[Amps] Replacing 811A with 572B

[Joe Subich, W4TV]

> 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.

If the AL-811A plate voltage runs that high it is exceeding the 
"Maximum Plate Voltage" rating for the 811A tube and is a bad 
design.  The RCA data sheet calls for 1500 V maximum in ICAS 
service and I would not want to push an 811A harder than that.
 
Unless the newer tubes are designed for higher plate voltages 
the 572B would be a safer choice.  However, I don't see any 
really reliable 572B.  I would certainly would not use a generic 
$39 Chinese 572B. 

73, 

   ... Joe, W4TV 
    


> -----Original Message-----
> From: Scott Manthe [mailto:scott.manthe at gmail.com] On Behalf 
> Of Scott Manthe
> Sent: Wednesday, April 09, 2008 10:08 PM
> To: Joe Subich, W4TV; Amps at contesting.com
> Subject: Re: [Amps] Replacing 811A with 572B
> 
> 
> 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 
> >  
> >
> >   
> >>