[Amps] [AMPS] Drake L-4 negative grid current reading on meter

[Steve Scoggins]

Hello Kevin

A common failure in 3-500Z tubes is a filament to grid short which may be 
intermittent.
This type of failure may be detected by the incidence of negative grid 
current (usually some plate current) during standby. Observing the safety 
procedures in your manual. Remove the tubes then use a VOM to check the 
tubes for grid to filament shorts.

Good tubes will have infinite resistance from each of the grid pins to the 
filament pins.
You can tap the tube lightly while doing the check for intermittent shorts.
The tube check procedure described above is from the Drake L-7 manual in 
section 5-8.

Also, check the 120V bias resistor labeled R11.
This is the 5K 7Watt power resistor located on the bottom of the PS.

The 120Vdc is switched in via Pin 5 of the RF Deck connector
when not transmitting in order to reduce the idle currents in the 3-500Z.
Pin 6 is the negative return for the plate HV and the 120Vdc bias.

Steve

----- Original Message ----- 
From: "Kevin G. Wagner" <w2fa at comcast.net>
To: <amps at contesting.com>
Sent: Wednesday, November 21, 2007 10:52 AM
Subject: [Amps] [AMPS] Drake L-4 negative grid current reading on meter


> All,
>
>    I'm having a problem with my Drake L-4 amplifier. A few weeks ago
> the .082 resister blew because one of the cap's let go in the power
> supply. I replaced all the cap's, diodes and the .082 resister in the
> power supply and whet to check out the amp. When I turned it on I
> have a negative grid current reading on the meter. It slams to the
> left at rest and when I key the amp it reads zero. I have purchased
> new tubes (3-500Z's) and I still have the problem. I then tried to
> calibrate the grid meter using the procedure in the Drake manual with
> no luck. I'm thinking it's a meter problem.
> Please help.
>
> 73,
> Kevin
> w2fa
>
>
>
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