[Amps] Tetrodes

[m.ford]

Hello Guillermo,

    A couple of years ago I set out to test a few dozen 8122's
that I had accumulated for my four NCL-2000 amps. I had both
RCA and BURLE but no EIMAC and the date codes were from
mid 60 up to 93. I researched the "amps" archives and found a
lot of advice on how to bring them to life before applying any
drive. I used one of my amps to burn the filaments for at least
24 hours and another amp to apply low anode voltage. If the
anode voltage held then I would key the amp (no drive) and let
it cook with very little idle current (adjust the bias to suit) for a
while. I then incremented the idle current over a few repetitions
until I reached about 125 ma. The next step was to apply high anode
voltage and repeat the process. It took a long long time to finish the
51 tubes that I had accumulated but in the end I had boxed and labled
18 well matched pairs which I now rotate into and out of service
every few months. I autopsied the duds and kept notes on what I found.
I also made notes on the good ones and track their performance.

    Here are some of my findings from the process:

    If the tube exhibits any positive screen current at idle it is dead
    or will be shortly and may take something else with it. Toss it.

    Slight negative screen current is ok and can be made less with
    additional burning if it makes it to the high anode voltage burn.

    Physical appearance is meaningless. I had one tube that was "blued"
    from overtemp and the cooler stuffed with rat hair but in the end
    became one of my best tubes.

    Watch out for undersize pins. I have a few (burle) and they would
    not make contact even in a new socket. Tinning with silver solder
    will solve the problem.

    If the plate relay trips upon application and the screen current meter
    slams up against the stops don't give up. It may just be one of those
    gas atoms exploding in a vacuum or something like that but it ain't
    necessarily the end of the tube or the meter. Burn the filament for
    another few hours and try again.

    My personal twist on the process is to divide the initial filament burn
into 2 or more sessions allowing the tube to cool in between. I figure the
temp change may stir things up inside the tube.

Good luck,  Mike k1ern




----- Original Message ----- 
From: "GGLL" <nagato at arnet.com.ar>
To: "Amplificadores Lineales" <amps at contesting.com>
Sent: Tuesday, July 12, 2005 7:02 PM
Subject: Re: [Amps] Tetrodes


Please, let me jump in here, with my usual question: for my two, nos RCA 
8122
(they have been stored many years) that I plan to put in use in a friend's 
NCL
2000 linear, will suffice to turn on heaters only, during say, 24 Hs?, or I
need also to make them work at low plate voltage after that 24 Hs. 
preheating?

TIA
Guillermo - LU8EYW.