[NJARC] Tubes for clinic
AMCMATADOR at aol.com
AMCMATADOR at aol.com
Fri Feb 19 23:43:55 EST 2010
Al,
I shoot for approx 30ma per tube. The orig. Fisher Spec calls for 80MA
per pair. At 40MA those tubes are running VERY hot, and dont last too
long. At 30, you get full rated power ,and low distortion, and the tubes last a
VERY long time! I put one cathode resistor on each tube, that way, not
only do they act as fuses, but they allow you to measure the current at each
tube, and then, match the tubes , so each pair, is closest in current draw.
When I restore a set, I actually select closely matching tubes, but that's
not really necessary, as long as they are close. As for the phase inverter,
Fisher calls for an IM distortion analyzer. I think I used to do it that
way. I think its a better method to use a 1000 CPS tone, and a scope and load
resistor. I then adjust the phase inverter for the cleanest clip and most
balanced sine wave. I cant count how many I have done this way, and had
very good results.
Darren
In a message dated 2/19/2010 8:43:53 P.M. Eastern Standard Time,
al at ar88.net writes:
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_________________________________________________________
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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