The Last Bass Outpost
Gear Discussion Forums => Bass Amps & Effects => Topic started by: georgestrings on December 16, 2014, 04:30:41 AM
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I have a Mesa 400 on it's way to me, and am considering tube options - it already has 6l6s in it, but I'm pretty sure I want to put kt88s in it, instead... The preamp tubes will probably be left alone, unless there's an issue with them...
Any thoughts???
- georgestrings
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PBG and others much more technical than I will surely reply, but swapping 6L6s for KT88s is a sometimes you win, sometimes you lose thing. Some amps handle it just fine, sometimes you put a lot of stress on your transformer. I would see how it sounds as is for a while.
Rick
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PBG and others much more technical than I will surely reply, but swapping 6L6s for KT88s is a sometimes you win, sometimes you lose thing. Some amps handle it just fine, sometimes you put a lot of stress on your transformer. I would see how it sounds as is for a while.
Rick
Yeah, I'm definitely looking forward to PBG's thoughts on the matter... My research (so far) says that the Mesa 400 was originally designed to run 6550s, and that the use of 6l6s in them was due to the lack of a reliable source for 6550s *at the time* - which resulted in a slight downgrade in power, and different musical characteristics... I also haven't discounted going with 6550s, FWIW...
Thanks for your input,
- georgestrings
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I'll just chime in here until PBG posts..
Just curious, what kind of upgrade are you after, soundwise?
I have not used a mesa 400 since 1993, and I did not experiment with tube swapping during the time i played it.
Usually, when people upgrade to 6550 or KT88's, it's usually because they want more clean headroom.
If they just want a somewhat different sound, they might experiment with different manufacturer of the same tube.
In many cases, they end up spending a lot of money (KT's are expensive - at least here in Europe) for not quite the
most impressing results. More often than not, a 'stock' amp sounds best with the tubes it was designed for, if it is serviced/set up/biased correctly. I see many cases where owners have worn out 2-3 different sets of output tubes, with the original driver tube still in. So the ouput stage may not work properly with any output tubes.
That said, there is no reason not to experiment, but I'd check that the power supply and output trannies can handle the tube, and of course check that there is enough negative supply available, and that you will not exceed the total current draw the unit was designed for.
Perhaps not the answers you want, but I'm sure someone here have tried this and can post something on how it sounds :)
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Is it a 400, or a 400+? The original 400 ran (6) 6550, while the "+" ran (12) 6L6.
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I'll just chime in here until PBG posts..
Just curious, what kind of upgrade are you after, soundwise?
I have not used a mesa 400 since 1993, and I did not experiment with tube swapping during the time i played it.
Usually, when people upgrade to 6550 or KT88's, it's usually because they want more clean headroom.
If they just want a somewhat different sound, they might experiment with different manufacturer of the same tube.
In many cases, they end up spending a lot of money (KT's are expensive - at least here in Europe) for not quite the
most impressing results. More often than not, a 'stock' amp sounds best with the tubes it was designed for, if it is serviced/set up/biased correctly. I see many cases where owners have worn out 2-3 different sets of output tubes, with the original driver tube still in. So the ouput stage may not work properly with any output tubes.
That said, there is no reason not to experiment, but I'd check that the power supply and output trannies can handle the tube, and of course check that there is enough negative supply available, and that you will not exceed the total current draw the unit was designed for.
Perhaps not the answers you want, but I'm sure someone here have tried this and can post something on how it sounds :)
I'm definitely looking for more clean headroom, which a change from 6l6s to 6550 or kt88s will provide, according to all accounts I've read thus far... Also, AFAIK the 400 WAS originally designed to run 6550s - so I doubt that'll be a problem... I also know of several people that are running kt88s in their 400s with excellent results - but I believe they had bias pots installed...
At any rate, I appreciate your contribution..
- georgestrings
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Is it a 400, or a 400+? The original 400 ran (6) 6550, while the "+" ran (12) 6L6.
It's a 400, not a 400+... and as you say, the 400 originally ran 6550 power tubes...
