[NJARC] Tubes for clinic

[John Ruccolo]

Hi Darren,

I beg to differ with your thiughts regarding bias an B+.

There's really little or no current draw from the bias supply, so the bias voltage should theoretically be the same, even with *no* output tubes plugged in. (But I agree that the selenium bias rectifier has to go!).

As for the plate voltage, it's not gonna "soar" with two output tubes pulled, although it may go a bit higher, of course. I think the regulation in the PS is decent enough that the voltage is not gonna spike.

I worked on a nice Sherwood years ago that had a blown 7868, a tube essentially identical to the 7591 except for its novar base. After recapping, I tested one channel at a time with two tubes with no problems.

Your idea regarding those cathode resistors is very good though.

Sorry you can't make it == how's that National tuner doing?

JR

--- On Fri, 2/19/10, AMCMATADOR at aol.com <AMCMATADOR at aol.com> wrote:

> From: AMCMATADOR at aol.com <AMCMATADOR at aol.com>
> Subject: Re: [NJARC] Tubes for clinic
> To: njarc at mailman.qth.net
> Date: Friday, February 19, 2010, 4:58 PM
> Just remember 
> Reply = Poster
> Reply All = Everyone
> 
> _________________________________________________________
> 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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