Thanks for the response.
I certainly do believe you, but as I said, I've never seen a non hand-waving explanation as to why it's bad to operate a tube amp without a load.
The principals at Rogue Audio have unquestioned integrity so I certainly believe what they say also- I would just like to know why.
Let me describe a set of circumstances.
1.The Rogues are used to play organ music on the Montis with sections of essentially only sub 300Hz music.The speakers present a load of 3kohms or there about under those circumstances. The organ is played very loudly and at frequencies which use large voltage excursions of the amp.
2. The system is tested for frequency response- the sweep is in two parts, one being from 1kHz to 10 Hz and is fairly lengthy in duration.
3. The room is tested repeatedly for resonances at sub 250Hz frequencies.
All of these actions apply high voltage sub 300Hz frequencies for lengthy periods of time, and although the panel is still connected there is no signal present at frequencies that would excite it and it is effectively open circuit.
The Rogues do not blow up, or even seem to suffer any damage. Why is this?
This is not a hypothetical situation in my home. All of these circumstances have happened many times, and yet the output stages haven't even changed bias current. No failed transformers, no wrecked KT120s.
Transformers and output tubes do just fail- output tubes all too often. In the case of the failed amps that you repaired why was the diagnosis that the cause was an open circuit? What is your diagnosis as to the actual failure process/mechanism?
What impedance doesn't count as an open circuit- for example if I was to place a permanent 300 ohm 5W resistor across the 4ohm output terminals of each of the monoamps would that be sufficient? Is 3k sufficient ? How about 1Mohm?
I've run LTSPICE simulations on Pentode and Triode output tube amps (using models for KT120s and KT88s) with no load and seen nothing that would cause me to believe that the output components might fail. Maybe I'm looking for the wrong thing...
All the best,
Wyn Palmer
Ok Wyn, I will try to answer you in the best way I can from real world experience.
Some examples : First was an observation of the aftermath of a tube amp connected to it`s proper load driven to max unclipped power on a test bench, by a previous employee that I was hired to replace..
This story of the first event, was told to me by my new boss in 1978.
The tech was running up a Fender guitar amp after repair to confirm it`s performance, when he some how unplugged the load resistor(dummy load while preforming the test. @ 1,000hz, he had his Simpson 260 analog meter connected to the amps output transformer at the time when the event happened..
Well Sir. when the load was removed the output transformer`s primary voltage soared as it probably went in to oscillation as well
The output transformer`s primary human hair diameter coated formvar(the varnish that is commonly used to coat bare wire for coil wire) became stressed with the sudden application of a very high voltage, and shorted it out from the spike of much higher than it was rated for insulation thus trashing the output transformer, plus the Simpson meter sustained serious damage as well !!
The reason that I learned of this event, was the prior tech left the Simpson 260 meter when he quit, and my new boss offered to give it to me, thus telling me the story of the no load output transformer story, and the pretty smoked meter guts, and it looked like lightning struck the rotary selector switch and a few precision resistors in which I eventually rebuilt the meter..
Another personal experience concerning this situation concerns a smart dummy load made by Sencore PA 81 that is not tube amp friendly, unless modified..
It was designed for SS amps..
If it even thinks whatever amp is connected to has DC, or a very low frequency component feeding it, it promptly disconnects the selected load resistor from the amp, with a LED indicator & clunk of the heavy duty relay disengaging the load!!
Great, if your testing a SS amp, but a real significant emotional event if your testing a tube amp of any type under power, as the output transformer instead of making it`s normal sine wave singing that can be heard on most output transformers while running the amp in to a load resistor, makes instead, a screeching sound, that I don`t want to hear, as I know the high voltage feeding it is soaring to very dangerous output transformer primary damaging levels..
There are more stories, but typing with one finger, because of my partial paralysis to explain what I know in real world experience is tiresome, and more than I feel like doing..
Your tube amps, do/experiment as you please, I will respect my conservative methods/reasons, as they have served my customers & me very successfully for over 4 decades.
Take care,
Kind regards, OKB