1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  
    TalkBass.com has been uniting the low end since 1998.  Join us! :)

A couple of Mesa 400+ tube questions for PBG or anyone else (re: tubes and biasing)

Discussion in 'Amps and Cabs [BG]' started by yoss, May 19, 2004.

  1. yoss


    Oct 1, 2003
    I've been reading through threads on Mesa 400/400+ tube issues and just wanted to make sure I have things straight in my head and to get any additional advice specific to the 400+. Sorry if I'm being a bit redundant.

    I've just purchased a new 400+ which should arrive in a week. I assume the tubes won't need to be replaced right away, but I'd like to upgrade to high quality tubes either sooner or later. So correct me if I'm wrong but:

    The tubes I'll want to replace are the 12 6L6's. However to do so the amp will need to be modified to include an "adjustable bias control". Once this is done I can put better tubes in without fear of hurting the amp, and in the future tubes can be easily switched out, and and the amp adjusted accordingly, or fixed, with less hassle and cost. Is this pretty much correct?

    And with the above in mind, what tubes should I use to replace the stock 6L6's and where can I get them for the lowest price? Also, should I replace the 4 12AX7 tubes as well? And for the bias mod I'm looking at about $200 bucks for that on top of tube cost?

    Anything else I should know about Mesa 400+ amps and tube issues?

  2. Before I re-tubed my X year old Mesa 400+, I emailed Bob Pletka from Eurotubes about this exact issue. Here's his verbatum reply:

    I hope this helps you out.

    When I re-tubed my amp (without biasing) it sounded absolutely incredible.
  3. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Ditto that recommendation. Go with the JJ's. They're very nice sounding and evenly balanced tonally, and matched quads can be had for excellent prices. If your amp is biased cold (ie stock), you should be able to put in the JJ's without any concern for rebiasing. Generally speaking they like to run a little cooler than the usual 35 mils, they usually end up around 25 in my experience. But you don't have to worry about that, just remember to put the members of the matched sets/quads on opposite sides of the OT, and you should be fine.
  4. Yep, and Bob was a pleasure to work with.
  5. Beefbass

    Beefbass Guest

    Feb 4, 2001
    Hey There, I've read a lot of posts about tube amps. I'll admit, I'm no amp tech. But if you search this question, you'll find that PBG is gonna tell you to always have your bias checked when changing tubes. That whole thing about tubes being inverters doesn't matter. You'll see why if you read some of what Psycho has said about it in the past.

    Since he fixed these things for a living for 13 years, it's a safe bet he knows what he's talking about.

    Just put Psycho bass guy into the search function, and those threads will come right up.
  6. I don't know...I'd probably show up here asking what to do and you'd tell me to put an adjustable bias mod in.

    Giving Bob the benefit of the doubt, I don't think he'd intentionally tell me NOT to do something that would eventually damage my amp or diminish the sound quality. He's in the business of selling tubes, and in order to establish repeat customers, he'd want his tubes to be represented the best way possible. If the best way possible was to take it to a tech and have it biased, then he'd probably suggest that.

    He's got nothing to gain by having me swap out tubes, sound like crap, and never buy from him again.

    Other tube manufacturers and tube retailers might say completely different things. I'm sure Svetlana, Panasonic, and Electro Harmonix tubes behave differently than JJ/Tesla. I was just sharing my experiences.
  7. Ive got an Old 50 watt Valve amp that my Step Dad built a few years ago. Ive recently replaced the original sovtek EL34 valves with a pair of JJ EL34s and they are a really good valve. I Know we are discussing 6L6s here but the JJ valves are that much better.
  8. Paul A

    Paul A

    Dec 13, 1999
    Hertfordshire U.K!
    Another vote here for JJ's - best you can get imho. :)
  9. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    People make entirely too much of this bias thing. Biasing a tube amp is like adjusting the idle in your car. It's a calibation, but there's a pretty wide range within which the car will work properly. A few mA's more or less isn't going to matter, unless you're far away from the midpoint of the operating range to begin with. The idea is that you have to set your idle "somewhere in the middle" to get your push pull circuit to operate correctly.

    If you really want to be a stickler about it, there's probably a "theoretically optimal" bias point (which can be found using an analysis of the crossover distortion), but if you want to get it "dead on" then all of a sudden you have all kinds of problems with drift and whatnot. IMO, Fender had the right idea with their early tube amps, which is to require a minimum of non-user-serviceable controls and functions. Most of the early tube amps had no bias adjustment, the bias voltage was fixed at the factory ("somewhere in the middle of the range") and the user could swap tubes as needed without ever having to worry about tweaking a control afterwards.

