Mark IV losing sound occasionally

[shredding]

Hi,
I'm running my mark IV to a 16 ohm cab through the 8 ohm output. Last night all of a sudden the amp went dead silent. Checked the cables and everything and still no sound. Turned the amp off and back up, the sound came back but faded out in 5 secs. Tried this several times same issue happens.

Then I switched channel (to chan 1) and the sound came back. When the sound's gone, you can hear a faint hum from the back of the amp when I pluck the guitar.

I checked the tubes and they all lit up allright. I wonder what is wrong since I'll freak out if this happens on stage!

Thanks!

 

[jvk]

Seems like you might have cranked your amp and damaged the tubes, and quite possible the output transformer. I'd stop using the 16ohm speaker. To test if your output transformer is fried, first connect up a speaker with matching impedance, and pull all the tubes and replace with a set known to be working (you can use only 2 in the inner sockets instead of 4).

What you are doing is mismatching impedances between the amp with a higher speaker load impedance. Doing this places a greater strain on the power stage. The overall tone changes and the output power to the speakers will be reduced, and then there's the issue of reflected load, which means your output tubes run at a higher voltage, and if you run the amp loud than a higher signal voltage is generated through then tubes and output transformer which will exceeds their maximum ratings and can cause an arc-over in your tube sockets - this will make shish kebab out of your tubes and worst still DAMAGE YOUR OUTPUT TRANSFORMER (ouch! expensive...).

As a side note, even though I don't recommend it (stick to the right rating for the amp) - If you decide to mismatch the speaker impedance by using higher speaker loads than the rating of the amp, you can potentially avoid trouble by playing at lower volumes. That is if you HAVE to mismatch. Or use one with a lower rating, it's safer.

 

[jvk]

it's definitely worse to go 8 --> 4 than 8 --> 16 (that would be an unsafe mismatch).

No I'm afraid that's incorrect mate. I don't know what Mesa recommends, but I'm pretty sure that if the cab's impedance is higher than the amp output, you get flyback voltages (reactive load) coming back to the OT. A higher impedance mismatch is harder on the amp than a lower mismatch. Driving 8ohms through a 16ohm load produces a larger flyback voltage than driving 8ohms thru a 4ohm load, placing more strain on the output tubes/transformer.

--

Driving 8ohm through 16ohm speaker load reduces the overall volume by 3db, and there would be some bottom end loss.

It's generally considered safe to mismatch by a step, because an OT is usually over-engineered so it can take it. But I hope your problem isn't with the output transformer. I know people who mismatch their Fenders and have been running them for years without any problems, but there are others who do the same and have blown up tubes/sockets/transformers. A one step mismatch shouldn't blow up an amp, provided the amp isn't cranked.

If you want to safely match your amp with a 16ohm load with the 8ohm output, pull the two outer power tubes, this will be a closer match.

 

[shredding]

Guys, I appreciate your prompt replies.

First, Rabies, I read the same thing in Mesa's manual saying that I can "up" match. (8 going to 16 ohm cabs). And they said Mesa amps are not very sensitive to mismatching. It's perhaps how mesa built their amps. I do know that down matching is a big no no.

Jvk, thanks for your valuable info too. I've never heard of reflecting fly-backs. On my cab it does have a 2 8 ohms stereo option (which can run parallel safely into my 2 4 ohms jacks in the back. But one channel constantly drops off and that's why I'm using the 16 ohm mono now.

Rabies, I'm running my amp full power. Simu class on pentode. The master's usually set at 5 and L channel's at 2 to 3.

Tonight we gigged again and i was using tweed power mode and have a whole night of gig without dropping out. So Jvk i guess the output tranny's good right? I just ordered 4 matched JJ 6L6's and it will come in a week. I guess the amp will be stable enough till then. (my old tubes are only 2 months old though).

HOw much do you think the output transformer costs?

Thanks again for all of your helps guys.

 

[dodger916]

it's definitely worse to go 8 --> 4 than 8 --> 16 (that would be an unsafe mismatch).

No I'm afraid that's incorrect mate. I don't know what Mesa recommends, but I'm pretty sure that if the cab's impedance is higher than the amp output, you get flyback voltages (reactive load) coming back to the OT. A higher impedance mismatch is harder on the amp than a lower mismatch. Driving 8ohms through a 16ohm load produces a larger flyback voltage than driving 8ohms thru a 4ohm load, placing more strain on the output tubes/transformer.

--

Driving 8ohm through 16ohm speaker load reduces the overall volume by 3db, and there would be some bottom end loss.

It's generally considered safe to mismatch by a step, because an OT is usually over-engineered so it can take it. But I hope your problem isn't with the output transformer. I know people who mismatch their Fenders and have been running them for years without any problems, but there are others who do the same and have blown up tubes/sockets/transformers. A one step mismatch shouldn't blow up an amp, provided the amp isn't cranked.

If you want to safely match your amp with a 16ohm load with the 8ohm output, pull the two outer power tubes, this will be a closer match.

There was a thread a while back that discussed this too. (thanks again, AdmiralB) Page two is where the controversy begins.

http://forum.grailtone.com/viewtopic.php?t=17767&postdays=0&postorder=asc&highlight=lsc+impedance&start=15

I was skeptical and confused about this also. Here is the technical reference from R.G. Keen (some amp guru?) that refutes Mesa's safe mismatch suggestions:

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch