[NJARC] Tubes for clinic
AMCMATADOR at aol.com
AMCMATADOR at aol.com
Fri Feb 19 20:03:20 EST 2010
The way to adjust the bias in the 500C is to change the resistor in the
bias supply circuit that goes to ground. I dont have a schematic in front of
me, so I cant tell you the specific resistor number. If you look at the
circuit, you will see that it shunts the voltage to ground, I adjust the value
of it, according to the current I read on the cathodes, I feel that's a
better route, than adjusting for voltage. Also, when you put the output tubes
in, if you add those cathode resistors, you can measure between the 2 tubes
in each pair, and mix and match them for best match. In theory , that will
help lower the IM distortion, as well as help your tubes last longer.
Darren
In a message dated 2/19/2010 7:50:12 P.M. Eastern Standard Time,
salb203 at aol.com writes:
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Darren,
I mentioned that to John in a email yesterday.No bias adjustment in that
receiver. I have a 500C I am restoring for a friend, I'll bring the quad
from that amp (as soon as I test them) with me tomorrow.
To bad you can't make it tomorrow... By any chance, does that robot have
any vacuum tubes in it...
Sal
-----Original Message-----
From: AMCMATADOR at aol.com
To: njarc at mailman.qth.net
Sent: Fri, Feb 19, 2010 4:58 pm
Subject: Re: [NJARC] Tubes for clinic
Just remember
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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:
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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
> > ______________________________________________________________
> > NJARC mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/njarc
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:NJARC at mailman.qth.net
> >
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> >
>
>
>
>
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