    But, the behavior of your particular amp will depend on the way it was designed. If it doesn't have a bias control, it probably doesn't need one. If it does have such a control, then it's likely that it could benefit from a tweak after swapping the tubes. The main thing is to get well matched sets of output tubes, ultimately that'll be much more important than whether your aggregate bias is a few mils up or down. Fender even went so far as to eliminate the bias control entirely in their later silverface amps, opting instead for a "bias balance control" to address the difficulty of obtaining properly matched sets of tubes in the 70's and 80's. In these latter amps, the aggregate bias is fixed, but the percentage of that which is directed to each tube can be varied.

    There's all kinds of schemes that'll work. My guideline is, make sure the amp isn't sounding horrible (and unlike it did before) after swapping the tubes, and then make sure the plates of the new tubes aren't glowing cherry red after a few minutes of normal operation. That'll test the endpoints of your operating range. If you're within the range, you're perfectly fine from an electrical standpoint. The rest of it has to do with how your amp sounds and responds, and I know plenty of people who tweak their bias controls by ear till they get the sound they want (obeying the guidelines I mentioned, but without regard to where the final bias setting ends up).
  10. Paul A

    Paul A

    Dec 13, 1999
    Hertfordshire U.K!
    One thing to bear in mind is that you will pay a premium for "Matched" sets from suppliers.
    I've always found with JJ's that they have been within a few mv of each other unmatched!
    Maybe I was just lucky though .....
  11. My Trace Elliot has a Visual bias device on the back of it. Three LEDs indicate the state of the power valves bias. I dont really know how accurate it it is but it does wander a little particularly when cold, when playing all three are on and off, its like an Xmas tree! Sometimes itl stay cold when operating at low levels, recording etc yet other times under the same conditions, will flick straight to green. I totally agree with nonsqtr with his statement. Be aware of it but dont mither about it either. If your valves arent glowing red, then you are propbably fine.
  12. yoss


    Oct 1, 2003
    Here's what Bob at Eurotubes told me regarding retubing my Mesa 400+:

    Your Mesa 400+ uses a matched set of twelve 6L6's and four 12AX7's....The JJ 6L6GC's and ECC83S preamp tubes work very well in the Mesa 400+ amps and will warm up and add thickness and definition to the tone.

    These amps are fixed bias at a cold setting so I always use a pretty warm grade of the JJ 6L6GC's which will get the bias up to a fair level as the stock tubes do so you don't have to worry about biasing the amp. You will however get better performance by modifying the amp for adjustable bias and dialing it in, but a word of warning, this will void your warranty!

    I would use balanced and matched ECC83S preamp tubes because the 400+ uses these as phase inverters and balanced tubes will help the sound to be very full on all notes.

    A full retube would run 217.00 and I can ship and insure them by Priority mail for 8.00 with a 3 day delivery anywhere in the US.

    $217 doesn't sound too bad to me, but again I know nothing about tube prices. How does this sound to you guys?
  13. Not with high powered bass amps: The idle in your car isn't at 5000 rpm. That's analogous to what idle dissappation of a tube is in a big bass amp since they're running much higher plate voltage and current than your typical guitar amp. I've heard that retarded idle analogy so many times, I'm sick of it. It applies in only the MOST vague sense, and while it looks/sounds like a logical bit of misinformation, it's still misinformation.

    Without paying tens of thousands of dollars for a distortion analyzer, you can get close enough with a scope to determine a pretty good "optimal bias." Yes, with age, the tubes will drift, which is EXACTLY why you need the ability to adjust the bias. I know that crossover distortion biasing is frowned upon by most guitar amp gurus, but I don't care. It works, and when I run dissappation calculations after the fact they're usually higher than the recommended 75% maximum disappation, but somehow the tubes last just fine.

    That's because a lot of the very early Fender amps used cathode bias, which doesn't need adjusting. Fender's early 60's and later larger amps all had bias/balance controls for p/p adjustment. So which amps are your referring to?

    Ummm, no. Say for instance a particular amp, like say a Bass 400+, was designed so that instead of having a bias control, anytime the tubes shifted draw, you needed to buy a NEW set of output tubes "matched" to the "rating" of the amp's voltage rather than adjust the amp for its existing and perfectly usable tubes. Tubes make a LOT more money than amps, so it's in Mesa's or any tube dealer's interest to encourage a "rating" sytem because it will mean more tube sales because every time the draw shifts, rather than just adjust for it, you have to buy a whole new set of output tubes.

    That's an EXTREMELY misleading statement. Bias balance works exactly like a normal bias control, just that you have both halves shared rather than separate. Fender had it LONG before the Silverface amps.

