----- Original Message -----
From: "Bill Fuqua" <wlfuqu00@uky.edu>
To: "Paul M Dunphy" <amplifier@ve1dx.net>; "AMPS" <amps@contesting.com>
Sent: Wednesday, April 09, 2008 11:34 PM
Subject: Re: [Amps] Replacing 811A with 572B
> Speaking of that. I belive the Johnson 500 ran about 2kV on the plate of
> the
> 4-400A and the same on the 811A modulators. Now that is pushing it.
True but when driven by a single ended Class A 6B4 they werent run out of
their dissipation. Since Johnson was a large and reputable company I would
have to assume that they received RCA's blessing to run them at 2KV. Just as
Collins did for the 30L1 in SSB service since the RCA spec sheet Im looking
at has a 1963 revision date.
Carl
KM1H
>
> 73
> Bill wa4lav
>
>
> At 10:47 PM 4/9/2008 -0400, you wrote:
>>At 11:07 PM 4/9/2008, you wrote:
>> >The AL-811H runs about 1600-1800 volts HV, not 1400, which is why it's
>> >not a bad idea to run the 572Bs in that amp.
>> >
>> >While the cost difference seems substantial, you'll probably never have
>> >to replace a 572B, imported or otherwise, when run in an AL-811H. RF
>> >Parts lists the Marshall 811A at $29.45 each and Chinese 572Bs at $39
>> >each, so the price difference at this point is substantial, but not so
>> >much so, if you consider the increased life expectancy.
>> >
>> >The size difference between the two tubes does not present a problem in
>> >the AL-811H amp.
>> >
>> >73,
>> >Scott, N9AA
>>
>> Agreed. If one just considers the initial cost of the tubes,
>>the 572B alternative is more expensive. Given that you likely will
>>never have to change a 572B in an AL-811H (as opposed to Chinese
>>clone 811As that have a half life of 30 days), that alone justifies
>>it. We are not talking $1000 Eimac 3CX1500A7s here, but $20-$40
>>glass tubes. You absolutely cannot get any NEW glass tubes that are
>>made in NA. The best you might hope for are NOS 20+ year old
>>ones. All new glass tubes are made overseas, and they are of
>>inferior quality, even the 572Bs. The reason they last forever in an
>>811A application is because they are running at 2/3 their
>>capacity. It's not because they are better built, just taxed far less.
>>
>> Putting aside the cost difference of four tubes, the real issue
>>here is: Do you want to take the 19 screws out of the AL-811H cover
>>every month and change a tube or two? That's why I switched to
>>572Bs. You are in the middle of a contest, chasing DX, or just
>>rag-chewing and "BANG!" Get out the screwdriver and spend 30-45
>>minutes figuring out which one (or ones) blew, replace it, and then
>>get back to whatever you were doing. Or put 572Bs in and forget them.
>>
>>73/DX Paul VE1DX
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>
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