PAL Electronics 351 BDX Base Linear

W5HRO

Amateur Station
#1
Picked up this old PAL Electronics 351 BDX base linear. This was from the same company who made the PAL VFO's back in the late 1960's to the mid 1970's. It's good for 300W carrier out on AM or 600W of average voice peaks if the source and amp are tuned together correctly. These things also worked all the way down to the bottom end of 15-meters stock. 8950's were such great tubes! You could make really small compact amps using them, but get really high power out and all the way up to 6-meters.

PAL_351_BDX_01.jpg

Below is a pic of the inside. If the transformer is still good then there is enough room for a 40 and 75 meter coil. Then increase the tuning C and adding parallel C to the loading cap would be easy. Then respray the cover with black wrinkle finish and bake it. Would make the Mars Thunderbird Mobile transmitter really scream! :p

PAL_351_BDX_02.jpg

I was also thinking, some people use to add a 5th 8950 tube in these because it has the extra place for it, but it probably wasn't too good on the transformer. That and using five 6V tubes today won't add up with the 12V filament winding. However, maybe adding the extra tube, but using a 12JB6A for the driver instead might work. Less drive, but then the filament voltages will add up. The only thing is it would use up the extra space needed for a 40 and/or 75-meter modification. Something to think about and consider for sure.
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W5HRO

Amateur Station
#2
Here is the schematic below. The amplifier used four 8950 tubes, one driving three and it still has the original tubes installed. I will convert it to 6LF6's if the transformer is still good. Would need to also increase the cathode and plate choke values to move the thing further down the HF spectrum, but it would still be broadband. I like the way they used the 6.8V zener to ground on the 3rd 27uH choke to clamp the cathode voltage. It keeps bad things from happening. The Palomar amps like the 300A for example had a bad habit of constantly blowing the tubes because it needed a zener in the same spot. Instead, it used a small unregulated negative halfwave voltage made from the filament voltage on the control grids which was a mistake. They probably thought it would keep the cathode from rising too far above the filaments, but not true. Negative grid bias just doesn't work well in cathode-driven grounded-grid sweep-tube amps. Cathode bias always worked best in these old 8950 linears and regulation at the final stage like in this PAL amp was important. People always had a habit of overdriving them is why and that's when the bad things always happened. There's a reason why this old PAL amp still has the original 8950 tubes in it. Any old Palomar brand amp still in good condition you can find today probably has about the 50th set of tubes it's had in the amp's lifetime.
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W5HRO

Amateur Station
#3
Picked up a lot of NOS 6LF6 tubes. These are the short version which take up less room. This is the best option to replace the old GE 8950 tubes. Many people today use 6LB6's, but the filaments draw almost an extra half an amp per tube compared to 6LF6's. Placing two 6LF6 filaments in series will actually draw 200mA less current than two of the original 12V 8950 filaments did in parallel. Plus 6LF6's have more power dissipation than 8950's and handle higher voltages.

6LF6_Tubes.jpg
 

W5HRO

Amateur Station
#4
I ordered 10 of the Hammond 1537M high current 250uH RF chokes below to get started. I'll replace all of the ones in the amp. They make several, but the 250uH ones should make the amp work all the way down to the bottom of the HF spectrum after doing the other. I was going to try the 100uH ones first, but I might as well go all the way because the size difference between the two is meaningless. These are exactly the type of high current RF chokes that should work best in these sweep-tube amps just due to the nature of them being high current pulse tubes. The diameters are a bit large and I may have to maneuver things around a bit so they fit, but in a home-brew amp where you can have more space they would probably be ideal. I'm tempted to build a big one up using like 6 or 8 tubes in parallel for the final and then add some other single 100W tube or higher as a driver with tuned inputs and outputs so it's done the correct way. Maybe a future project down the road.

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