Wrong ohm setting

[dashizzler]

OK. Let's say some prick used my stuff at our shared practice spot and he hooked his 8ohm cab up to my head which was set at 4ohm. He played it for 15 or so minutes before someone noticed(could've been longer, who knows) When I got there tonight the kid told me and I bit my lip and plugged everything in. It warmed up and everything was OK, as far as I know. It still plays loud enough and sounds fine. Now is there anything I should be looking for as far as signs of damage, screwed up tubes etc.? Thanks for your comments.

 

[plexifier]

Well, the power tubes and possibly the output transformer were stressed but there's a very good chance that no permanent damage was done to the amp. Some of it depends on how the bias was set prior to this and how loud the amp was used when the load didn't match, etc.

I would watch the power tubes for a while when playing the first time to make sure they don't red-plate (large metal parts glow orange or red, not just the typical heater glow in the middle). Make sure the amp has the typical volume you expect also.

 

[dashizzler]

So if the tubes do glow orange or red, what does that means and what can I do? Should I have it looked at or just replace the tubes?

 

[Ken]

So if the tubes do glow orange or red, what does that means and what can I do? Should I have it looked at or just replace the tubes?

Just replace the tubes, and get the amp biased. 15 min. on a mismatch isn't SO bad if it was the only time it was done; I wouldn't lose sleep over it if the amp is sounding the same as it was. But the asshole should foot the bill if the tubes are bad.

Ken

 

[lucidspoon]

Yeah. I wouldn't worry a ton. Hell. I've had my DSL on for 4 or 5 minutes while trying to play with the speaker lead plugged back into the head before I realized my mistake, and it still works.

Disclaimer: Don't try that at home. You will smell burning.

 

[plexifier]

If they do red-plate, they are probably ruined. I doubt they will though. Just something easy to watch for. I doubt you have anything to worry about.

Running a higher load than the amp is set for usually stresses the screens in the power tubes the most and when that happens the tubes can arc between pins at the base, which will blow a fuse. The screen resistors can be damaged too. Since no fuses apparently blew, I doubt very much that the tubes or amp are damaged.

 

[dashizzler]

Ok. Thanks guys. I'll keep my eye on the tubes. I put new kt77's in a few months back, I'd hate to buy new ones. I appreciate the comments. Keep em coming if you think of anything else.

 

[jvm210guy]

Just punch the fucker in the face and call it even...

 

[tresmarshallz]

I know this is not recommended, nor is it how I do it with Marshalls....I was under the impression that a lot of people think that is a 'safe' mismatch'. I don't want to start a firestorm of debate though. I think many people, for different reasons run this way ALL the time, like head at 8ohm with 16ohm cab, or like your friend did with head at 4oh into 8ohm cab.

I think it would take extended use at LOUD volumes for that to do any damage whatsoever.

 

[plexifier]

I think it would take extended use at LOUD volumes for that to do any damage whatsoever.

I tend to agree with you, except that the added stress on the power tubes *may* cause problems fairly quickly. I'm not saying it's a good idea to mismatch impedance but I do think it gets blown out of proportion sometimes. We have to look at the whole situation and not just automatically say "get new tubes" or "your OT is fried".

Some very knowledgeable techs have told me that a one step mismatch is basically safe. I've also known guys who have done this for years with no problems, at least not yet. Again, not a good idea but...

 

[Ken]

I tend to agree with you, except that the added stress on the power tubes *may* cause problems fairly quickly. I'm not saying it's a good idea to mismatch impedance but I do think it gets blown out of proportion sometimes. We have to look at the whole situation and not just automatically say "get new tubes" or "your OT is fried".

Some very knowledgeable techs have told me that a one step mismatch is basically safe. I've also known guys who have done this for years with no problems, at least not yet. Again, not a good idea but...

When I first got my Marshall I used an 8 and 16 ohm cabinet together at 4 ohms for a short time, and even defended the practice. I no longer defend any mismatch as in the long run it can definitely shorten the life of your tubes an OT, but deep down I'm not losing sleep I was 1.333 ohms too high.

Ken

 

[tresmarshallz]

I think we are all in agreement that we would not recommend it, bu to the OP, don't worry about your amp, it is FINE!

As a weird side note: my Mesa manual says that a 1-step mismatch is fine and the hardware will take it, I even called them and told them I had to run head at 8 with a 16 ohm cab, they said do it, you won't have any problems. I am sure Marshall does NOT make this claim though so you wouldn't see me doing it on any of my Marshalls...

 

[Alabama Thunderpussy]

Some very knowledgeable techs have told me that a one step mismatch is basically safe. I've also known guys who have done this for years with no problems, at least not yet. Again, not a good idea but...

It depends on the design. 6L6 output sections tend to fare better under load mismatches than EL34 designs. Maybe it has something to do with higher a-a impedance on the 6L6.

 

[TradAmpGuy]

It depends on the design. 6L6 output sections tend to fare better under load mismatches than EL34 designs. Maybe it has something to do with higher a-a impedance on the 6L6.

Thats what I have also heard... Fenders seem more tolerant of a load mismatch one notch either way... I'd try and avoid it though. The same sources that say this also do not recommend it for Marshalls.

 

[Joey Voltage]

Meh you guys worry too damn much. Generally speaking there is quite a large bit of wiggle room in either direction before you would really have cause for concern. In other words there is quite a large range of acceptable, which relies on many, many determinates all of which havebeen hashed out to death.

