[NJARC] Tubes for clinic
Al Klase
al at ar88.net
Sat Feb 20 00:03:54 EST 2010
Don't go out of your way. We have a good sounding pair of modern
speakers in house that I was planning on bringing out.
Al
On 2/19/2010 10:09 PM, salb203 at aol.com wrote:
> I'll also bring a small pair of speakers for the Fisher 500C...
>
> Sal
>
> -----Original Message-----
> From: Al Klase <al at ar88.net>
> To: njarc at mailman.qth.net
> Sent: Fri, Feb 19, 2010 8:43 pm
> Subject: Re: [NJARC] Tubes for clinic
>
> Just remember
>
> Reply = Poster
>
> Reply All = Everyone
>
>
>
> _________________________________________________________
>
> Sal,
>
>
>
> Glad to hear you can bring some 7591's. I share Darren's concerns. Do
>
> you want to handle this customer?
>
>
>
> Darren,
>
>
>
> What's your approximate target for zero-signal plate current? I think
>
> you're saying 28-30 mA per tube. The way I read the data Sylnania data
>
> sheets they want you to run 60-75 mA. I like your 10-ohm stunt. Do I
>
> understand correctly that that's one resistor per tube, rather than
>
> shared for the pair.
>
>
>
> Do you have any advise on adjusting the phase inverters?
>
>
>
> Al
>
>
>
>
>
> On 2/19/2010 8:03 PM,AMCMATADOR at aol.com <mailto:AMCMATADOR at aol.com> wrote:
>
> > Just remember
>
> > Reply = Poster
>
> > Reply All = Everyone
>
> >
>
> > _________________________________________________________
>
> > 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 <mailto:salb203 at aol.com> writes:
>
> >
>
> > Just remember
>
> > Reply = Poster
>
> > Reply All = Everyone
>
> >
>
> > _________________________________________________________
>
> >
>
> > 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 <mailto:AMCMATADOR at aol.com>
>
> > To:njarc at mailman.qth.net <mailto:njarc at mailman.qth.net>
>
> > Sent: Fri, Feb 19, 2010 4:58 pm
>
> > Subject: Re: [NJARC] Tubes for clinic
>
> >
>
> >
>
> > 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 <mailto: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 <mailto: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 <mailto:philvourtsis at gmail.com>> wrote:
>
> >>
>
> >>
>
> >>> From: Phil Vourtsis<philvourtsis at gmail.com <mailto:philvourtsis at gmail.com>>
>
> >>> Subject: [NJARC] Tubes for clinic
>
> >>> To: "NJARC Reflector"<njarc at mailman.qth.net <mailto: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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> >>
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> >>
>
> >>
>
> >>
>
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> --
>
> Al Klase - N3FRQ
>
> Jersey City, NJ
>
> http://www.skywaves.ar88.net/
>
>
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--
Al Klase - N3FRQ Jersey City, NJ http://www.skywaves.ar88.net/
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