Tempting Fate?

[nevetslab]

I'm sure I'm not the only one who's done this.....powered up the amp head I just made changes on yesterday when I came in this morning, and got initial plate, screen and cathode voltage & current readings, and trimmed them up to where I had been running them. 20 minute later, after typing in my notes, I then realized I didn't have the dummy load moved over and connected. Tube Amp. Though, not driving signal...just sitting at idle....but, WITHOUT A LOAD CONNECTED. Quickly moved my small load bank over and connected it.

Under these NON-signal drive conditions, with a stable amplifier, can this cause any major harm?

[g1]

Hasn't when I've ever done it.

[Jon Snell]

The only harm will come from a signal driving at a reasonable level or an oscillating amplifier.
No harm came from your ommision.

[g1]

Also consider whether the speaker out jack has a shorting switch or not. If it does, you will get away with a lot more mistakes.

[nevetslab]

The Speaker Output jacks do NOT have shorting terminals engaged, as both jacks are in parallel. The Normals are on the jacks, just not wired that way, as we find used in Fender amps. Standard practice is ALWAYS having a load on the output xfmr.

[g1]

Standard practice is ALWAYS having a load on the output xfmr.

Agree, but do you always remember to terminate the reverb send transformers? They are meant for 8 ohm and we often run amps with the reverb circuits disconnected.

[Leo_Gnardo]

Under these NON-signal drive conditions, with a stable amplifier, can this cause any major harm?

No harm done, no foul. Most Fenders will tolerate an open load on the output, many other amps too. Long time ago I noted in a Mesa Mk II (around 1986-7) output tubes gave a pulsating blue glow - indicating I guess a low frequency oscillation - if it didn't see a load.

[Helmholtz]

There can't be a general answer. Some amps tend to self-oscillate with open output - others don't.

I'd say if the amp produces sustained full output, everything should be ok.

[nevetslab]

Agree, but do you always remember to terminate the reverb send transformers? They are meant for 8 ohm and we often run amps with the reverb circuits disconnected.

Good point. I must confess when I've removed the chassis from a Combo Amp over to my service cradle, leaving the combo cabinet on the other bench, I rarely removed the tank over to the service cradle. So in those cases, that transformer isn't loaded. When driving signal from there, I couple the cabinet speakers via long extension cable, but never do with the Reverb. Luckily, the Reverb Circuit still works when the chassis has been moved back into the cabinet. I'll say here, I've never given it a thought. Opps.

[pdf64]

If the amp has a NFB loop and a reasonable margin of stability, then provided your test signal didn’t overdrive it, the NFB should work to keep everything good.

[35L6]

I blew up an OT testing without load ( accidently ) . I think when I touched the plate with my probe that was enough signal to do it .

[Helmholtz]

If the amp has a NFB loop and a reasonable margin of stability, then provided your test signal didn’t overdrive it, the NFB should work to keep everything good.

Quite a few conditions.
What do we know about margin of stability without a load here?

Often global NFB is actually the reason for instability and removing the load considerably increases loop gain and NFB ratio.
I'd expect an unloaded OT to have its main resonance around 1kHz .

[Chuck H]

Wow I dropped some likes on this thread! All good observations. Especially the point about a no load condition potentially changing the phase relationships and causing instability. That probably would have been detectable in testing under zero signal circumstances, but still an important observation. And bolstered by another poster mentioning that it actually happened to them when the meter probe was connected. And...

I have absolutely failed to connect a load to an amp under test. And with signal. And at full clip!!! Fortunately it was my own amp and it's using a 16XX series Hammond OT. I think those things only fail if two hobbits toss it into Mount Doom.

[drewl]

Before I knew what I was doing with tube amps, I did some stuff that definitely should have fried them, but got lucky!


i used to use my '73 Marshall 1987 to drive tube power amps, WITH NO LOAD on the Marshall!

I did fry my buddy's Vibro Champ when I connected a 15" speaker in parallel with the cabinet speaker.

Went downstairs to dinner, came back and my whole room was filled with putrid fried trans smoke.