[NJARC] Tubes for clinic
salb203 at aol.com
salb203 at aol.com
Fri Feb 19 22:07:12 EST 2010
Al,
If this 500C is from the public, I don't think we should go nuts modifying the bias circuit, adding cathode resistors etc.
BTW 10 ohms 1 percent is a great value as you can easily measure the current through each output tube plus if you use 1/4 watt, it will act as a fuse in case a tube fails and not take the audio transformer with it. You can also add a 500K pot to balance the current through the push-pull pair but that is another issue. I have done that on Dynaco ST-70's.
We should just check it out, replace the coupling capacitors and maybe some electrolytics and troubleshoot what is wrong with it. If the owner want a a complete rebuild, the clinic is not the place to do it.
Regards,
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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