Wow, great info from both of you!!?
lou, do you still have any of those power supply boards for sale? Sounds like something handy to have.?
I had read about the power supply being in that air cooled compartment under the RF deck and the first time I had the cover off, I just Took a long stare at the RF deck and wondered how this top cover that all these parts were bolted to would come off. I also wondered why their bottom of the cabinet didn’t have a removable panel.?
I figured the big parts like the tapped tank circuit and band select could and should remain buoyed to that HV compartment lid. I would just need to remove sever connections. Some leads could remain soldered to the RF deck components by simply removing the bolted in terminal to the variable tune cap. The leads that built to the variable load cap would be impossible to get to as the load cap is under the tune cap and so they would need to be on soldered from the components on the RF deck. Tubes parasitics all that would need to be removed. ?The band select has a coupling clamp and may need only one end of the clamp removed. May be a good idea to mar the blame and shaft so they align back to avoid having to re index the band select positions.?
There’s also a little hole with a wire that comes through that panel somewhere in the area between the tube and plate choke. Mrobably should also remove the?
horizontally, mounted, choke, may or may not be necessary, but this was my thinking at the time before reading Lou’s comments.?
my Vp also reads low. On the Vp meter, I think each little line is 60V and so mine is reading roughly 2,175Vp.?
I believe the inaccurate low Vp reading is do to out of tolerance resistors that are likely on the power supply board, 3 at 1meg, and 1 at 150K, all 2W.?
I really enjoy having accurate meters. I have some other amps and I enjoy calculating the plate impedance, input power and efficiency.?
plate impudence = Vp / Ip
input power watts = Vp X Ip
efficiency = output watts / input watts X 100
?
so I never can tell what’s really happening with my commander until I can replace those resistors.?
Have always been interested in how the novice back in the day kept their home brew rigs compliant to 75W input and even the General and up classes back when the legal limit was 1KW DC input to the final amp.?
the Heathkit SB 220 amplifier really amazed me on how they designed this for hams to tune up. This was very clever.
I learned a few interesting facts about the SB-220 and some operational history on how they use to legally tune it up in the old days when the law was 1000W DC input to the final amp. Back in the 60’s when they designed it, the heathkit engineers were clever.?
Because the duty cycle of voice modulation is 50% or less, you could legally operate at 2KW input on SSB. But how do you tune up the amplifier to 2KW without exceeding the 1KW limit??
This is how the engineers made it possible. By using 2 high voltage settings that differ by the square root of 2 (1.4X)
When you tune, the amplifier output network, you are transforming the load impedance to a much higher plate impedance. That transformer and penis remains the same regardless of the plate voltage. Power is related to the square root of the voltage. ?If you increase the voltage by 1.4, you increase the power by 1.4 squared (2X) so you tune in the lower voltage tap to 1KW PEP and then switch to the 1.4x high voltage tap, the SSB position. ?You would only need to possibly increase your exciter drive level to match the same plate impedance but you would not need to adjust the load or plate on the amplifier.
Here is the example of how the engineers intended for everyone to tune up.?
In the CW/Tune position, tune up with your CW key. Stop whenever you want at whatever power out you want and if your using a scope, get your trapezoid linear with the proper amount of loading and dip your plate and all that good stuff. Now that you’re done, make a note of your voltage on the plate and your plate current.?
Vp= Plate Voltage
Ip= Plate Current.?
So if I have the following parameters:
Vp= 2190V
Ip= 450mA
Then I need to know my plate impedance.?
Vp / Ip = plate impudence?
2190/.450=4,867 Ohms.?
Remember this plate impudence because this is about the plate impedance we will need to put the amp at in the SSB high V tap.?
Now switch to SSB high tap and key down fast and take note of the Vp under load.?
Let’s say your Vp=2780V
Vp / plate impedance = the Ip you need to be at in the SSB high tap.?
2780/4867=0.571?
You need to key down in SSB and increase exciter drive lever until your at Ip 571 or close. Say past the 5 and between the 3rd and 4th line is 570mA Ip.?
Ok, your now at:
Vp= 2780
Ip= 570mA
That’s it, your done! You do not need to adjust your load or dip the plate with the tune. You should now be at the same plate impedance in the high tap as you were when you tuned and dip the plate in the low tap.?
Let’s see. Remember our plate impedance in the low tap tune position was 4867 ohms.?
Vp / Ip = plate impedance
High tap values
Vp 2780 / Ip 570mA = 4869 plate impedance. Same impedance you already tuned and dipped your plate in the low tub tuneup.?
Of course you don’t have to do any of that now with our current power allocations but I just love the history of exactly how they designed this amplifier.
some may already know this but I have had many people swear this is incorrect. One friend of mine who
is a brilliant buddy from AM said impossible, makes no sense, not possible. About an hour later he wrote me back and said the numbers and math definitely match up, but I don’t know how this is possible, I’m still trying to figure out and get my head around it, I must be missing something. I have run this test hundreds of times with a scope and every time I have done it, it’s dead on and everything is perfectly loaded and linear with proper load coupling, and the plate dipped and the tube resonant. This is how it was designed and why.?
anyway, I would love to have all metrics super accurate because it’s part of the enjoyment of operating
