[Johnson] Thunderbolt Q & A

[Mahlon Haunschild]

Hello, list.  Just subscribed the other day (don't know why it took me
so long to do it).  Was browsing the archives and saw a couple of
Thunderbolt-related questions:

"I just picked up a Thunderbolt..... looks good.  What is the best way
to drive it with my Ranger?  Thanks from Ron VE4SR"

The best way is to build and use the 6dB swamping attenuator described
in the Thunderbolt manual.  Reason:  You have to tune/load the
Thunderbolt at full power, and then, for AM service, somehow back off
the drive by 6 dB to allow room for the modulation envelope.  The
attenuator allows you to do that with the turn of a switch.  Simply
backing off the drive control on the Ranger after tuning/loading the
Thunderbolt will degrade the modulation quality, as well as make the
6146 operate in a region where the plate tank Q is considerably
different from design.

"What is the reason for the bias connecting cable shown in the manual 
between the Ranger and the Thunderbolt?  Is it necessary for AM
operation?  Thanks.... Ron"

The bias control cable is VERY necessary with the Thunderbolt.  It is
connected to the SPDT contacts on the TR relay & switches the amplifier
grid bias from standby to operating.  NOTE:  If you have an older
Thunderbolt (made with 4-400As from the factory) you MUST have this
cable and the TR relay connected before you turn on the amplifier & put
it in "Linear" mode; else there will be NO BIAS on the grids, the
screens will then conduct heavy current (far more than the supply can
provide).  This makes the tubes unhappy, the screen supply unhappy, and
eventually the operator VERY unhappy.

To correct this, simply install R118, part of the design changes that
Johnson made to the later Thunderbolts:

1.  Secure one (ea.) type 270 K ohm, 1 watt resistor (I'm using a 2 watt
part because it is On Hand);

2.  Connect aforementioned resistor across contacts 5 and 9 of the Mode
switch (see schematic for proper identification).

3.  Rest easy, knowing that your tubes will never be without bias again.

Here are the other design changes noted between the two schematics:

-  Early Thunderbolts used 866As and 4-400As, newer Thunderbolts used
3B28s and PL-175s.  While the 3B28s are a drop-in replacement for the
866As in this case, you can't use 4-400As in a Thunderbolt designed for
PL-175s without "undoing" some of the changes made to the grid bias
circuit  (the PL-175 requires a higher grid bias voltage for AB1/AB2
service than the 4-400A).

-  R3, the 350 ohm Globar grid resistor, is replaced with three 1K
non-ind. resistors of unknown wattage.

-  V109 was a VR75, now is a VR90 to accommodate the different bias
requirements of the PL-175s.  R116 changed from 2K to 5.6K, as
appropriate.

-  SW102, the "Plate" toggle switch, is changed from SPST to a DPST
unit; the second pole is wired to switch the other (previously
unswitched) side of the AC line to the plate transformer.  (NOTE: this
is a very good mod to make in the interest of safety).

-  C101, the 8 uF 2 kV oil cap, is replaced with a series string of six
80 uF 450V electrolytics, each bridged with a 120K 2W resistor.  All of
this is mounted on a circuit board located in the space vacated by the
oil cap.

-  R101 - 106 are changed from 4K to 5K (probably to accommodate the
approx. 3 mA current being eaten by the cap bank resistors).

-  C105, a .01 uF disc ceramic, is connected between T101 secondary
center tap and ground.  The cap is actually located on the back of the
LV filter/bleeder circuit board.  If you can come up with an
explanation  for this, please let me know...

That's what I've discovered during my Thunderbolt restoration project. 
The manual on BAMA is for the early version, and I have an original
manual for the later version.  Still need to scan it and post it to
BAMA....

regards,

Mahlon - K4OQ






>
> -  T103 is now corectly drawn on the schematic with a tapped primary (0
> / 110V / 120V).  NOTE:  if yours was assembled with the 110V tap
> connected, you might want to change it to the 120V tap.