Testing for grid current on sb-200

[Mark Graalman WB8JKR]

> I am still unclear the safe way to test for grid current on the
> SB-200
> that I am working on.  For those that have not read my threads, this
> amp
> lost its grid current all of a sudden. I suspect a component went
> bad as
> tubes are new Taylor 572b's.

  The DC path back to the cathode for the grid is through R21/22,
RFC2, R15 and the meter, R16 and then to ground.
 With R15 being the shunt for the grid metering.
 When you key the amp with NO RF drive do you get about
90 ma. of plate idle current?

> So in checking, I cannot find D18 as recommened to check by a member
> here.  The amp came with all Harbach mods so was D18 removed when
> soft
> key went in?

  D18 is the ALC diode and it has or had nothing to do with grid
current.

> I get 1.077 volts dc at the amp relay jack.  This was described as
> grid
> bias also?

  I have no idea what the Harbach mods are, but in a stock SB200
you'd have about -100 volts at the relay key jack.
 Which is also the cutoff bias voltage when the amp isn't being
keyed.

> Please tell me the safe (realtively) place to check the grid current
> and
> bias voltage.  At the base of the tube at the ground to grid pin?
> Is

  With the amp keyed but NO DRIVE you should measure about
-2 volts or so at the junction of R21 and R22, When the amp is
NOT keyed, you should measure about -100 volts or so, but with
the Harbach mods, hard telling.

  BE VERY VERY CAREFULL!!!!!! The 2400 vdc in the plate supply of
this unit will kill you quickly!

 Without knowing what the harbach mods are, its difficult to give
accurate advise as to what to check, if I wre you, I'd take out
the Harbach mods, put the amp back to stock and use a small
5v dip relay to key the amp. These amps worked for 25 years without
mods and it takes nothing more than a relay to remove the high
switching voltage from your rig.

  73,  Mark  WB8JKR

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