Damage to valve amp when powered up but disconnected to speakers?

[Horsebox]

Hello all,

Well, I'm hanging my head in shame. I've gone and swapped over some speakers and I left the amp on in the process. Complete twit, but it's done now.

The instruction manual for the amp specifically warns of damage if this is done.

Having connected the amp back up to the new speakers there is a slight hum coming from the speakers that wasn't there before. It doesn't vary with volume. You can also hear the faint metallic sounds of the valves as they warm up, and some scratching. The pre-amp valves produce a sharp sound when tapped lightly.

Does anyone have a clue what, if anything, I may have cooked? Anyone know what damage to inspect for specifically?

Thanks for reading and any words of wisdom. I'm off to the naughty step.

[kevinkr]

In this case probably nothing. Usually the damage that occurs is a fried output transformer which is stunningly obvious. (Smoke, small fires, blown fuses, bad or no sound) The damage occurs when the amplifier is playing loudly and the speaker is disconnected.

I suspect all of the noises were also present with the previous speakers, but if they were significantly less efficient would be less noticeable.

Sounds like no harm done. All noises you describe are pretty usual, and are exacerbated by more efficient speakers.

[DF96]

When people listen for a difference they are very likely to hear a difference, even if it is not really there. In this case a change of speaker is very likely to make a real difference. As kevinkr says, the most likely fault is clearly not present so don't worry. Try swapping back to the original speakers - but switch off this time!

[Horsebox]

Thanks for the replies. My apologies, I should have said in the OP that the first thing I did when I noticed the hum was revert back to the original speakers which also then revealed the hum. I can categorically say it wasn't there before as I made a point of listening for it when I first received the amp.

The speaker change was from a £100 pair to a £350 pair of B&Ws which are supposedly revealing.

I guess it's possible that as the amp "beds in" from new that it's character may change a bit, but it seems too much of a coincidence given that I mishandled it at the same time.

At the end of the day though it still works ok (no smoke, explosions or otherwise!), but I just have this niggling doubt that I may have subtly knackered it a bit. Perception is everything though, perhaps I'm just not that impressed with the new speakers!

So the specific risk of leaving on with no speakers connected is to the OPTs?

I'm tempted to take the bottom off to see if there's anything obvious. Or maybe I should get out more!

Kind regards.

[kavermei]

As long as your amp was not actually outputting any music, no harm done. Don't lose any sleep over it!

[kavermei]

Originally Posted by Horsebox

So the specific risk of leaving on with no speakers connected is to the OPTs?

Primarily to the OPTs (internal arc-over due to high induced voltages) and secondarily the output tubes. Depends on where the high induced voltage sparks over first.

But this is only when your're actually playing loud music over the amp while its output is disconnected.

In the end, tube amps can stand quite some amount of abuse, so I'd be surprised if any lasting damage is done in this case.

[Horsebox]

Thanks for the reply kavermei (and others). I set the volume to zero before swapping the speakers so hopefully no damage done.

Lightly tapping any of the eight valves is reproduced through the speakers. It's far more pronounced if the pre-amp valves are tapped compared to the KT88s. I don't know if this happened before as I didn't test it. Would you say this is normal?

Apologies for the newbie questions, and thanks for the help.

[kevinkr]

Originally Posted by Horsebox

Thanks for the reply kavermei (and others). I set the volume to zero before swapping the speakers so hopefully no damage done.

Lightly tapping any of the eight valves is reproduced through the speakers. It's far more pronounced if the pre-amp valves are tapped compared to the KT88s. I don't know if this happened before as I didn't test it. Would you say this is normal?

Apologies for the newbie questions, and thanks for the help.

Yes, this is completely normal.. Most tubes are microphonic to a degree and some much more so than others. It is also expected that the more gain is present after the tube that is tapped the louder that noise will be. (i.e. tubes earlier in the signal chain will contribute more to the microphonics in general all other things being equal.) You can select tubes from a small lot to find less microphonic samples and also experiment with alternate brands to see if you can find ones that are less microphonic and/or sound preferable to you. (aka tube rolling)

[trobbins]

Further to kevinkr's comments about microphonics - all tubes are microphonic - and every valve amp will add its own blancmange of microphonic distortion to the output. To alleviate this distortion you would mechanically (and sometimes acoustically) decouple the valves/valve bases/amp chassis from room and speaker vibrations. Sometimes a particular valve is annoyingly the worst offender - but it doesn't sound like you have such a problem at the moment. Most times people don't notice that distortion is being added, because it is below their perception threshold.

Ciao, Tim

[Miles Prower]

Originally Posted by Horsebox

Thanks for the replies. My apologies, I should have said in the OP that the first thing I did when I noticed the hum was revert back to the original speakers which also then revealed the hum. I can categorically say it wasn't there before as I made a point of listening for it when I first received the amp.

The speaker change was from a £100 pair to a £350 pair of B&Ws which are supposedly revealing.

If you are absolutely certain that the hum wasn't there originally, and that it's not just a case of more revealing speeks, well, revealing more, tneh you might want to see if the filter capacitors might have been partially poofed as a result of overvoltage due to running the OPTs without a load.