why didnt my Amp Blow up? Idled with no load

[Reeltarded]

Oh I see what you are saying.

Anything that drives output is off-idle. I bet you are right.

The only easy way I know to test this is not going to happen.

[es345]

Regarding NFB and oscillations here is my observation after successful building tube amps (as a hobby) for about 40 years:

- for sure you can get oscillation caused by NFB, I had it more than enough especially in early times
- there were different root causes: e.g. wrong cabling structure, too much of feedback, feedback over to many stages etc.
- in all of the cases I had observed this was happening both with or without load in the same way, would be interesting to see a case where it is different
- in all cases it was a bad design - done by myself or by the designer of the schematic I had copied -.
- the good thing: I have learnt a lot .

There is another interesting method to handle the "no load" situation:
Look at the Fender 400PS. It has 3 separate OT secondary windings each connected to a separate output jack . The cathode to ground connection of a tube pairs is disabled if there is no plug in the jack.

[Reeltarded]

I have seen massive electron bloom in an unloaded amp's output before. That would be anecdotal evidence to support most all of the above 5 or so posts, no?

Something is making the funny noise.

[martin manning]

Regarding NFB and oscillations here is my observation after successful building tube amps (as a hobby) for about 40 years:

- for sure you can get oscillation caused by NFB, I had it more than enough especially in early times
- there were different root causes: e.g. wrong cabling structure, too much of feedback, feedback over to many stages etc.
- in all of the cases I had observed this was happening both with or without load in the same way, would be interesting to see a case where it is different
- in all cases it was a bad design - done by myself or by the designer of the schematic I had copied -.
- the good thing: I have learnt a lot .

In all of these cases your negative feedback was turned into or was accompanied by positive feedback, which is required for oscillation. Again, if the output jack is shorted, there is no feedback from the output.

There is another interesting method to handle the "no load" situation:
Look at the Fender 400PS. It has 3 separate OT secondary windings each connected to a separate output jack . The cathode to ground connection of a tube pairs is disabled if there is no plug in the jack.

Ampeg did something similar by lifting the PI tail, I believe.

[es345]

In all of these cases your negative feedback was turned into or was accompanied by positive feedback, which is required for oscillation.

You are right, I should have been more precise: the design faults has lead to a condition of a total phase shift of 360 degree while having an amplification >=1 at the oscillation frequency