Why not inexpensive GI-7B?only needs add a 6V filament transformer.
Hsu
> ----- Original Message -----
> From: "Tom W8JI" <w8ji@w8ji.com>
> To: "Hugh VA3TO" <hduff@cogeco.ca>; <amps@contesting.com>
> Sent: Monday, July 16, 2007 5:14 AM
> Subject: Re: [Amps] AL-811H conversion ?
>
>
>>
>>>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 ?
>>
>> Hugh,
>>
>> 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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