Tung Sol Reissue 7581A in Mesa D180

[BoogieMan]

Howdy. My Mesa D180 seems short on power as it is only as loud as a GK MB200. The tubes are the original fitment Mesa 6L6 GC STR415 the tough ones from Sylvania which were getting silly expensive until they ran out. They lasted! So, I don't know much about tubes but I'm learning. ..

I wish there was better info cached on D180. From what I can gather they were originally designed for 6550 then the quality of those nosedived so the next best thing was the overspec 6L6. I can't see how the 6L6 can survive where a 6550 was meant to be without lowering the plate voltage. They monkied with the driver tubes a couple of times and added graphic EQ which is capable of a bit of a level boost. I have no time line or poweramp schematic.

This amp has a strange current limiting master volume section. Correct me if I am wrong here but I think that it brakes the voltage hitting the power tubes causing them to distort with less drive from the pre amp.

After only a few years and less than 1000 units sold Mesa abandoned the fancy Master and turned it into a regular one. At the same time rumour has it they went back to 6550 tubes and called it Bass 400, before again reverting to 6L6. I'm guessing that nasty Chinese tube quality  meant too many failures and it was anaemic competition for the SVT so they doubled up on tubes for the 400+

The 400 schematic shows 540v applied across 3 tubes, is that 180v each? I think not! But 540v is way more than any 6L6 is rated at. A little knowledge is a dangerous thing.

Given the tube store's glowing review of their new tung sol 7581A as the duck's nuts 6L6 I figure it should be a good substitute for the original STR 415. I have no idea what to ask for in the way of bias current.

Note, installing bias trim pots isn't an option as there is no tech support here.
 

[Psycho Bass Guy]

Unless you can tweak the bias, you're wasting money to buy real 7581's (and I have a NOS matched set of JAN (milspec) rated I could sell you) or ANY new output tubes. Mesa tube bass amps are astoundingly conservative and biased extremely cold. That's how they run their stupid tube ratings game and that's why they stress their "dynamic headroom;" the only way you'll ever make rated power is to crank the pre-gain to hell and then it will be dirt city.

All Mesa bass amps find a much more useful life with 12AU7 driver tubes (I call it "un-guitar-amping" them); even though they are never shown on the schematics, they make WAY more sense than the 12AX7's Mesa did use. That's an easy swap, nothing other than letting the amp cool and changing the tube. You'll lose gain overall, but the amp will stay cleaner longer, have MORE power since the 12AU7 will drive more current into the output section, you'll be able to push the input harder and have it get louder cleaner, and you won't have to change anything else (provided your amp isn't skating on the edge of bias redline which if it's anemic sounding, it isn't). 

The output rail is indeed 540vdc, which is right in line with the ratings for a 6L6GC, the standard to which almost all modern generic "6L6's" are made: http://tdsl.duncanamps.com/show.php?des=6L6GC
Take a look at how high the voltage ratings go for class B and C power. 40 volts is no big deal in the Class A/B  D180. I almost guarantee the actual plate voltage measurement will be significantly higher than that anyway, and you've never known.

I'll also almost guarantee that rather than new tubes, you need ONLY a bias adjustment, but new tubes with higher idle emission put in the amp will "fix" the problem. If you do swap the tubes, hold on to the STR's. Odds are, they're fine, just "broken in."

[BoogieMan]

Thanks, a 12AU7 would be a cheap enough experiment.

Any thoughts on how that driver tube interacts in a current limiting master volume setup? To get cleanish tone they all require master over 9. Mesa instructs for bass that you leave it on 10. Believe it or not it was sold as dual purpose guitar amp suitable for bass. The less than clean sounds are neato stuff

No chance of adding bias. One trip to tech costs like 2 sets of tubes. The 7581A I am looking at is not vintage NOS btw. http://blog.thetubestore.com/tung-sol-introducing-impressive-7581a-vacuum-tube/

http://www.thetubestore.com/Tubes/6L6-5881-Tube-Types/Tung-Sol-7581A

What do you think of my conjecture that it should hold up similarly to the old STR415 set?

