I second Jim's advice. I know from personal experience how a "simple
tube swap" can turn into the creeping-spend project from hell. I think
I have the only $3K SB200 on earth - that sits on the shelf now
collecting dust as it's too unreliable and I've got tired of working on it.
Swap a decent tube in (the 400 are easier to find cheap), fix the few
problems back to OEM and be done. The AL80 is a good amp overall.
Worth the restoration.
In any event, good luck! If nothing else, it's a great learning
experience although given your inventory of tubes, maybe you are not
hearing anything you don't already know.
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
On 10/29/2024 8:40 PM, jim.thom jim.thom@telus.net wrote:
Date: Tue, 29 Oct 2024 12:56:46 -0400 (EDT)
From: Paul Decker <kg7hf@comcast.net>
To: amps@contesting.com, amps-request@contesting.com
Subject: [Amps] 3-500z substitutes
<Hey gang.
<I'm the proud owner of an AL-80A that's an absolute basket case. Tube is
gone, HV <section looks like it needs a rebuild.
<My "in stock tubes are: Qty 6 - GI-46B, Qty 6 - 3cx800A7 and U7, Qty 6 -
3cx400A7 and <U7, Qty 2 -TH308, Qty 2 -TH328, Qty 2 - 8791, Qty 4 - 7213,
Qty 2 - 7650, Qty 2 - <4cx1500B, (4cx350/250/150, 2c39, some other power
tubes I don't really know) and <sockets for all except the GI-46B and
4cx1500B.
<The goal is "fix'er up" with buying as little as possible. I'm leaning
towards the GI-46B <and the other triodes since there is no need for any
extra screen bias voltages or <control. The pair of 3cx400U7's (or 800's)
would fit in real nice too, except it then <requires some kind of forced
air cooling mechanism.
<It looks like it's using a voltage doubler for a no load voltage of 3000V
and loaded <voltage of 2700V. I could pull a few windings off the
transformer to lower it down a bit for <the 3cx400/800 tubes, or just run
at 2700.
<What do you all think would be the best fixer up route?
<Paul.
Just replace the 3-500Z...done. You can also stuff a 3-400Z / 8163 in
there...and also a
4-400A/B/C...and also a 4-250A.... + a couple more.
IF you swap to a different tube type, you will also have to add a separate
fil xfmr.....which won't fit inside the AL-80A. The xfmr used in the
AL-80A/B contains the HV and also the filament winding + the low vac for
the relay supply etc.
The input Z of your proposed alternate tubes will be different =
re-designing ALL the tuned inputs from scratch..which will take a long
time. The 3CX-400/ 800 tube will require grid current overload
protection, and also a 3 min timer for the fil. Alternative tubes will
require replacing the existing fan with a squirrel cage blower....which
will not fit. Then toss in the socket, and chimney.
If you want to use alternate tubes, any existing issues with the AL-80A
will have to be fixed 1st.
So you may as well just fix / repair any existing issues 1st...then install
a new 3-500Z..and be done with it.
As others have stated, sell off one or more of your existing tube
stock....and buy a new 3-500Z. Buy it from W8JI at CTR engineering, then
you know it's good....and tested 6 ways to sunday. Tom hi pot tests em,
plus emission tests, plus measures the tube C's, etc.
IF you try and use an alternative tube....... you will be in for a rude
awakening...spend a lot of money, and months of your time. You will have
'saved'..... nothing.
Jim VE7RF
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