[AMPS] Trouble shooting 8877 amp

[Ian White, G3SEK]

G0MJW wrote:
>
>> When fired up and keyed, the amp draws no plate current, and at the
>> slightest hint of RF the grid current climbs and quickly trips out.  Our
>> first thought was the HV had gone away, but it is still there.  I didn't
>> see it, but was told there is a slight twitch or rise in the plate current
>> as the grid shoots up.
>
>Maybe the HV has gone away after all. It could be that there is some high
>resistance in the circuit so as soon as you try and draw any EHT the volts
>fall rapidly. A burnt out secondary might achieve this. Also check the EHT
>is getting to the valve - I assume you measured it on the anode and not at
>the PSU.

 It certainly sounds like there's no HV getting to the anode. Possibly
the RF choke could be burnt out, or something else between the HV meter
connection and the tube itself.

> Ians' (G3SEK) Triode board has a check for this sort of thing.
>
Not right at the anode - the best you can do is to connect the HV
sensing divider as close as possible. As in Steve's amp, the grid
current trip is there as a backup to the HV sensing. Also there has to
be some way to remove the RF drive when a grid current fault is detected
(usually by switching the amp into bypass mode).


73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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