capt205 said:
Sure, they
WERE pretty good 30 or 35 years ago. But it's not just the mileage, it's the years, too. The 1972 Ford T-Bird was pretty good, but try to find one that's driveable now.
Odds are that you have BOTH age issues AND 'mileage' issues, wear and tear that always occurs in normal use.
It has one short, fat 'baldy' tube that serves as the driver. A tube that small calls for a radio that's pretty small to drive it. The Johnson should fit that requirement. If that tube is good, great. If it's not, you will need one.
The two large tubes with the plate-cap connections should deliver 250 to 300 Watt peaks, depending on which version of the "6LQ6/6MJ6" tubes you have, and how many miles have already been used up on yours. If they are no good, that won't be a large surprise. Some 'average' wattmeters will show no more than 50 to 75 Watts of 'swing' when you modulate. A peak-reading meter will show more.
capt205 said:
The radio and mic work, but the amp won't key.
The remaining, smaller 6BQ5 tube is used only to key the relay. If you hear the relay go "CLICK" when you key the mike, that tube is doing its job. If you don't hear anything, it doesn't mean the 6BQ5 is bad. Doesn't mean it's good, either.
The five 450-Volt electrolytic filter capacitors are probably bad by now. They aren't meant to last 20 years, let alone 30 or 35. Unless all five of them have been replaced recently, you can assume they are bad. Even if they check okay, they will "remember" how old they are once you run the unit for a week or so, and break down then.
Seems to me that they used cheap bleeder resistors on the five filter capacitors. If you decide to replace the filter caps, replace the bleeders at the same time. Cheap insurance.
The relay is fairly stout. If the mileage is low, cleaning the oxide from the contact points with a solvent-soaked strip of paper will be necessary. JUST DON'T USE A FILE OR ABRASIVE. If the contacts are worn, you'll need a relay, too. If a shop takes this job, they will probably use a 12-Volt DC relay in place of the original 110-Volt DC relay, and will use a transistor to key the relay in place of the 6BQ5 tube. And if the old relay and tube work okay, there's nothing broke to fix in that circuit. The original relay gets harder to find every year.
The Mach 3B is a borderline "splatter box" and tends to cause interference in phones, baby monitors, televisions, stereos and toaster ovens. Just don't try to use a "big" radio with it. Newer base stations will overdrive that short driver tube.
73
