This is a question regarding the popular Admiral 7" TV

Sammy says under a section labeled Production Changes:

"Early sets used a 5U4 rectifier tube, Later a 5Y3"

One way to tell is measure between pins 4 and 6 of the rectifier socket 150 ohms is 5U4, 175 ohms is 5Y3.

My transformer measures 137 ohms using a known accurate Fluke.

The set works better with the 5U4 seeming starved for B+ with a 5Y3.

I am aware the 5U4 draws about twice the filament current.

Sam's mentions a red dot on the lamination's which I don't see.

Tube layout says 5Y3.

My set has a vertical linearity pot which his not present in the Sam's

Waddaya all think ?

Jim

If you think you need more B+ voltage, you could use either a 5V4 or a 5R4, both of which have the same 2 amp filament current of the 5Y3.

Based upon your resistance measurements one would reasonably conclude that the transformer in your set was intended for use with a 5U4. If it doesn't get too hot in use I wouldn't be concerned about using the 5U4. Usually the power transformers in these little sets run fairly cool.

Try a 5T4 if you have one. If the tube layout specifies 5Y3 that's what I'd use absent any reason to believe the chassis is not original to the cabinet.

It's actually 50% more 3A for 5U4 vs 2A for 5Y3...

The 5R4 Dennis suggested is a safe bet... Has a 2A filament, doesn't drop as much voltage as the 5Y3 and are easy to find in the ST-16 envelope if you want BIG... The 5V4 is a heater cathode type will also have less voltage drop, and give some delay in building B+ if that is a concern...

Somewhere I posted the results of B+ given by various 5v rectifers at something like 100ma current, I'll see if I can locate that post... The 5Y3 did give more drop than any of the other four or five I tried...


Found it



Tom

Keeping in mind the Sam's Photofact for the Admiral 19A1 chassis has a paragraph in the "production Changes" stating that early versions used a transformer designed for a 5U4.

They say “if you use a 5Y3 with a 5U4 transformer the B+ will be too low”.

I discovered this after I had the set together on a shelf for morning news testing.

My transformer's secondary HV resistance is closer to the 5U4 measurement than the 5Y3. 150 ohms for a 5U4, 175 ohms for a 5Y3, mine measures 137 ohms). They also say there is a red dot somewhere on the 5U4 transformer (I haven’t looked yet)

When I first started tinkering with this set I was using my Heathkit PS-3 bench supply for B+ because I wasn't getting sufficient High Voltage. When I had the B+ cranked up to about 250 volts things worked better. Resistors and capacitors are OK in the B+ string.

It was quick and easy to stick a meter in pins 4 and 6 of the rectifier socket and check the resistance of the transformer.

I plugged in a 5U4 and have been running the set in the cabinet with the back in place and monitoring the transformer temperature being aware of the higher 5U4 filament current.

The set performs considerably better with the 5U4 probably due to B+ being about 20-30 volts higher making the High Voltage probably 500 volts higher.

The tube layout on the bottom of the Bakelite cabinet is destroyed although I can make out 5Y3 and the set had a 5Y3 in the socket when I bought it.

This little TV set has become the restoration from Hell and I still need a Fine Tunning Knob.

Jim

60 years later it would probably be impossible to prove the chassis was original to the cabinet. While you would like to think they would always remain together, it was not unheard of for repair shops to "swap" a working chassis, or one that had already been repaired but the customer never came back to pick the set up, for one that needed extensive work. A previous TV collector/owner could have also done the same thing.

Admiral was also well known for making running changes along the way, it is entirely possible they could have mismatched a chassis/cabinet/tube layout label at the factory.