[ETO_Alpha] FS: New unopened 3CPX800A7

Mon Mar 23 16:51:07 EDT 2015

At the first reading I also noticed what Jan had said about higher 
CURRENT  values for the tube,  but i dismissed it as his push to get 
something listed and not to hopefully overstated the value or features 
of the CPX tube..

Beginning in May of last year I went on a  "update" of my Alpha 77Sx  
Soon like most things, the project went big or more. Followed by a new 
HV filter cap and a slight increase in capacitance as well operating 
voltage. Then came the main Xformer followed by a new HV rectifier board 
that has been totally redesigned and updated. All the while the moment 
of effort and truth was fast approaching.

I was yellow, and punted to Eimac for help.  In two separate calls with 
direct application engineers for the 8877/3CX1500A-7 tube they never 
budged on the HV top rating of 5000 vdc.. My amp was now making 4950 
idle plate voltage, yes sure they tested it higher, but would not move 
even for idle voltage. Finally we discussed real operating conditions 
and characteristics of the amp proper. This is a conversation in it's 
own self.

As an off hand comment it was proffered, had i considered the 3CPX 
version of the 3CX tube  I said no never heard of it.  Soon enough as 
much data as they would release was here on my computer for review. Yes 
the outward tube design for the CPX gives the white ceramic area about a 
3/16-1/4" taller profile than the 8877/3CX model.  Outwardly that is it. 
No more.
I was struggling within myself about and even though it was at idle the 
higher right at 5Kv voltage where before the amp had run like 3.8Kv at 
idle, looking at the Plate meter "pegged" or very damn near left me no 
room for comfort and seriously questioning my effort and how i had lost 
the original objective and was now lost in the forest of higher Plate 
Voltage..

I like the 77Sx  and was just not warming to the idea of the 8877 and 
5kv and operating right at or on the edge..

The operating and performance spec's for the CPX tube had immediate 
appeal and the 10kv plate voltage spec I was in love with and as Jim 
states it was only for the HIGHER plate VOLTAGE pulse and as it was 
explained to me indeed only the Ceramic body length was increased to 
allow for a slightly higher grid inductance and plate voltage.
I bought into the CPX version and went on the hunt for a pair of the 
damn things.  Soon enough that was accomplished and at what I thought 
was a decent enough price of $375 per tube and while not new did come 
from a medical application and checked almost identical to new tube 
testing numbers.
As for operation is was text book if not easier and quicker in the 
Loading and Tuning to get the amp up and performing, an absolute breeze, 
I had total confidence from the outset and that was not misplaced. I 
have had the 77 since it was new in 85' . It has always loaded and tuned 
well, but now the SWEET spots are or seem very sure and easy to walk 
into and rock back and forth between tune and load and just feel and 
know you are at the best efficiency of all the design parameters ...

Soon after I had the thought of spare tubes and that i wanted a set,  
none were showing up in the usual haunts, only ebay, United RF seems to 
have as I recall at the time 56 of the CPX tubes listed.  I wanted the 
personal attention and knowledge of the tube so i called and soon enough 
we had agreed on $380.00 per tube incl shipping and test results, so i 
have a set of spare tubes now as well..

closing notes:  my 77 had the original Eimac 8877 tubes as supplied when 
delivered new and had become weak and not holding the same numbers of 
recorded settings i have recorded for various band freq and antennas SWR 
points .. so that is what started the whole circus and $2,400 later not 
including the spare set of tubes.  If the amp last me the rest of my 
operating life, that i have over invested in the amp will be of small 
concern, reflecting back to 85'  i had to hock my ass to buy the thing 
in the first place.

Final thought:  I like using the CPX version tube and i don't drive the 
amp any more or harder than i previously did..

The fellow who is offering the replacement HV boards did a splendid job 
and that was the second 'goal' to replace or just assure the performance 
of the HV rectifier board. over time i had already replaced and upgraded 
the other smaller ckt boards.

HV board: John Stanford, email is wb8svn at gmail.com
Xformer:   Hammond using the Peter Dahl design  using DigiKey
HV Filter Cap:   Condenser Products
other small nuances and tricks i had to use upon request

Warm regards:  mac/mc  w5mc

On 3/23/2015 10:55 AM, MU 4CX250B wrote:
> I was under the impression that the pulse rated tubes were identical
> internally to the non-pulse-rated  tubes, except for slightly longer
> ceramic insulation that increased the peak voltage rating (and added
> more inductance to the internal grid wires.) I know the tubes are
> specified for high peak currents, but i thought that was just an
> additional specification, not a change in tube construction. If I'm
> mistaken, I would welcome being corrected.
> 73,
> Jim w8zr
>
> Sent from my iPhone
>
>> On Mar 23, 2015, at 7:32 AM, Jan C. Robbins <swanman at cfu.net> wrote:
>>
>> If you're running a 9500, here is your chance to get that extra tube at a REASONABLE price.
>>
>> I have one brand new purchased 2015) Penta pulse-rated 3CPX1500A7 (8877), capable of handling much higher plate current than the standard Eimac or Penta 8877 you buy from, e.g., RF Concepts, Ameritron or RF Parts (all of which sell Pentas, which are superb tubes). I sold 3CPXs to hams for nearly 20 years, and they are THE BEST tubes for very-high-power amps.  The 3CPX1500 normally sell for $775 and up (way up), but I can ship this one anywhere lower 48, fully insured, for $595. PayPal preferred, other methods acceptable. Tks es 73 N0JR
>>
>>
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