The radio is a Canadian Addison model 57, sorry, folks, it's a plain Jane woody, manuf 1952.
Challenge #1, the back label shows a 50B5 power tube, but the tube is a 50L6. It looks original, with the Addison logo. As far as I can tell the radio left the factory with this modification. They were using up surplus octal tubes I suppose.
But ... #2 somebody has monkeyed with the 12AT6 detector circuit, adding a resistor, which is now burnt beyond recognition.
The mystery resistor goes from the 12AT6 pin2 (cathode) to pin 8 on the 50L6. From this pin 8, a .01 mfd cap goes to pin 3, thence a wire to the output transformer. I found a schematic for a Model 55 here.
http://pacifictv.ca/schematics/addison55.pdf
To that schematic, convert the power tube to a 5oL6.
Like other circuits with 12AT6 tubes I've looked at, it shows the cathode pin 2 going to ground. I'm thinking I should do the same here.

If the cathode bias resistor (now burned up)in series with a .01 cap is a valid hookup, what would a typical value be? I might try that.

:The radio is a Canadian Addison model 57, sorry, folks, it's a plain Jane woody, manuf 1952.
:Challenge #1, the back label shows a 50B5 power tube, but the tube is a 50L6. It looks original, with the Addison logo. As far as I can tell the radio left the factory with this modification. They were using up surplus octal tubes I suppose.
:But ... #2 somebody has monkeyed with the 12AT6 detector circuit, adding a resistor, which is now burnt beyond recognition.
:The mystery resistor goes from the 12AT6 pin2 (cathode) to pin 8 on the 50L6. From this pin 8, a .01 mfd cap goes to pin 3, thence a wire to the output transformer. I found a schematic for a Model 55 here.
:http://pacifictv.ca/schematics/addison55.pdf
:To that schematic, convert the power tube to a 5oL6.
:Like other circuits with 12AT6 tubes I've looked at, it shows the cathode pin 2 going to ground. I'm thinking I should do the same here.
:
:If the cathode bias resistor (now burned up) to pin 8 with a .01 cap to pin 3 is a valid hookup, what would a typical value be? I might try that.
:
Well I think I have it. The mystery resistor is a bias R for the 50L6, so could be 150 ohms. Pin 2 of the 12AT6 needs to be tied to B-. Then that was a convenient point to connect the power tube bias resistor.
Now why the resistor burned? Pin 2 was not grounded (to B-).

If this resistor is the cathode bypass resistor on the 50L6. The resistor could burn up if the 50L5 has a nasty heater to cathode short.

:If this resistor is the cathode bypass resistor on the 50L6. The resistor could burn up if the 50L5 has a nasty heater to cathode short.
:

Thanks, Warren. I will check the tube. I don't have a tube tester anymore, but a short should be easy to detect?
That resistor was replaced once and burned again, with the original power tube still in place.

Pin 2 of the 12AT6 is indeed wired straight to the B-, so it is biased correctly after all. They tied pins 2 and 5 together and there is a small black wire from pin 5 to a terminal on the volume pot, which is at B- potential.
(After I printed off a schematic and changed the pin numbers, it began to make more sense :>)

::If this resistor is the cathode bypass resistor on the 50L6. The resistor could burn up if the 50L5 has a nasty heater to cathode short.
::
:
:Thanks, Warren. I will check the tube. I don't have a tube tester anymore, but a short should be easy to detect?
:That resistor was replaced once and burned again, with the original power tube still in place.
:
:Pin 2 of the 12AT6 is indeed wired straight to the B-, so it is biased correctly after all. They tied pins 2 and 5 together and there is a small black wire from pin 5 to a terminal on the volume pot, which is at B- potential.
:(After I printed off a schematic and changed the pin numbers, it began to make more sense :>)
:

Update - the tube does not indicate a dead short between the heater and cathode when cold. When hot, maybe. When I first checked it out, I saw the black resistor, but the heaters warmed up ok, before the tendrils of smoke told me to shut her down.

