1935 GE Model A-53 AM/SW Tombstone Radio

[vortalexfan]

Originally Posted by dieseljeep

I can't see why a 470 ohm, 1w resistor wouldn't work. It's in the cathode circuit, of the audio output circuit. The rectifier tube can be a 5Y3, 5W4 or another 5Z4.
I have one just like it! It's a strange non-AVC superhet, with the volume control that isn't in the detector circuit.

Well, this one had a 5U4GB in it in place of the 5Z4 Rectifier tube, and all of the tube substitution books I have show that the 5Y3 and the 5W4 are not substitutes for the 5Z4 tube, and neither is the 5U4.

Is it common for those old flexible resistors to just blow apart? I'm asking because this resistor's outer cloth shell was completely disintigrated and all that was left was the resistor's wire element which then shorted to the chassis and left a burn mark on the chassis where it shorted to the chassis!

Also as for the replacement resistor I did come across in my parts stash 2 270 Ohm 2 watt flameproof resistors that I tied together in series to make a 540 Ohm 2 watt resistor (the original resistor had no tolerance markings listed so I figured that that would be close enough.

the original 6A8 tube (this radio had all metal cased tubes which were brand new at that time) was completely dead according to my tube testers (I tested it on my Knight Model 600 Tube Tester, my Sencore Mighty Mite VII Tube Tester and on a B & K Model 606/666 Tube Tester and all three of them tested the tube as bad/zero emissions), thankfully I had a NOS 6A8GT tube that I was able to stick in there, hopefully it won't affect the performance of the radio with it not being a metal cased tube like the rest of the tubes.

You said this radio doesn't have AVC or a standard Volume Control configuration, how does this radio perform compared to others in your opinion? I like to use these radios for Night Time DX'ing.

[dieseljeep]

Originally Posted by vortalexfan

Well, this one had a 5U4GB in it in place of the 5Z4 Rectifier tube, and all of the tube substitution books I have show that the 5Y3 and the 5W4 are not substitutes for the 5Z4 tube, and neither is the 5U4.

Is it common for those old flexible resistors to just blow apart? I'm asking because this resistor's outer cloth shell was completely disintigrated and all that was left was the resistor's wire element which then shorted to the chassis and left a burn mark on the chassis where it shorted to the chassis!

Also as for the replacement resistor I did come across in my parts stash 2 270 Ohm 2 watt flameproof resistors that I tied together in series to make a 540 Ohm 2 watt resistor (the original resistor had no tolerance markings listed so I figured that that would be close enough.

the original 6A8 tube (this radio had all metal cased tubes which were brand new at that time) was completely dead according to my tube testers (I tested it on my Knight Model 600 Tube Tester, my Sencore Mighty Mite VII Tube Tester and on a B & K Model 606/666 Tube Tester and all three of them tested the tube as bad/zero emissions), thankfully I had a NOS 6A8GT tube that I was able to stick in there, hopefully it won't affect the performance of the radio with it not being a metal cased tube like the rest of the tubes.

You said this radio doesn't have AVC or a standard Volume Control configuration, how does this radio perform compared to others in your opinion? I like to use these radios for Night Time DX'ing.

The 5Z4 is a real odd-ball tube! The first ones looked like a ballast tube, with a perforated metal housing and two sealed capsules in it. They only made it for a short time, as they were failure prone. The newer 5Z4's were like a metal 6F6. As I stated, the 5Y3 has a 5volt 2amp heater and the same pin-out. The 5U4 has a 3amp heater and too much current for the power transformer.
Regarding the flexible resistor, they all crumbled after so many years. I would use two resistors to equal a value closer to 450-470 ohms.
When DXing with a TRF or a non-AVC superhet, you have to have the volume control set to max, to have the sensitivity to seek out distant stations.
If the radio squeals or motorboats, you might have to use a tube shield on the 6A8GT.

[vortalexfan]

Originally Posted by dieseljeep

The 5Z4 is a real odd-ball tube! The first ones looked like a ballast tube, with a perforated metal housing and two sealed capsules in it. They only made it for a short time, as they were failure prone. The newer 5Z4's were like a metal 6F6. As I stated, the 5Y3 has a 5volt 2amp heater and the same pin-out. The 5U4 has a 3amp heater and too much current for the power transformer.
Regarding the flexible resistor, they all crumbled after so many years. I would use two resistors to equal a value closer to 450-470 ohms.
When DXing with a TRF or a non-AVC superhet, you have to have the volume control set to max, to have the sensitivity to seek out distant stations.
If the radio squeals or motorboats, you might have to use a tube shield on the 6A8GT.