Alot of this worry you read about is based on many of assumptions, and sweeping statements. Are all amplifiers designed in a certain magical mandated way in the first place, and follow an ideal cookie cutter paradigm concerning operating conditions are used and what reflected load has to be presented to the driving stage? - hell no! who's not to say one particular amp by design is not already teetering "too close for one techies comfort" at either extreme, even when your using what you think is the correct output tap for the load you have connected.

Now the load. to add to the above, even If if there was a perfect magical set in stone guidline that was mandated, and every manufacturer follows, are you driving a completely perfect linear resistive load? - you most likely wont be. an 8r speaker load could be 6 ohms at 300hz but then be 40ohms at 75hz, and 5Khz etc.. making the load presented to the driving stage very eliptical - This will of course change depending what cab you use.

I say if it didn't go up in flames, and seems to be working fine now. I wouldn't lose any sleep over it.

It depends on the design. 6L6 output sections tend to fare better under load mismatches than EL34 designs. Maybe it has something to do with higher a-a impedance on the 6L6.

Marshall generally, with maybe the exception of the lower voltage JMP 50W and early 80's 2204's (and a few other models) tend to use OT's that some would consider (all things equal of course-see above) to reflect a bit on the low side for EL-34's particularly under certain operating conditions of these amps, even when considering a purely resistive load especially if you have a pretty stiff screen supply. They seem to have arbitrarly pulled the 3.4K/1.7K figure straight from Phillips datasheets,(or bought what they could find) that reccomended this load when considering a class B push pull design with a 375V plate and screen voltage (I think, I would have to look at it again, but I seem to remember this from the sheet).

Some of this started when Randall Aiken posted on a few DIY forums two years ago or so that he noticed this with a few Marshalls he had on his bench one of which he was fixing for a friend. (it might have been people at the Metro Forum having this problem, but I know he posted this in other places). He posted a few loadlines he drew using the typical 1.7K/3.4K a-a load, because he noticed that even under larger signal conditions, the screen supply seemed to be a bit stiff in the amps he had on his bench, especially considering the plate voltages used, which was causing the load line to pass above the maximum dissipation curve for much of the operation, especially at higher volumes. I think he might have updated his site with this info at one point, but thats all I remember him saying in the other forums at that time. I believe he also made comment that he thought that the Lossier Iron that was used in older Marshall's may have been their saving grace in this regard - which he deffinitely has a point on.

 

[john l]

Your amps probably fine. If in a mismatching scenerio yours is the lesser of the two evils IMO... he deffinately deserves a good smack in the back of the head for not asking you if it was cool to use your amp though.

Step up mismatch IE 4ohm tap to 8 ohm cab
This way the tubes have it easier but your OT "can" be stressed due to flyback voltage....google it.

Step down mismatch IE 8 ohm tap to 4ohm cab
OT has it easier but your power tubes are run harder (alot harder) which can cause red plating and eventually tube failure. Depending on the severety of the short it can take out other components along with it causing even more damage.

Obviously its better to just match the impedience your amp wants to see but for what its worth Ive step up mismatched plenty of times over the years and never had an issue.

EDIT
These are just POSSIBLE outcomes and by no means allways happen Im just saying if the conditions are right blah blah blah Ill shut up now

 

[Ken]

who's not to say one particular amp by design is not already teetering "too close for one techies comfort" at either extreme, even when your using what you think is the correct output tap for the load you have connected.

.

I love you Joey, but I have to say this kind of refutes your point. If amps ARE teetering on one of two edges of too low or too high, a mismatch in the wrong direction will be MORE likely to push it over the edge and BOOM, so a mismatch in general is throwing the dice.

While my anecdotal belief is the same as yours, that most amps including Marshalls have some leeway, I also think it's not wise to put this to the test with an intentional mismatch in case it's teetering as you suggest. After all, we do have posts from people who have had catostrophic damage after a mismatch.

Ken

 

[Joey Voltage]

I love you Joey, but I have to say this kind of refutes your point. If amps ARE teetering on one of two edges of too low or too high, a mismatch in the wrong direction will be MORE likely to push it over the edge and BOOM, so a mismatch in general is throwing the dice.

While my anecdotal belief is the same as yours, that most amps including Marshalls have some leeway, I also think it's not wise to put this to the test with an intentional mismatch in case it's teetering as you suggest. After all, we do have posts from people who have had catostrophic damage after a mismatch.

Ken

Ken,

It doesn't refute anything I hope, maybe it was the way I might have worded it, and my response was also anecdotal, and hypothetical, to prove a point, but the point I was trying to convey with what you quoted, was just because the user thinks they have matched the driver to the appropriate load, (ie they use the 16ohm secondary setting, when connecting to a cab that is rated for a nominal 16ohms) this doesn't mean that electrically it is what "some" would call an appropriate match, or an automatic middle ground per se, it really does depend on many determinates as stated including the intended use.

 

[Les Moore]

Been wondering about something. If a mismatch occurs, or you forget to connect the cab, does the OT wear or does it just stop working one day? Does a transformer wear over time or does it just break suddenly? If it wears out, how does that effect the sound?

 

[john l]

The only blown OTs Ive heard sounded like just like frying bacon with faint guitar going on it the back ground but they can just stop functioning all together. Yes you can stress them and just like anything if you stress them hard enough they can be permanently damaged or break, if however you didnt have any load connected at all you could fry one in a minute or so if you had the amp turned up high and were putting a signal through it