Impedance mismatch

[nsubulysses]

I have a scenario where 2 of 4 power tubes have a blackened logo. As if either two power tubes lost bias, or one tube had a runaway bias condition and redplated then failed, then possibly the other remaining tube on that side of the push pull had to handle double current then the others tube got the blackened logo ?

The amp is not in my possession. It's my friends amp, he Thought the amp sounded less loud when playing through an orange 2x12 and used it this way for a couple practices by turning it up extra loud. Then he tested it through his marshall 4x12 and thought it worked fine and was loud again. Turns out the orange had 1 blow. Speaker so instead of two 16 ohms in parallel for 8 ohms it was just one 16 ohm speaker. Head set to 8 ohms.

Would this impedance mismatch possibly damage the power tubes or would they in theory just be fine and just be current limited and put out less power because of the impedance mismatch ?

Amp still seems to work fine and be loud, but two logos are blackened as if it was redplating. He also never noticed the power tube logos darkened. I noticed when I saw him at a show. He said when playing through his marshall 4x12 it seems "normal"

[catalin gramada]

High voltage + low primary impedance=sure recipe for redplating. I.ve noticed is happen all times on the side which is drived hard so try to low grid leaks power resistors a bit to see if help. Otherwise anodes are hefty sheet of iron. I.ll be not worried if it toasted a bit. Need to be measured anyhow to be sure the screens was not damaged.

[Helmholtz]

Higher than nominal speaker impedance increases high power screen current and dissipation. Lower than nominal speaker impedance increases plate dissipation and lowers screen dissipation.

[nsubulysses]

So if the amp is set to 8 ohms and the cab is 16 ohms then there would be higher screen current.

But I doubt (just guessing) higher than average screen current could heat the tube to the point that the logo on the glass starts to darken. Wouldn't it basically have to redplate to do that?

This person lives out of town but is playing here again August 24. I told him he should stop by the shop before thier show if they have time because I put the power tubes in the amp in March and although he says it still works fine and sound "normal" through his Marshall 4x12, 2 of the 4 power tubes shouldn't have blackened logos. He is doing a tour August 30-September 14 or so and I don't want the tubes to fail on his tour since he is my old friend and also I know he will not be bringing a back up amp because they tour in a minivan and there's no room.

Maybe there is also some kind of intermittent bias issue or possibly a tube just had a problem. Anyway just trying to get some theories going as I wonder about this scenario so thank you

[g1]

There is nothing to tie the blown speaker event to the tube's blackening is there? I don't think he noticed them so could have been that way for awhile?
I guess it's possible you will find it's only running on 2 of the 4 tubes.

[vintagekiki]

Impedance mismatch cannot damage the power tubes. Many amplifiers 100W turn off duet tubes when operating in half power mode.
Power tubes damage occurs when the amplifier operates without a speaker connected.

[nsubulysses]

Higher than nominal speaker impedance increases high power screen current and dissipation. Lower than nominal speaker impedance increases plate dissipation and lowers screen dissipation.

could this also be stated as higher than nominal (ideal?) OT primary impedance increases screen current and dissipation and lower than nominal OT primary impedance increases plate dissipation but decreases screen dissipation, or is the concept different?

[Helmholtz]

could this also be stated as higher than nominal (ideal?) OT primary impedance increases screen current and dissipation and lower than nominal OT primary impedance increases plate dissipation but decreases screen dissipation, or is the concept different?

Correct. Higher speaker impedance just reflects as higher OT primary impedance.

[nsubulysses]

Impedance mismatch cannot damage the power tubes. Many amplifiers 100W turn off duet tubes when operating in half power mode.
Power tubes damage occurs when the amplifier operates without a speaker connected.

Well, there is another tidbit. He did say that he thought hte amp was cutting out, but then realized it was the orange cab that was cutting out. I take that to mean that it actually wasn't "cutting out" but maybe a speaker in the orange 2x12 had an intermittent connection or was intermittently not working and it was dropping down in volume as it was becoming a 1x12. maybe this intermittent back and forth for a little bit of going between 16 ohm total load and 8 ohm total load before the 1 speaker totally stopped working had something to do with the power tubes getting damaged. Can a speaker still work intermittently if hte voicecoil burns open? Maybe with all the vibration it could intermittently re-connect momentarily?

But I guess it's all speculation til I actually get to see the amp in person Aug 24.

[Helmholtz]

Impedance mismatch cannot damage the power tubes.

This I doubt. In any case it means increased stress/reduced lifetime for the power tubes.

Many amplifiers 100W turn off duet tubes when operating in half power mode.

But if you read the amp's manual you will find that when doing so output impedance needs to be re-adjusted.

[vintagekiki]

Can a speaker still work intermittently if the voice coil burns open? Maybe with all the vibration it could intermittently re-connect momentarily?

Unfortunately the interrupting speaker cannot be repaired by itself.
Open the speaker box and check the speaker wiring. Maybe voice coil isn't burns, maybe it's cold solder in speaker wiring. If you find that voice coil burns it is safest to fix it

[vintagekiki]

But if you read the amp's manual you will find that when doing so output impedance needs to be adjusted.

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/Mesa%20Boogie/Mesa_Boogie_Mark_I_Schematic.pdf

https://schematicheaven.net/boogieamps/boogie_mkii.pdf
60/100 switch in cathode

[J M Fahey]

Can a speaker still work intermittently if hte voicecoil burns open? Maybe with all the vibration it could intermittently re-connect momentarily?

Not if it actually burns, but thereŽs many other failure modes possible.

If tinsel wires fatigue crack, which is quite common, they create the King of Intermittents, because a speaker moves by definition, so frayed copper edges touch/separate when "the other" speaker pushes its cone by air pressure.

Turn all room lights off and youŽll see tiny blue sparks in the middle or one tinsel wire.

Or better remove speaker from cabinet, put it face up on table, drive it with 1kHz with an SS amp so disconnection is no problem and tap cone with your fingertips; sound should come on/off and sparks appear at the breakage point.