Retubing 2Chan Mesa Dual Rec help!

[ryan776]

Hey guys! Finally a proud owner of a mesa!!!

I'm planning on retubing my 2chan dual rec this week (electro harmonix (2)5u4g / (4)6l6 with (5)jj 12ax7) and I had a quick question. I know that it self biases and all but when putting in new tubes do you need install the matched pairs as:

1 and 4
2 and 3

or can you just put them left to right?

Also, one of my preamps has a metal casing around it I think. Can anyone tell me why? lol

Thanks in advanced for the help!

 

[echoes420]

You are correct... It is the 2 inner and 2 outer tubes that need to be matched to each other.

And the metal thing around V1 is just to reduce microphonics.

 

[domct203]

Your amp is not self-biasing. It is a fixed bias amp which means the bias voltage is at a fixed value and requires tubes selected to run at those voltages.

Did you have your tube supplier pick tubes selected to run at Mesa bias specs?

Dom

 

[ryan776]

Do I need to keep that metal casing on the preamp?

Your amp is not self-biasing. It is a fixed bias amp which means the bias voltage is at a fixed value and requires tubes selected to run at those voltages.

Did you have your tube supplier pick tubes selected to run at Mesa bias specs?

Dom

Currently it has 12ax7's 6L6s and 5U4Gs in it (the way i purchased it). It is a second hand amp but in mint condition and he said hes only used those tubes types but hasn't replaced them in over a year. This shouldn't cause any problems as long as the switch is on 6L6 and i insert them in the correct order right?

Also, do the preamps have an order?

 

[domct203]

Wrong! Just because it is a 6L6GC does not mean it will work correctly in the amp.

All tubes are not created equal. That is why we must put them into matched sets before use. Tube bias conditions can and will vary greatly even among the same manufacturer and type. Unless the amp has been modified to adjustable fixed bias you must use tubes that are selected to work in a Mesa amp.

If the tubes run out of spec on the cold side of bias you will have an amp that is sterile and stiff, with a nasty overtone when pushed and not developing full power. If the tubes run too hot you risk a tube failure and or amp component failure.

I advise you educate yourself on tube amp bias and what it means to the life of your amp.

The metal cover on the preamp tube is to help with microphonics. You don't need to use it, but it was put there for a reason.

Dom

 

[domct203]

Also, the preamp tubes do not need to be in matched sets. You can safely mix and match 12AX7 tubes. However, they each serve a different function in the amp, and will affect the character, tone and response of the amp in different ways. Refer to the Tube Task Chart in your manual for a description of each preamp tube's function in your amp.

Dom

 

[ryan776]

I'm planning on using the exact same tubes but using JJs and Electro Harmonix brand leaving the switch on the back to 6L6. Would this would cause a problem? They also gave me similar but different brands on http://www.thetubestore.com when i entered that I have a Mesa Dual Rec.

EDIT: The tubes I've bought are in matched pairs already if thats what you ment. But how would I go about it if I were to buy singles of the same tube? Is there any difference?

 

[domct203]

Not only do power tubes need to be in matched sets, those matched sets must fall within the amp manufacturer's operating specs (cathode current draw).

Chances are that if you ordered them for a Mesa Rectifier you should be OK.

I still advise you educate yourself on tube amps.

Dom

 

[ryan776]

Any recommendations for where to read up? I appreciate all your help.

 

[domct203]

Start with the last few sections of the 3-channel Dual Rectifier manual (download from mesaboogie.com). There is a section on the basic operation of tubes very well written in non-tech terms.

Once you digest most of that you can start searching on the net for various articles and papers written on electron tube operation. The info in theBoogie manual is a great primer IMO.

Dom

 

[94Tremoverb]

Also - not mentioned so far - when you change the tubes, *do not* throw out the old ones! They are known to be working and if they're only a year old the chances are they're still in good condition unless the amp has been really thrashed... unlikely if it's mint. It's a total myth that tubes need changing every year - they need changing when they fail or wear out, and the two things are not necessarily related at all... tubes do not have a 'best before' date, if the amp has been only lightly used they will be as nearly good as new.

Even if they are well worn they are still a known working set and thus are valuable as spares - *all* tube amp owners need spares, since you have no idea if or when one may just fail, sometimes for no obvious reason... and being new is no guarantee at all that this won't happen (in fact, to some extent the opposite - tubes tend to go through a short period when new when they are *more* likely to fail). But it's still worth buying the new ones anyway, if you don't already have spares. You will also need spare fuses since a blown power or rectifier tube will usually take it out. Get at least two (or a pack of five, or whatever) of the value specified on the back of the amp and *no other* - including the F or T which indicates fast blow or time lag... T for Mesa amps usually.

By the way, ideally you should be buying a matched quad not two matched pairs (it isn't the same thing - a quad means all four valves are matched, whereas two pairs may be different from each other even if the pairs are matched) and then it doesn't matter which positions they go in.

 

[screamingdaisy]

When ordering from The Tube Store you need to enter that you're ordering for a Mesa when ordering tubes so that they will select tubes that fall within the correct operating range of a Mesa amp. You can do this via the comments section on their online order form.

Assuming you didn't do this, they'll just send you any old matched pairs. Once the arrive check the number they write on each tube and email The Tube Store to verify that those numbers fall within Mesa's specs.

 

[ryan776]

I appreciate everyones help! thanks again.