I did the same thing (placing a NOMEX plug and socket in the faceplate wiring). It sure made things easier to manage.And a few more for those interested.
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Nylon 6/32 screws under filter bank to support the Harbach Soft Start module. Nylon stand off's were used under the chassis to hold the circuit board. One of the reasons the filter bank was installed at least 3 times (maybe 7 or 8!)
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New "Fish Paper" (insulator) being glued to the chassis after all fasteners behind it were secure.
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Connector added to all front panel wiring to facilitate front panel removal for service, adjustment etc without a soldering iron.
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This is what I called the "Beauty Shot". Full filament voltage to the 50 year old tubes.
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New Parasitic Suppressors installed
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Air conditioner coil cleaner was used on all the aluminum panels to bring them back to their original glory. Got to work fast with a scotch brite pad while wet or the panels get a white stain on them from the acid.
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You and Rich (AG6K) discuss here, mocking-up the cathode-to-grid "load impedance" using a resistor between the cathode and grid at the tube sockets.... I'm assuming that would be a 100 Ohm resistor in each tube socket to simulate 50 Ohms.. Is this correct?Continuing on with the Tuned Input tuning.......
Thankfully this amplifier is 50+ years old. There has been much written and documented to help me along the way with this amp. I could not have done it if not for all of those before me.
Keeping in mind when this amplifier was developed most if not all transceivers were tube final radios and input matching was not near as critical as todays transistor final radios.
This is the area of interest for this portion of the project. This picture only shows 3 of the 5 input circuits but the modification is very similar for each band.
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The goal is to increase the "Q" of each circuit. Too much "Q" and the bandwidth will be too small - SWR will be very flat on very narrow portion of the band.. Too little "Q" and the tuning match will be too wide and no amount of turning the inductor slug will bring the input SWR into an acceptable range.
Rich Measure's states this better than I can.
(Below copied from AG6K)
Increasing both capacitances will lower the operating frequency of the tuned-input, so L must be decreased to bring the frequency back up to where it started. This can be accomplished by removing turns of wire from L, and by adjusting the inductor's tuning slug.
It is important to keep in mind that the matching ratio of a tuned input circuit can not be changed by adjusting the slug-tuned inductor alone. In order to arrive at the optimum SWR, Two components must be adjusted in order to change the matching ratio of a tuned input circuit.
The inductor for each band was removed one at a time and in most cases turns were removed from the coil and the input and output capacitors were (in most cases) increased in value.
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The assembly was installed and the circuit was tested using the VNA at the input connector and a dummy load at the output connector. Resistors were temporarily installed from the cathodes of the tube sockets to the grid pins of the socket to simulate the loaded resistance of the tube. The appropriate band was selected and swept for the frequency involved. The tuning slug was adjusted for the best match as reflected on the VNA. This was all done without powering up the amplifier but will need slight adjustment under full power when all other work is done.
Below is the input section after the 80M, 20M and 15M coils and capacitors were modified. 40M on the top right and 10M on the bottom right.
View attachment 49627
Now for a few "results" pictures.
In all cases, each vertical graduation on the VNA is .2 SWR.
i.e. The first horizontal line from the bottom of the scale represents 1.2:1. The second horizontal line is 1.4:1, the third is 1.6:1 etc
80 meter result
View attachment 49628
40 Meter result
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20 Meter result
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15 Meter result
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10 Meter result
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I agree with @nomadradio. A PITA!!
On to the next episode.
|811A, 572B||220 ohms|
Thanks a bunch! You cannot know how grateful I am for that info! I truly have looked high n low for it, and at every chance I had. I'll give that a try later on this evening or tomorrow. I'll need to study Rich's treatise on the topic you linked to also.For various tubes the following cathode resistances are used at each tube:
811A, 572B 220 ohms 3-500Z 100 ohms 4-1000A 90 ohms 3-1000Z, 3CX1200
Above from W8JI. (His site is also linked)
I am pretty sure I used 390 ohm resistors (I did not have any 400 ohm resistors) from each cathode pin to ground to arrive at the 100 ohm (plus or minus) cathode resistance mentioned by Tom.