[Amps] Testing 8874s

[jeremy-ca]

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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