I recently returned to amateur radio after a 17 year absence; i.e. from approximately April 2000 to January 2018, during that time there were many changes to the hobby. One thing that seemed to survive is the Globe King series of transmitters; I was gratified to hear that so many of them were still on the air. My Globe King 400B is circa 1951 and has been subjected to a number of modifications. One of the things that I like about the Globe King 400B is that it is a simple, link coupled, basic design that lends itself to justifiable improvements and modifications.
a) The V70D’s have been replaced with 8005’s; including modifications for filament voltage, (10 vac), improved fixed and grid leak bias.
b) The final tuning capacitor and output coils, (B&W TVL series), have been spray coated with an insulating varnish, Glypital-1201A, to address flashovers.
c) The output link has been spray coated with the same insulating varnish and outfitted with a series tuning capacitor to improve loading.
d) I have a spare GK 400B RF deck which is modified to use 812A’s.
The main power supply is basically unmodified except for some additional capacity, (15 uf), for the RF B+. Some day I may replace the 5U4 and 866’s with solid state rectifiers to reduce heat.
A separate “Heising” deck, with the three diode method of ultra modulation, has been inserted between the modulators, (yes, there are two GK modulators), and the RF section. This deck, or coupling unit as it is referred to, contains 40 henries of inductance and 20 uf of coupling capacitance along with three brick diodes and a 5000 ohm 200 watt loading resistor; about 200 vdc is supplied from a separate power supply for the keep alive voltage. There are few advantages to this method of coupling between the modulators and RF deck: First - no DC passes through any modulation transformer secondary, the benefits of which have been extensively explored in many AM forums and publications. Second – the ultra modulation circuit prevents over modulation in the negative direction; a major source of splatter. And third – increased modulation in the positive direction can be realized; i.e. 125%, 150% or higher with compression can be typically realized, (OK … I tend to hit it a little hard … but that’s radio!).
a) The GK 400B deck is only slightly modified; i.e. increased audio coupling capacitors, some negative feedback at the 6F6 drivers, 811 output tubes, and some global negative feedback.
b) A GK 500 modulation deck was completely stripped down to the chassis and rebuilt using a modified Williamson design; i.e. two 6SN7’s into beam power drivers, (a pare of 6F6’s, 6V6’s, 5881’s, 6L6GC’s, or EL34’s can be used), driving a pare of 811A’s or 572B’s running at 1750 vdc on the plates. Bias for the final modulator tubes is switch selectable at: 0 vdc, 5 vdc, or 10 vdc using 5 watt Zener diodes. Plate stopper resistors are used on all driver and output tubes for stability. There are three feedback loops: First - the current 5881’s are connected in an ultra-linear configuration using a UTC S-19 modulation transformer as the driver transformer, the screens are connected to mid point taps. Second – there is balanced feedback form the secondary of the S-19 driver transformer to the grids of the 5881’s, this helps flatten out the S-19 and lowers the effective plate impedance of the drivers. Third - there is some global DC feedback from the secondary of the modulation transformer to the cathode of the input stage. Feedback is like salt, (for normal people that is, I really like salt!), just the right amount can be good and too much can make things worse; obviously an area of great discussion and opinion.
All audio is processed via a separate audio chain composed of four or five commercial components. The source of RF drive is a Collins 310B connected into the VFO input jack of the Globe King.
Not sure if ur QTH is in 3-land or not... but if so when are we going to hear you on?
Thank you for the comment, I am located in Cinnaminson, NJ since 1977, and am on the air just about every day on 40 meters or 75 meters.
How about 75m AM?? Can't recall if we've worked or not??
Pretty sure that N2DTS is in your area... he's on 75m quite a lot.