You have to remember Bill Orr was talking about these tubes/Amps running in
Amateur Service ..Not Commercial, therefore there is a big differance how
far you can push these tubes in the two services. Most commercial broadcast
transmitters are designed with a great deal of head room.
But some are not.I can remember one of the 1st Gates FM transmitters I
maintained was a 5KW using a 4CX5000A. That transmitter went through them
far to quickly, so we changed the chimney out and switched to a 4CX10,000D
and it pretty much ran on that one tube the rest of it days.The 5000 should
have been fine, but it wasn't. Another transmitter I used to maintain was a
20 KW Gates on Channel 8..It used and RCA Tetrode and ran it way to hard and
used go through them like popcorn and usually on a weekend. They ran so hot
, that the ceramic usually failed. Of course trying to get management to buy
buy a bigger transmitter was like pounding you head against the wall. That
was 20 years ago and I am sure it is still running, bet the engineer there
now can't wait for Feb 17th,2009...My point is there is a big differance
when we are talking amateur use. Take a look at all the little amps built
with sweep tubes, man talk about abuse!!
W6RD
From: "John Lyles" <jtml@losalamos.com>
To: <amps@contesting.com>
Sent: Thursday, July 24, 2008 9:10 PM
Subject: [Amps] tube output power
> When I worked for BE, I designed the cavity amplifier that was put into
> production in the FM3.5A and FM5A FM transmitters. The 'standard' tube
> used for 3 - 5 kW FMs that used tetrodes was the 4CX5000A, in RCA and
> other makes of power amplifiers. George Badger of Eimac showed us their
> new shortened 4CX3500A, which broke the traditional wisdom, and it ran
> beautifully in the 5 kW model (actually 5.5 kW with 110% power type
> accepted). While this isn't 2 x the dissipation rating, it does show that
> the tube chosen for a particular power level depends a lot on other things
> like cathode emission, expected lifetime, class of operation, screen
> dissipation, as well as plate dissipation. We did built a triode
> transmitter as well, using the 3CX1500A7/8877, although it was only
> running 1750 watts output. And the 8990/4CX20,000A was used in the FM30.
> In no case am I aware that we actually built PAs that produced output
> powers that were exceeding twice the plate dissipation. We were fairly
> conservative, but we had a lot of customers and didn't want to fail in the
> marketplace at the time.
>
> John
> K5PRO
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