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Re: [Amps] AL-811H conversion ?

To: "Hugh VA3TO" <>, <>
Subject: Re: [Amps] AL-811H conversion ?
From: "Tom W8JI" <>
Date: Sun, 15 Jul 2007 17:14:22 -0400
List-post: <>
>I have the opportunity to aquire an Ameritron AL-811H that 
>needs a new
> transformer for the HVPS.
> I could get a replacement xformer but before I do that, 
> I'm wondering if
> I could use it as a basis to something else with.  What 
> I'd *really*
> like to do is make it into a 6m amp but as I understand it 
> the 811 tubes
> don't work well up to 6m. Could I change the amp to run a 
> single 3-500z
> or is the power supply not big enough ?


Having spent many hours looking at 881's and such, I'm going 
to disagree with some responses.

The 811A is a terrible tube, as is the 572B, on six meters. 
They are unstable enough on 30MHz, let alone six.

Now I'm not saying through some luck and twiddling you can't 
make one work. On a bet with Denny Had of Dentron I made 
four 6LQ6 sweep tubes work on six meters, but it isn't a 
GOOD design and it was not as stable or reliable in 
production as it should have been. It was unreliable in 
production and field use.

The problem with the 811 and 572 is the long skinny grid 
wire that comes down to one pin. This causes isolation 
between the filament and anode to be poor. This is why 
Heathkit, Gonset, and Ameritron neutralize the 811A's, and 
why the 811A Collins that does NOT neutralize has all sorts 
of stability issues on 15 and 10.

The same is true for 572B's, two tubes are about the limit 
in stability before you have to neutralize.

Of course you can add intentional loss to gobble up power 
and avoid neutralizing but it's probably better, if 
possible, to neutralize.

My advice is unless you are used to twiddling and cutting 
and trying, to just use a stable tube.

You might consider a swap to a 4CX250B, or even two or three 
4CX250 tubes. They would run OK at 1500 volts and they would 
be very stable if you built the amp right. Other VHF tubes 
would work also, the higher the usable frequency the more 
stable and easier to work with the become.

If it was my amp, I'd get a transformer and repair it. I'd 
put the correct line fuses in the amp and sell it, or use it 
myself. I'd build a new six meter amp out of a tube that is 
reliable and good on six, not a tube that tends to oscillate 
near 100MHz.

73 Tom 

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