My main worry is if the tube is rated for a higher 35w power output it may jigger the balance of the Master vol system further than it already is. Flipside, if it would soften the descent into ramptant fuzz instead, that would be great! Damn Mesa for ''losing'' the schematic. Note I don't expect 180w to turn into 210w so to speak.

[Psycho Bass Guy]

Any thoughts on how that driver tube interacts in a current limiting master volume setup? To get cleanish tone they all require master over 9. Mesa instructs for bass that you leave it on 10. Believe it or not it was sold as dual purpose guitar amp suitable for bass. The less than clean sounds are neato stuff

That's basically how and why they used the 12AX7's for drivers. Current drive is much more demanding on an amp and stresses everything exponentially more, especially so in tube amps. By hedging their bets and making the amps hard to actually drive to the rails, it's a bit of built-in insurance against tube defects and parts limitations. Their output transformers are relatively small for their rated power. Rather than being big low end brutes like most other bass amps, they followed their guitar line's philosophy of preamp voltage drive/overdrive being run into a "cold" output section. Until their new Strategy and Prodigy amps came out, not a single Mesa tube bass amp actually put out anywhere near the power rating implied by the model names in stock form. Even the Bass 400+ with 12 6L6's is only 225 watts and the Bass 400 with 6550's was likewise; an Ampeg SVT eats them in volume any day of the week but runs a hell of a lot hotter, is harder to service, and weighs twice as much.  All of their s/s output-sectioned amps are a completely different sound and ballgame because it's MUCH easier to make s/s stable for low end.

My feeling is that Mesa in the early days didn't think bass players would be very interested in what they offered and they couldn't compete against Ampeg, Sunn, or Fender's then-current and well-selling tube bass amps just in parts costs. To this day, ALL Mesa tube bass amps have a high pass rolloff in the stage coupling to the output section that starts in the second octave. I've discussed that here in other threads a bit. The dual purpose bit, IMO, was because Mesa (Randall Smith) didn't think that bass players would want his product, so he was playing it safe and sticking close to his guitar amp market roots.

No chance of adding bias. One trip to tech costs like 2 sets of tubes.

What is your location? You need to find a better tech.  For all intents, most Mesa bass output sections are just modified silverface Fender power sections and any decent tech can do the work easily. It's also not a good idea to blindly swap output tubes, regardless of any BS "ratings."  Installing an adjustable bias pot or even just swapping out bias resistors will also allow for an examination of the amp and may uncover otherwise hidden potential problems. ANY D-180 is over 30 years old now. Parts get old and heat from use breaks things down. The "marketing" answer is to tell you to buy new output tubes. The "engineering" answer to your amp losing apparent power is to have it checked out. Engineers keep amps running. Marketing guys just want to sell you something.

The 7581A I am looking at is not vintage NOS btw. http://blog.thetubestore.com/tung-sol-introducing-impressive-7581a-vacuum-tube/

http://www.thetubestore.com/Tubes/6L6-5881-Tube-Types/Tung-Sol-7581A

What do you think of my conjecture that it should hold up similarly to the old STR415 set?

New Sensor (who actually makes those) puts out some VERY good tubes. It's likely those even surpass the old STR spec, which was a Sylvania specification developed for Fender in the 70's. It means "Special Test Requirement" and came from CBS Fender's (for the time) high-powered tube designs.  Mesa just glommed onto the name because those were the tubes that were plentiful when their amps were first being made and made it a marketing gimmick. Their "STR" numbers mean dick.

My main worry is if the tube is rated for a higher 35w power output it may jigger the balance of the Master vol system further than it already is.

Tubes are voltage amplifiers which operate based on the circuit they're in. It doesn't matter what the "potential" output of a tube is, unless the circuit can drive it to that, it won't put it out.

Flipside, if it would soften the descent into ramptant fuzz instead, that would be great! Damn Mesa for ''losing'' the schematic. Note I don't expect 180w to turn into 210w so to speak.

It's a modified Fender Studio Bass/Super Twin output section with Mesa's cascading preamp. The 12AU7 driver will definitely be a step in the right direction, but I still say you need to have it looked at. There may be something wrong with it. When I still did tech work, not a single time that an amp was brought to me for tube replacement was it ever actually necessary, but I usually found other stuff wrong that once repaired brought new life back into old warhorses that just needed some attention. Tube amps are much tougher than advertised, but they DO need maintenance.

[BoogieMan]

If I understand you, subbing a 12au7 in V4 would limit the power output???

Meantime it sounds really good just not as loud as I expected. I haven't owned it long. Replacing the original 30 year old tubes seemed a reasonable proposition. It would be a three day affair to get a tech on it in between the driving. Courier service is more risk than warranted. If I knew of a truly worthy tube tech to take it to I could plan vacation around it. Mesa NZ told me to play it until it acts up.

[Psycho Bass Guy]

If I understand you, subbing a 12au7 in V4 would limit the power output???

No. Compared to the 12AX7 in there now, a 12AU7 has a much lower voltage gain factor, so the gain value for that stage is reduced, but a much higher plate current/output power capability, which in turn will drive the output tubes harder, and with Mesa's insane amounts of preamp gain, it's not like the amp actually ever gets quieter. You'll notice the noise floor of the amp at idle will drop and it will have more punch and lose less low end at high volume, and because the drive section now has greater power, it will drive the power section to greater output power. The difference isn't earth shattering; it's more a change in the feel of the amp, but it will be noticeable.

Meantime it sounds really good just not as loud as I expected. I haven't owned it long. Replacing the original 30 year old tubes seemed a reasonable proposition.

The thing is, the modern accepted concept of "short" lives of tubes is little more than repeated marketing hype started way back when folks like Groove Tubes and Mesa figured out that they could convince folks to pay a premium for ordinary tubes with fancy labels and performance ratings derived by measuring their gain at a set bias current and plate voltage, which is generally much lower than the actual operating conditions, ergo the ratings are almost meaningless.  Power tubes, even driven very hard, generally have the same life expectancy as power transistors: let that sink in for a minute. How many people with solid state amps talk about "retransistoring" their amps other than replacing defective ones? The first things to need replacing in most tube amps are the electrolytic capacitors and large wattage resistors, as they change value with heat and age. The fact that tubes have sockets makes them an all too tempting catch-all for any kind of amp ill, but the REAL truth is that if a power tube is bad or worn out in a tube amp, something else in the amp killed it, and until that problem is addressed, dropping in new tubes is just as likely to make things worse or at best case, ruin a new set of tubes.

It would be a three day affair to get a tech on it in between the driving. Courier service is more risk than warranted. If I knew of a truly worthy tube tech to take it to I could plan vacation around it. Mesa NZ told me to play it until it acts up.

Mesa's "service" network is based on sales, not product expertise, and their answer reflects that. Any competent tech who knows his way around any old tube Fender bigger than a Bassman should have no trouble measuring the output power, installing a bias adjustment, and setting the bias for maximum clean power. He can also tell you just how healthy your output tubes actually are.  I know a REALLY good tech in Australia; I can ask for a recommendation from him for someone in New Zealand.

...and back to my point about Mesa's exaggerated ratings, the fact that your roughly 100-120 watt tube bass amp has an equivalent volume to a s/s GK MB200 is normal. The things I am telling you about will enable you to truly hear what the amp is capable of doing and if you like the amp's tone now, you'll like it even more once you tweak it a bit.

[Pilgrim]

You're right that marketing today would have you think that tubes have a short service life.  My 1967 blackface Bassman amp still has most of the tubes in it that were in it when I got it (in 1967).  It has been used occasionally, on and off for the past 48 years.  I did have all the caps replaced in 1998.