Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pepi, Nov 20, 2005.
Can you run an amp without a load if the tubes are removed?
What usually kills a tube amp's output transformer is the flyback voltage that's generated with too high or no load on the amp. However, with no output tubes to supply a signal to the output transformer, no flyback voltage can be generated....so it should be safe to run a tube amp with no output tubes and no load.
A load is not required with power tubes pulled.
More than "no signal", the OT primary is disconnected from the rest of the amp when you pull the power tubes.
Party true. If you pull the output tubes, the amp's power supply is still applying high voltage to the output tube sockets via the output transformer and screen resistors. Even though there's no current being drawn through the output transformer & screen resistors, if you go poking around the inside the amp, you can still get a good jolt from the tube socket terminals.
Agreed, I should have been more clear in stating that all of the killing voltages are still present. It is the signal path that is interrupted by pulling the tube (which does, for all intents and purposes, eliminate the need for a load on the amp)
For what purposes would one want to pull the power tubes and run the amp? Is it for testing purposes?
Can't speak for others, but I pull power tubes all the time during bench testing/troubleshooting. It helps to isolate problems and also saves the power section/speaker from abuse when the preamp is doing evil things, when you're troubleshooting the bias supply, etc.
I agree with Wakarusa that it would be fine in a few cases (not for extended periods) but again the question is...
Why would you need to do it? other than testing?
I must disagree! When you flip the Standby to On the surge creates a spike in the Output XFMR. When you switch it off the same is true.
With no tubes there is no load so voltage is very high- hello arc?
Too scary for me...
Seems to me that the instantaneous voltage on the OT primary when you flip the STBY switch is the same with or without power tubes installed - so the risk of an arc would also be the same in both situations.
I'll go one further and suggest that a tube socket and OT in good condition should never be at risk with rated unloaded B+ applied. The arcs that you do see tend to be caused by poor condition (failing OT insulation, crap on the output socket, etc.) or by the dreaded flyback from impedance mismatches -- and flyback voltages are far in excess of anything the PT can put out.
An amatuer speaking here, I always test for shorted power tubes when a fuse is blowing by removing the power tubes and powering up to see if the fuse stays intact.