Impedance matching to cabinet

[dopplerblueshift]

Out of interest, I know that turning on a head without an impedance load trashes the power tubes but what is the consequence of running the head with the incorrect impedance setting on the back panel for the cab (e.g. the cab is 8ohms and the switch is on 16 ohms)?

[fiveightandten]

Impedance mismatch doesn't generally hurt the tubes. It does however put stress on the output transformer. Running the amp on 16 ohms into an 8 ohm cab will burn out the output transformer. It's just a matter of how long it will take in order to do that.

-Nick

[bassdrop]

I think Andy said you can sometimes hear a gentle whistling from the OT itself when its severely stressed

[dopplerblueshift]

Thanks Guys,
Would this cause fuses to blow before damage was done?

[fiveightandten]

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by dopplerblueshift</i>
<br />Thanks Guys,
Would this cause fuses to blow before damage was done?
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

Not usually. It does stress the tubes a little, so one of them may short out by chance and pop the HT fuse...but the chances of that happenning are pretty slim.

You'll know the transformer is on the way out when you get volume drops. Then the volume will drop down for good. .

-Nick

[Andy H.]

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">I think Andy said you can sometimes hear a gentle whistling from the OT itself when its severely stressed<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">Ah, that was more of a memorable incident my old amp tech had. He was working on an old AC50 and being a smarty pants was running it briefly without a load while testing it. (The theory being that if there is no signal then there should be no problem.) However there was some sort of internal noise, a self oscillation somewhere, so the thing basically went into meltdown. Yes, first he knew about it was he heard a LOUD whistling from inside the amp, which was the output transformer going ballistic. Lots of power developing with precisely no where to go. He switched off FAST, and luckily there seemed to be no permanent damage... But by the time that sort of thing happens it is often (usually!) too late.
http://www.thegearpage.net/board/showth ... adid=37983

Re impedance mis-matching, his normal advice is that it is best to avoid it if at all possible. The amp will develpo its full power and usually sound its best into the correct impedance load. However for a healthy valve amp with a strong output transistor then a 2x mismatch either way is usually tolerable. So for an amp set to 8 ohms that would mean a 4 or 16 ohm load. More then that is asking for trouble. (Fenders are more tolerant of mis-matching generally then Marshalls he says BTW.)

It is safer for the amp if the cabinet impedance is LOWER then the amp setting rather then the other way around. This will tend to wear the power valves more then normal, but is less likely to damage the OT then going in to a higher then set load would. OT damage is still possible, but if risking a mis-match then a lower load then amp setting is the way to go.

My word of warning is with the '90s reissue OR80s and OR 120s. These had a different output transformer from the '70s originals, and were <i>much less tolerant of mis-matching. Quite a few blown trannies have shown this to be the case!!!</i> (Otherwise they are also excellent amps, but it is worth being aware of this and making sure you always set the impedance selector correctly.)

Andy.

PS I'm off to Bonnie Scotland in the morning for a couple of days / jars of ale. Have fun all!

[ceddyluv82]

safe trip Andy!:)

[dopplerblueshift]

Thank you all for that!