[Johnson] Thunderbolt question: Matched finals needed?

[Mahlon Haunschild]

Steve,

I see that others have answered your basic questions.  I'd be surprised
if Johnson went to the expense and trouble of using matched pairs of
4-400As in the Thunderbolt; it just doesn't fit in with their
"Chevrolet" design philosophy (as opposed to Collins and one or two
other contemproary manufacturers).  You're safe to replace just the one
tube, IMHO.  I have a used one that I can sell you inexpensively, if you
wish.

Before you do anything else, are you sure that your particular T-bolt
uses 4-400As?   Check the following pointers:

-  8 uF oil capacitor used for HV filter
-  Bleeder resistors behind the fan are 4K
-  V109 is a VR75/0A3

Then you have the early version of the T-bolt, which uses 4-400As.

If, however, you have:

-  Stack of six 80 uF electrolytic capacitors used for HV filter
-  Bleeder resistors behind the fan are 5K
-  V109 is a VR90/0B3

then you have the later version, which used PL-175s.  It can be
converted to use 4-400As by changing V109 and a couple of resistors
(R116 and R118, specifically).

I strongly recommend that you replace all three of the 30 uF 45OV
electrolytic capacitors in the screen and bias supplies with equivalent
Sprague Atoms.  These capacitors always run hot, and they were never
very good to begin with.

See the recent archives for some additional comments from me regarding
the Thunderbolt.  

-- 
regards,

Mahlon - K4OQ

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> Message: 1
> Date: Thu, 12 Jun 2003 19:22:33 -0700 (PDT)
> From: Stephen Victor <victorsj@yahoo.com>
> To: johnson@mailman.qth.net
> Subject: [Johnson] Thunderbolt question:  Matched finals needed?
> 
> Hi
> My Thunderbolt arrived, unfortunately with one broken
> 4-400A (plate cap broken off).  Can I replace just
> that one, or is a "matched pair" needed?  Does anyone
> know where I can get a 4-400A?
> Also, does anyone know where I can get the 4-pronged
> bias plug to fit the socket in the back of the
> chassis?
> Many thanks
> 73
> Steve Victor WA2DTW