[Amps] Re: SB200 Grid Current...

[R.Measures]

>Roy, and the group: It's entirely possible the tubes are at fault  because
>they may be shorted only when the filament is hot (energized)  the only way
>to know is with known good tubes.!!

**  Or by measuring Vg-fil with a high-potential tester.  

>Also, you better check all the metering
>resistors, grid and plate, and also , as mentioned, the plate voltage, with
>the proper meter and ALL safety precautions.!!!I would bet that either the
>tubes are bad or you've had a parasitic osc. that killed the metering
>resistors. Check the continuity from the pwr. supply to the plates.  I'm not
>the ultimate expert on these amps but I have repaired a few.I think the
>SB200 is a very good amp, but let's face it, it needs to be up-dated as any
>amp from way back when needs to be.!!! there several things that need
>up-grading.
>     carl / kz5ca
>----- Original Message ----- 
>From: "Roy Koeppe" <royanjoy at ncn.net>
>To: "Amps Reflector" <amps at contesting.com>
>Sent: Wednesday, November 26, 2003 8:34 AM
>Subject: [Amps] Re: SB200 Grid Current...
>
>
>> Re:
>>
>> "Hi,
>> I hope someone can give me some hints on isolating this problem in a
>> SB200.
>> I don't know if there were any symptoms prior to the failure since I
>> just
>> got this amp.
>> The SB200 has excessive grid current and zero plate current. When I key
>> the
>> amp the grid current jumps up to more than 200 ma in the grid meter
>> position
>> (full scale +).  The plate should read about 90 ma but it is zero. I
>> then
>> pulled the 572b's and when keying the amp I get zero grid current
>> (except
>> for a slight bump of the meter reading while the relay operates). I also
>> got
>> zero plate current. High voltage looks OK.  I checked the tubes for
>> shorts
>> with an ohmmeter and they were OK.  Put the tubes back in one at a time
>> and
>> with either tube or both the amp reads the excessive grid current and no
>> plate current.  I checked the grid bias and when keyed it was -2 vdc as
>> it
>> should be.  The six 30 k ohm resistors in the PS run very hot and have
>> toasted the circuit board, but I guess that is "normal".  During this
>> testing I get almost no output from the amp.  With 10 watts drive I get
>> maybe one or two watts output.  I and able to peak the small amount of
>> output which make me thing the tank circuit is resonating correctly.
>> Thanks for your help,
>> Dan AF9R"
>>
>> Dan,
>>
>> Sounds like your HV is not reaching the plates. Check the plate RFC,
>> etc. Measure HV at the plate caps--but only if you have the proper
>> safety knowledge.
>>
>> Good luck,       Roy   K6XK
>>
>>
>>
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>
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