[NJARC] Tubes for clinic
John Ruccolo
jr6v6gt at yahoo.com
Sun Feb 21 10:49:54 EST 2010
Hi Jim,
Good point, but I was only tellling what it says on the resistors: 8 Ohm NON-IND.
I'm out of my league with all the highly mathematical engineering stuff, but I will speculate: a speaker VC is really a *solenoid* winding of sorts, (around the PM core), where a normal wire-wound power resistor is merely an air-wound coil with ceramic filling replacing the air. So, their characteristics must be a little different.
Perhaps there is a frequency range where an 8-ohm WW resistor will look like a dead short to the amplifier (at "resonance?"), instead of an 8-ohm load. I imagine that would be a problem, *especially* for a solid-state amp.
Regards,
John
--- On Sat, 2/20/10, Jim Whartenby <antqradio at sbcglobal.net> wrote:
> From: Jim Whartenby <antqradio at sbcglobal.net>
> Subject: Re: [NJARC] Tubes for clinic
> To: njarc at mailman.qth.net
> Date: Saturday, February 20, 2010, 9:56 AM
> Just remember
> Reply = Poster
> Reply All = Everyone
>
> _________________________________________________________
> Why does one even worry about non-inductive load resistors
> when testing audio amps? Isn't the voice coil of a speaker
> a pretty good inductor?
>
> I'm willing to wager a couple of big pin receiving tubes,
> NOS if I have them, that the VC inductance plus the speaker
> wire inductance provide a reactance close to the DC
> resistance of the speaker for any reasonable frequency,
> speaker power level and wire length. Or to put it another
> way, any wire wound power resistor is as good as the speaker
> it replaces. Any takers?
> Jim
>
>
>
> ----- Original Message ----
> From: John Ruccolo <jr6v6gt at yahoo.com>
> To: al at ar88.net; njarc at mailman.qth.net;
> salb203 at aol.com
> Sent: Fri, February 19, 2010 9:46:26 PM
> Subject: Re: [NJARC] Tubes for clinic
>
> Just remember
> Reply = Poster
> Reply All = Everyone
>
> _________________________________________________________
> 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
> > >>>
> > >> >
> >
> ______________________________________________________________
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> > >> >
> > >>
> > >>
> > >>
> > >>
> > >>
> > >
> >
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> > --
> > Al Klase - N3FRQ
> > Jersey City, NJ
> > http://www.skywaves.ar88.net/
> >
> >
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