Mesa Express 5:25 Bias Resistor

[FXR]

Hi Folks,

Been reading here quite a bit since I got my Express 5:25. I have a love hate relationship with it however.

Anyway, right now what I'm trying to do is locate the bias resistor for the output tubes on the Mesa Express 5:25. Does anyone know which it is? I can't find a schematic and Mesa won't come off of it while the amp is in production. I do not have the amp open at this time either, but have some good pics here from a time I looked around. (FWIW) http://www.cpurigs.com/forums/vbpicgallery.php?do=view&g=176

My love hate comes from the output tubes after maybe months of trying different OTs. The original Mesa tubes seem good. But, 3 sets of tubes I just got from Mesa are junk already and going back. All three sets went bad in 4 to 8 hours of use. While in use they seemed to overdrive very easily and compress into what is not a very desirable sound. Hell, a couple even red plated while in use.

I got the amp off the floor and just figured hey, they're old, let's replace them for GP. Bad idea. It put me into all kinds of problems with sound for too long before I realized, guess what, Mesa tubes are burning down fast and sound like crap despite the claim of "plug and play".

I have no way to test tubes, so I have no way of knowing what the specs of these 4 sets of Mesa EL-84 tubes may be. All I know is that the original set is labeled AC-10-WHT and the 3 sets Mesa sent are AC-10-GRY. If I did my research right, the GRY have less overhead? But beyond that, I don't think there is much to the so called plug and play spec of the tubes. Hell, I specified I had WHT and got GRY. An issue Mesa will hear about as I get my ducks in a row.

I have a set of JJ EL84s that are rated 31ma and a set of RI Mullard EL84s rated about 34ma. The Mullards I'm not crazy about, the JJs seem the closest to the original tubes as far as when they break up and distort. I'm thinking I want to go down to 27.5 for 10.5 watts. Right now, if in spec, the V on the tubes in PP should be 382. So, with the 34ma the math says 12.98 watts, too high IMO. The 31ma says 11.84 watts, right on the edge. 27.5ma or so would be down at 10.5 watts.

So, since I've been a tech for years, my plan is to take some readings and buy tubes aftermarket implementing some math to choose them. In my research I discovered checking plate voltage, current draw at idle and the bias resistor which can be replaced to make the current draw adjustable with the changes in bias voltage. (did I get that right?)

So, there's my story and why I'm searching for a Mesa Express 5:25 schematic and/or the location of the Mesa Express 5:25 cathode bias resistor to take some measurements and do some math. I don't think I'll be modding it physically until the warranty is up, but I want to do the measurements for selecting tubes with some overhead.

Thanks for taking the time to have a look at my questions. Although I've worked with 6BQ5 in RF applications and worked on countless other analog electronic component based electronics, I'm just delving into the AMP stuff.

Greg

 

[J.J]

The White code tubes are Mesa's hotest. I'd actually say they did you a favour with the Gray ones.

The 5:25 seems to be biased fiarly well in that it is not hot or cold. My amp came with Yellow grades and the bias measured around 24ma (I'd have to check my notes) and 10.5 watts. When I replaced the tubes I was given a Blue set that are apparently bettween you white and grey tubes.

None of the Mesa tubes will take run hotter than the tubes specs. If you got all your sets at the same time then maybe the whole crate of tubes was dropped and they were all damaged. So far, I have had zero trouble with the Mesa tubes. I changed my first set after 11 months and I am not afaid to abuse them. On average the amp is used 2 hours a day and gets a good workout at jams and rehersals (Ear plugs needed). I thought the Mesa tubes came with a 6 month warranty, so you should be able to get something done if the tubes are the issue.

The hotter tubes are supposed to break up sooner and the Blues certainly do before the yellow ones did, but I have not rechecked the plate current to see what the difference is.

Those other tubes that you have that are rated at the 34 and 31ma, I don't think that will actually tell you what they will do in the Express. Those rating will be based and a specific plate voltage and bias voltage that will almost certainly not be the same as the 5:25. The only way to know for sure it to put them in your amp and measure the plate current.

Persally I wouldn't change the bias though others here have so other tubes could be used. All the best sorting it out.

 

[FXR]

Thanks for the reply. I sometimes over think things. This might be one of those times. I'm not doing a bias mod, or any mod actually.

You did clear up a misunderstanding though.

The hotter tubes are supposed to break up sooner.

It explains some things I couldn't reconcile because of my confusion.

I'm thinking at this point I should get a bias tool and see what's going on. But, like the above, first I see a current only one tube device, then a two tube device then current and voltage type device. I'm not so sure which to get. If I have to measure plate voltage direct I can, but the head has to come out of the box. But since I'm not modding, I think that is a constant and I could use a tool that does current only. A dual is about 60$ shipped for current only, about 80 with current and voltage. Decisions.

Thanks for the reply and letting me think out loud.

