newby needs bias/tube tech info

[greg_moreira]

Hi guys! Im brand new to the board! Ive actually always been a mesa fan, but just recently discovered this site and Im hooked! Lots of great info on here! Hope you guys dont mind helping a noob out!

Anyways, here is my situation. Ive got a mesa triple rectifier solo head and Im curious about some things. I bought it used less than a year ago, but overall its not very old. As is, its still all stock. Its got mesa 6L6 STR 440 power tubes as well as the factory preamp and rec tubes.

Im a little curious about the bias situation. Ive heard a few people successfully running the bias switch in EL 34 mode WITH 6l6 tubes in order to heat it up a bit. Actually when I bought it....this is how it was sitting at the music store! I bought it from guitar center, got it home and tinkered with it for about a half an hour while going through the manual. While reading and tinkering...I ran into the bias section in the manual. Checked the back of the head and sure enough I got 6l6 tubes and somebody had it flipped to EL34. No clue how long it was set like that but no damage done. Anyways, I hadnt messed with it long enough to actually be sure that I like that sound or not.

Here is my actual question though. Im interested in ordering up a bias probe so that I can check my bias to see where its at. But what do the numbers mean to me, and how can I be sure if a tube is safe to run at a certain bias?

Ive heard that the bias on mesa's is set a little cold, so the 6l6 might survive on the EL34 bias(obviously mine did). But If I were to try this and experiment with other tubes in the future...I want to be SURE its right! Is there a chart somewhere that tells you what bias a tube type should run at to work right.....or how does all of this work? Im not what I am supposed to look at when matching up a tube with the "fixed bias" in my head for best results.

Thanks guys!

Edit: one last thing(not sure if it matters). On the bottom of the power tubes, it says "10 AC YEL". Not sure if this helps in telling how hot or cold these tubes are. Thanks again!

 

[clutch71]

Running the bias in EL34 when you have 6L6's in there will reduce your tube life (short term) and continual use will potentially damage your amp (long term)

There are no bias test points on the outside of the chasis nor am I sure how you would check it.

Welcome to the board!

 

[ja22y]

You can get the tube probes like Weber Bias Rite, Amp Head, Bias King. These probes allow you to see the plate current of your tube. On the sites they should have charts or links to tube specs (the tube is rated by how much power (watts) it can handle), and formula to set the bias current.
All the Mesa amps I came across have fixed bias though.

 

[greg_moreira]

Yup mine is fixed as well. I think I will leave it that way, but my main concern was being able to actually see what the bias is doing......and then reference that number to something in order to tell if its safe for my tubes.

I dont need tubes, but I want to experiment anyways just to see what kind of sounds I can coax out of this thing.....but I wanna do it safely!

I did a little searching around....and apparently the number/letter combo on my tube means its a yellow color code. Ive read that this means its one of the coolest mesa tubes that are still in mesa spec(at least this is what I read from many sources). Ive also heard some guys flip it to EL34 mode with the colder 6L6 tubes to get em a little hotter...and it stays safe since its a cold tube to begin with. Like I was sayin, I bought my head used, and it was set this way with no apparent damage, but once I realized this I flipped it back to where it needs to be til I can "learn myself" a bit.

This is one thing Id like to experiment with, but not without checking the bias. So I guess Im gonna need to invest in one of these bias probes, try out a few things and then see if I can find a chart or some kind of data somewhere that tells me if the bias is safe for the particular tube(along with any other brands I might try and experiment with). Thanks for the input guys!

 

[ja22y]

greg,
I just created a new post tube data sheet. Hope it helps

 

[greg_moreira]

Good stuff! Thanks for that link!

 

[Juancito12]

Modding your triple to adjustable bias is pretty easy but dangerous for do it yourself types because of high voltages (even when the amp is unplugged because capacitors retain high voltage that can electricute you). A good tech can do this mod for $45-60 and install a bias pot and test leads. The test leads would allow you to use a good (Fluke brand) multimeter to more safely measure your tube bias and adjust the bias pot to optimal depending on the output tube type you're using. You don't necessarily need the test leads but its easier to deal with.

If you wanted to avoid even opening your chassis up to set bias you could have the tech install the bias pot and test leads on the back of the chassis. Although this requires drilling new holes in the the chassis which is too drastic for my taste but would be safer.

You can get the adj bias mod and a fluke multimeter for the same price as a bias rite or other measuring device. Measuring bias is helpful if you want to try out different tubes but if you can't adjust it to the tubes you like then you're stuck using only certain tubes. Adj bias would be the way I would go and in fact I am in the process of getting the mod done myself.

A lot of amps are fixed bias today and there's nothing wrong with that approach. But if you want to experiment with different output tubes and not spend a fortune finding ones that work with the fixed bias setting or just want your amp to idle at an optimal setting no matter what tubes you use, then adjustable bias is the way to go in my opinion.