Wawasee JB-2000 issues

[nomadradio]

It's a finely-balanced design. The driver is a 500-Volt tube with 1200 Volts on the plate. By rights it should go poof the first time it's keyed. Had a memorable conversation with an "engineer" at Wawasee Electronics many decades ago. Asked him how they get away with that. He explained that keeping the screen grid at 120 Volts DC or so was what made it possible, along with a zener that biases the driver tube near to cutoff. The big tubes have zero bias, which makes them more sensitive to a low drive carrier. Good thing, since the AM carrier has to be turned way down in the driver to keep it from cherrying up and failing too soon. The 3-500Z tubes tolerate zero bias in part by keeping the anode supply at around 2400 Volts. And by keeping the carrier below 150 Watts or so.

It's a hot-rod design. Things we do to it are meant to minimize seized rings and dropped valves. But push it too hard and you'll wrap it around the tree at Dead Man's Curve.

73

 

[Danzik]

This thing is still operating but I now know why they named it the blackcat because it sure sounds like one every half hour or so !!!! Just like the firecracker, it doesn't pop the breaker on the rear since I have replaced it and doesn't pop the 3 amp HV fuse either but boy it cracks, scared the crap out of me last night so something is still up and it still makes me think it is the transformer shorting out every now and then but its cool not hot so I don't know ! Last week it took out the coil windings on the HV relay, the power switch, and popped the 5 watt 0.5 ohm resistor on the meter, so right now I am at a loss and need a transformer unless it is a tube doing it, so I guess the only option I have is to pull it out of the cabinet in the middle of my garage, put a couple security cameras on it and key it up and talk away on it till it goes pop again " somtimes it takes a hour of using it ! " and check the cameras and see where the actual flash is coming from, its loud. Knowing that the driver is already bypassed anyone know what a good replacement transformer would be for it ???? Would a SB-220 work ? Do you have any Nomad ? Thanks

 

[nomadradio]

First thing I would do is unhook the HV secondary wires from the transformer and test for insulation breakdown to ground. An insulation tester good for at least 3000 Volts is not a Home Despot pegboard item. Likewise, testing the breakdown voltage of each tube cold, from cathode to grid might reveal a gas problem. Saw an insulation tester on Ali Express for around 45 bucks. Goes to 4000 Volts. Can't remember the model number. I bought one and it seems to work okay. I'll find the brand and model next time I'm at work.

The only single transformers I remember to run two Zs went into Henry desktop 2K linears. Everybody else used a separate filament transformer for a 2-tube model. Or you could see if Ameritron can still sell you a transformer for the AL-80B. You would need to remove one tube. That's all it's meant to power. We have used that one to rescue Heathkit SB-1000 amplifiers. The factory transformer in that model is famous for going "grunt" and failing on standby. One tube would be better than zero. The only tricky part is to work out the buck/boost winding combo for 240 Volts.

It's been years, but we used a Peter Dahl HV transformer meant for the SB-220 and found room for a separate filament transformer in a JB2000 decades ago. Just no economical solution to a failed JB2000 power transformer. Hopefully it's a tube that's going poof. Have a close look at the filament pins on the tubes. Any sign that the solder has melted should prompt a VERY close look at the filament spring contacts on the sockets. If they are burned, they'll ruin a good tube. High contact resistance will release heat sufficient to melt the solder from the filament pins of a new tube.

The Z is famous for loose grid wires. Connecting a meter set for continuity beep from cathode to grid may reveal this problem. You'll need to turn the tube on its side and slap it gently while rotating it to find this fault.

And if I had a suitable transformer I would just lie and say "no".

73

 

[Danzik]

First thing I would do is unhook the HV secondary wires from the transformer and test for insulation breakdown to ground. An insulation tester good for at least 3000 Volts is not a Home Despot pegboard item. Likewise, testing the breakdown voltage of each tube cold, from cathode to grid might reveal a gas problem. Saw an insulation tester on Ali Express for around 45 bucks. Goes to 4000 Volts. Can't remember the model number. I bought one and it seems to work okay. I'll find the brand and model next time I'm at work.

The only single transformers I remember to run two Zs went into Henry desktop 2K linears. Everybody else used a separate filament transformer for a 2-tube model. Or you could see if Ameritron can still sell you a transformer for the AL-80B. You would need to remove one tube. That's all it's meant to power. We have used that one to rescue Heathkit SB-1000 amplifiers. The factory transformer in that model is famous for going "grunt" and failing on standby. One tube would be better than zero. The only tricky part is to work out the buck/boost winding combo for 240 Volts.

It's been years, but we used a Peter Dahl HV transformer meant for the SB-220 and found room for a separate filament transformer in a JB2000 decades ago. Just no economical solution to a failed JB2000 power transformer. Hopefully it's a tube that's going poof. Have a close look at the filament pins on the tubes. Any sign that the solder has melted should prompt a VERY close look at the filament spring contacts on the sockets. If they are burned, they'll ruin a good tube. High contact resistance will release heat sufficient to melt the solder from the filament pins of a new tube.

The Z is famous for loose grid wires. Connecting a meter set for continuity beep from cathode to grid may reveal this problem. You'll need to turn the tube on its side and slap it gently while rotating it to find this fault.

And if I had a suitable transformer I would just lie and say "no".

73

Yeah, 10-4 on all this Nomad. Could be still somewhat gassy tube i spose, but they are new tubes but possibly NOS so who knows how long they sat around, they were arching at first on power up on standby! They no longer due that though after I cooked them in slowly for 2 weeks. The tube sockets are brand new and are making good contact, no solder melting on pins and I did add a belly fan just like what is recommended which worked out real well. I am now kinda wondering now if it isn't one of the tubes after they get a certain temp or whatnot, maybe before I do anything else I may pull tubes from another box and go from there, I never have noticed the transformer getting warm or humming or anything like that and the POW comes right out of the blue during keyup so maybe it makes more sense being a tube, back to the bench lol I will be looking into the tubes more I think about it now, I will try the continuity test again rolling them and tapping however I have checked that before when this first started. Thank you ! Let me know on that insulation tester though, could come in handy I don't have a hi pot