OK - so here is the way I am going to do the work on other peoples' radios re: fuses. Sorry if I am boring you all, just like to get your opinions for the last time.

For AC/DC radios:

I think I am going to install a fuse in the line inside the chassis to protect the 35W4 tube (or other rectifier) and associated components from overcurrent damage in the event of an electrolytic short or other short to chassis. The fuse will be in a holder for easy replacement and will be a slo blo, sized at 2X the receiver current rating.

For transformer - powered radios:

The same rules apply as above, with the fuse rating set as a slo blo, at 2X the receiver current rating.

I will be putting a sticker underneath the case on the outside indicating that a fuse has been installed in this chassis.

Thanks All for letting me bore you some more. Any final thoughts?

Lou

Unfortunately a fuse in an AA5 would not protect the 35W4. Excessive current draw would blow the heater tap before any fuse could act. For transformer sets a fuse would protect the transformer.

OK - thanks Warren

So it appears that the AA5 is self protecting (but sacrifices a 35W4).I did have one here that caused a B+ resistor to smoke but the 35W4 did NOT fail.

Lou

:Unfortunately a fuse in an AA5 would not protect the 35W4. Excessive current draw would blow the heater tap before any fuse could act. For transformer sets a fuse would protect the transformer.
:

If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.

OK Warren

So I guess this carbon resistor failure would protect the tube and surrounding circuitry.

Since this is a 1 watt resistor unit (at 1200 ohms) , then the current would only have to climb to greater than 1 milliamp to exceed the 1 watt dissipation level. A fuse would not protect the unit.

Hmmm, seems most of the current to run the radio (radio draws 250 ms, or 30 watts) is in the FILAMENTS!!

Lou

:If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.
:
:

A 1200 ohm resistor dissipates 1 watt with 29 ma through it. If you want to protect the rectifier tube you can rewire the circuit so that the tap is not used to feed the rectifier plate. Then a fuse can be selected that will protect the whole set.

:OK Warren
:
:So I guess this carbon resistor failure would protect the tube and surrounding circuitry.
:
:Since this is a 1 watt resistor unit (at 1200 ohms) , then the current would only have to climb to greater than 1 milliamp to exceed the 1 watt dissipation level. A fuse would not protect the unit.
:
:Hmmm, seems most of the current to run the radio (radio draws 250 ms, or 30 watts) is in the FILAMENTS!!
:
:Lou
:
: :If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.
::
::
:
:

PS: THANKS WARREN for clarifying this.

Lou

:If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.
:
:

Warren - I am wondering....

Did the design engineers at RCA actually go thru this kind of failure analysis when they designed the All American Five?

Curious
Lou

:If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.
:
:

Lou, my 1960 or 62 Japanese AGS is a series string 7 tube AM/FM with a silicon diode AND a fuse.

http://www.radioatticarchives.com/radio.htm?radio=10915

The fuse is on one leg of the power cord.
I hadn't swapped the filter caps, just burned them in with the variac, I know, my bad. Had the radio playing for about 3 hours when there was a flash and the 2 amp fuse blew. (A 1 amp slo-blo would work, but I only had a 2.) The cathode bypass cap and another cap beside it were cooked, so I replaced them along with the filter caps. A wirewound 150 ohm resistor in the power supply end was scorched but survived. The diode was fine. I think at that time, a silicon diode was an expensive part so the designer felt it was worthy of fuse protection?

I have always assumed fuses were left out of radios because they were expensive, especially in the 30's and 40's.

:Warren - I am wondering....
:
:Did the design engineers at RCA actually go thru this kind of failure analysis when they designed the All American Five?
:
:Curious
:Lou
:
::If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.
::
::
:
:

interesting John

Well the point is - if I was doing this for myself at home, I can monitor the radio and I know what happens if it fails. Someone "out there" would now know what to do,may leave the radio on and it could cause a fire. In this case, a silicon diode may NOT fail and the circuit would draw heavy current as it did.

So it looks like we have several categories of circuits:

1. Transformer powered - which would benefit from a fuse to protect the transformer and prevent a fire

2. "Traditional" AA5 with a 35W4 or equivalent tube which will fail with the overcurrent

