[Johnson] Invader 2000

Wed, 28 Apr 2004 18:26:01 -0700

Mahlon-You are correct, I did inadvertently swap the two.  Which begs the
question, why did Johnson use a -75 volt bias in the Invader 2000 (using
PL-175's) and -90 volt bias in the Thunderbolt using the same tubes?
According to the voltage tables in the manual, it looks like the 4-400
version of the Thunderbolt employed element voltages that were almost
identical to the Invader with PL-175s, while the Thunderbolt with PL-175's
used a lower screen voltage and higher grid bias than the Invader.

Scott
N7NB

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Mahlon Haunschild
Sent: Wednesday, 28 April, 2004 04:10
To: [email protected]
Subject: [Johnson] Re: Invader 2000

SCott, I'm not surprised that the bias voltage is the only change in the
Invader 2000, but I would be surprised if the tube swap was the only
circuit change.  Also, I believe you've got your bias voltaged flipped.
  I have both Thunderbolt manuals, and I own a 4-400A Thunderbolt plus a
PL-175A Thunderbolt carcass.  I'm not familiar with the Invader 2000, so
all I can speak to is what's in the Thunderbolt:

In a PL-175A thunderbolt, the regulator tube is a VR90 and the dropping
resistor is 5.6K 2 watt.

In a 4-400A Thunderbolt, the regulator tube is a VR75 and the dropping
resistor is 2K 10 watt.

So, unless the transformer voltage changed, you may need to change the
dropping resistor too.

regards,

Mahlon - K4OQ

> --__--__--
>
> Message: 7
> From: "Scott Prather" <[email protected]>
> To: "Kenneth" <[email protected]>,
> 	"George Shapow" <[email protected]>,
> 	<[email protected]>, "Roy Morgan" <[email protected]>,
> 	"Ed Wirtz" <[email protected]>
> Subject: RE: [Johnson] Invader 2000
> Date: 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

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