An SB-220 needing lots of attention

[Crawdad]

Great thread and a beautiful result. Impressive work sir. Love the black.

73

 

[GlocknSpiel]

And a few more for those interested.
View attachment 49695
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!)
View attachment 49696
View attachment 49697
New "Fish Paper" (insulator) being glued to the chassis after all fasteners behind it were secure.
View attachment 49698
Connector added to all front panel wiring to facilitate front panel removal for service, adjustment etc without a soldering iron.
View attachment 49699 View attachment 49700
View attachment 49701
This is what I called the "Beauty Shot". Full filament voltage to the 50 year old tubes.
View attachment 49702
New Parasitic Suppressors installed
View attachment 49704
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.
View attachment 49705
73
David

I did the same thing (placing a NOMEX plug and socket in the faceplate wiring). It sure made things easier to manage.

 

[GlocknSpiel]

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.
View attachment 49623
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.

View attachment 49624

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
View attachment 49629
20 Meter result
View attachment 49630
15 Meter result
View attachment 49632
10 Meter result
View attachment 49633

I agree with @nomadradio. A PITA!!

On to the next episode.
73
David

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?
I've read about this procedure elsewhere on the interwebz, but nobody has actually outlined the actual procedure. This sure would help me with a present issue with getting 20M and 10M networks down below 4:1 INPUT SWR! Any help here would be greatly appreciated.

 

[Dmans]

Tuned input circuit

Tuned input circuit of amplifiers and what the tuned input circuit does. How the tuned input improves efficiency and reduces IM distortion.

w8ji.com

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 3CX800A7	68 ohms

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.

73
David

 

[GlocknSpiel]

Tuned input circuit

Tuned input circuit of amplifiers and what the tuned input circuit does. How the tuned input improves efficiency and reduces IM distortion.

w8ji.com

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 3CX800A7	68 ohms

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.

73
David

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.
I've never had to deal with this issue on other amps because of where other amps place the input network. Powered input tuning of the SB220 scares me during alignment because of it's placement. Most other amps are safer... and this one 220 is acting very strange on 20M & 10M.
EDIT: Just a quick update...... Yesss!! I temporarily slapped the 4ea 390 ohm resistors in there, hooked my VNA up to the input, pushed the relay closed and bam! She jives with what I'm seeing on the input SWR meter when powered-up with 25W exciter results on 80M --- exactly!! All of the work still needs to be done to 20 & 10M band positions (coil windings and/or cap changes to bring those bands into resonance) --- but that should be a breeze now. I have an easy direction to proceed in, and no frickin faceplate flopping around while I set it up the rest of the way! I'm stoked!
I owe you an adult beverage and lunch, David!