[Johnson] Attenuator, connectors for Thunderbolt

[Mahlon Haunschild]

Just a few comments to Glen's message:

Glen Zook gzook@yahoo.com 
Tue, 27 May 2003 08:01:05 -0700 (PDT) 

<snip>

NEVER run a Thunderbolt (don't even turn it on!) without the bias plug
being inserted and an external relay connected properly to switch the
bias between standby and transmit.  Both modes require a bias, it is
just a different amount.

Oh man, is this ever important!  I alluded to this the other day, but to
elaborate, if you turn on an early Thunderbolt in "Linear" mode without
the proper bias control relay/wiring, the 4-400A tube grids will FLOAT,
the screens will draw current well in excess of the capacity of the
screen supply, the screen supply will become unhappy, and the tubes
might become unhappy (excessive screen dissipation).  This will happen
whether or not the HV is on.  This was fixed in the later Thunderbolt
with the addition of a 270K 1W resistor across the standby bias
(effectively cutting off the tubes in absence of any other bias
connection).  If you have two bias connectors, you might want to wire
one to a SPDT switch for testing purposes.  Check my posting to the list
last month for more details.

<snip>

As for the filter capacitors:  The oil filled seldom go bad.  But, if
they are the dry versions and if the
rig hasn't been used for a while, they are definitely suspect.  You may
have to change them out.

I've seen three Thunderbolts recently, and all of them needed to have
the bias & screen supply capacitors replaced.  All of them had blown
their seals.  Suggest that T-bolt owners replace these as a preventive
measure.  Again, see my posting to the list last month for further
details.
---
regards,

Mahlon - K4OQ