I worked through the power supply problems on a seven-tube 1949 Stromberg-Carlson AM-FM. I replaced a selenium rectifier with a diode, replaced the filter choke and an open resistor. I also recapped the entire thing. Now it's up and running, but it has a hum. I notice that if I touch the 12AV6, which serves as AM detector-AVC-AF, that the hum goes away. Is that a clue to the problem? Also, according to schematics of several editions of this model (1204), the 12AV6 should be a 6AQ6. I found the radio in a junk store, and it had the 12AV6 in it. I'm confused that the 12AV6 would even work, thinking that the filament voltage would be wrong. My goal is to eliminate the hum. Any thoughts?

:I worked through the power supply problems on a seven-tube 1949 Stromberg-Carlson AM-FM. I replaced a selenium rectifier with a diode, replaced the filter choke and an open resistor. I also recapped the entire thing. Now it's up and running, but it has a hum. I notice that if I touch the 12AV6, which serves as AM detector-AVC-AF, that the hum goes away. Is that a clue to the problem? Also, according to schematics of several editions of this model (1204), the 12AV6 should be a 6AQ6. I found the radio in a junk store, and it had the 12AV6 in it. I'm confused that the 12AV6 would even work, thinking that the filament voltage would be wrong. My goal is to eliminate the hum. Any thoughts?

:I worked through the power supply problems on a seven-tube 1949 Stromberg-Carlson AM-FM. I replaced a selenium rectifier with a diode, replaced the filter choke and an open resistor. I also recapped the entire thing. Now it's up and running, but it has a hum. I notice that if I touch the 12AV6, which serves as AM detector-AVC-AF, that the hum goes away. Is that a clue to the problem? Also, according to schematics of several editions of this model (1204), the 12AV6 should be a 6AQ6. I found the radio in a junk store, and it had the 12AV6 in it. I'm confused that the 12AV6 would even work, thinking that the filament voltage would be wrong. My goal is to eliminate the hum. Any thoughts?

Mike: This is an AC/DC set, right?

I think the 12AV6 should work in a series string as a replacement for a 6AQ6 - they both have the same basing and the same heater current (0.15A).

I can't explain why touching the tube would eliminate the hum - unless somehow the tube is vibrating internally? Weird. Did it have a hum before you recapped it?

P.S. When you replace a selenium rectifier with a silicon diode, the B+ often goes up to high - as a result of the lower forward drop through the silicon diode. Might require a dropping resistor if the B+ is way to high. I don't visualize this as the cause of the hum.

:Mike: This is an AC/DC set, right?
:
:I think the 12AV6 should work in a series string as a replacement for a 6AQ6 - they both have the same basing and the same heater current (0.15A).
:
:I can't explain why touching the tube would eliminate the hum - unless somehow the tube is vibrating internally? Weird. Did it have a hum before you recapped it?
:
:P.S. When you replace a selenium rectifier with a silicon diode, the B+ often goes up to high - as a result of the lower forward drop through the silicon diode. Might require a dropping resistor if the B+ is way to high. I don't visualize this as the cause of the hum.
:
:
Mike,

Have you checked to see if the tube is missing a tube shield? Probably can look at the tube socket to determine this. Some tube shields just snapped on to metal shoulders that are part of the socket.

Radiodoc

::Mike: This is an AC/DC set, right?
::
::I think the 12AV6 should work in a series string as a replacement for a 6AQ6 - they both have the same basing and the same heater current (0.15A).
::
::I can't explain why touching the tube would eliminate the hum - unless somehow the tube is vibrating internally? Weird. Did it have a hum before you recapped it?
::
::P.S. When you replace a selenium rectifier with a silicon diode, the B+ often goes up to high - as a result of the lower forward drop through the silicon diode. Might require a dropping resistor if the B+ is way to high. I don't visualize this as the cause of the hum.
::
::
:Mike,
:
:Have you checked to see if the tube is missing a tube shield? Probably can look at the tube socket to determine this. Some tube shields just snapped on to metal shoulders that are part of the socket.
:
:Radiodoc
:Thanks, guys. Yes, it is AC-DC. I'll check the B+, and that's a good thought about the tube shield. I know that the tube doesn't have one.

Mike, check your connections to the 12AV6. Bad pin connection, loose, ground connections. By touching you are moving? Bad contact.
Bill VA

:::Mike: This is an AC/DC set, right?
:::
:::I think the 12AV6 should work in a series string as a replacement for a 6AQ6 - they both have the same basing and the same heater current (0.15A).
:::
:::I can't explain why touching the tube would eliminate the hum - unless somehow the tube is vibrating internally? Weird. Did it have a hum before you recapped it?
:::
:::P.S. When you replace a selenium rectifier with a silicon diode, the B+ often goes up to high - as a result of the lower forward drop through the silicon diode. Might require a dropping resistor if the B+ is way to high. I don't visualize this as the cause of the hum.
:::
:::
::Mike,
::
::Have you checked to see if the tube is missing a tube shield? Probably can look at the tube socket to determine this. Some tube shields just snapped on to metal shoulders that are part of the socket.
::
::Radiodoc
::Thanks, guys. Yes, it is AC-DC. I'll check the B+, and that's a good thought about the tube shield. I know that the tube doesn't have one.