- georgestrings
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For a bit more clean headroom with 6L6s you could try a set of JJ Teslas or whatever they are called now. I have done side by side comparisons between JJs , old Motorolas and Winged Cs. The clean headroom winner was definitely the JJ Tesla. Warm low end break up was a tie between the other two. If I was retubing a guitar amp I would probably go with the Winged C or old Motorola(if you could find them). For a bass, JJs would be my choice.
Rick
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For a bit more clean headroom with 6L6s you could try a set of JJ Teslas or whatever they are called now. I have done side by side comparisons between JJs , old Motorolas and Winged Cs. The clean headroom winner was definitely the JJ Tesla. Warm low end break up was a tie between the other two. If I was retubing a guitar amp I would probably go with the Winged C or old Motorola(if you could find them). For a bass, JJs would be my choice.
Rick
Thanks for weighing in...
- georgestrings
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And while you're at it, ask your supplier for a strong set! When shopping matched sextets or octets of tubes, there are not always much to choose from. But if your supplier/shop is kind, let them pick a strong set for you. Either brand (and I point to the WC/JJ comparsion here) can have early or late breakup. Yes, there are differences between tubes, but a working tube can be weak or strong. A/B tests (in my ears/opinion) of tubes/breakup is only ever valuable if they are equally strong (within reason). That said, even as a WC endorsee, I say thumbs up for JJ too - good and reliable for the most part!
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Change the driver tube from a 12AX7 to a 12AU7 if you find you need more headroom. It will lower the overall gain of the amp a bit, but that trade will give you more useable volume and headroom. I'd also go with 6550's or KT88's, whichever you can find the cheapest. Mesa output sections are designed for lots of distortion and "dirty" power and tend to be a bit muddy but their preamps are very bright and high gain. Any of the big beam tetrodes, 6550's/KT88's, are going to have lower plate impedance which will shift the tonal balance even further towards lows, but in the case of the Mesa amps, it increases low-end extension and rolls off the highs just barely inside the range of human hearing. The 400 was designed around GE 6550A's, which is what all modern 6550's copy. It's a 200 watt amp, but it's good sounding 200 watt amp. I'm using a 400+ right now with a 12AU7 driver and while it will never have the punch of a beefier tube amp, it is a nice flavor tonally that cannot be derived any other way.
Basically, Randall Smith took what he had learned about making guitar amps and translated it directly to bass amps. There's an interview with him in the new Bass Gear magazine about his design philosophy that confirms everything I have been speculating about Mesa's tube bass amp design philosophy for years. They're not my favorite, but they are distinctive.
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Change the driver tube from a 12AX7 to a 12AU7 if you find you need more headroom. It will lower the overall gain of the amp a bit, but that trade will give you more useable volume and headroom. I'd also go with 6550's or KT88's, whichever you can find the cheapest. Mesa output sections are designed for lots of distortion and "dirty" power and tend to be a bit muddy but their preamps are very bright and high gain. Any of the big beam tetrodes, 6550's/KT88's, are going to have lower plate impedance which will shift the tonal balance even further towards lows, but in the case of the Mesa amps, it increases low-end extension and rolls off the highs just barely inside the range of human hearing. The 400 was designed around GE 6550A's, which is what all modern 6550's copy. It's a 200 watt amp, but it's good sounding 200 watt amp. I'm using a 400+ right now with a 12AU7 driver and while it will never have the punch of a beefier tube amp, it is a nice flavor tonally that cannot be derived any other way.
Basically, Randall Smith took what he had learned about making guitar amps and translated it directly to bass amps. There's an interview with him in the new Bass Gear magazine about his design philosophy that confirms everything I have been speculating about Mesa's tube bass amp design philosophy for years. They're not my favorite, but they are distinctive.
Thanks for weighing in - I was hoping you would... I'm leaning towards a set of JJ KT88s from eurotubes, and having my amp guy bias them properly - although eurotubes state that their tube sets are a direct match for my amp, and no re-biasing is necessary... Any thoughts on that???
I was already planning on trying the 12AU7 driver tube swap... Also, agreed that my 400 is quite bright - rolling off some highs will not be a bad thing for me... While the 400 isn't quite as powerful as an SVT-II(my other tube amp choice), it's much lighter - and I tend to prefer the Mesa sound over the Ampeg sound - so far, I'm fairly happy with my purchase...
