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04-11-2022, 11:18 AM
Hi Ken
Welcome to the forum, Ken.
Your Q could go into Personal projects. I'll add some repair-oriented sections.
When you have a unit that has been "sleeping" for a while, you really should approach it as "broken equipment" in need of repair. In that case, you remove the power tubes and do a visual inspection inside the chassis. Screen-stops usually need amending to a flame-poof type and often a larger value.
The unit should be powered through your Power Limiting Safety Socket (PLSS) which uses incandescent bulbs in series with the unit to protect what isn't broken yet. It also gives old caps a small chance to reform in a controlled manner. The first lamp should be low-wattage.
Amps that have slept as long as your are likely to need new filter caps. DO NOT waste money on oil-filled caps, paper dielectric, or other voodoo things. There may be enough space inside the chassis that new modern caps can be installed there, if you want to leave the original; can caps in place for appearance. Buy caps that are 10k-hour or more life expectancy. Snap-mount caps can be mounted on perf-board, or be simply tied in place with the pins wired where they should go.
Check that the bias voltage is present right at the tube socket. This is the most important voltage in the chassis and it is crucial that it be high enough in value to control the tubes. If there is a pot to set it, adjust for maximum negaitive voltage at the tube socket. Only after this step is it safe to install the power tubes.
If you have a scope, you can test the amp with no load and see that a clean sine wave will pass through. The tubes only need a few milliamps bias to do this. This step needs a larger bulb in the PLSS.
When it seems like everything is working okay, you can bypass the PLSS and just use the amp's fuses for protection.
These old tube PA amps are likely to also need coupling caps and cathode-bypass caps replaced. There may also be resistor positions that should be renewed as carbon resistors are notoriously unreliable, especially 20%-tolerance cracked-carbons - just throw those in the garbage !! The all-important grid-bias path for the output tubes should be replaced with metal-film resistors. The symptom you describe may be a resistor problem, if there is a resistor feeding the screen node of the supply from the plate node. Basically, there is a lot of stuff that needs to be attended to before you even power the unit the first time.
Have fuun
Welcome to the forum, Ken.
Your Q could go into Personal projects. I'll add some repair-oriented sections.
When you have a unit that has been "sleeping" for a while, you really should approach it as "broken equipment" in need of repair. In that case, you remove the power tubes and do a visual inspection inside the chassis. Screen-stops usually need amending to a flame-poof type and often a larger value.
The unit should be powered through your Power Limiting Safety Socket (PLSS) which uses incandescent bulbs in series with the unit to protect what isn't broken yet. It also gives old caps a small chance to reform in a controlled manner. The first lamp should be low-wattage.
Amps that have slept as long as your are likely to need new filter caps. DO NOT waste money on oil-filled caps, paper dielectric, or other voodoo things. There may be enough space inside the chassis that new modern caps can be installed there, if you want to leave the original; can caps in place for appearance. Buy caps that are 10k-hour or more life expectancy. Snap-mount caps can be mounted on perf-board, or be simply tied in place with the pins wired where they should go.
Check that the bias voltage is present right at the tube socket. This is the most important voltage in the chassis and it is crucial that it be high enough in value to control the tubes. If there is a pot to set it, adjust for maximum negaitive voltage at the tube socket. Only after this step is it safe to install the power tubes.
If you have a scope, you can test the amp with no load and see that a clean sine wave will pass through. The tubes only need a few milliamps bias to do this. This step needs a larger bulb in the PLSS.
When it seems like everything is working okay, you can bypass the PLSS and just use the amp's fuses for protection.
These old tube PA amps are likely to also need coupling caps and cathode-bypass caps replaced. There may also be resistor positions that should be renewed as carbon resistors are notoriously unreliable, especially 20%-tolerance cracked-carbons - just throw those in the garbage !! The all-important grid-bias path for the output tubes should be replaced with metal-film resistors. The symptom you describe may be a resistor problem, if there is a resistor feeding the screen node of the supply from the plate node. Basically, there is a lot of stuff that needs to be attended to before you even power the unit the first time.
Have fuun
