[NJARC] Tubes for clinic
John Ruccolo
jr6v6gt at yahoo.com
Fri Feb 19 22:46:26 EST 2010
Sal,
That's fine. I'm bringing four 8-ohm 20-watt non-inductive resistors if you would like to use them as "dummy loads." That way nobody gets their ears assaulted during initial testing. Once it's up and running we'll all be anxious to hear it through the speakers.
JR
--- On Fri, 2/19/10, salb203 at aol.com <salb203 at aol.com> wrote:
> From: salb203 at aol.com <salb203 at aol.com>
> Subject: Re: [NJARC] Tubes for clinic
> To: al at ar88.net, njarc at mailman.qth.net
> Date: Friday, February 19, 2010, 10:09 PM
> Just remember
> Reply = Poster
> Reply All = Everyone
>
> _________________________________________________________
>
> 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
> 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
> 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
> > To: 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
> 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
> >>>
> >> >
> ______________________________________________________________
> >> > NJARC
> mailing list
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> >> >
> >>
> >>
> >>
> >>
> >>
> >
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> --
> Al Klase - N3FRQ
> Jersey City, NJ
> http://www.skywaves.ar88.net/
>
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