MKIIIC EL34s for outer pair, and Fan diagnostics?

[Emoto]

Hi guys,

Have a MKIII 1/12 that I bought new sometime in the mid-80s. I've used it in a few bands over the years an have always lived the sounds it could make.

Any comments of running 6L6's for the inner pair and EL34's for the outer?

The other day, one of my outer pair tubes committed suicide and started glowing orange and putting out a tremendous amount of heat and noise with wildly fluctuating volume. Yet, the fan in the bottom of the cabinet never came on. Any thoughts on something controlling the fan that "always goes bad" that I should look at?

 

[topcat0399]

Hi guys,

Have a MKIII 1/12 that I bought new sometime in the mid-80s. I've used it in a few bands over the years an have always lived the sounds it could make.

Any comments of running 6L6's for the inner pair and EL34's for the outer?

The other day, one of my outer pair tubes committed suicide and started glowing orange and putting out a tremendous amount of heat and noise with wildly fluctuating volume. Yet, the fan in the bottom of the cabinet never came on. Any thoughts on something controlling the fan that "always goes bad" that I should look at?

ahhh tube death. I can't address your ??? on the tubes since I own a Mark IVa but;

I'm pretty sure that fan should ALWAYS be running. check the power connector on the fan or make sure its spinning free of obstructions.

good luck.

 

[Emoto]

Thanks, Bottle Rocket!

So you think the fan is supposed to run all the time? :shock: I don't think I have ever seen mine run.

Any other MKIII owners care to comment about when the fan runs? It spins freely (OMG IT SPINS! LOL) and the leads are connected at both ends...

 

[>Photi G<]

Yeah, the fan is plugged into an AC plug at the bottom of the chassis, next to the PT. This plug is attached to the On/Off switch on the front of the amp, so it should power up when you turn on the amp. To check if it's running, unplug it from under the chassis, and plug it into the Accessory Outlet on the back of the amp, which is unswitched (Make sure the amp is plugged in first).

As far as swapping power tubes, first things first. Your amp must be a Simul-Class to be able to run EL34s in the outer sockets. You should be able to tell this by the wattage switch on the back (or the front, if you have no EQ). If the switch's upper position says Simul-Class, then EL34's are a go. If it says 100 RMS, then 6L6's only.

Also, since the tube died from Red-Plating, Pull the chassis out of the cabinet and make sure that no resistors are burned out on the power tubes. Once the amp passes your visual inspection, put in a new pair of outer tubes. Or, replace all the power tubes if they're older than a year.

 

[Emoto]

Thanks, Photi G!

I pulled the cord running to the fan and put my continuity meter across it. Guess, what? The cheesy plug that goes into the chassis never made contact with wire on one side, so this thing has NEVER worked from day one. I tried re-seating the plug on the wire a couple of times to no avail. I will hit the hardware store and get a new plug and wire it up right.

What would a burned out resistor look like?

I have a local friend who can do any work on it that is needed, as he does a lot of guitar amp work for local musicians...

 

[>Photi G<]

Burnt resistors are usually easy to spot. No complicated diagnostics required. A quick visual inspection should reveal any.

As practice, try to locate the 2 burnt resistors in this photo:

See? not that hard. Replacement should be simple, since most Mesa's were made to be easily serviced (PTP power amp, components on the topside of the pre-board). There may not even be any in your amp at all, so that's one job out of the way.

 

[Emoto]

Burnt resistors are usually easy to spot. No complicated diagnostics required. A quick visual inspection should reveal any.

As practice, try to locate the 2 burnt resistors in this photo:

See? not that hard. Replacement should be simple, since most Mesa's were made to be easily serviced (PTP power amp, components on the topside of the pre-board). There may not even be any in your amp at all, so that's one job out of the way.

2nd and 3rd from the left?

 

[Emoto]

Ok, nothing looks even remotely toasted in there. What a relief.

Off to get a new plug.

Thank you so much to all of you for stepping up with great info!

Bob

 

[Emoto]

Ok, the new plug has been installed on the fan wire, and presto-chango, the fan runs whenever the main power switch is on.

I have to wonder how tube life and tone may have been affected all these years of running with no fan. :roll:

At any rate, with the new EL34s in there, the amp sounds fantastic. Really crisp with a touch of sparkle, and some hair when I lean into a note. I couldn't be happier.

Thanks again.

Bob

 

[topcat0399]

Thanks, Bottle Rocket!

So you think the fan is supposed to run all the time? :shock: I don't think I have ever seen mine run.

Any other MKIII owners care to comment about when the fan runs? It spins freely (OMG IT SPINS! LOL) and the leads are connected at both ends...

glad you got it sorted out.

BTW:

from one "bottle rocket" to another.....the names topcat0399!

 

[Emoto]

Thanks, Bottle Rocket!

So you think the fan is supposed to run all the time? :shock: I don't think I have ever seen mine run.

Any other MKIII owners care to comment about when the fan runs? It spins freely (OMG IT SPINS! LOL) and the leads are connected at both ends...

glad you got it sorted out.

BTW:

from one "bottle rocket" to another.....the names topcat0399!

Oh, duh. Sorry about that. Should have look higher.

Apologies all around.