1950s or '60s vintage Zenith TVs, Need some info

[dieseljeep]

Originally Posted by Kevin Kuehn

I sawed the top off my cans and re-used the old terminals. Any chance you can post a close up of the underside of your chassis around the 6AQ5, including your filter caps? Sometimes it helps to have a fresh set of eye's look things over.

Rather impressive! How do you cut the cans so perfect?
Bought and sold a set like that about 60 years ago. Mine was an earlier model that used a 3BY6 instead of the better circuit using a 3BU8.
The outward appearance was similar.

[Kevin Kuehn]

Originally Posted by dieseljeep

Rather impressive! How do you cut the cans so perfect?
Bought and sold a set like that about 60 years ago. Mine was an earlier model that used a 3BY6 instead of the better circuit using a 3BU8.
The outward appearance was similar.

Thanks. I actually removed these and used my band saw. Then a few passes over 220 grit on a flat surface squares the end. Seems like a lot of work, but by using a very large (radiator sized ) soldering iron I'm able to heat the ground lugs and squeeze them straight with a flat nose pliers, then carefully wiggle them free. Space permitting you certainly could hack them off using a Zona saw while they're still attached to a chassis.

[vortalexfan]

Originally Posted by Kevin Kuehn

Thanks. I actually removed these and used my band saw. Then a few passes over 220 grit on a flat surface squares the end. Seems like a lot of work, but by using a very large (radiator sized ) soldering iron I'm able to heat the ground lugs and squeeze them straight with a flat nose pliers, then carefully wiggle them free. Space permitting you certainly could hack them off using a Zona saw while they're still attached to a chassis.

OK, so I looked over my chassis after you mentioned the missing wire and sure enough, I'm not sure how I missed it but I forgot to put that wire back in when I removed it from the old can cap to install the new one, I now have the voltages back in the audio section, and its functioning as it should.

But now I have a new problem, one of my Video IF tubes was bad (which is why I'm not getting anything from the tuner) which is a 3CB6 and I tried subbing in a 6CB6 (because I have a whole butt load of them) and it seems the TV doesn't like them, because they just glow white and then just die after 3 seconds (and it doesn't matter which 3CB6 socket its stuck in, it does the same thing in all 3 sockets) and I was hoping I could get away with not having to order a new tube for the TV (as I don't have any 3CB6 tubes in my stash.)

Is it normal for a 6.3V Tube to glow white hot and then burn out after 3 seconds in a socket originally meant for a 3V tube in a series string set?
I was thinking that it was possible to install a higher voltage tube in a lower voltage tube spot in series string radios and TVs (like how you can sub in place of a 35L6 a 50L6 on old Series string radios and it would run fine.)

Am I missing something here?

[Kevin Kuehn]

Originally Posted by vortalexfan

OK, so I looked over my chassis after you mentioned the missing wire and sure enough, I'm not sure how I missed it but I forgot to put that wire back in when I removed it from the old can cap to install the new one, I now have the voltages back in the audio section, and its functioning as it should.

But now I have a new problem, one of my Video IF tubes was bad (which is why I'm not getting anything from the tuner) which is a 3CB6 and I tried subbing in a 6CB6 (because I have a whole butt load of them) and it seems the TV doesn't like them, because they just glow white and then just die after 3 seconds (and it doesn't matter which 3CB6 socket its stuck in, it does the same thing in all 3 sockets) and I was hoping I could get away with not having to order a new tube for the TV (as I don't have any 3CB6 tubes in my stash.)

Is it normal for a 6.3V Tube to glow white hot and then burn out after 3 seconds in a socket originally meant for a 3V tube in a series string set?
I was thinking that it was possible to install a higher voltage tube in a lower voltage tube spot in series string radios and TVs (like how you can sub in place of a 35L6 a 50L6 on old Series string radios and it would run fine.)

Am I missing something here?

3CB6 is 600ma filament vs 6CB6 300ma. In other words your entire series string of tubes should be 600ma type, so trying to pull 600ma through a filament intended for 300ma isn't going to cut it. Your 6AQ5 is a 450ma tube, so you're really pushing that one as well.

Congratulations on a major victory putting that missing B+ wire back in place. Although I can't believe how much time we've been chasing tail because of that very simple oversight. You really need to post more photo's of your handiwork when asking for advise. My suggestion to you is to take very detailed pictures before you remove any original components so you can refer back to them. And you would do us all a big favor by getting a couple books on understanding very basic DC circuits so you have a better grasp of how to trace voltages in a power supply circuit. Read up on Ohms Law and understand how to apply it.

[jr_tech]

“ Your 6AQ5 is a 450ma tube, so you're really pushing that one as well. “

For grins, I connected a 6AQ5 to a constant current power supply set to 600ma and measured the voltage drop across the heater. The heater voltage stabilized at about 10.5 volts! This is indeed really pushing it for a 6.3 volt tube... I suggest that proper 5AQ5 be ordered as well.

jr

[vortalexfan]

Originally Posted by jr_tech

“ Your 6AQ5 is a 450ma tube, so you're really pushing that one as well. “

For grins, I connected a 6AQ5 to a constant current power supply set to 600ma and measured the voltage drop across the heater. The heater voltage stabilized at about 10.5 volts! This is indeed really pushing it for a 6.3 volt tube... I suggest that proper 5AQ5 be ordered as well.

jr

Ok, I'll have to get the proper tubes ordered I guess. Hopefully they aren't going to be too expensive.

[Tube TV]

Originally Posted by vortalexfan

Ok, I'll have to get the proper tubes ordered I guess. Hopefully they aren't going to be too expensive.

Those series string tubes are usually quite cheap. If you shop around $3 more or less. Especially the IF tubes.

X2 at what everyone said abot taking pictures beforehand. Saves a lot of headaches. It sure beats the old days of writing notes down on paper.

[vortalexfan]

Originally Posted by jr_tech

“ Your 6AQ5 is a 450ma tube, so you're really pushing that one as well. “

For grins, I connected a 6AQ5 to a constant current power supply set to 600ma and measured the voltage drop across the heater. The heater voltage stabilized at about 10.5 volts! This is indeed really pushing it for a 6.3 volt tube... I suggest that proper 5AQ5 be ordered as well.

jr

OK well I also noticed that the previous owner of this TV had installed a 6BU8 in place of a 3BU8 tube in the TV I'm guessing that should also be replaced with the proper tube as well?

[old_coot88]

Originally Posted by vortalexfan

OK well I also noticed that the previous owner of this TV had installed a 6BU8 in place of a 3BU8 tube in the TV I'm guessing that should also be replaced with the proper tube as well?

First, you might want to see if someone's put a shunt resistor across the 3BU8 heater (pins 4 and 5). It'd be a low value resistor, something like 20 ohms. Without the shunt for protection, seems like the 6BU8 would be lighting up like that 6CB6 did.

[vortalexfan]

OK, so the tube in the TV that I thought was a 6BU8 was actually a 3BU8 tube it was a GE branded replacement tube dated 39th Week of 1963, there was a bunch of dirt buildup on the tube designation number and the way GE does their tube numers in a "stencil" font the dirt buildup made the 3 look like a 6.

And then as far as the 5AQ5 goes, I took it and rechecked it in another tube checker I had laying around (a B & K DynaJet Model 666/606 Tube Tester) and the tube checked fine (no shorts in it anymore) so I wonder if I somehow blew off whatever was causing the short in the tube, I know that can happen sometimes.

So it looks like all I need is that 3CB6 tube and a 6CN7 tube which I don't happen to have in my stash anywhere, the 6CN7 tube in the TV which is an original Zenith badged GE tube dated the 20th Week of 1956 only had one section that tested good, the other 2 sections tested completely dead.

[old_coot88]

Originally Posted by vortalexfan

Is it normal for a 6.3V Tube to glow white hot and then burn out after 3 seconds in a socket originally meant for a 3V tube in a series string set?

Yes.

A little factoid worth bookmarking:

A series circuit is constant current thru the whole string.

A parallel circuit is constant voltage across the whole (i.e., the voltage across each component is the same, tho each may draw different current).

I was thinking that it was possible to install a higher voltage tube in a lower voltage tube spot in series string radios and TVs

A higher-voltage rated heater has higher resistance, thus runs hotter in the constant-current string (that's why the 6CB6's heater was actually seeing closer to 12 volts and blew its cookies).

(like how you can sub in place of a 35L6 a 50L6 on old Series string radios and it would run fine.)

Am I missing something here?

The 35 and 50 volt heaters are both designed for the same current (.15 A).