[NJARC] Tubes for clinic

[AMCMATADOR at aol.com]

Hi All,
 
     Just a note on running a 500C minus 2 tubes. By  removing 2 of the 
output tubes, there will be an excess of B+ as well as an  excess of bias 
voltage. The combination of the 2, will make it difficult, to  really know if the 
set is functioning correctaly.  The biggest problem,  with the 500C , is 
that as it shipped from the factory, the 7591s are running  far hotter than 
they need to be for minimal distortion. Add to that, a  nearly 50 year old 
selenium rectifier, and you have a great recipe for roasted  7591. That's why, 
many 500Cs are found with charred 7591 tube bases! If the  set is going to 
see any real use, I would suggest, replacing that selenium with  a silicon 
rectifier, and biasing those tubes somewhere in the 28-30 milliamp  current 
per tube range.(you can adjust the value of the resistor that goes to  ground 
in the bias supply).  I like to replace the cathode grounding link  on the 
7591s with a 10 ohm half watt resistor. That gives you an easy way to  
measure tube current, (measure the voltage across that resistor and use ohms  law) 
and an added means of protection. 7591s are known to short from time to  
time. A shorted tube can easily take out a power or output trans. With a 1/2  
Watt resistor on the cathode, it will pop (like a fuse) and protect a more  
costly transformer . Finally I would change the 4 .047 output tube coupling  
caps, they often go bad.  I really wish I could be there on Saturday, this  
would be fun, but I unfortunately am under the gun at work, to have a robot 
 completed , which means I must work Saturday:( 
 
    Hope this is of some help
 
                Darren 
 
 
In a message dated 2/19/2010 4:27:58 P.M. Eastern Standard Time,  
philvourtsis at gmail.com writes:

Just  remember 
Reply = Poster
Reply All =  Everyone

_________________________________________________________
That's  right, we can check one channel at a time. I've been working on too
many  crappy series string sets lately.

Phil V.

On Thu, Feb 18, 2010  at 7:10 PM, John Ruccolo <jr6v6gt at yahoo.com> wrote:

> We can  still test one channel at a time, at the very least.
>
> 7591's  *are* expensive!
>
> One of my secret pending projects is a  substitution for the 7591's in my
> own Fisher 500.
>
> ---  On Thu, 2/18/10, Phil Vourtsis <philvourtsis at gmail.com>  wrote:
>
> > From: Phil Vourtsis  <philvourtsis at gmail.com>
> > Subject: [NJARC] Tubes for  clinic
> > To: "NJARC Reflector"  <njarc at mailman.qth.net>
> > Date: Thursday, February 18, 2010,  3:39 PM
> > Just remember
> > Reply = Poster
> >  Reply All = Everyone
> >
> >  _________________________________________________________
>  >  There is a fellow who wants to have a Fisher 500C checked
> > out on  Saturday.
> > Unfortunately he broke two ofthe output tubes. I told  him
> > we may not have
> > those tubes available for sale,  but I would check here
> > first. Does anyone
> > coming to  the clinic saturday have a couple of these to
> > sell or borrow so  we
> > can test out the amp? He knows they are expensive and  would
> > be willing to
> > buy them.
> >
>  > Phil V
> > Expres- NJARC 1998-2008
>  >  ______________________________________________________________
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