[Johnson] Invader 2000

Tue, 27 Apr 2004 18:38:26 -0700

I studied this aspect of the Invader 2000 quite closely at one time, and it
appears that the only modification necessary to use 4-400's in place of the
PL-175's is a change to the bias voltage.  In the Invader (and the later
Thunderbolt that used PL-175s) the bias voltage is -75, provided by an 0C2
regulator tube in the case of the Invader.  In the earlier Thunderbolt that
used 4-400's, the bias voltage is -90.  The Invader uses an 0C2, which is a
7-pin miniature tube, while the Thunderbolt with 4-400's uses the octal
VR-90. I don't know of a 90 volt regulator tube in a 7-pin miniature, so in
my Invader I simply cut the lead that grounds the plate of the 0C2 and
inserted a 15 volt, 1 Watt zener.  This provided the requisite -90 V,
assuming the OC2 is still good (my original 0C2 was regulating to a mere 68
volts).

Hope that this helps.

Scott
N7NB

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Kenneth
Sent: Tuesday, 27 April, 2004 06:27
To: George Shapow; [email protected]; Roy Morgan; Ed Wirtz
Subject: Re: [Johnson] Invader 2000

The below outfit has the 4-400's in several flavors, as for the price "call"
which means expensive.

http://www.rfparts.com/<http://www.rfparts.com/>

Kenneth A. Crips W7ITC, AMI 873
with Irish Terriers Chic' Beezie
and Cowboy. In this house Vacuum
Tubes still rule.

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