Now that I've purchased all the ones I could find under $10.00, I'll share a little known 12 pin sweep tube substitute with the forum, after some info. The tube I'll mention is best suited for amps that run under 800 volts on the plate. Many with a high low switch are using a lower plate voltage to obtain the low power setting. This tube works great here and still has enough gain to compensate for the lower voltage.
The reason this tube excels if you can keep the plate voltage down, is because it was designed to have the highest peak cathode emissions of any other sweep tube ever designed. It's a "current handler", able to sustain a continuous carrier current of 400ma with peaks up to 1.5 AMPS! Even the Maco, GE, M-2057 cannot peak this high. The tube is the 6KN6 and is was made by GE's competition mostly for the biggest Magnavox televisions.
The 6KN6 was made by Sylvania and RCA. If you run these things at 800 volts with 400ma carrier per tube and a 4:1 PEP ratio, they last forever in AM service where all of the others don't. Not many model TV's used them and linear builders were discouraged by the lower plate voltage. What that means is early production runs created an ample supply so that there were never any later "cheap" production runs in the 1980's.
Running the plate over 800 volts with both grids grounded will cause the plates to cherry red no matter what you do with the cathode bias, other than the deepest class C. Grid bias resistors will alleviate this to a point, but they are inherently unstable in their voltage drop with secondary tube emissions and often cause drift in power as you're keyed. You're best bet is to just stay between 750 and 800 volts B+.
If you're tempted to use this tube in an amplifier with more voltage, consider peeling off some of the HV windings on the plate transformer before risking the tubes. They are almost always the outer most wrapping just under the paper and do not require a complete transformer disassembly. Cleanly cut the paper in a spot where it can easily be taped back on when done, pull it out of the core and start unwrapping the thin HV turns, cutting them when they get too long to keep pulling through the core until you get to the right voltage.
You'll normally have about 660 VAC on the stock HV taps and you'll want to get this down to about 550 VAC, to hit your target after rectification and filtering. It is safest to do this on a variac at exactly 1/10 the line voltage and than the target voltage becomes 55.0 VAC versus the stock 66.0 VAC. Check the change in voltage after removing the first 10 or 20 turns so you can get an idea how many you will have to take off by calculating how many volts were removed versus how many turns were removed.
With the right cathode bias (diodes work), you should now be able to ground both grids and the power output will be 100% stable even when driven in conditions of secondary screen emissions. This also occurres with lower drive levels as the tube gets older. The ability to stabilize screen voltage allows you to to extend the useful end of life service from the tube without it becoming a "power drifter" first. Now don't gobble up every last 6KN6.
The reason this tube excels if you can keep the plate voltage down, is because it was designed to have the highest peak cathode emissions of any other sweep tube ever designed. It's a "current handler", able to sustain a continuous carrier current of 400ma with peaks up to 1.5 AMPS! Even the Maco, GE, M-2057 cannot peak this high. The tube is the 6KN6 and is was made by GE's competition mostly for the biggest Magnavox televisions.
The 6KN6 was made by Sylvania and RCA. If you run these things at 800 volts with 400ma carrier per tube and a 4:1 PEP ratio, they last forever in AM service where all of the others don't. Not many model TV's used them and linear builders were discouraged by the lower plate voltage. What that means is early production runs created an ample supply so that there were never any later "cheap" production runs in the 1980's.
Running the plate over 800 volts with both grids grounded will cause the plates to cherry red no matter what you do with the cathode bias, other than the deepest class C. Grid bias resistors will alleviate this to a point, but they are inherently unstable in their voltage drop with secondary tube emissions and often cause drift in power as you're keyed. You're best bet is to just stay between 750 and 800 volts B+.
If you're tempted to use this tube in an amplifier with more voltage, consider peeling off some of the HV windings on the plate transformer before risking the tubes. They are almost always the outer most wrapping just under the paper and do not require a complete transformer disassembly. Cleanly cut the paper in a spot where it can easily be taped back on when done, pull it out of the core and start unwrapping the thin HV turns, cutting them when they get too long to keep pulling through the core until you get to the right voltage.
You'll normally have about 660 VAC on the stock HV taps and you'll want to get this down to about 550 VAC, to hit your target after rectification and filtering. It is safest to do this on a variac at exactly 1/10 the line voltage and than the target voltage becomes 55.0 VAC versus the stock 66.0 VAC. Check the change in voltage after removing the first 10 or 20 turns so you can get an idea how many you will have to take off by calculating how many volts were removed versus how many turns were removed.
With the right cathode bias (diodes work), you should now be able to ground both grids and the power output will be 100% stable even when driven in conditions of secondary screen emissions. This also occurres with lower drive levels as the tube gets older. The ability to stabilize screen voltage allows you to to extend the useful end of life service from the tube without it becoming a "power drifter" first. Now don't gobble up every last 6KN6.