One other thing I did not mention - the 12AT6 has been switched out. It is actually a 12AV6. Would the slightly more powerful preamp tube affect the power tube bias resistor in a bad way?

And finally, I have a few 180 ohm resistors. Should I use that or go look for a 150 ohm?

The use of a 12AV6 in place of the 12AT6 would not effect the 50L6 in any bad way. You can use a 180 ohm resistor in place of the 150. Slight less gain. If the 50L6 does develop a heater to cathode short, it would make a hum though the speaker.

:The use of a 12AV6 in place of the 12AT6 would not effect the 50L6 in any bad way. You can use a 180 ohm resistor in place of the 150. Slight less gain. If the 50L6 does develop a heater to cathode short, it would make a hum though the speaker.
:

With the old tubes in, burnt resistor, electrolytics and paper caps replaced, the radio played well for 3 mins, then volume cut out to very faint and garbled. When I cut power, volume peaked for a second. I'll try some tube substitutions manyana. The new resistor stayed cool.

::The use of a 12AV6 in place of the 12AT6 would not effect the 50L6 in any bad way. You can use a 180 ohm resistor in place of the 150. Slight less gain. If the 50L6 does develop a heater to cathode short, it would make a hum though the speaker.
::
:
:With the old tubes in, burnt resistor, electrolytics and paper caps replaced, the radio played well for 3 mins, then volume cut out to very faint and garbled. When I cut power, volume peaked for a second. I'll try some tube substitutions manyana. The new resistor stayed cool.
:
Swapped all tubes, replaced a couple of dubious resistors. Radio played for 8 minutes then died to a very low volume. Is this a symptom of silver mica disease? Volume pot? One of the mica caps?

The silver mica disease normally is a crackling static noise. But could be the start of the problem. Sometimes tapping on the IF cans will stop or start the problem. One of the mica capacitors could also be leaky. You can use a can of compressed air upside down and use it as a freeze mist. Dribble the mist on a suspect mica cap. Such as C-8 C-15 & C-16. Also check the B+ on pin 7 of the 12AV6. R-7 330K could be up in value.

:The silver mica disease normally is a crackling static noise. But could be the start of the problem. Sometimes tapping on the IF cans will stop or start the problem. One of the mica capacitors could also be leaky. You can use a can of compressed air upside down and use it as a freeze mist. Dribble the mist on a suspect mica cap. Such as C-8 C-15 & C-16. Also check the B+ on pin 7 of the 12AV6. R-7 330K could be up in value.
:

Thanks, Warren, this helps a lot.
My patient is determined to die in the OR. The symptoms are worse every time I power it up. Earlier today, touching the slug on IF1 with a screwdriver brought the volume back for a moment or two. When that wore off, doing the same on IF2 had a similar but more short lived effect. Now, this no longer does the trick.
So on the bright side, this can't be a gradual corrosion on the silver/mica in the IF cans. That's my theory and I'm sticking to it (till proven wrong:>).
I found R7 was high at about 420K, just like you predicted, so I subbed it temporarily with a variable R set to 330K. C9 had been replaced before by a .01 mfd, so I replaced it with a .02 per the schemo.
Now, the radio is very quiet and no longer responding to stray capacitance or whatever was working before. 12AV6 plate voltage is about 63 vdc, 50L6 plate is about 83 vdc.

C15 and 16 and the resistor between them appears to be a small ceramic unit with 3 leads. I've been trying to ignore it, but since you are pointing in that direction, I will next try bypassing that with some discreet components.

