[Amps] Replacing 811A with 572B

[Scott Manthe]

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


Joe Subich, W4TV wrote:
> Why waste the money ... four 572Bs are half again to twice the 
> price of four 811s.  The 572B will not provide any more gain or 
> power output than the 811 unless you modify the power supply for 
> about 40% more voltage (2 KV vs. 1400 V).  
>
> The 572B is taller and wider which might represent a problem 
> (moving the plate caps too close to the metal case or overly 
> restricting air flow between the tubes).    
>
> If you are really concerned about reliability pick one of the 
> better 811 suppliers - perhaps Marshall (made in the USA) - 
> but I would not waste the money and effort on imported 572Bs. 
>
> 73, 
>
>    ... Joe, W4TV 
>  
>
>   
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