3. The silicon type AA5, which could draw HEAVY current as in your case - this may benefit from a fuse

Lou

:Lou, my 1960 or 62 Japanese AGS is a series string 7 tube AM/FM with a silicon diode AND a fuse.
:
:http://www.radioatticarchives.com/radio.htm?radio=10915
:
:The fuse is on one leg of the power cord.
:I hadn't swapped the filter caps, just burned them in with the variac, I know, my bad. Had the radio playing for about 3 hours when there was a flash and the 2 amp fuse blew. (A 1 amp slo-blo would work, but I only had a 2.) The cathode bypass cap and another cap beside it were cooked, so I replaced them along with the filter caps. A wirewound 150 ohm resistor in the power supply end was scorched but survived. The diode was fine. I think at that time, a silicon diode was an expensive part so the designer felt it was worthy of fuse protection?
:
:I have always assumed fuses were left out of radios because they were expensive, especially in the 30's and 40's.
:
:
::Warren - I am wondering....
::
::Did the design engineers at RCA actually go thru this kind of failure analysis when they designed the All American Five?
::
::Curious
::Lou
::
:::If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.
:::
:::
::
::
:
:

If you don't have a proper fuse how about a small value, low wattage resistor in series with the silicon diode? A resistor is often used in CFL lights as a fuse.

Regardless of what's used a radio will require service when it blows.

Norm

:interesting John
:
:Well the point is - if I was doing this for myself at home, I can monitor the radio and I know what happens if it fails. Someone "out there" would now know what to do,may leave the radio on and it could cause a fire. In this case, a silicon diode may NOT fail and the circuit would draw heavy current as it did.
:
:So it looks like we have several categories of circuits:
:
:1. Transformer powered - which would benefit from a fuse to protect the transformer and prevent a fire
:
:2. "Traditional" AA5 with a 35W4 or equivalent tube which will fail with the overcurrent
:
:3. The silicon type AA5, which could draw HEAVY current as in your case - this may benefit from a fuse
:
:Lou
:
::Lou, my 1960 or 62 Japanese AGS is a series string 7 tube AM/FM with a silicon diode AND a fuse.
::
::http://www.radioatticarchives.com/radio.htm?radio=10915
::
::The fuse is on one leg of the power cord.
::I hadn't swapped the filter caps, just burned them in with the variac, I know, my bad. Had the radio playing for about 3 hours when there was a flash and the 2 amp fuse blew. (A 1 amp slo-blo would work, but I only had a 2.) The cathode bypass cap and another cap beside it were cooked, so I replaced them along with the filter caps. A wirewound 150 ohm resistor in the power supply end was scorched but survived. The diode was fine. I think at that time, a silicon diode was an expensive part so the designer felt it was worthy of fuse protection?
::
::I have always assumed fuses were left out of radios because they were expensive, especially in the 30's and 40's.
::
::
:::Warren - I am wondering....
:::
:::Did the design engineers at RCA actually go thru this kind of failure analysis when they designed the All American Five?
:::
:::Curious
:::Lou
:::
::::If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.
::::
::::
:::
:::
::
::
:
:

Thanks NORM

I am just trying to prevent any kind of fire from occurring in AA5 radios that I fix for people. It seems that the circuit does not lend itself to the use of a fuse, BUT it does appear to be somewhat self-limiting.

Lou

:If you don't have a proper fuse how about a small value, low wattage resistor in series with the silicon diode? A resistor is often used in CFL lights as a fuse.
:
:Regardless of what's used a radio will require service when it blows.
:
:Norm
:
::interesting John
::
::Well the point is - if I was doing this for myself at home, I can monitor the radio and I know what happens if it fails. Someone "out there" would now know what to do,may leave the radio on and it could cause a fire. In this case, a silicon diode may NOT fail and the circuit would draw heavy current as it did.
::
::So it looks like we have several categories of circuits:
::
::1. Transformer powered - which would benefit from a fuse to protect the transformer and prevent a fire
::
::2. "Traditional" AA5 with a 35W4 or equivalent tube which will fail with the overcurrent
::
::3. The silicon type AA5, which could draw HEAVY current as in your case - this may benefit from a fuse
::
::Lou
::
:::Lou, my 1960 or 62 Japanese AGS is a series string 7 tube AM/FM with a silicon diode AND a fuse.
:::
:::http://www.radioatticarchives.com/radio.htm?radio=10915
:::
:::The fuse is on one leg of the power cord.
:::I hadn't swapped the filter caps, just burned them in with the variac, I know, my bad. Had the radio playing for about 3 hours when there was a flash and the 2 amp fuse blew. (A 1 amp slo-blo would work, but I only had a 2.) The cathode bypass cap and another cap beside it were cooked, so I replaced them along with the filter caps. A wirewound 150 ohm resistor in the power supply end was scorched but survived. The diode was fine. I think at that time, a silicon diode was an expensive part so the designer felt it was worthy of fuse protection?
:::
:::I have always assumed fuses were left out of radios because they were expensive, especially in the 30's and 40's.
:::
:::
::::Warren - I am wondering....
::::
::::Did the design engineers at RCA actually go thru this kind of failure analysis when they designed the All American Five?
::::
::::Curious
::::Lou
::::
:::::If the resistor you are referring to is the one in the pi circuit ( around 2K ) current draw after this resistor would cause this resistor to heat up. The hotter it gets the higher the resistance is. The 35W4 is somewhat protected by the resistor climbing in value. The resistor would finally fail and smoke. If you look at schematics for AA5 you will see the heater tap is connected to the plate.
:::::
:::::
::::
::::
:::
:::
::
::
:
:

The chances of a fire caused by an AA5 or any other kind of radio is about as likely as being hit by an errant asteroid.

:Thanks NORM
:
:I am just trying to prevent any kind of fire from occurring in AA5 radios that I fix for people. It seems that the circuit does not lend itself to the use of a fuse, BUT it does appear to be somewhat self-limiting.
:
:Lou

Jerry

That's usually right but I knew an Insurance Adjuster that went to homes after radio caused fires.

Norm

:The chances of a fire caused by an AA5 or any other kind of radio is about as likely as being hit by an errant asteroid.
:
::Thanks NORM
::
::I am just trying to prevent any kind of fire from occurring in AA5 radios that I fix for people. It seems that the circuit does not lend itself to the use of a fuse, BUT it does appear to be somewhat self-limiting.
::
::Lou
:
:

OK Guys

All the other types are covered.

So maybe I should let the AA5 types self limit and have either a power supply carbon resistor or the 35W4 filament fail. Sounds OK then?

Lou

:Jerry
:
: That's usually right but I knew an Insurance Adjuster that went to homes after radio caused fires.
:
:Norm
:
::The chances of a fire caused by an AA5 or any other kind of radio is about as likely as being hit by an errant asteroid.
::
:::Thanks NORM
:::
:::I am just trying to prevent any kind of fire from occurring in AA5 radios that I fix for people. It seems that the circuit does not lend itself to the use of a fuse, BUT it does appear to be somewhat self-limiting.
:::
:::Lou
::
::
:
:

If you really feel you want to help an AA5 out, this is what you can do. If the radio has a dial lamp. Shunt the lamp with a 47 ohm 1 watt resistor. This would protect the tap in the rectifier tube if the dial lamp burns out. Also with the cold start lamp flash. Does not decrease lamp intensity. A radio without a dial lamp, still shunt the heater tap with the same resistor. Ad a 33-ohm half-watt flameproof resistor in series with the cathode of the rectifier tube. This resistor is now the weak link. Goes out with a little poof if any overload on B+. The B+ is slightly reduced with this resistor.

Norm,

I wonder if that was a variation on the "electrical origin" fires. Knowing how hard it is to get wires hot enough to burn anything without something truly stupid like coins in fuse boxes, electric heaters with rags drouped over them, or the infamous curtain burners I'm always more than a little suspicious that when fire inspectors can't find an obvious cause "electrical" becomes the default.

:Jerry
:
: That's usually right but I knew an Insurance Adjuster that went to homes after radio caused fires.
:
:Norm
:
::The chances of a fire caused by an AA5 or any other kind of radio is about as likely as being hit by an errant asteroid.
::
:::Thanks NORM
:::
:::I am just trying to prevent any kind of fire from occurring in AA5 radios that I fix for people. It seems that the circuit does not lend itself to the use of a fuse, BUT it does appear to be somewhat self-limiting.
:::
:::Lou
::
::
:
:

It's not hard to get an extension cord smoking if you put a load on it, combined with a poor connection. The breaker or fuse in the panel won't pop if the current draw is under 15 amps. A radio power cord can start a fire, no doubt, but an AA5 doesn't draw that much current, less than a table lamp. Now if the cord insulation is rotten, the fuse or resistor won't prevent a cord fire.
Maybe that is what made the radios suspect, rotten cords.

:Norm,
:
:I wonder if that was a variation on the "electrical origin" fires. Knowing how hard it is to get wires hot enough to burn anything without something truly stupid like coins in fuse boxes, electric heaters with rags drouped over them, or the infamous curtain burners I'm always more than a little suspicious that when fire inspectors can't find an obvious cause "electrical" becomes the default.
:
:
::Jerry
::
:: That's usually right but I knew an Insurance Adjuster that went to homes after radio caused fires.
::
::Norm
::
:::The chances of a fire caused by an AA5 or any other kind of radio is about as likely as being hit by an errant asteroid.
:::
::::Thanks NORM
::::
::::I am just trying to prevent any kind of fire from occurring in AA5 radios that I fix for people. It seems that the circuit does not lend itself to the use of a fuse, BUT it does appear to be somewhat self-limiting.
::::
::::Lou
:::
:::
::
::
:
: