Bias mod on Mark V?

sick thread on LS bias mod with apparent good results: http://forum.grailtone.com/viewtopic.php?f=31&t=45632

would the Mark V benefit from a bias mod as well?

I thought I saw some Weber bias rite readings on here for a V and it was reading a little hotter than a IV or III.

I don't need 70% MPD, but 30-40% MPD seems wrong to me (unless you're really interested in lengthening your power tube lives...)

I really wish Mesa would offer this as an option on their amps, it seems like all the boutique amp makers (e.g. Diezel and Rivera) allow for adjustable bias such that you can plug in different families of power tubes. It's almost like Mesa doesn't believe their tubes and/or components (e.g. OT) are tough enough to handle "normal" bias setting in excess of 50 or 60 % MPD...

The thing to remember is that the average guitarist is really quite technically stupid, and a fair number of them (I'd argue the majority, but I've never conducted a poll or anything) don't even bias their amps when they swap tubes. The fixed bias scheme allows Mesa to compensate for their lack of knowledge/lack of inclination to hire a tech.

As for the cold bias... no idea why Mesa does that. I do know that my Recto used to burn through a set of power tubes in about a year, and if biasing hotter means I need to replace them every six months I think I'd rather just leave things as they are.

No need for the bias mod on the Mark V. They are plenty hot enough. http://forum.grailtone.com/viewtopic.php?f=33&t=38013&start=30 If I remember correctly the 6L6's were Yellow color code and the EL-34's were Grey.

so that's interesting that Mesa diverted from their cold-bias strategy on the V... sounds good w/ 4xEL34 probably, no wonder everyone running 4xEL34 is so popular.

It would be fine to have a bias control on the Mk.V to have the freedom of choice of power tubes ( e.g.NOS Siemens EL34s) and best bias settings.
The fixed bias scheme is absolutely fine for Mesa's tube biz, but a strict limitation for demanding players.
So any suggestions/instructions much appreciated,
Mason

I looked into a bias mod for my 295 when I owned one. The simul-class power section was causing fits for the tech - not sure though if the fits were due to simul-class or due to lack of good schematics. Maybe I'm missing something, but I don't understand why one couldn't bias the outer tubes with one pot and have another pot devoted to the inner tubes.

JOEY B. said:

No need for the bias mod on the Mark V. They are plenty hot enough. http://forum.grailtone.com/viewtopic.php?f=33&t=38013&start=30 If I remember correctly the 6L6's were Yellow color code and the EL-34's were Grey.

Click to expand...

I just bought a used H&K switchblade and re-tubed with eurotubes and had a discussion with one of their techs there:

We’ve seen the amps average around 460 plate volts. With 460 plate volts, 70% dissipation for an E34L’s, KT77’s or any other 25 watt tube would be 38mA.

You can easily measure the exact plate voltage of your amp using your Bias Rite. Once you have the reading you can use the bias formula:

The formula for biasing is the plate dissipation of the tube (25 watts for a 6L6 or an EL34 or KT77) divided by the plate voltage (lets use 460 for this example) times .7 will just get you out of crossover distortion and you can go as high as .8 percent.

25 divided by 460 = .0540 or 54 milliamps X .7 = .038 or 38 milliamps

Click to expand...

So if you use the same formula for the STR-440 6L6 quartet in the post/link above, you'll realize that even with the hotter biased pair (36mA), the MPD is ~64.7%.

Michael @ eurotubes stated that they like to bias their amps @ 70-80% MPD. This means that in many cases, even with the so-called "hot-biased for a Mesa" mark 5, it's still on the cold side by their standards. Now there are advantages with colder biasing (IIRC better clear tones and headroom? plz correct me on this if I'm wrong) as well as less "incidents" for Mesa to deal with in terms of fixing/replacing blown screen grid resistors and tubes under the gracious 5 yr warranty.

bottom line: they need to manufacture fixed-bias (for dummies, I mean come on, how hard is it to bias power tubes once a year with a bias rite?) as well as adjustable bias versions of their amps.

EDIT: just noticed this EL34 reading:

with all STR-450 EL-34's , EL-34 bias setting

Outer sockets 43 mA
Inner sockets 53 mA

Click to expand...

so the outer sockets is 77.4% MPD (that's in range for safe/hot)

the inner sockets is 95% MPD (not safe and may have issues or burn out/fail sooner)

so this is even worse.

i wonder how accurate the bias rite is? I've used one myself and it's pretty convenient but not sure how accurate...

ET recommends 70% dissipation.

The Weber BiasCalc also gives 38mA for EL34 at 460V plate voltage (70% dissipation). I'll go for a little cooler 35mA setting with a bias control, as soon as I get the mod done.
http://www.ionpool.net/arcade/tech/Weber_Bias_Calculator.html

any non-authorized mods will void warranty, enjoy.

rabies - if it weren't for non-authorized mods this forum wouldn't exist (or the Marshall one for that matter). If you don't know what I'm saying have a look at Mesa's company history (and Marshall's) :wink:

IIRC 70% plate dissipation is the upper limit of "safe".

I biased my Marshall to 60% PD from a factory setting of a mere 46% PD (amp had a cold, so to speak).
What I noticed was:
OD tones got considerably smoother, had more character and felt more alive. Whole thing felt fuller.
The clean tones got a weeeeeny bit darker but barely noticeable.

Does the Weber meter include the screen draw in it's measuring? If not, bear in mind that what the meter shows you is actually higher than the actual plate current, since the screens draw some of it (I don't know for 6L6s but EL34s draw approx. 3-5mA per tube on the screens).

One more thing - there is no "magic number" when it comes to amp biasing. Let MESA babble all they want, change bias=change tone. It may be a stupid piece of advice but try a couple of different bias settings and use the meter only for reference (match sides and to make sure you don't go too high) and use your ears to decide what works best for you. Who says running at 70% is the optimum? Sounds crap to you, sounds good to someone else.

Very old post....

I wanted to say that even though the Mark V and most Mesas are biased cold for the 6L6 setting, just use a hotter grade of tubes.
Eurotubes gave me a set that is graded rather hot do that it gets the bias where it should be for 6L6 tunes. Sounds better than using the stock Sovtek Mesa tubes.

Using EL34/6CA7, I got a coope grade from Euro since Mesa biases the EL34 setting, much hotter..
So, whether I am using EL34 or 6L6, I am in an optimal range.

That said, my Mark V gets really hot on the backside, and the ven the speaker cable lead, gets super hot.
Hotter when using EL34s.