[Amps] Testing 8874s

[Van K7VS]

Karls advice is very close to how Alpha recommends checking individual 
tubes.  They suggest one at a time and simply apply a small amount of drive 
10-20 watts into a single tube and compare with a good one.  One thing that 
Karl failed to mention is that you should stuff a rag of some type in the 
two emply tube socket holes so you have plenty of air flow around the tube 
you are testing.  Van, K7VS
----- Original Message ----- 
From: "jeremy-ca" <km1h at jeremy.mv.com>
To: "Joe Staples" <w5asp at earthlink.net>; <amps at contesting.com>
Sent: Thursday, November 01, 2007 5:09 PM
Subject: Re: [Amps] Testing 8874s


> First step is to check for hard shorts with an Ohmeter.
>
> I would then test them as a single tube amp on 40M (just a personal
> preference for testing one tube in a multi tube amp).
> Install fuses 1/3rd the value. Even tho there is a HV current limiter
> circuit there is no need to take chances.
>
> After the warm up delay is over very carefully turn on the HV and key the
> amp thru the normal Relay jack, NOT QSK. NO drive at this point but you
> should have idle current on the meter. If nothing nasty occurs insert 
> about
> 10W drive and tune for maximum output. Then slowly bring up the drive to 
> 30W
> while retuning and keeping the grid current in spec; 20ma is more than
> enough for evaluation. A good tube should produce 500-550W output.
>
> Many used 8874's come from VHF/UHF commercial service and are pretty close
> to dead by the time they are replaced. Shorted tubes usually come from 
> hams
> who have problems with overdriving and mistuning.
>
> Carl
> KM1H
>
>
>
> ----- Original Message ----- 
> From: "Joe Staples" <w5asp at earthlink.net>
> To: <amps at contesting.com>
> Sent: Thursday, November 01, 2007 3:28 PM
> Subject: [Amps] Testing 8874s
>
>
>> We have an Alpha 78 (3 holer) which has a bad (shorted) tube and need a
>> replacement.  We have found several 8874 tubes of questionable origin and
>> unknown conditon.  What is the best way to if one or more of them is
>> usable as a  replacement ?
>>
>> We have access to another Alpha 78 which appears to have three good tubes
>> (usually 1400+ watts into a Bird on 40 M).  Would swappng one of its 
>> tubes
>> with an unknown and checking the effect on the power output be OK ?
>> (We're concerned that if an "unknown" tube is shorted it will crater the
>> Alpha.)  Another alternative would be to remove all but one of the "good"
>> tubes, check the power out (with low drive) and then replace it with an
>> "unknown" ?
>>
>> Any suggestions would be appreciated.  Unfortunately the "sick" Alpha
>> lives in another country ... we need to known good replacement before
>> leaving.
>>
>> Thanks.
>>
>> Joe, W5ASP
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>
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