Hi Dave,
I too agree with Kijanki. While the chances of damage occurring as a result of running unloaded under no signal conditions are small, they are definitely not zero.
One example: Suppose one of the small signal tubes suddenly develops a problem such as a short during the process. That could put a "signal" through the amp, and chances are that with no speaker connected you would not have any indication that was occurring. That "signal" might persist for hours, until you concluded the variac procedure. Chances are that one or both output transformers would be damaged as a result.
The reason that particular example comes to mind is that when I purchased my present amplifier, together with a set of brand new tubes, after listening for about 2 hours, from one second to the next the amp suddenly went from producing beautiful music to producing a loud eruption of continuous static-type noise from one of the speakers. It turned out that a small signal tube had abruptly developed a short. There was no damage, of course, since a speaker was connected and because I shut the amp off within a few seconds. I subsequently found that the loud noise occurred even when no music signal was being sent into the amp.
If you want to use a resistor, this 8 ohm 20 watt non-inductive resistor from Radio Shack should be suitable, given that the intent is to use it under no signal conditions, and that your amps are rated at around 20 watts. Or you could order something comparable online from Digikey.com. One for each channel, of course. It would be prudent to touch the body of the resistor from time to time during the procedure, to make sure it isn't getting hot. Any sign of warming, beyond what might be caused by heat from nearby tubes, would indicate that the amp has developed a problem.
Best,
-- Al
I too agree with Kijanki. While the chances of damage occurring as a result of running unloaded under no signal conditions are small, they are definitely not zero.
One example: Suppose one of the small signal tubes suddenly develops a problem such as a short during the process. That could put a "signal" through the amp, and chances are that with no speaker connected you would not have any indication that was occurring. That "signal" might persist for hours, until you concluded the variac procedure. Chances are that one or both output transformers would be damaged as a result.
The reason that particular example comes to mind is that when I purchased my present amplifier, together with a set of brand new tubes, after listening for about 2 hours, from one second to the next the amp suddenly went from producing beautiful music to producing a loud eruption of continuous static-type noise from one of the speakers. It turned out that a small signal tube had abruptly developed a short. There was no damage, of course, since a speaker was connected and because I shut the amp off within a few seconds. I subsequently found that the loud noise occurred even when no music signal was being sent into the amp.
If you want to use a resistor, this 8 ohm 20 watt non-inductive resistor from Radio Shack should be suitable, given that the intent is to use it under no signal conditions, and that your amps are rated at around 20 watts. Or you could order something comparable online from Digikey.com. One for each channel, of course. It would be prudent to touch the body of the resistor from time to time during the procedure, to make sure it isn't getting hot. Any sign of warming, beyond what might be caused by heat from nearby tubes, would indicate that the amp has developed a problem.
Best,
-- Al