I have recently aquired a M-W 84HA AC/DC set with 12SA7,12SK7,12SQ7,50C5,35Z5 combination. The 35z5 has a pilot lamp filament tap. When I received it, it was dead and no tubes lit. I put in a new #47 pilot lamp and it flashed like a light bulb. I checked and pins 2.3 were open on the 35Z5. I replaced it and the pilot lamp. I plugged it in, and all seemed normal. Then, just in a few seconds, the lamp got bright, it burned out, then the 35Z5 got bright and burned out as well. What should I look for in the filament circuit that could cause this type of behavior? It is possibly a shorted filament in one of the other tubes or a bad capacitor or what is likely here? I would really appreciate some help before I go through a few more expensive "fuses"! Dennis

The 50 volt tube has an intermittent heater short. It's the next tube in-line from the 35 volt one. When the 50 volt tube shorts, the 35 volt one takes the full line voltage. This is the most likely problem.

:The 50 volt tube has an intermittent heater short. It's the next tube in-line from the 35 volt one. When the 50 volt tube shorts, the 35 volt one takes the full line voltage. This is the most likely problem.

Sorry, I meant to say that it was a 50L6, not 50C5. It seems to light up before the flash of the #47 lamp and then the open on pins 2 and 3 on the 35Z5. I will try another 50L6 to see if the problem goes away. This is getting to be expensive! Thanks.

Hi Dennis,
Recently I had a simlar problem. Type 35Z5GT are expensive and getting rare. I didn't want to keep blowing the things out until I had the problem under control so I substituted a 1N4007 and 22 ohm 2 watt resistor for the diode section and the remaining filament of the old tube for the 35Z5GT filament. It seemed to work OK. I had to disconnect some of the wires from the tube socket to do this.

Before you try this, though, I would check the filter capacitors. Leaks in them can cause this problem.
The 50L6GT can be tested for filament shorts using tube testers. If you know something aobut your tube tester you can try this.
Make the pin the tube tester uses to supply the 50L6GT with filament a no-connect instead of a filament setting. Rout the filament to the tube cap connection. 50L6GT of course doesn't have a cap so it is free to use the connection.
Then place an AC volt meter from the tube cap connection to the filament pin (usually 2) on any of the unused sockets. The current will be high at first, but should come down and level off around 150 milliamperes.
If our suspicion is right, the tube will pull much more than this when heated.

In my radio the problem turned out to be a short between the filament supply and the B+ caused by a resistor nudged out of place. I didn't even need a soldering iron to fix it. Finding it was the hard part.

Best Regards,

Bill Grimm

Hi Dennis

You mention a few seconds and 35Z5 burned out. Does this mean 10 or so after tubes start drawing plate current?

Quite common for a load on the B+ line to cause filament of 35Z5 and lamp to burn out.

Be sure caps 7A and B (20 mf) have been replaced. Also replace the cap connected to pin #5 of the 50L6, C5 .005mf.

http://www.nostalgiaair.org/PagesByModel/823/M0011823.pdf

Bring up your radio with a 40 watt light in series with AC line. This will act as a fuse and light if there is a short.

Norm

:Hi Dennis,
: Recently I had a simlar problem. Type 35Z5GT are expensive and getting rare. I didn't want to keep blowing the things out until I had the problem under control so I substituted a 1N4007 and 22 ohm 2 watt resistor for the diode section and the remaining filament of the old tube for the 35Z5GT filament. It seemed to work OK. I had to disconnect some of the wires from the tube socket to do this.
:
: Before you try this, though, I would check the filter capacitors. Leaks in them can cause this problem.
: The 50L6GT can be tested for filament shorts using tube testers. If you know something aobut your tube tester you can try this.
: Make the pin the tube tester uses to supply the 50L6GT with filament a no-connect instead of a filament setting. Rout the filament to the tube cap connection. 50L6GT of course doesn't have a cap so it is free to use the connection.
: Then place an AC volt meter from the tube cap connection to the filament pin (usually 2) on any of the unused sockets. The current will be high at first, but should come down and level off around 150 milliamperes.
: If our suspicion is right, the tube will pull much more than this when heated.
:
: In my radio the problem turned out to be a short between the filament supply and the B+ caused by a resistor nudged out of place. I didn't even need a soldering iron to fix it. Finding it was the hard part.
:
:Best Regards,
:
:Bill Grimm
:

:Hi Dennis
:
: You mention a few seconds and 35Z5 burned out. Does this mean 10 or so after tubes start drawing plate current?
:
: Quite common for a load on the B+ line to cause filament of 35Z5 and lamp to burn out.
:
: Be sure caps 7A and B (20 mf) have been replaced. Also replace the cap connected to pin #5 of the 50L6, C5 .005mf.
:
:
:http://www.nostalgiaair.org/PagesByModel/823/M0011823.pdf
:
: Bring up your radio with a 40 watt light in series with AC line. This will act as a fuse and light if there is a short.
:
:Norm
:
::Hi Dennis,
:: Recently I had a simlar problem. Type 35Z5GT are expensive and getting rare. I didn't want to keep blowing the things out until I had the problem under control so I substituted a 1N4007 and 22 ohm 2 watt resistor for the diode section and the remaining filament of the old tube for the 35Z5GT filament. It seemed to work OK. I had to disconnect some of the wires from the tube socket to do this.
::
:: Before you try this, though, I would check the filter capacitors. Leaks in them can cause this problem.
:: The 50L6GT can be tested for filament shorts using tube testers. If you know something aobut your tube tester you can try this.
:: Make the pin the tube tester uses to supply the 50L6GT with filament a no-connect instead of a filament setting. Rout the filament to the tube cap connection. 50L6GT of course doesn't have a cap so it is free to use the connection.
:: Then place an AC volt meter from the tube cap connection to the filament pin (usually 2) on any of the unused sockets. The current will be high at first, but should come down and level off around 150 milliamperes.
:: If our suspicion is right, the tube will pull much more than this when heated.
::
:: In my radio the problem turned out to be a short between the filament supply and the B+ caused by a resistor nudged out of place. I didn't even need a soldering iron to fix it. Finding it was the hard part.
::
::Best Regards,
::
::Bill Grimm
::
Thanks to all of you for the kind replies. Now, I have some ideas on what to try next. I really appreciate the help. Dennis

Dennis,

What Mr. Leal refers to in putting a light bulb in series with the input cord is what is commonly referred to as a "dim bulb tester". You can see an assembled homebrew one at: http://www.oldtestequipmentarchives.com/item.htm?item=120

Clifton

::Hi Dennis
::
:: You mention a few seconds and 35Z5 burned out. Does this mean 10 or so after tubes start drawing plate current?
::
:: Quite common for a load on the B+ line to cause filament of 35Z5 and lamp to burn out.
::
:: Be sure caps 7A and B (20 mf) have been replaced. Also replace the cap connected to pin #5 of the 50L6, C5 .005mf.
::
::
::http://www.nostalgiaair.org/PagesByModel/823/M0011823.pdf
::
:: Bring up your radio with a 40 watt light in series with AC line. This will act as a fuse and light if there is a short.
::
::Norm
::
:::Hi Dennis,
::: Recently I had a simlar problem. Type 35Z5GT are expensive and getting rare. I didn't want to keep blowing the things out until I had the problem under control so I substituted a 1N4007 and 22 ohm 2 watt resistor for the diode section and the remaining filament of the old tube for the 35Z5GT filament. It seemed to work OK. I had to disconnect some of the wires from the tube socket to do this.
:::
::: Before you try this, though, I would check the filter capacitors. Leaks in them can cause this problem.
::: The 50L6GT can be tested for filament shorts using tube testers. If you know something aobut your tube tester you can try this.
::: Make the pin the tube tester uses to supply the 50L6GT with filament a no-connect instead of a filament setting. Rout the filament to the tube cap connection. 50L6GT of course doesn't have a cap so it is free to use the connection.
::: Then place an AC volt meter from the tube cap connection to the filament pin (usually 2) on any of the unused sockets. The current will be high at first, but should come down and level off around 150 milliamperes.
::: If our suspicion is right, the tube will pull much more than this when heated.
:::
::: In my radio the problem turned out to be a short between the filament supply and the B+ caused by a resistor nudged out of place. I didn't even need a soldering iron to fix it. Finding it was the hard part.
:::
:::Best Regards,
:::
:::Bill Grimm
:::
:Thanks to all of you for the kind replies. Now, I have some ideas on what to try next. I really appreciate the help. Dennis
: