When to change tubes on mesa boogies?

[Mango Moon]

Hello Everyone,

I've got an older (ca. ~1990), all original, rack-mounted Mesa Boogie Studio Preamp and a Mesa 50:50 Stereo Power Amp that I purchased new as a pair back in 1991. Both have had 'minimal' use over the years (probably less than 50-100 hours total), and visually speaking, both are in great shape. In addition, both seem to operate 'fairly' well. However, both units have been moved around and exposed to the elements over time, and I'm told by others that to optimize tone and insure that they're operating at tip-top performance, I should probably replace most or all of the tubes.

On the face of it, given the sheer cost of quality tubes these days and lack of availability in certain instances (i.e., out-dated tubes), is this good advice? In other words, would I gain anything meaningful from doing so?

The Studio Preamp has two sets of 12AX7's (7025 STR) with one 12AT7 (STR), while the fixed bias 50:50 power amp uses three 12AX7's and a quartet of (essentially unavailable) 6L6GC's (STR 420). I suspect the entire set of 12 tubes would run me in excess of $400. Does it make any sense? Worse yet, could I end-up with potential compatibility problems?

Suggestions or feedback anyone? Thanks All!

[mikeboone]

Do you like the way it sounds??? Your ear is usually a good judge and if you live in a region where tubes arent readily available, I'd leave it alone.

Just curious... Where do you live that tubes arent available to you? There are several online retailers.

Unless you are looking to buy some esoteric NOS tubes, a matched quad of 6L6's should run about $100USD or so. Each of the preamp tubes will run $15-20USD. I think you are looking at about $260USD.

[Mango Moon]

Mike

Thanks for your reply - I appreciate it. I'm afraid you've misunderstood me a bit on the "availability" issue; what I meant by "lack of availability" wasn't geographic in nature, but rather that my current (original) 6L6GC power tubes (Mesa STR-420) are no longer available from Mesa or anyone else (as best I can tell), anywhere in the world. And with a fixed bias, its difficult to determine what I can (and should) use as replacements. For example, the widely acclaimed Sylvania STR-415's are literally impossible to find and their counterparts (the Sylvania 7581A's) run $100 per tube!

More importantly, even if I chose RCA Blackplates (at ~$60 a pop), I don't know enough myself to determine whether they or any of these others would cause damage to the amp? And I'm sure you know the drill . . . because of the fixed bias, everyone says "stick with Mesa tubes."

As for "the way it currently sounds", I play a strictly clean style, and to my ear, its not quite as crisp or 'crystalline' as it once was. In addition, its not as quiet - its got a bit more crackling goin' on at times. Otherwise, I'd say there's little difference over the years. The original Mesa Owner's Manual says to "expect 6-months to 2-years (or more)" from the STR-420's, so I figured, even though the usage has been light, at nearly 18-years, its probably due.

[BackwardsBoB]

The tubes aren't magic. In fact, if you can get the right Mesa tubes, they'll run cooler than most people here at the forum would run them, slightly improving life, but sacrificing tone.

You can find various tube vendors on the web. Any that answer the phone can supply tubes that will bias OK in your amp. Eurotubes, for example, answers the phone, will prepare a kit for you at low cost, and has instructional videos. Unfortunately, they only sell JJ's tubes. Like all tubes, you'll find conflicting reviews. I think the tubes are average, but as good or better than the Mesa tubes, and the support and shipping from Eurotubes are first-rate. If you like a clean, transparent sound, you might like the JJ's tubes very much. Another choice, with a wider selection of tastier, pricier tubes would be Angela Instruments, www.angela.com. The Tube Store (www.thetubestore.com) can probably set you up at a good price too, and the Canadian location makes for little hassle. Shop around.

You don't have to play an amp for it to need service. Socket contacts corrode, pots get scratch, jacks get corroded or loose. A good tube tech will probably charge you a bit more for tubes, and charge for his time, or a minimum fee, but he will check and clean the amp, and he'll be able to verify the power-tube bias (you can too, if you have at least a voltmeter than can measure mA).

If your pots aren't scratchy, and the jacks are performing well, you can get by with socket contact cleaning when you change the tubes. The videos at Eurotubes show a method (put contact cleaner on the new tubes' pins, and slide them in and out of the socket maybe three times), or you can get the little brushes they sell now for cleaning between teeth, and some contact cleaner, and brush up the contacts in the sockets when you change the tubes. Some contact cleaner will damage some pots. The folks at CAIG laboratories sell good contact and pot cleaners, though they are a bit pricey, www.deoxit.com.

I only change preamp when the amp tone changes, but its OK to change them with the power tubes. Any 12AX7s/12AT7s will work since the circuits are self-biasing. Just order "typical" tubes. Eurotubes and others use the european part numbers, ECC83 and ECC81. Same thing, different name.

Don't forget to turn your amp off with standby off (that is, running), then unplug it and wait 20 minutes or so before you change tubes, and work with one hand in your pocket. A search on "tube amp safety" ought to find you a few guidelines than can help make the process safe.

Good luck!

[mikeboone]

I think there is a limit to quality vs. cost and the tubes you are referring to are past that limit for a guitar amp. They are tested to very low noise and distortion characteristics over the full audio spectrum.

I'd put them in a Macintosh, but not a Mesa.

Just an opinion.. Good luck.

[Mango Moon]

Thanks gentlemen!

Bob, I'm afraid the more I look into this, the more confused I get. I just spoke with Mesa Boogie directly and they tell me that my 50:50 stereo power amp with original Mesa 6L6GC STR-420's is indeed "fixed" bias (i.e., its not self-adjusting), and hence, I can not simply install different tubes like the highly acclaimed STR-415 or even the RCA Blackplate variety.

Therefore, at least according to them, I have but two choices:

1) Purchase (and install) directly from Mesa Boogie, the recommended and somehow properly screened (??) Chinese-made Mesa 6L6 STR-440's (which you seem to be saying will sacrifice tone); or

2) Risk the purchase of four of the other (superior) alternatives (e.g., Sylvania STR-415's, 7581A's, RCA Blackplates), and be forced to locate and pay a trained technician (which I don't really have access to) to have those high-dollar tubes evaluated relative to the amp's fixed output?!@#$%^&*?.

Good Lord! Does it get any worse? Someone wake me when its over! Are these truly my only two options for this once fine little amp? If so, am I going to notice a big tonal difference between the original STR-420's and the Chinese made 440's?

[dai h.]

if the 6L6GC-STR are these:

http://www.pbase.com/fhludlow/toobs

then those are a cheap Russian 6L6 type that should be available from somebody (I doubt they are anything close to unobtainum, more of the opposite I would think). Not really a desired 6L6 type by tube connoiseurs although they might work fine and sound good in your amp. (Showing a tube vendor a picture of the exact type might help in IDing what you want.)

- a pot to adjust the fixed bias (allow adjustment of bias voltage) can be installed (by a qualified tech). Should not be anything close to impossible by someone competent.

- if the amp hasn't been used much the tubes could be just fine. Typically they do not wear out just sitting there but wear out the more and harder they are used (example: soft playing would wear them out less, circuit specs operating closer to limits would wear them out faster).

- some parts (such as aluminum electrolytic capacitors) can and do degrade from non-use, so if there are problems (abnormal buzz, hum) it probably could use servicing. Clean, contacts, pots,etc.

[BackwardsBoB]

I've actually got a pair of matched NOS in-box 7581As of my own, so if you'd like to pay $400 for power tubes (if you can find them), I can't call you crazy. But if you've got to find ones with the right bias current for a 50:50 without a bias tweak, I can only wish you luck.

I'm pretty sure that the original tubes didn't have any great mojo. We're talking Chinese tubes from 1990, right? There are lots of nice tubes out there, even from current production, that will sound nicey-nice in your amp. You could get a set of Winged-C 5881s and live happily ever-after.

Real, serious tube vendors measure the bias current of all the power tubes they receive, and if you simply tell them that you have an old 50:50, they'll ship you tubes that will be just fine, and if you want premium tubes, they can do that too. You're going to get more bang increasing your idle current above the low value that Mesa ships than you're going to get by going to some expensive NOS setup.

Think about it. Mesa has to buy or special order tubes and screen them. Fallout costs them big money, so they need the amp to accept a fairly wide range of production variation in the tubes. I can't believe that the recommended Mesa tubes are going to be as closely matched as the tubes matched by a decent vendor, and Mesa under-biases a bit so that production tubes that run a bit hot won't fry. Tube sellers can provide you with a closely matched set that predictably bias a bit hotter than the average stock Mesa tubes, you'll still get decent tube life, and you'll get better tone.

Don't be nervous. You can get some nice tubes, have an amp tech install them, measure the bias current to make sure you're OK, and get your amps cleaned and checked too, and still save lots of money over what you're talking about.

And no, you don't have to replace your tubes because they're old. The most common failure is a filament failure, and you don't run around changing your home's light bulbs every year do you? But a socket contact failure, a cap failure, or a loose connection, can make the smoke come out really quick, and an 18-year-old amp, regardless of mileage, needs some love to reduce this risk and keep things running tippy-top.

If I bought your amp, I might re-tube it as a starting point, but I'd certainly test and keep your tubes. I even buy used tubes on eBay.

I buy old Carvin x-series amps. Lots of them. I get them in, like the minty grey-fuzzy XV-112 I got today for $325 shipped, and I run them off to storage, sometimes without taking them out of the box. Gets wifey really mad. They're from around 1990, and I'm not even going to plug them in until I get around to doing a little maintenance. I find all kinds of nice OOS tubes in them.

[Mango Moon]

Bob,

I understand everything you're saying here, and again, I thank you, but my problem is, I don't have ready access to a qualified tech to . . . "install them, measure the bias current to make sure it's OK", etc., etc.

Perhaps more importantly, I can (and do) safely install tubes in amplifiers on a regular basis myself (don't need a tech for that), and I've got a Bias-Rite with multimeter sittin' right here to allow for idle current and bias measurement of the original or replacement tubes. Problem is, I don't know what that idle current is supposed to be on a 50:50 and I wouldn't know what to do if it was too hot or too cold. Hence, I'm still missin' something, somewhere.

