[Amps] 3-500z substitutes

[Jeff Blaine]

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 at telus.net wrote:
> Date: Tue, 29 Oct 2024 12:56:46 -0400 (EDT)
> From: Paul Decker <kg7hf at comcast.net>
> To: amps at contesting.com, amps-request at 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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