Fifty/Fifty & matched quad tubes...

[aebah]

Hello everyone!

It's time for me to change the tubes in my Mesa, problem is I've never done this before. I recently bought a matched quad set of 6L6s. It's my understanding that you can't exactly bias my power amp. So... what do I need to do? Does it matter what order I put them in since they're matched? All I have to do is push the metal discs down pop the tubes out and then line the new ones up with the key and tighten the metal disc?

Advice?

 

[Triaxstasy]

That´s basically the way to go....but with the fixed bias circuit the tubes run pretty cold (somewhere between 17mA and 22mA)

Crane came up with a real simple but effective workaround by replacing just one resistor in the bias circuit with a cermet trimpot and so giving the 50/50 the ability to to adjust the bias (which is totally simple btw)

check here

 

[crane]

No matter what order you put the tubes as they're a matched quad. Tubes are arranged in a AABB order in this unit, so if you had two pairs you should put one pair two the left, the other to the right (in case you change another time).
Be carefull if you plan to do the bias mod as the capacitors remains charged for a few minutes after shutdown, and their discharge on your finger could simply be lethal.

 

[RectoStudioGuy]

I've done a few of the bias mods over the years...great results.(the two I use now) But, you could just put hotter tubes in and 'to some degree' achieve a good results.

 

[aebah]

Thanks everyone. Very useful information and great tutorial.
I started a hackersapce at my university and this is definitely going to be a project that I will be working on there tomorrow. I can't wait.

Thanks again!

 

[zodiac272]

Yeah it's pretty easy... grab the base and rock them in a circular motion while pulling. When the sockets are empty, flip the tabs back so they grab when you slide new tubes in.

scott