[Amps] 3CV1500A7

[Paul Christensen]

> "A very good friend and my elmer has an Alpha PA-70V and he needs to 
> replace the famous and almost disappear 3CV1500A7. Does anyone knows where 
> we could find one or how this amps can be modify to accept another 
> available vacuum tube?"

Juan,

The few new tubes that exist are locked up in our safe-deposit boxes.    The 
3CV1500A7 is electrically and in most respects, identical to the 3CX1000A7. 
The only difference is that the CV has a boiler fitted onto the anode for 
vapor cooling.  But the socket, and all other physical and electrical 
properties are the same.

There are several possibilities:

1) Send the tube to Econco for a rebuild estimate.  In speaking with 
Econco's John Cavenary a few years ago, the success rate of a rebuild is 
less than 50%.  The tube structure of that small power tube just does not 
hold up well to the forces required in the rebuild process.   Econco should 
be consulted first before sending them the tube.  Their policy may have 
changed since the last time I spoke with them;

2) Modify the amp with a 3CX1000A7 and add air cooling.  This involves 
extensive metal work, but many 70V amps were converted by the Alpha factory 
and their customers back in the mid '70s.  "Instant-On" feature is 
maintained; or

3) Modify the amp with an 8877.  Same air cooling caveats apply, only a 
time-delay system will need to be incorporated.  Not too terribly difficult, 
but challenging for some.  "Instant-On feature is lost."  Before Bob 
Douglas, W5GEL became SK a couple years ago, he modified his 70V with the 
8877 and we was very pleased with the results.   And, he did the mod at near 
90 years of age.

I just recently purged the H2O system on my amp and it's as strong and 
silent as ever.  Quick horror story: I purchased a gallon of "Brand X" 
distilled water while standing in line at the local Walgreens.  Purged the 
water and added the Brand X water.  Powered-up and Leakage current went 
off-scale.  Either I got a bad jug or that company is re-selling tap water 
as "distilled."  Purged the amp again three times and added ZephyrHills 
brand and leakage current was back to zero!

FYI - for those interested in what the inside of a vapor-phase-cooled amp 
looks like, pictures of my 70V can be seen on Alpha's web site.

http://www.alpharadioproducts.com/html/70v_pix.htm

Paul, W9AC