{Collins} Re: 572-B for 30L-1



Harold,

Back when the 30L-1 was designed, the 811As were a good choice.  However, 
like so many other things, operating conditions have changed over the 
years.  In particular, the applied AC power line voltage.  I don't know 
the story at your QTH, but my normal line voltage in the shack is 122 to 
124 volts.  That produces a standby plate voltage of just about 2000, as 
read on the panel meter. 

If you go and look up the ratings for 811As, the max plate voltage is 
1750, if I recall correctly.  (I have in front of me a 1990 "ARRL 
Handbook", and it lists the max 811A plate voltage at only 1500 V!  I 
believe that to be a bit low.)  The max plate voltage rating for a 572B is 
2750 volts.  Sit down and grind the numbers.  In my book, the ratio of 
115V (the design input) to 122V (what I actually have) should be the same 
as the ratio of 1800 V (what the schematic lists for B+) to a new voltage. 
 That new voltage calculates out to about 1910 VDC, which given the 
accuracy of the analog meter and change of multiplier resistor values over 
time, agrees very well with the "just about" 2000V reading I get on the 
meter of my 30L-1.

You are absolutely correct about not getting any extra output power from 
the amp with 572Bs, but when the present 811As get a little weak in the 
knees, you really might want to consider the 572Bs for "peace of mind" 
replacement.  Considering the age of original 811As that might still be 
running in some of the 30L-1 amps that are out there, I must say that the 
quality of the vacuum pulled on them at time of manufacture was very good, 
as are the quality of the seals and getters.  However, facts are that 
these bottles are not getting any younger by the day.  As they age, a few 
more gas molecules are bound to float around inside the envelope.  Get 
enough of those and you might have a nice light show sometime.

I am not sure what you mean, Harold, when you suggest buying a "heavy duty 
set of matched 811As" in you rpost below.  I always thought that 572Bs 
WERE heavy duty 811As.  I am not aware of an 811B, for example, that is 
any better than an 811A.  I have never seen 811-series bottles with thick 
carbon plates, like the 572Bs have.

I am no stranger to "pushing" final amp devices a bit to get whatever out 
of them.  In fact, when I got my 30L-1 about 18 months ago, I would have 
left in the unmatched set of 811As that were working just fine, but one of 
them was an oddball bottle made in the UK and having the internal tube 
structure rotated 90 degrees from that of the RCA design.  Thus, that one 
bottle has its plate in the dreaded horizontal plane.  I did not have any 
811As on the shelf, but I did have a set of NOS, NIB, Cetron 572Bs.  End 
of story.  I am not sorry I made the change.

73, Dale
WA9ENA 

 



"Harold Hairston" <k4hca@xxxxxxxxxx> 
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Subject
{Collins} Re: 572-B for 30L-1






Steve;

Every new 30L1 owner asks that same question sooner or later----usually 
sooner. Can it be done? Absolutely.

 First question  that begs an answer is, "Why would you want to switch"? 
You 
can not run any more power with the 572B's. The 30L-1 Power Supply 
Components simply won't support it. The 572B's cost more and there is 
little 
evidence that they last that much longer than the 811"s.

If Art Collins had thot that the 572B's were better, he would have 
designed 
the 30L-1 accordingly. Forget the idea of switching to the 572B's and 
invest 
those bucks in a heavy duty set of matched 811A's and enjoy your 30L-1.

Harold


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