Factory Mesa 2:90 Bias Setting

[QuantumDot]

Do you guys know what the standard Mesa bias setting is for the 2:90 poweramp? Is there such a thing or is every 2:90 set differently when it comes to the bias resistor?

Since Mesa is using color codes ranging from Red (lowest idle plate current rating around 20mA) to White (highest idle plate current rating aroudn 60mA) and is telling us users that we can mix any color as long as the tube type remains the same, is it the case that Mesa biases their fixed bias poweramps for their middle-of-the-road tubes like the green or gray tubes, so that red tubes will run slightly colder and white tubes will run slightly hotter or are they biasing their amps for the hottest (white) tubes so that any other color you put in will never cross that upper limit?

 

[Henz]

Based on the schematics, - 47vdc for the outer and - 56vdc for inner (470 plate resistors). I'm assuming that's for both versions but the tube layout may be different in version two. The tubes with the 2.7k' s plate resistors are the lower voltage (-47). But there is a general tolerance usually around 10%. These are US measurements. As for your follow up question regarding tube colors, I can't give any advice. Perhaps call customer service for info. Probably need different tubes for the proprietary voltages. Blessings.

 

[QuantumDot]

Has anybody here measured the plate voltages of their Mesa 2:90 with any Mesa 400 series tube with a specific color code?

 

[Henz]

I'm no amp tech but have lots of hands on experience. Please, any tech correct me . I'm a sponge wanting to learn. I believe the voltage present on pin three will be the same with or without a tube in.

 

[QuantumDot]

I'm no amp tech but have lots of hands on experience. Please, any tech correct me . I'm a sponge wanting to learn. I believe the voltage present on pin three will be the same with or without a tube in.

I'm no amp tech either, but if the plate voltage stays the same, no matter the power tube you put in, you could most definitely calculate the definitive plate voltage. If you have the plate voltage, it's easy to get the idle plate current a tube must have to work correctly with a bias setting:

I know that the Mesa 2:90 is basically the power amp section of the Mesa Mark IV. I read somewhere that the plate voltage with Mesa 440 6L6GC tubes was around 460V. According to this bias calculator website (https://www.tedweber.com/bias-calc/ , Class AB, 6L6GC tubes and 460 plate voltage), we'd have to use a bias around 44.6mA, which means that Mesa tubes with an idle plate current around 45mA should be optimal, no? This would probably mean that Mesa biases their amps for their green tubes, as they should have an idle plate current reading of about 45mA. Is this wrong?

 

[Henz]

Great reply. You're very well versed. That formula for determining MPD seems pretty easy. I used it for my C+ . it always came up a bit cold. I believe 6L6's are 25 watts and EL's are 30 . I read were mesa likes to try to reach the 70% MPD. Some ppl think that's a bit hot. I guess it comes to personal subjectivity. In my C+, the EL family tubes run a bit cooler to touch than 6L6's. The low end may be a bit tighter with the el's. One of the pit falls of guitar players is that it can be costly with certain things as some are not returnable like tubes. I've been tube rolling for years and I'm still searching for the " perfect" tone.

 

[QuantumDot]

I received the "gray" tubes today, which draw a little bit less idle current than the blue tubes, and I noticed something strange.

I powered on the 2:90 without any preamp signal just to see if the tubes are working and, during the warm-up phase in standby mode, the tubes looked normal to me, they all were glowing red as you can see in the first picture. As soon as I flipped the standby switch and put the amp into operating mode, I noticed a blue glow all of a sudden. The blue glow pictures were taken only seconds after the standby mode and the volume pot was at noon and the channel A speaker out was connected to a Suhr reactive load (Channel B was off with level set to 0 and presence to 10).

Is this a normal glow for a new set of 6L6GCM and 6L6WGC tubes or does it mean that the new tubes are running way too hot?

I never saw the previous Mesa 420 set glowing blue, they were always red, whether in the warm-up stage or the operational stage.

 

[Henz]

I'm 99.999999% certain that its normal for some power tubes to give off a blue hue. I have a set of Preferred Series power tubes that give off a blue hue. Though yours are a lil vivid, I think they're fine. This is normal . Its the electrons inside the tube . Even tho your previous 420's didnt do it, the new ones might be made from a different manufacturer. From my experience with my boogies, if the p tubes are running a bit hot, when I power down, there's a bit of a poof sound thru speakers. How do the new ones sound?

 

[QuantumDot]

I'm 99.999999% certain that its normal for some power tubes to give off a blue hue. I have a set of Preferred Series power tubes that give off a blue hue. Though yours are a lil vivid, I think they're fine. This is normal . Its the electrons inside the tube . Even tho your previous 420's didnt do it, the new ones might be made from a different manufacturer. From my experience with my boogies, if the p tubes are running a bit hot, when I power down, there's a bit of a poof sound thru speakers. How do the new ones sound?

I tested the two channels with different powertubes today and I didn't see any red plates during the 20 minutes of play time. If the amp was indeed biased too hot for these tubes, I would have probably seen red plates by then and not just blue/red glow Fortunately this wasn't the case.

As far as tone is concerned: Maybe it's because the 420s were already old and worn out but the two TAD sets that I have in there now, a 6l6GCM set on channel A and a 6L6WGC on channel B, both sound better than the Mesa 420s. My favorite is the 6L6WGC because it's a bit more mid-focused and great for Marshall based preamps (Im using the Synergy preamp with the Friedman HBE and the Vai module). The 6L6GCM are probably better for mid-scooped stuff, but if you like a modern kind of vintage with a more solid bottom end and sweet highs, the 6L6WGC redplate tubes are great!

 

[Henz]

You're good. It only takes em a few min to redplate. I'm glad it worked out well for you. I may have to try the WCG's because I love more mids. Every now and then peek back when powered up just to make sure things are groovy. As you probably know, when one does redplate, you'll hear that hum thru the speakers. Head for the power switch asap!! lol. Anyway, rock on brother!!

 

[Henz]

QuantumDot, I hope all is well. hey brother, I wonder if you can do me a huge favor?