- georgestrings
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Thanks for weighing in - I was hoping you would... I'm leaning towards a set of JJ KT88s from eurotubes, and having my amp guy bias them properly - although eurotubes state that their tube sets are a direct match for my amp, and no re-biasing is necessary... Any thoughts on that???
Mesa's tube bass amps ARE basically slightly revoiced guitar amps from the ground up and it's easy enough to figure out what tube current disappations correlate to Mesa's "rating" system, and Mesa runs their power tubes very cold but does not drive them with power, but rather cascading voltage gain, hence the massive power distortion and bright sound. The best thing to do with the rating system is to ignore it. Tubes change in current output over their lifetime and many good tubes have been trashed that needed only a simple bias adjustment.
The preamp's coupling caps can be changed for better low end, (Mesa's preamp begins rolling off bass frequencies very steeply around 80 Hz - because they are guitar preamps.) but it ends up re-voicing the entire amp and changing all the tone controls and the end product is very "Fendery" which is where the circuit started out anyway. Have your tech install bias pots and bias the amp for maximum stable clean output. A good tech could add a negative feedback loop to the output transformer to get even more clean power out of it, but it's going to seriously alter the sound, much in the same way that the 12AU7 driver tube swap does. One positive to this however, is that if you DO go this route, it's an easy and completely reversible mod and using the 12AU7 will help to get the most out of the power section if you do. 12AU7's have a much higher plate current capacity than 12AX7's. That translates into more clean power to drive the output section. These are all things that will make the amp have much more punch and a better low end, but it will change the tonal character of the amp. IMO, that change is for the better, but lots of other folks feel otherwise.
I was already planning on trying the 12AU7 driver tube swap... Also, agreed that my 400 is quite bright - rolling off some highs will not be a bad thing for me... While the 400 isn't quite as powerful as an SVT-II(my other tube amp choice), it's much lighter - and I tend to prefer the Mesa sound over the Ampeg sound - so far, I'm fairly happy with my purchase...
Note that the changes I am recommending are going to make the amp sound more like the Ampeg, as it has more clean power on tap and was designed that way. With the 12AU7, the preamp voicing will be the same, complete with all the over-the-top gain, but the passive EQ center frequencies will change. The graphic EQ is separate s/s circuit and will be unaffected. The Master Volume knob will become much more useable, and the Volume controls will lose much of their "fuzz boxy-ness" and actually perform as advertised.
Just to give you an idea of what to expect, there's absolutely no harm in swapping back and forth between the 12AX7 driver and the 12AU7 without doing anything else to the amp, so you can literally A/B that part of the mod yourself. Let me know what you think about the changes in sound if you decide to try it. From what I remember on Talkbass before I was banned, everyone who tried the mod loved it, and then after I was banned, several "expert" critics emerged from the woodwork to 'refute' me. I doubt most, if any of them, even had a Mesa tube amp.
My Mesa still sounds like a Mesa, but it has a bit more punch. However, the light weight is because the amps use relatively small power and output transformers and the amp will never be more than a 225 watt amp; it will just be a cleaner 225 watt amp with better lows. I think my preference for setting up my tube bass amps for the maximum potential is why my views on tube bass sound conflict with so many other folks' ideas; they're used to buzzy overdrive from cold tubes being driven with insufficient power. There IS a "danger" to what I do: you can't get away with using crap tubes, but aside from that, the only thing you will notice is a slightly physically hotter amp and power tube life of a couple fewer decades, so you may end up needing to change them if you gig every day...in about ten to twenty years. 8)
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Thanks for the additional info, PBG - I really value your experience on this...
I'll be sure to post updates as I tweak this amp to best suit my use/tastes...
- georgestrings
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Hi Physco. Long time! I finally came around to wanting a tube amp and found another one with 12 Wing "C" 6L6GC power tubes and 4 JJ 12AX7 pre. I don't have a 12AU7 around but I tried a JAN 5751GE and a Sylvania ECC81 and I like the Sylvania better. It did calm the gain down a little and bass is better.
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No love for a 12bh7 driver then?
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I still have some 12HB7 tubes. Some vintage and a new EH.
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I just picked up a EH 12AU7 and the controls have more usable range. I don't know if it sounds much different. Maybe lost some honk. Input 2 is still quite touchy.
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12BH7's would be more of the same effects as the 12AU7 except that the 12BH7 is not a direct drop-in replacement for the 12AX/AT/AU7 because it requires double the heater current, but it should work OK with no issues. There's not enough clean consistent power in the supply rails to make the 12BH7 any better than the 12AU7 in practice anyway. The 12BH7 does have more output power to drive the power section, but no matter how much harder the output stage is driven cleanly, the Mesa is 225 watt amp.
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Fascinating thread. I always wondered about the 12AU7 driver in my SVT III Pro.
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Thanks PBG. Will the 12AU7 make the tranny run hotter than a 12AX7 ?
225 Is way plenty for me these days. We have a laid back blusey band now (Allmans, Steely). The Alice In Chains tribute is over (Ear Plugs!).
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Isn't 225 watts from a tube amp kind of a lot of power anyway? :-0
I mean, that's less than an SVT but a lot more than a V4... How many other amp choices were there in that power range back when the Mesa came out?
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There were always big tube amps around. I think My Mesa dates around (2000). I also have a V-4 (Magnavox 1972) and a Sunn 2000S (1969) that I could use and have more than enough power for me, but the size of the heads and antiquity make them stay home.
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Thanks PBG. Will the 12AU7 make the tranny run hotter than a 12AX7 ?
No. It's a direct replacement.
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Nice. Thanks.
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One more question. I have 3 - JAN GE 5751 tubes just sitting. What could I expect with the 5751 in v3 AND 12AU7 in v4?
Found this in the SD forum.....
Default Re: The Phase Inverter tube. How to choose?
A decent P.I. tube makes a big difference to my ears. Amazingly, those cheap, crappy Sovtek 12AX7As and 12AX7Bs make pretty good ones. As Myles Rose noted, JJ ECC83S tubes do something in the P.I. slot that is not pleasing. I dont' think they do good service there. I think of the GE 5751 as the gold standard P.I. tube. In an amp calling for a 12AX7 P.I., the GE 5751 will bring out the best presence and detail in your amp. That's just my experience.
Lots of folks push the 12AT7 as an ideal P.I. tube. It depends. In some amps, it sounds great in that role. In amps that want a 12AX7 there, the 12AT7 just sounds like it is lacking punch. Let your ears be the judge.
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You'll have a bit less gain and not much else difference aside from whatever tonal curve that make has (brands tend to have their own tonal characteristics). Keep in mind the SD forum is geared to guitar players who equate gain with "fullness" when the two are not necessarily the same or even related in many cases. The Mesa preamp is already so over-gained that taming it too much just makes the amp less powerful, but I don't think a 5751 would sound bad. IIRC, I tried them in mine and the difference wasn't worth keeping them in there. The first stage and tonal gain stages of the Mesa actually NEED the gain of a 12AX7. Also keep in mind that there are different types of phase inverter circuits and it sounds like the link you provided is describing a Fender-style guitar amp with an overloaded preamp driving a slightly lowered gain inverter section for an SRV-type tone. That kind of tone sounds great for guitar, but is not good for bass.
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I got a Sylvania 12AX7 in V1, a JJ ECC83 in V2, a JAN 5751 in V3, a Sylvania 12AU7 in V4.
I got a chance to dig in a little today running Ch1 at 5, Ch2 at 2.5 and master around 4 all knobs pulled and a smiley EQ, B at 7, T at 6, and M at 5.
I tried it with D410-XLT and a Berg NV215. Oddly the Eden seemed more controlled.
Kinda liking what I am hearing. Of course the real acid test will be live with the Bag End stack. I'll decide eventually between the BagEnd and Berg and dump one.
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The Eden sounding better than the Bergantino makes sense because the Mesa's innate tone and output transformer pretty much roll off the bottom octave pretty steeply and the XLT cabs shine in the low mids.