Mark iv tube problem and question

[blacky656]

Hello,

I just recently acquired a Mark iv B head.. everything was fine until I blew a fuse.. replaced it and it was working great until last night.. here's what happened:

We were using the silent recording and had the recording knob on the back cranked and the master in the front around 8.. I noticed some noise coming from it after an hour or so then an "Arcing", puff of smoke.
the amp was still on and when we looked in the back I noticed the last tube on the far right was glowing red hot and the others were dim..

I shut it off and powered it back on and noticed the same thing.. the amp still played as I did a little test but felt like it had 1/4 of the power and volume..

According to the manual it may be a blown power tube.. so I pulled them and looking at them I see no color codes that the manual and mesa tube troubleshooting video seem to indicate..

They all say Mesa 6l6 and on the base reads 05 AC GRN..

2 of the tubes have Gold lettering on the glass but the other is white lettering.. I'm not sure why the lettering color would be different?

I'm looking to buy all new tubes and hope it's nothing worse.. any advice would be greatly appreciated!

Thanks

 

[thunda1216]

"GRN" means the color code is "Green."

 

[MrMarkIII]

The "gold" lettering may be due to excess heat oxidizing the white lettering. Just like making toast.

FYI, when you blow a fuse, that's means there's something wrong with the amp.
Blowing the fuse is designed to prevent more expensive problems down-line of the fuse.

Merely replacing the fuse doesn't fix the problem.

As far as the tubes go, remember that the four big power tubes work in pairs. You have two pairs, an "A" pair and a "B" pair. They are installed in the chassis like this:

A - B - B - A

So, there is an OUTER PAIR (the two "A" tubes),
and an INNER PAIR (the two "B" tubes).

I'm betting the two tubes with the new and improved gold lettering are the "last tube on the right" (an "A" tube), and its mate on the far left (the other "A" tube).

With any luck, you know which one of these was "glowing red hot".

When you replace power tubes, you should replace them in pairs.
The color codes enable you to find a matching pair.
In other words, get two green, or two red, or two of any color.

 

[blacky656]

Thanks for the response..

a friend of mine mentioned the Gold color is probably from the heat as well.. and those were the ones glowing red hot.. specifically the last tube on the right.

I'm going to order a new set of tubes to test before I ship this to Mesa..

The question is with the "matched pair" is confusing since all the 4 power tubes are GRN.. how do I know which one goes in A or B? when I got the amp they pulled all the tubes and I didn't notice and codes so I just installed them.

Thanks

 

[MrMarkIII]

It's cool, if all four tubes are green (or yellow, or blue, etc.), then they all match each other, so you can put them in any of the sockets.
But if you had, say, two green and two red, you would install them like this:

GREEN - RED - RED - GREEN
-or-
RED - GREEN - GREEN - RED

What you should avoid is this:

RED - GREEN - RED - GREEN

It won't kill the amp, but it may not sound as nice as matching the pairs.

Remember:
The four big power tubes work in pairs. You have two pairs, an "A" pair and a "B" pair. They are installed in the chassis like this:

A - B - B - A

Both "A" tubes need to be the same color.
As long as they match each other, both "B" tubes can be any other color.
The "A" tubes do not need to be the same color as the "B" tubes.

Do you know which tubes from your amp are "A" and which are "B"?

 

[blacky656]

Since they are all GRN I don't know about A or B.. I looked at the sockets and they are clean but no label A or B.. the plate inside just says ABCD for tube placement..

I'm going to try and embed a couple pics.. you should see the gold lettering and flip it over and the 2 end tubes have a dark burn mark..

Shot at 2012-07-08

Shot at 2012-07-08