[NJARC] Tubes for clinic

Al Klase al at ar88.net
Fri Feb 19 20:43:01 EST 2010 

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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