Mesa Mark IV with 6L6GC tubes? Why are they light-bulb like?

[Mach1andy]

I went to look at an amp tonight that had the original (1997) Mesa Boogie 6L6 GC powertubes in them. I noticed that they are kinda 'bulb-like' and not a straight cylinder like most tubes.

One of the tubes was microphonic and I know I can't just replace one tube.. at least two so do I:

A) Replace all 4 power tubes

B) Replace 2 outer power tubes (the one that was bad was on the far left)

or

C) Replace the outer tubes with EL34 tubes (I have 2 of them laying around).

What would the advantages/disadvantages be to each?

Thanks for all of your help!

 

[Ronnie_Rocker]

the tube are possibly Mesas old 6550 6L6. they sounds awsome!! i have tried to buy those for a long time, but havn´t fund them yet. i hade 6550 in my old Mark IV head. gr8 stuff.

 

[jbird]

Pre-amp tubes go microphonic, not power tubes! :idea: And I'd replace all the pre-amp and power tubes if ages are unknown, or over two years of age!

 

[Russ]

I am considering swapping out some power tubes myself in my Mark IV. I too have some extra matched sets of power tubes lying around. Before you go swapping be sure that the tubes you are going to use are still spec'd for use in the Mesa. Go get them on a meter because if they are not within Mesa spec you run the risk of damaging your amp. It would be easier to just go get a matched quad and have an emergency spare set that are relatively matched still (from your 3 good power tubes). This way if you have a tube failure you can swap in a matched pair. This will eventually lead to you having 2 matched pairs in your amp sometime down the road. At some point you will have a matched quad again but it is a good idea to keep spares handy and a matched pair is preferred to unequally worn spares. This is not to mention it is hard to find Mesa's older tubes. If you do eventually get a pair of EL34's in your amp you will get a sound that has a bit of english grit to it. It will not be completely english like a Marshall, but if you want that go get one. Many people like the sound of it. I personally am partial to the all 6L6 configuration. I still have to try the 6V6 loaded configuration but that is not in your post.

So bottomline:

Swap for a matched quad and save the 3 good ones so you have a matched pair and a spare for the matched pair should one of your matched quad go bad you can swap in a matched pair and have a spare for each matched pair with two matched pairs in operation.

Once you have a good working all 6L6 configuration try a matched pair of EL-34's that are within Mesa spec. This will also allow you to sell off a pair if you wish to recoup some of the cost, trade off a pair for something else, or you can just hold on to the tubes as you will eventually need to retube again.

In any event it would be a good idea to retube anyway and use any good tubes as spares to ensure your reliable operation.

 

[Mach1andy]

I left the 2 6l6's in the middle and put in 2 EL34s in the end sockets-- It sounds great! But...

The two tubes in the middle are a bit weak-- I mean, i can tap the tubes and hear the impact coming through the speaker-- not a great thing Im sure. I don't want to lose the sound as I know those tubes are 6L6 GC Str420 and the new ones are Str 440. They're a different shape but thats as much of a difference as I know. Does anyone know what the differences are?

Again, the tubes were used barely over 9 years-- probably a total of 10 times. Replace all, power, none --- suggestions?

 

[Russ]

First do not tap power tubes. From the amount of use that you are talking about I would say that the tubes should be fine. However, if the tubes sound weak then swap them out. The old 420 was based upon one 6L6 tube design that had a particular bottle shape. The newer 440 is based upon an EL34 design that obviously has a different bottle shape. There are many differences between a 6L6 and an EL34 physically but I would be more concerned with the sound differences as that is why we would buy one over the other. The biggest difference in sound is that the 6L6 is going to be more crisper and have a more American feel and flavor and the EL34 will sound a little more British. Think Fender (American) and Marshall (British). Mesa takes tubes and tests them to ensure that they are operating within their spec then relabels them and sells them as their own. This increases the end cost to the consumer but provides a level of convenience and reliability upon purchase. Their screening maintains the amp's designed operation. This is not to say that you cannot go buy GT or other branded tubes. Just be sure that you know what you are getting and that they are compatible or you may have issues. There are some posts here that indicate the correct GT hardness ratings. As far as using other brands you need to either know what your amp's designed threshold is for its power tubes or buy from a vendor that does. There are many tube types that you can use for your amp and there are a few different shapes as well within those types. I wouldn't worry so much about the shape of the tube so long as it physically fits in your amp and works. I would be more concerned with the sound of the tube.