Other than TV power transformers don't remember any radio transformer with 2 - 6 volt and no 5 volt winding? Wonder if this was a Zenith replacement for a bad transformer? Might have been easier to use a special transformer then rewire for 5 volt rectifier? Maybe 6X5's are being lit off of 5 volts?

Wouldn't surprise me. It might have been the only possible expedient at the time.

Larry

The radio is at a different location and I don't remember the model number. I was surprised when I looked at the schematic and saw that the 6X5's had a separate filament winding. I didn't measure the voltage.

RRM

If that's the way the schematic had it, with the heaters tied to the cathode in 5AR4 style, they were certainly playing it safe...

Larry

Found the data. The radio is a 10-S-567. The chassis is the same as the 10-S-566. Fixed the heater to cathode short problem.

RRM

It came with 6X5 filaments connected to cathode on a separate winding. Shown as 1940-1941 radio. Wonder why we haven't seen more of this model? Maybe due to WW2?

http://www.nostalgiaair.org/PagesByMode ... 024677.pdf

That's a easy one Norm, don't have to fix 'em if they ain't broke...

Tom

Certainly by that time Zenith was well aware of their serious engineering mistake on the 1940 models, as many power transformers had failed within several months of the radio being sold to a customer. My parents had a 12-tube 1940 Zenith and the transformer burned up shortly after the 90 day warranty expired. Makes sense that Zenith tried to modify the design, but you will also notice that they started changing over to 5 volt rectifiers in some sets. Tying the rectifier cathode to the heater winding only solves the problem of H-K shorts, and does nothing to address P-K or P-P shorts which were also happening in various designs of 6X5 tubes.

By 1943 Zenith had published a "fix" for the problem, which advised dealers to install common dial lamps in each HV lead going to the rectifier socket to act as fuses, claiming they had never had a transformer fail a second time in a set with the lamp/fuses added.

Here's the factory bulletin, the info on the 6X5 is in the right hand column near the bottom. ARF member Ed Engelken posted the bulletin originally. http://www.gvtc.com/~edengel/ZenithWW-II.jpg

Eventually Zenith supplied a transformer from a later model radio having a 5 volt rectifier filament winding to use in the sets that were built originally with two 6X5's. They must have given up on trying to make the sets work reliably using 6X5's, or they ran out of original transformers to sell.

I have been using a single 6AX5 to replace the two 6X5's, which requires rewiring one of the sockets and blanking off the other one, but it works perfectly and is not prone to catastrophic failure.

I didn't see any pics of the original offending 6X5G Zenith tube so here's some. BTW, this tests ok, but I wouldn't use it. Plate structure looks identical the 84/6Z4.

All thanks for chipping in, AOR . I've never had one of these "first offender" G tubes. I wonder if all G's had that stacked-plate arrangement.

It should still be good for light applications. The 84 /6Z4 I put into that power supply I mentioned above also has the stacked plates, and it's been there since 1988 with lots and lots of hours logged...

Larry

That was used in a lot of farm radios since the heater draw was much lower than most other rectifiers.

Scott Todd

But since farm radios were usually battery powered, why would it need a rectifier?

Guessing you mean in a vibrator P/S??

Tom

Central Electronics used a pair of 6AU4GTA's for the 600V+ HV to the pair of 6550 finals in the 100V and 200V transmitters running 100W output on CW and SSB. Very reliable and one of the few damper tubes Ive seen used in that service. Mine were in service from 1959 to last year when I started a complete overhaul of recapping, etc.

Carl

Right. These were usually 32V farm radios with vibrator supplies for the B+.

Scott

Yes, let's stay on track. When pushed, the 6X5 was notorious for failure, not so much 6X5GT. Used properly, OK. I have a couple of test instruments that are OK after 50 years of service. Same with 6X4 and a couple of more varients of the offset wing design common cathode rectifiers like 6CA4 whihch threaten to outlast me.

This thread is a great example of why I like the new "tubes and parts" category so much.

Long after we're gone the debate will rage on. Matters not tha a pair of diodes for a few pennies and a resistor can do the same job. I worry more about house fires and s**t like that anymore.

Glad to have launched it, Bill . Always glad to oblige.

Larry

I am restoring a Zenith 8S463 which has the dreaded 6X5G tube. Is there a preferred method for solving this problem? I see reference to using a couple diodes and a resistor. How should I calculate the value of the resistor if I go this route?

Thanks,

Barry

Several choices here- you could put some low drain flashlight lamps in series with each of the HV leads to the plates (something like a #49); upgrade the tube to a 6X5GT since it's much improved over the pain G version; go with silicon diodes and a series resistor to keep the voltages in line with what they'd be with a tube in the socket.

Scott Todd