SB400 - Low Grid Drive on 40 Meters

[Mark Johnson]

Hi Ken, thanks for your input on this problem. In tune, on 80, 20, and 15, I 
can get full scale, or better on the grid current reading. On ten meters, 
about 1/4 to half scale. On 40, depending on where I am in the band, I get 
zero or slight movement on the grid current. As mentioned, on all bands, I 
get good output, that is anywhere from 100 plus watts on 80 meters to 50 
plus on meters.

6EW6's are good and the rig seems to work OK....

She's a bit of a mystery to me Ken.

Mark V Johnson VE3DJU/VE3DDI
----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: <HEATH at LISTS.TEMPE.GOV>; "Mark Johnson" <mvjohn at sympatico.ca>
Sent: Thursday, August 13, 2009 2:54 PM
Subject: Re: [HEATH-TEMPE] SB400 - Low Grid Drive on 40 Meters


> Mark:
>
> I have been reading the mail here for some time concerning this
> problem.
>
> I have a question that I hope you don't mind answering:
>
> 1) In the TUNE condition, how much grid drive does your meter
> show on the other bands?
>
> (I am suspecting that any metering resistors included in the grid-
> current part of the circuit have gone bad.
>
> 2) I had a somewhat similar problem with my never-used SB-401
> some time ago. However, in MY case, I had no grid drive on 40
> meters at all.
>
> The problem was eventually traced to an internal short in one of the
> two 6EW6 mixer tubes, specifically, V-4. The symptoms were
> VERY odd, in that grid-drive and output on all other bands was
> absolutely correct, the HET oscillator output for 40 meters was the
> correct level and frequency, and everything else appeared to be
> completely correct. I simply had no 40 meter output.
>
> However, I DID have output at about 8.6 Mhz.
>
> At one point,I even removed the 8.6 Mhz trap and still had the
> problem.
>
> In your case, having approximately correct 40 meter output, I am
> suspecting the grid metering circuit is at fault.
>
> BTW, those 6EW6s are somewhat special in that they exhibit an
> unusually high transconductance. I understand from other sources
> that they are also somewhat "delicate".
>
> Ken Gordon W7EKB
> 

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