    The "sound" test only works if you know what the amp is SUPPOSED to sound like. My V9 came from a guy who thought it sounded great and half of its output tubes were completely dead! Also, regarding "cherryplating", suppose one of your grid line gets shorted; according to your system, then it's time for new tubes. The bottom line is tubes are electrical components of a circuit and that circuit's operation needs to take the tubes into account. I don't hear anyone talking about swapping transistors and "not worrying about a few mA" because there aren't any transistor dealers encouraging people to do so "for the sake of tone." I don't buy into the voodoo of "this sound" or "that sound" from running tubes at a certain disappation. I run tubes for maximum clean output, just like I run transistors because they do exactly the same thing. I've never had a guitar player complain about the way the amp sounds after I worked on it either.

    What "range?"

    Only because they KNOW hat it's supposed to sound like. I do that as well, but the difference is I double check using the scope.
  14. I got a Mesa 400+ bout less than a month ago maybe. It's the most awesomest.

    I was just wondering how much a dozen of thos J/J Tesla 6L6's goes for?
  15. yoss


    Oct 1, 2003
    Bob at Eurotubes quoted me $217 + $8 shipping for a full retube of a 400+. This would mean all 12 power tubes and the 4 preamp tubes. I think the 12 6L6's alone would probably be a little bit under $200 from him. Scroll up a few posts in this thread and you'll see what I posted from him about retubing my Mesa and the costs to do so.
  16. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    217 bucks is a good price for a full retube. By comparison, matched quads of JJ's on eBay go for around 50 bucks, and the ecc83s's maybe 8 bucks apiece. And by the time you add in the shipping and all that, and adjust for the risk factor on eBay, 217 bucks sounds pretty attractive.

    Let's see, PBG seems to have a differing opinion on the bias issue, which is fine, I'll respect that 'cause it seems he's been doing the tube thing for a long time too. It's hard to fault someone for erring on the side of safety and precision, and if you want to rebias your amp every time you change tubes, by all means do so. Being comfortable with your amp is important, from a player's standpoint, and if the bias thing causes you concern or distracts you while you're playing, definitely do the adjustment (or have someone do it for you).

    In my own amps, which as PBG pointed out are mainly the smaller guitar type tube amps (although I do own an SVT and a 300PS, both of which have relatively more intricate biasing systems), I prefer the "ordinary" bias control, and I've modded all my silverface amps to remove the bias balance control in favor of the blackface-style control (that would be a Twin Reverb, a Super Reverb, and a Bandmaster). The downside of that is that I have to use matched pairs or quads of output tubes, 'cause mismatched tubes in this type of circuit can sound pretty bad. Also it makes the adjustment a little harder to do by ear. On the other hand, matched sets of output tubes are (thankfully) pretty easy to get again these days, and I've been doing this tech stuff for about forty years now, and over the years my ear has gotten pretty good in that regard. And I've never lost or damaged an amp by using my ear, and I"ve done literally hundreds of them this way. Whenever I swap output tubes, I listen first, and if the amp sounds okay I don't rebias. If I feel it could benefit from a bias tweak, I'll try it by ear first, while carefully watching the tubes. If I can't results that way, only then will I break out the bias gadget and multimeter. I usually bias the quick and dirty way, 'cause IMO it's not worth spending the time to use a scope and a signal generator to look for a crossover notch. Really the main issue is idle current, and in my experience 99% of the people out there can't tell the difference sonically between two otherwise identical amps that are idling at 15 mA and 30 mA. The useful idle range for 6L6's is probably between 10 mA and about 40 mA, with a nominal plate voltage of around 400 volts (the range can be recalculated for different plate voltages). Biasing anywhere within that range will get your circuit operating "correctly" from an electronic standpoint, although biasing more precisely can get you some benefits in terms of tube life, and also debatably in terms of sound. I'm just saying this to let you know what "I" do, obviously YMMV and all that. Also note that when it comes to a customer's amp, I'll do whatever the customer wants me to do (as long as I think it's safe). If the customer wants to pay me for an extra hours' time to bias his amp with a scope and a signal generator, I'll do that. If he wants to pay me for fifteen minutes to pull the chassis and use a bias gadget and a multimeter, I'll do that. If he doesn't care, I'll let him listen to the amp after I'm done with it, and if he has any issues with the sound, I'll offer to rebias it for free using one of the quantitative methods. I've only had one person take me up on that offer in the last ten years, and most of the time I'll tell people not to waste their money needlessly on elaborate biasing methods, since mA-level precision isn't really needed in the vast majority of cases (the exceptions being oddball circuits like the Fender 400PS, which has a lengthy factory procedure for biasing correctly, and failure to adhere to it can easily damage the amp, and in some cases can actually melt the 6550 output tubes). But I don't know of any 6L6 type amps that require precise biasing like that. And again, if rebiasing "is" needed (electrically speaking) "or" if it makes you feel better from a player's standpoint, by all means do it, and use whatever method your prefer.

