Friend of mine dropped off a pride for me to look at. Said that when amp is keyed, you can hear arcing. I haven't hooked it up yet, but I took the cover off to investigate. The band switch is heavily pitted. The tank coil used for 10 meters is pretty much black on 4 of the turns where it is tapped. Burned the enamel right off. I am thinking that this amp is very unstable and is oscillating. Probably needs neutralized. Does anyone know where I can get a new bandswitch? Or does anyone have a junked out pride laying around? I haven't played with tets too much. On the neutralization, I know it will be a small value. Something between 10-30pf maybe. How or when do you know when you have the right value installed? Any and all suggestions are welcome. Guy wants to keep it multiband. So making it a monobander is not really an option. He wans to use it on 80 meters as much as 10 meters.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Pride KW-one
- Thread starter Crusher
- Start date
If you can't find an original bandswitch, pick the closest model Ameritron and order its bandswitch. It will have more contacts on it than you need for input circuits and 160 meters but you can get one that has much higher RF current ratings.
Circulating currents in the tank coil are greatest on the highest frequency band. Compound that with 11 meter users sharing the 10 meter tank coil and you have heating problems. I'd change the 10 meter coil to some copper tubing like the smallest refrigerator water line stuff.
Increasing the diameter of the conductor used to form the tank coil will require a longer length of conductor to reach the same value of inductance. Usually adding approximately 1 turn on the 10 meter coil will allow you to bend the coil to reach the match.
The MFJ is your friend here. Tune the amp for maximum output on any band you can get it to work at. Once you got it tuned for full wood, power everything down. Remove the tube from the socket and replace it with a 5000 ohm carbon potentiometer from the plate lead to ground.
Keep the wires from the plate clip and the chassis as short as possible when connecting them to the 5K pot. Connect the MFJ to the output of the amp set it to the frequency the amp was just tuned on. With no power to the amp, manually engage the RF relay and adjust the pot for minimum VSWR.
With the MFJ off, you can now measure across the 5K pot with the DVM and find the resonant load impedance of this amp. Any changes you make to the tank circuit should always allow a match back to this impedance. Now you upgrade the 10 meter coil and check it for a match without AC, RF or DC applied!
Just keep the pot in place and adjust the 10 meter coil, tune and load controls for a perfect VSWR on the MFJ. The coil should be bent so the plate tune adjusts close to its normal spot for the band in use.
If this amp is going to be driven with an HF rig, you can skip the neutralization. The control grid is already swamped to ground through a 50 ohm 100 watt resistor. The tetrode has now lost most of its gain over a triode. It should run stable without neutralization although IMD would be improved if it were added.
The biggest problem with adding it is the amp uses no input circuits. Without them you have no means to invert the neutralization signal and no ability to provide the correct level of feedback across the amps operating bandwidth.
I'd skip the neutralization in this case and favor adding some regulation to that poor screen supply. Especially if it's going on 80 meter SSB. That alone will make a big improvement in IMD and linearity. The zener diode across the base collector junction of an NPN transistor is a good way to make a heavy duty zener with improved regulation.
You may also notice this amp uses a Pi-L tank circuit. That helps reduce harmonics and is a real good thing on the lower HF bands where the harmonics would land in an upper ham band. The "L" section usually does not have RF heating problems. When choosing a bandswitch, make sure you pick one that can accommodate this circuit.
Circulating currents in the tank coil are greatest on the highest frequency band. Compound that with 11 meter users sharing the 10 meter tank coil and you have heating problems. I'd change the 10 meter coil to some copper tubing like the smallest refrigerator water line stuff.
Increasing the diameter of the conductor used to form the tank coil will require a longer length of conductor to reach the same value of inductance. Usually adding approximately 1 turn on the 10 meter coil will allow you to bend the coil to reach the match.
The MFJ is your friend here. Tune the amp for maximum output on any band you can get it to work at. Once you got it tuned for full wood, power everything down. Remove the tube from the socket and replace it with a 5000 ohm carbon potentiometer from the plate lead to ground.
Keep the wires from the plate clip and the chassis as short as possible when connecting them to the 5K pot. Connect the MFJ to the output of the amp set it to the frequency the amp was just tuned on. With no power to the amp, manually engage the RF relay and adjust the pot for minimum VSWR.
With the MFJ off, you can now measure across the 5K pot with the DVM and find the resonant load impedance of this amp. Any changes you make to the tank circuit should always allow a match back to this impedance. Now you upgrade the 10 meter coil and check it for a match without AC, RF or DC applied!
Just keep the pot in place and adjust the 10 meter coil, tune and load controls for a perfect VSWR on the MFJ. The coil should be bent so the plate tune adjusts close to its normal spot for the band in use.
If this amp is going to be driven with an HF rig, you can skip the neutralization. The control grid is already swamped to ground through a 50 ohm 100 watt resistor. The tetrode has now lost most of its gain over a triode. It should run stable without neutralization although IMD would be improved if it were added.
The biggest problem with adding it is the amp uses no input circuits. Without them you have no means to invert the neutralization signal and no ability to provide the correct level of feedback across the amps operating bandwidth.
I'd skip the neutralization in this case and favor adding some regulation to that poor screen supply. Especially if it's going on 80 meter SSB. That alone will make a big improvement in IMD and linearity. The zener diode across the base collector junction of an NPN transistor is a good way to make a heavy duty zener with improved regulation.
You may also notice this amp uses a Pi-L tank circuit. That helps reduce harmonics and is a real good thing on the lower HF bands where the harmonics would land in an upper ham band. The "L" section usually does not have RF heating problems. When choosing a bandswitch, make sure you pick one that can accommodate this circuit.
Ok thank you shockwave. I'm gonna post some pics in the next day or two. Its in rough shae and yes the guy bought this amp from a CB'er. This amp was built to take a HF 100w rig or so I was told, I haven't looked at bottom yet. Work has been keeping me busy. 29.5 hours in last 2 days.
You should find a bunch of 100 ohm 2 watt resistors wired together to make one big 50 ohm 100 watt dummy load that soaks up all that drive power before feeding the grid. Without a tuned circuit in the input, the RF voltage feeding the grid can't rise up as it would if it were feeding a high impedance circuit. That is why this 4XC250B can be driven with 100 watts.
It looks like after the bandswitch crapped out, it tried going thru another part of circuit. I have seen worse, but I'm not a fan of thr prides.
Well I found out what happened to amp. Guy was using a RCI 2995 for a radio. Evidently radio was peaked out. Was doing over 200W pep. Yup that will kill an amp quick. And yes, it was on 11 meters.
Double your input, does he use a meter and was it still swinging forward wattage with that much drive. I feel sorry for the amp!! Ouch. Sounds like you nailed the issue though. Guess you will have some more work as it sound like you will need to de tune the power on the radio as well. Good thread and goes to show how many threads I have seen lately about this same issue, over driving amps.
Well I talked to guy today. He decided against multiband and just had me convert to monoband. I didn't have anything that would do 100 or less. Best I could do was about 120 pep. 10W carrier. Get around 100w carrier out and almost 700 pep. I ended up using some capillary tubing used for refrigeration. Its about twice the diameter of 10 gauge wire. Seems to work good. The guy I repaired it for only plans to use a 1 transistor amplifier as a driver. Input was nice and low. The sure used a ton of resistors to pad the input. But the board is nice and neat. Had to use a shop vac and a small paint brush to clean out blower. Why no one ever seem to clean these is beyond me.
