Tube amp math

zingo

I need the mathematical justification for why it is bad to run a tube amp without a load. I know it's a strange request, but can anyone help?

unc2701

No load means that when the High voltage side on the output transformer swings there's nothing pulling the generated EM field off, so it goes from being a transformer to being an electromagnet. It'll melt the primaries in your output tranny... but I think this is usually just a guitar amp phenomena- most home audio gear has protection circuits.

mhardy6647

Here's a pretty good explanation in this PDF from One Electron:

http://www.one-electron.com/Trans/UBT2_10.pdf

to wit: "As with any output transformer, the UBT-2 [transformer] should always have a load connected when driven. If there is no load and the output tube is over-driven, the high inductance of the primary will cause extremely high voltages to be produced, damaging either the output tubes or the transformer."

Where it gets tricky is that "over driven" statement. In some amps, this is (so to speak) a self-inducing condition. Some amps will self-destruct OPTs and or output tubes within seconds of no-load operation; others don't really seem to mind at all. I suspect it is related to the amp's architecture and specifically the amount and or execution of negative feedback. I am just guessing, though.

EDIT: Take a look a the "feedback" section of this page on a simple little series-circuit amplifier from the late, great Fred Nachbaur's website: http://www.dogstar.dantimax.dk/tubestuf/4x4howit.htm

zingo

Thanks

I-SIG

mhardy6647 wrote: »

Here's a pretty good explanation in this PDF from One Electron:

http://www.one-electron.com/Trans/UBT2_10.pdf

to wit: "As with any output transformer, the UBT-2 [transformer] should always have a load connected when driven. If there is no load and the output tube is over-driven, the high inductance of the primary will cause extremely high voltages to be produced, damaging either the output tubes or the transformer."

Where it gets tricky is that "over driven" statement. In some amps, this is (so to speak) a self-inducing condition. Some amps will self-destruct OPTs and or output tubes within seconds of no-load operation; others don't really seem to mind at all. I suspect it is related to the amp's architecture and specifically the amount and or execution of negative feedback. I am just guessing, though.

EDIT: Take a look a the "feedback" section of this page on a simple little series-circuit amplifier from the late, great Fred Nachbaur's website: http://www.dogstar.dantimax.dk/tubestuf/4x4howit.htm

Sounds almost like a ferroresonant condition.

Wes