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Both tubes light...I have an "old" set of 572B's that also exhibit the same
symptom but they were old and I was getting suspect of how much life they had
left which is why I replaced them. They went out of service working fine,
though. I'll check the tubes with an ohmmeter to be sure.
W6NQ wrote:
> Hi Chuck:
> Previous comments have given you have a wealth of good info to explore.
> One thing not mentioned is the possibility that you might have a
> shorted or gassy tube at this time.
> Shorted tubes usually fail due to broken filament.
> You can check for this and possible direct grid-filament short with an
> Ohmmeter.
> A gassey tube you can't tell for sure without a hypot tester.
>
> Did both tubes light?
>
> If tube looks good--
>
> Replace the 33 Ohm resistor before further tests.
> Install one of your original tubes into the socket that lost the resistor.
> Also run the new tube that didn't loose it's resistor.
>
> See what happens.
> Then sub the second new tube in place of the the first new one.
> Any change?
>
> This might be clue-- i.e.:
> Does the one tube that oscillates (or could be shorted/gassey)
> carry it's problem along where ever it goes?
>
> Regards, Merit W6NQ
>
> ----- Original Message -----
> From: "Chuck Mills" <millscl+@pitt.edu>
> To: "Gary J. Ferdinand" <W2CS@bellsouth.net>
> Cc: <amps@contesting.com>
> Sent: Monday, March 04, 2002 6:36 PM
> Subject: Re: [Amps] SB-200 Problem
>
> > Yikes....lots of good info here. I replaced all of the blown components I
> could
> > find (RF Choke, the 33ohm from lug to ground) and threw the antenna switch
> ...
> > it still pegs and the 33 ohm resistor will blow again (or throw the
> breaker at
> > the panel...). This is on the bench with NO rf into it.
> >
> > Chuck
> >
> > "Gary J. Ferdinand" wrote:
> >
> > > > Heathkit SB-200 Built circa 1977. Currently in need of a minor
> rebuild
> > > > for maximum life which I plan to undertake (Harbach power supply, new
> > > > antenna relay, replace some ancient caps, etc...). Currently has new
> > > > 572B's with < 5 hours operating time on them.
> > > >
> > > > Symptom:
> > > >
> > > > While transmitting a while back, I let the smoke out of a resistor
> that
> > > > is part of the RF choke on the anode clip (one of the 80 5w wirewound
> > > > guys with the coil wound around it). Obtained new resistor and
> soldered
> > > > it in.
> > > >
> > > > Proceeded to put amp back in service. Upon hooking it up, found a
> clear
> > > > frequency and got ready to transmit. As soon as the antenna relay
> > > > clicks shut, the plate and grid current peg and another resistor self
> > > > destructs. This one is off of lug 2 of V2 to ground (right most tube
> > > > socket when looking at the back of it).
> > >
> > > Chuck,
> > >
> > > Were you feeding power to the amp when it pegged or did you just throw
> it
> > > into transmit position but without the exciter putting any power into
> it?
> > >
> > > This sounds like a classic amp-taking-off parasitic problem. It would be
> > > interesting to note if it were happening by its lonesome or whether you
> fed
> > > 100W into it when it happened. It would also be interesting to know
> what
> > > band it was being tuned to.
> > >
> > > Others here are far more expert than I, but if I were you (and ONLY if
> the
> > > amp misbehaved when feeding power to it), I would feed a very small
> amount
> > > of power to it (barely cause the plate current to go above resting Ip)
> and
> > > determine if it is possible to find a point where it is resonant. Also
> > > determine whether it resonates on ANY band. One failure mode of the
> SB200
> > > is the bandswitch wafer. Another are tank circuit connections (solder
> joint
> > > failures). If the amp is not seeing a 50 ohm load (because of a screwed
> up
> > > tank circuit due to either of the above), all bets are off as to
> > > whether/when it remains stable.
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
--
==============================================
Charles L. Mills, CCNP
Network Analyst
University of Pittsburgh Network Services
3943 O'Hara St.
Pittsburgh, PA 15260
Voice: (412) 624-6030 Fax: (412) 624-4774
==============================================
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