::The silver mica disease normally is a crackling static noise. But could be the start of the problem. Sometimes tapping on the IF cans will stop or start the problem. One of the mica capacitors could also be leaky. You can use a can of compressed air upside down and use it as a freeze mist. Dribble the mist on a suspect mica cap. Such as C-8 C-15 & C-16. Also check the B+ on pin 7 of the 12AV6. R-7 330K could be up in value.
::
:
:Thanks, Warren, this helps a lot.
:My patient is determined to die in the OR. The symptoms are worse every time I power it up. Earlier today, touching the slug on IF1 with a screwdriver brought the volume back for a moment or two. When that wore off, doing the same on IF2 had a similar but more short lived effect. Now, this no longer does the trick.
:So on the bright side, this can't be a gradual corrosion on the silver/mica in the IF cans. That's my theory and I'm sticking to it (till proven wrong:>).
:I found R7 was high at about 420K, just like you predicted, so I subbed it temporarily with a variable R set to 330K. C9 had been replaced before by a .01 mfd, so I replaced it with a .02 per the schemo.
:Now, the radio is very quiet and no longer responding to stray capacitance or whatever was working before. 12AV6 plate voltage is about 63 vdc, 50L6 plate is about 121 vdc. (Correction)
:
:C15 and 16 and the resistor between them appears to be a small ceramic unit with 3 leads. I've been trying to ignore it, but since you are pointing in that direction, I will next try bypassing that with some discreet components.

R13 measures 39K ohms and there is infinite R across the caps, C15 and C16.
:

By touching the slug in the IF can with a metal screwdriver, the metal screwdriver become part of the ferrite slug. This detunes the IF or in this case may have tuned it closer to the 455KC. This would indicate the IF might have the silver migration problem. The silver mica migrates and shorts the primary to the secondary winding of the coils. You can disconnect one side of the IF transformer, read with an ohm meter on high scale from primary to secondary. Any reading would indicate a short from primary to secondary. This would cause a silent radio.

:By touching the slug in the IF can with a metal screwdriver, the metal screwdriver become part of the ferrite slug. This detunes the IF or in this case may have tuned it closer to the 455KC. This would indicate the IF might have the silver migration problem. The silver mica migrates and shorts the primary to the secondary winding of the coils. You can disconnect one side of the IF transformer, read with an ohm meter on high scale from primary to secondary. Any reading would indicate a short from primary to secondary. This would cause a silent radio.
:
:

What was odd, was that the volume stayed up after removing the screwdriver, then died away after 2 minutes. It's a plastic handled screwdriver with about 2 inches of steel blade. This PM, I wasn't able to recreate this phenomenon. The radio still plays a strong local station, but is very weak.
These are the mini IF cans with only one slug and they are suspects, all right. I'll check them both for shorts.

Slugs are top and bottom. On some IF's the two slugs are tuned by a long adjustment plastic tool with the hex at the end. This way you can tune the top and bottom slugs by sliding the tool down into the bottom slug.

:Slugs are top and bottom. On some IF's the two slugs are tuned by a long adjustment plastic tool with the hex at the end. This way you can tune the top and bottom slugs by sliding the tool down into the bottom slug.
:
Thanks, I see that now. I was able to get volume to kick in with a plastic screwdriver on the IF top slug last night. Then I was able to get the same effect by standing back with my hands in my pockets. :>)
I loosened the clip on the IF#2 and reclipped it to the chassis. I held a cold butter knife from the freezer against the various mica caps to no effect. Sorry it did not have a catalin handle. (It had a metal handle that could have killed me. Kids, don't try this.)
I went over some solder joints and cleaned the volume pot and sockets again. The poor thing was badly out of alignment and the antenna trimmers were cranked in tight. It is now playing for extended periods under observation. I will give it a proper alignment and that should do it. Thanks for your help. PS, I found no short in IF 1.

::Slugs are top and bottom. On some IF's the two slugs are tuned by a long adjustment plastic tool with the hex at the end. This way you can tune the top and bottom slugs by sliding the tool down into the bottom slug.
::
:Thanks, I see that now. I was able to get volume to kick in with a plastic screwdriver on the IF top slug last night. Then I was able to get the same effect by standing back with my hands in my pockets. :>)

Update - the culprit was a cold solder joint, where 4 components and a lead from IF 1 all come together on a terminal strip. Although I'd resoldered that point, the lead from the IF secondary was making intermittent contact.