiency, will produce about 83 watts of rf out while dissipating 45 watts as heat in its plate structure. Additionally, it will do so for what is considered a long life in commercial service. An amplifier with three (3) of these tubes could produce 250 watts output and four (4) tubes could produce 334 watts, all enjoying the same long life in CCS.
 RCA rated the tubes higher in ICAS, recognizing that amateurs did not generally use tubes in the same manner and would accept shorter life in absolute terms. Even then, the life was considered adaquate as it was spread out over several years of activity and inactivity. Thus, in ICAS the tube could be run at 65 watts plate dissipation, running 187 watts input and about 122 watts output. A three (3) tube amplifier could run 366 watts output and a four (4) tube amplifier could run 488 watts output, assuming 65% plate efficiency in both cases.
 As I remember, the Collins 30L-1 amplifier had four (4) 811A's in it and was rated at 500 watts, a pretty honest rating. Word was, if you saw orange coloration in the plates while operating, you were driving them too hard. Dull red was considered proper and that was with RCA's.
 From these ICAS ratings, folks took liberties. CW is a 50% duty cycle mode, therefore we should be able to raise the power input/output some to compensate for the fact that the tubes will not be at work full time while transmitting. SSB is an even lower duty cycle mode, perhaps 20-25%,therefore we should be able to drive them even harder and still not cause anymore heating. With speech processing, the duty cycle climbs towards 50% and higher.
 RCA 811A tubes are gone, practically speaking. Sure there are some NOS tubes, but how many? Most users are using Chinese tubes. These are not thought to be high quality tubes. Is it honest to even believe these are as good as RCA's were? I think not. RCA's had small horizontal fins attached to the vertical plates or perhaps they were merely punched from the vertical plates and bent horizontally. In any event those horizontal fins distinguished the 65 watt ICAS tubes from the 60 watters. In all likelihood, the Chinese tubes are no more than 55-60 watts in rating, no matter what.
 How about the cooling? Does the cooling in these amps meet the standards RCA called for to cool the glass envelopes and plate seals? I don't know but I do know that some have reported that the failures among their tubes always seem to be a particular one. It reminds one of the days when Volkswagon engines ran hot on number 3 cylinder!
 All of the modes of thinking that suggest we can drive these tubes well beyond the ICAS rating are pushing inferior (inferior in the sense that they don't measure up to the RCA's) tubes to the ragged edge. Its no wonder that holes appear in the plates, glass envelopes melt, elements short, etc.
 Even though you bought tubes that were matched or thought they were matched, can you really be sure that they were/are matched over the entire conduction curve from cutoff to saturation? For that matter, can you be certain that they remain matched after some use? W8JI says not!
 Even Heathkit never claimed an output rating of 800 watts PEP for the Warrior HA-10. For Ameritron to do it is entirely bogus. In fact, the only reason an amplifier can produce more power output on ssb than on cw is because the filter capacitors don't discharge to quite as low a value of plate voltage during low duty cycle voice peaks. For the 811A's to produce 800 watts pep output, they would have to produce 1200 watts input. Its pure baloney.
 If you want your tubes to last don't look for more than 500 watts cw output or perhaps 550 watts pep on ssb out of a four (4) tube 811A amplifier. Don't look for more than about 375 watts cw or 425 watts pep out of a three (3) tube model. And on RTTY, which is a 100% duty cycle mode, don't look for more than 350 watts or 275 watts respectively.
 The ratings I've suggested are approximately 1 db down from claims for cw output and only about 1.5 db down for ssb output. To put a db in perspective, it is the smallest change in power that is barely discernable, and then not by everyone.
 Another possibility to consider is to change the tubes to 572B's. These used to be known as 572B/T160L's. Their rating is 160 watt plate dissipation each. Some have suggested that the Chinese versions may only have a proper rating of 130 watts each. Even then, they sound like a perfect replacement for the 811A's. I haven't checked the specs. for the plate-grid capacitance and don't know if they can be neutralized without modification in the AL-811/AL-811H. That might be inportant up around 15/12/10 meters.
 One more thing-If the manufacturers produced the 3-500Z amplifiers with the same specsmanship applied to 811A amps, we'd see single 3-500Z's rated for 1600 watts pep out on ssb and 1150 watts out on cw. 2 tube models would be rated at 3200 watts pep ssb and 2300 watts cw. Seems they're much more conservative when the tubes get a lot more expensive. I guess they just expect that you'll be willings to buy 811A's more often.
 

Alan Applegate:
There are a few more considerations.

The Ameritron amps use an automatic bias which shuts down the tubes during hesitations in drive. This helps, but is not a cure all especially when one uses compression and all of the background noise keeps the tubes in conduction.

Further, almost without exception, amateurs over drive these amps. Four 811As only require about 65 watts of drive for full output. Driving them with 100 watts is asking for short tube life. What's more, the ALC must be used in most cases even with just 65 watts of drive. These two facts are outlined in the owner's manual.

Replacing the tubes with 572Bs doesn't mean there will be more output. About all that happens is, the users over drive the amp which they think is okay now that the tube plate ratings are much higher than before. What they forget about is, the power supply dynamics are inadequate for the service they're trying to put the amp through.

Overdriving, regardless of the tubes used, is a sure way to increase IMD, and award your fellow amateurs with all manner of splatter and distortion.

If you want more power, buy a bigger amp!

Alan, KØBG
www.k0bg.com

Roy Lincoln:
Alan,
The AL-80B uses the bias scheme you mentioned but the AL-811 and AL-811H do not.

It stands to reason that 100 watts will seriously overdrive this class of amplifier because the tubes are are already overdriven at 65 watts.

572B's are just right for resolving the plate dissipation problem of the 811A's, if they can be neutralized.

Michael S. Higgins:
Why would agrounded grid 572B amplifier need to be neutralized?

Roy Lincoln:
Neutralization is necessary to cancel the effects of interelectrode capacitance between the plate and control grid of power amplifier tubes especially at 28 mhz, even in cathode driven amps. The symptoms of neutralization being necessary would be amplifier instability and possibly actual oscillation.

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