    I'll only comment on one little thing in PBG's post, about the "bias balance control" on the silverface Fender amps (not to be a contrarian, just in the way of education). My statement on the topic was 100% correct and was in no way misleading. These "bias balance" controls are very different from the "ordinary" bias controls that one finds on the earlier blackface amps. The difference is, that the ordinary bias controls adjust the "aggregate" bias, in other words both tubes go up and down together, and it's not possible to adjust each tube individually with these controls. On the other hand, the bias balance control does not alter the aggregate bias at all, and there's no way to move all the tubes up or down "together", the only thing you can do is to adjust the "balance", which does absolutely nothing for the aggregate bias but does allow the user to control each tube individually. As far as I know, the bias balance control was a silverface era invention, I'm not aware of any 60's blackface amps that use it. The history of Fender (in print) supports my claim in regard to the purpose of this change in the bias circuitry.

    For the early Fender amps that used factory preset bias, I can cite as examples the Bandmaster 5E7 and 6G7, the Concert 6G12, the Pro 6G5, the Showman 6G14, the Super 5F4, the Twin 5F8-A, and the venerable Bassman 5F6-A (the latter being two of the holy grails of tube guitar amps), all of which have grounded cathodes and factory preset bias voltages. You can find the schematics and layouts on schematicheaven.com.

    (If you want to compare these circuits with an "ordinary" blackface-style adjustable bias control, look at the Bassman AA864 circuit. If you want to see an example of a silverface-style "bias balance" control, look at the 135 watt ultralinear Twin Reverb).

    Anyway, Bob at EuroTubes sounds like a knowledgeable guy too, and if he says he's done the tube replacement without rebiasing in other similar Mesa amps, then I'd give that a shot. Do watch the tubes for a while after installing them though, occasionally a new tube will go bad after just a few minutes of use, and you want to catch that early if it happens, so you can minimize the risk of losing eleven other brand new power tubes (and note that rebiasing will not mitigate this portion of the risk). I've had very good experiences with JJ 6L6's, and in my experience the odds are overwhelming that everything will work properly and you won't have to jump through any hoops after installing the new tubes.
  17. yoss


    Oct 1, 2003
    Excellent info guys! Thanks a bunch!
  18. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Can't speak for Mesa (obviously), but I will guarantee that you need to check bias on any tube output SVT when changing tubes (vintage, II, IIPRO Cl, etc).

    There is a lot of available power in the power supply, and you can easily bias to a point that will be too hot. Also, on the units with the "protective" circuitry, excess bias may make that too sensitive.

    Some amps are made with fixed bias because they are intended to use selected tubes within a certain gain range. All tubes within that range will then have acceptable bias.

    Others are low enough power or supply voltage not to be much of an issue. Some use tube types with less sensitive biasing requirements.

    But typically amps with supply voltages pushing the tube ratings will have and need bias adjustments. Any tech who says the adjustment doesn't matter is one I would not recommend going back to.

    I'd generally second what PBG said, although nonsqtr has some decent points.
  19. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Interesting posts, as always. These are the reason I keep floating around in Amps from time to time.

    How much can you expect a tubes bias to drift? By reading some of these posts, it seems like they are constantly in a tiny state of flux. Are the constantly drifting back and forth, or is it a gradual thing over a long period of time? How far does a tube have to drift before it's too far out of spec? How long can you expect that to take?
  20. mgmadian


    Feb 4, 2002
    Austin, TX
    That does seem to be the central issue re: whether it's advisable to change the bias (or even install an Adjustable Bias Control) in a fixed bias amp.

    The way I see/understand it (as a Bass 400+ owner who's presently contemplating new tubes): the interplay/relationship between the amp's Bias setting and the tubes themselves is analogous to trying to multiply 2 numbers together to produce the result "12": if the first number is 2, the second number needs to be 6. But... if the first number is 3, then the second number must be 4 (bear with me here).

    Likewise, if the desired/optimal outcome is to evoke a "good sound" from the amp, OR to set the amp's operating characterstics a certain way, it seems you could achieve this with either a Fixed Bias or an Adjustable Bias. You can either fix the bias and adjust the tubes to yield this optimal outcome (i.e. make sure you install tubes with the right specs), or you could install any tubes (within reason), and modify the Bias value to yield the same outcome.

    When installing new tubes, you can select tubes having the desired specs. The question I have is: will tubes really 'shift' or 'change' enough over their expected lifetime such that it might make sense to keep them (i.e. they're not in need of replacement altogether) and merely tweak the bias to bring the amp back to its 'optimal setting'? If so, what about simply having a new (fixed-value) bias resistor installed?

    Any thoughts on the logic (or lack of) here?