[Johnson] Thunderbolt Q & A

Sun, 13 Apr 2003 15:41:41 -0500

Well, by Golly I know where I'm going with my Johnson questions from now.
Thanks for an informative post, Mahlon! All of us on the list appreciate it.

73, Jerry W5KP   (Valiant, two Rangers, two Ranger II's, and hoping to find
a Thunderbolt and/or a 500 one of these days)

----- Original Message -----
From: "Mahlon Haunschild" <[email protected]>
To: <[email protected]>
Sent: Sunday, April 13, 2003 3:14 PM
Subject: [Johnson] Thunderbolt Q & A

> 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.
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