[Amps] Replacing 811A with 572B

[Paul M Dunphy]

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