 

[take5]

Mesa bias their amps within a certain range and then tests and color codes their tubes that fall within this range. This allows you to select tubes to suit your playing style.......do want more dirt sooner or more headroom? Here is the info that I have on Mesa's color coding for their tubes:

The colors are in order of headroom, Red breaks up the earliest, White has the most headroom. YMMV.

Red
Yellow
Green
Gray
Blue
White

I actually got this "ranking" a couple of years back off the BB with the explanation that the hotter the color the hotter the tube. Makes sense to me......I am not an electronics guy.

Feel free to chime in if any of this is incorrect.

take5 AKA Greg

 

[FXR]

See, now that's what I thought too. But, the Mesa guy told me the opposite Thursday and I was too distracted to call him on it and have a discussion.

They sent me Gry and they are so compressed it sounds like junk.

So, three sets of useless output tubes from Mesa and I have to use their tubes or lose my warranty. I'm falling out of love with Mesa. Not to mention the cheap 12ax7 JJs they sent that the glass tips break off when you breath on them.

 

[take5]

I don't think the power tubes are your issue....it's your preamp tubes. I have a DC3 & DC5, both amps were sounding kind of tired and I retubed them with Mesa SPAX7's in all the positions......the ones with the grey plastic sleeve. They are a lot less cash than buying NOS "boutique" tubes and I really like them. Both amps definately "idled" quieter, the reverb sounded great, nice cleans & smooth distortion, no harshness.....huge improvement, I recommend them.

Greg

 

[take5]

Oh yea, I think that the Mesa guy you spoke with has his color coding backwards!

 

[FXR]

Well, I'm chasing ghosts it seems. We had the amp just sounding amazing today and then, out of nowhere, mud. It was like what the hell just happened? It was like someone changed the guitar cable to a speaker cable or something. It's been on going that we play and things are just so sweet and then bam, mud. I give up after trying all the tubes Mesa sent and still having an expensive amp that goes to crap with no explanation no matter what set of preamp and output tubes are in it.

I'm pretty annoyed right now. Playing when you know you're compromised just isn't the same. I'd feel much better if it just broke then having to ship it and say it works great until somewhere in the middle of needing it.

,

 

[J.J]

A bias tool will probably just let you know that the bias is correct so I'm not sure it would be value for money, unless you do want to modify your amp.

Do you use any pedals or anything else that could go wrong, or just straight into the amp ?

New production tubes are not great for sure, but they are pretty cheap considering that they are actually quite difficult to make. I'd be happy to pay double for a high quality tube sounds good, has high reliability and tight manufacturing specs.

Since you have been done all the tubes, it does sound like there is some other problem with your 5:25. If you leave it on , but without playing it will it start to "mud" out after a time ? If so, I'd be taking it in for a warranty job letting them know how to make the problem happen. Coupling caps and the like can be bad from new. An older amp I got cheap would red plate one of the power tubes after about half an hour. Everthing measured fine in testing, until I made some measurements whith the amp running and found that the coupling caps leackage increased with temperature ! That in turn would cause the bias voltage to increase until the plates became nice and red. Im not saying that your amp has a similar fault, but rather that simple things can cause very stange results, even starting with the best quality components. That fault took me a long time to find and I nearly gave up on that amp. It had been to a tech before I got it and that tech was not able to find that fault.

I can see why people give up on tubes and go solid state

Hope you get it all solved soon. I am sure if you keep at it you will get it sorted and hopefully be happy for a very long time.

 

[Don]

It's been my experience that modern EL84 tubes are lousy and it seems that amps that use them push them really hard!

Also, my amp is a 5:50 with 6L6s, but when I replaced the original tubes which were marked yellow with a set that were marked white, the amp ran considerably hotter- hot enough to smell, even after the tubes were run for quite a while!

 

[FXR]

Thanks for the replies. This is certainly a problem that is intermittent and I'm pretty darn sure its not tube related. I too think it's a bad cap for the reason that I stated, it's like suddenly a cord went bad. In fact, we were switching cords, going direct, using pedals on and off, all with the same results.

On the bias testing, yeah, it would be money spent to monitor what should just work. It's not like I'm going to mod it or anything in the near future. I'll use the money for shipping and or tubes.

In the next day or two I'm going to jam it into an external speaker and make sure it's not speaker related. But, based on two things Mesa said the first time I called, I think I should have just shipped it. But the thought of waiting six weeks wasn't pleasant. But, It's time to move past this or my wife is going to kick me in the backside for talking her into letting have the money for this Mesa. In her mind, we got screwed.

 

[h_ruukki]

Sorry if I'm repeating something that got said - but did you already check the very basic things like cables, voltage, impedance, stuff like that? Also, did you test the amp without any pedals and so forth? What is certain, in any case, is that you definitely did NOT get screwed, no matter what your wife may say. Not with a 5-year warranty.

I got my own 5:25 just now and just love the sound. Wouldn't play anything but a Mesa now that I have one again (I used to have a Mark II combo 10 years ago).

(Edit: Oh wow. I'm replying to posts several years old. Way to go. But the 5:25 is still great!)