You said . . . "you're going to get more bang increasing your idle current above the low value that Mesa ships than you're going to get by going to some expensive NOS setup", and I follow you completely on that, but that's the point! How am I going to do that (i.e., increase the idle current) with a fixed bias amp?

For example, if the idle current on a 22w Fender '65 Reissue DR with a pair of 6V6's is supposed to be 24-28 mA, and I install a matched set of 6V6's rated/biased at 26 mA, I can bump the idle current up with a screwdriver until they run hotter and warmer at (say) 30-32 mA. But how am I going to control this in a similar fashion with a fixed bias amp like the 50:50?

[vintagekiki]

Power Tube Maintenance
by Mike Bendinelli / Boogie Tech

Sory for unsatisfactory copy

[Mango Moon]

Yup, makes perfect sense to me! Thanks Vintage!

[hasserl]

First of all, understand that "fixed" bias means that the tubes are supplied a fixed voltage from a bias voltage supply. It does not mean that the bias is not adjustable, it just means that the bias voltage does not vary during use. it stays at one particular voltage where it was set, thus it is fixed. Some fixed bias amps have a small potentiometer insatlled in the bias voltage supply to allow for easy changes of the "fixed" voltage. Other amps do not have the potentiometer, so to change the "fixed" voltage you have to change a resistor. This is the way that Mesa Boogies are built. The bias is fixed, it is adjustable, just not with a potentiometer, you'll have to change a resistor to change the bias. A potentiometer can usually be added to amps like this to make future bias adjustments easier to do.

What BackwardsBob was telling you is that these tube dealers, like Eurotubes, have dealt with so many of these amps they have a pretty good idea what the tube requirements are for these amps and they can usually supply you a set of tubes that will run just fine in your amp just as it is without you having to worry about adjusting the bias. That seems like a very reasonable route to take. Mesa would love to keep you as a tube customer, so they are going to tell you this can't be done and you must buy your tubes from them. Tell them to shove their tubes where the Sun don't shine.

You said you have a Bias-Rite and a multi meter. So get the tubes from someone like Eurotubes, install them in your amp ands check the bias. If they are too hot, call Eurtotubes back up and tell them, I'm sure they'll swap you those tubes for another set that will work. It's not that big of a deal, it's not like landing on the moon, we can actually do this. Or, you could change the bias resistor yourself to get the new tubes biased up properly.

[Mango Moon]

Many thanks Hasserl:

Some of it works, some of it doesn't:

You said . . . "First of all, understand that "fixed" bias means that the tubes are supplied a fixed voltage from a bias voltage supply. It does not mean that the bias is not adjustable, it just means that the bias voltage does not vary during use. it stays at one particular voltage where it was set, thus it is fixed. Some fixed bias amps have a small potentiometer insatlled in the bias voltage supply to allow for easy changes of the "fixed" voltage. Other amps do not have the potentiometer, so to change the "fixed" voltage you have to change a resistor. This is the way that Mesa Boogies are built. The bias is fixed, it is adjustable, just not with a potentiometer, you'll have to change a resistor to change the bias. A potentiometer can usually be added to amps like this to make future bias adjustments easier to do.".

Perfectly understood! Not going to be adjusted short of a soldering job. Ask for "hot" and let the chips fall where they may.

You said . . . "Mesa would love to keep you as a tube customer, so they are going to tell you this can't be done and you must buy your tubes from them. Tell them to shove their tubes where the Sun don't shine."

Spoke with them earlier this morning for clarification and they said . . . "put whatever 6L6 tube you want into it and it'll be fine." Maybe not perfect or even optimal, but "fine" (i.e., no damage to be expected). So that's what I'll do.

You said . . . "If they are too hot, call Eurotubes (whomever) back up and tell them, I'm sure they'll swap you those tubes for another set that will work. It's not that big of a deal, it's not like landing on the moon, we can actually do this.".

Wrong! If only the world were that Poly Anna. Return policies are not what they used to be, and more importantly, shipping charges to where I'm at can run more than the cost of the tubes. I ordered a low noise NOS Mullard 12AT7 (4024) a week ago at $33.00 and the shipping alone was $24.95 UPS Ground. Nice thought though!

Thanks.

[mikeboone]

A word from the man himself on biasing. This article simplified my life with regards to bias.

http://mesaboogie.com/US/Smith/biasadjust.html

Some folks spend way more time worrying about bias than they actually need to, IMHO.

If you dont want to modify your jewel by placing a trim pot in it, you can purchase a decade box from many suppliers to substitute resistor values until you get your bias technically correct.

I like the ear method. I start with a reference voltage as Mr Smith suggests, then tweak it until it speaks to me. I then check the current to make sure its not something totally whacked out and roll with it. I dont care how often I have to replace tubes if I run them in the ground as long as they sound good on the way out!

Have fun guys...

[rabies]

think about how you don't get a QC certificate in your box of mesa (re-branded) tubes. then think about how bullshit robotube really is...