Well it seems that this radio may not work with an unshielded 6A8 tube in the set as the radio just hums/motorboats really badly when fed a signal from my signal generator or any other kind of signal and unfortunately I don't have any tube shields that are large enough in diameter to fit over an octal tube and short enough to go over a 6A8GT tube so that the gridcap can go over it...

when I have the original 6A8 metal tube in the radio it doesn't do anything (no audio or anything) but then when I turn off the radio I can hear my signal generator coming through loud and clear and undistorted (no hum/motorboating), which tells me that they wired this radio up so that it could only use metal cased tubes I think...

As for the resistors, I already have the 2 270 ohm 2 watt reistors installed in the radio, and I don't really want to have to go back in and remove it and reinstall new ones again as it was a pain in the butt to install them the first time around, is there any sort of problem with having a resistor that's 70 ohms higher than the original? The original resistor as I said previously didn't have any sort of tolerance listing listed for it in the Rider's Service Data which usually means its at least 20% tolerance or more, and 20% of 470 is 94 ohms which means that 540 Ohms is well within the original specs...

[dieseljeep]

Originally Posted by vortalexfan

Well it seems that this radio may not work with an unshielded 6A8 tube in the set as the radio just hums/motorboats really badly when fed a signal from my signal generator or any other kind of signal and unfortunately I don't have any tube shields that are large enough in diameter to fit over an octal tube and short enough to go over a 6A8GT tube so that the gridcap can go over it...

when I have the original 6A8 metal tube in the radio it doesn't do anything (no audio or anything) but then when I turn off the radio I can hear my signal generator coming through loud and clear and undistorted (no hum/motorboating), which tells me that they wired this radio up so that it could only use metal cased tubes I think...

As for the resistors, I already have the 2 270 ohm 2 watt reistors installed in the radio, and I don't really want to have to go back in and remove it and reinstall new ones again as it was a pain in the butt to install them the first time around, is there any sort of problem with having a resistor that's 70 ohms higher than the original? The original resistor as I said previously didn't have any sort of tolerance listing listed for it in the Rider's Service Data which usually means its at least 20% tolerance or more, and 20% of 470 is 94 ohms which means that 540 Ohms is well within the original specs...

That statement usually pertains to carbon resistors! Wire wounds are usually pretty close. The flexible ones are wire wound.

[old_coot88]

Sounds like there might be a 'floating grid' condition with the 6A8. That's when the DC path to the control grid is open (like an open in the RF coil or bandswitch). What resistance do you measure from the grid cap to ground?

(Since there's no AVC bus, the coil ties directly to ground according to the schematic.)

[vortalexfan]

Originally Posted by old_coot88

Sounds like there might be a 'floating grid' condition with the 6A8. That's when the DC path to the control grid is open (like an open in the RF coil or bandswitch). What resistance do you measure from the grid cap to ground?

(Since there's no AVC bus, the coil ties directly to ground according to the schematic.)

Its 5.5 Ohms from the 6A8 Grid Cap to chassis ground.

Is that what its supposed to measure, or do I have the aforementioned "floating grid" issue you spoke of?

[old_coot88]

Originally Posted by vortalexfan

Its 5.5 Ohms from the 6A8 Grid Cap to chassis ground.

Is that what its supposed to measure, or do I have the aforementioned "floating grid" issue you spoke of?

It's definitely not floating. A few (or several) ohms sounds right. Floating would be infinite resistance.

[vortalexfan]

Originally Posted by dieseljeep

That statement usually pertains to carbon resistors! Wire wounds are usually pretty close. The flexible ones are wire wound.

I have since removed the series 270 Ohm 2 Watt resistors and replaced it with a 270 ohm 2 watt and a 180 ohm 2 watt resistor in series to make a 450 Ohm 2 Watt resistor.

[dieseljeep]

Originally Posted by vortalexfan

I have since removed the series 270 Ohm 2 Watt resistors and replaced it with a 270 ohm 2 watt and a 180 ohm 2 watt resistor in series to make a 450 Ohm 2 Watt resistor.

In a self-biased output tube circuit, it's best to be close to the original cathode bias resistor. It'll save a distortion issue, later on!
You seem to get some, not-to-common sets to repair, restore!