Help... SPAX7

[Russ]

Does anyone know why Mesa SPAX7 tubes don't test so good? I just bought a handful for my Mark IV and before I installed them I tested them to blueprint my amp as I do my other amps. I found that the two triodes don't test very close across the board. I mean they test good on one side and new on the other. This isn't even consistent from tube to tube. On some it is the first triode that is weaker and others it is the second. Listening to them after installing them, I found that they don't vary much to any appreciable amount whether they are stronger on side a or b. I think out of the half dozen that I bought only one was close enough to be considered slightly matched.

I did find that the first triode was getting killed in the old tubes. That was the reason I got new tubes. Am I to assume that the front of V2 and V3 just get brutally punished. The old tubes still tested new on the second triode. They sounded ok still even with a severely bad test result. Should I put the tubes that are balanced front heavy in those positions? The old V1 was balanced as was the V5 (PI). I found that they were just marginally good tubes still. I am just curious because it seems that this Mark IV is an odd creature to say the least.

Out of curiosity I ran the bad tubes in my Marshall and it sounded like @ss. I am assuming then that the circuit is less sensitive to bad tubes in the Mesa. Although a high gain circuit should be. None of the tubes were microphonic amazingly, just not very strong or anymore.

Has anyone else had this phenomenon with their Mark IV or with Mesa SPAX7s?

I was going to try some nice 1960 RCA 12ax7s that I have in this Mark IV but now I am a little afraid to being that I would hate to ruin the tubes. They will last much longer in my Marshall and not get hammered on one side only.

The old 12ax7s were a mix of JAN Phillips and EH. I am sad to see that the JAN tube was so severely worn on one side. It would have been nice to put in the Marshall.

Also is there any real difference between the regular Mesa (Sovtek) 12ax7s and the SPAX7s? I had assumed that they were tested and graded higher but the one plain Mesa 12ax7 I got tested better than the majority of the SPAX7s.

I asked Mesa but didn't get much of a response. They just said that they tested them to make sure that they work.... That didn't instill much confidence in their tube testing and grading.

On a side note, I also picked up a few GTs. I got a Russian and a Chinese and both were closely matched and tested new. I was surprised that their typical tube tested so well after seeing Mesa's standard for the SPAX7.

Would it be wise to skip the Mesa 12ax7s altogether and just get the GTs?
Price point is comparable so there isn't much to compare dollars and cents. I know there are alternatives like ordering handpicked tubes and whatnot but I am looking for a reliable source that is convenient. It would be a shame to have to order tubes ahead of time and pay more and have to hold a reserve stash. I don't mind holding a full set, but still. It would be nice to be able to run into a local shop and just take one off the shelf and know it is good. I definitely won't be switching to Mesa branded 12ax7s for my Marshalls. I just thought that the Mark IV with Mesa's convenience would be nice. I also assumed that Mesa's grading was tighter than it appears to be. Why sell them as new if they don't test as new? Why not sell them as a regular run of the mill tube also. Why label it Special Requirement and charge more for it? I think I will be staying with GT for the convenience and quality factor. The salesman at the GC I got them from wants me to bring the tester in so he can see the bad quality personally. I was offered the chance to individually test the remaining stock and handpick my own set but I just am not into the inconvenience of dragging the tester over there and having to test all their tubes to find acceptable ones. Isn't that what we are supposedly paying for by buying their tubes and paying more for the SPAX7 as opposed to the regular stock? I am now inclined to check my next power tube purchase even closer. If they can't get the preamp tubes right what is to say that they can get the power tubes right either? I am now curious to see what their actual specs are for their tube grading.

I read in one of the posts here that a microphonic tube is ok in the PI? To me that kind of makes sense but who would want a bad tube in a critical position like the PI? But then again the PI in these is not as critical as other amplifier designs.

It looks like I will be blueprinting this amp for a while. Any suggestions or comments would be very appreciated here. I just want the amp to sound its best without having to resort to buying all NOS for it all the time.

Now it has me wondering if I should look into ordering tubes that are purposely overly heavy to the front side and just good on the back.

Help...

 

[Boogiebabies]

Well, you put a ton of effort in this post, but the SPAX7 is just a regular Sovtek/EH that tested better than others for microphonics. Mesa does not test for gain or matched triodes. In all of my amps, I use the GT 12AX7M
abd they generally test 110/110 in gain and have been very lucky to get such strong tubes. I have had a lot go bad on me quickly, but GT replaces them without a problem. I use the M in the V1 and V5. The rest are Ruby or GT 12AX7 C-9's. They have great drive and are quiet and durable. They dont come off like some of the old silver specials with buzz saw high end and almost no vacuum. Ruby has remade the Silver Special in a 7025 designation. In some cases they sound extremely good for high gain applications. I have tried a ton of NOS and although most are superb sounding preamp tubes, I hear a negligble difference and stick with the new stuff. I will openly admit a quad of STR-415 Phillips and 5 RFT 12AX7's is beyond belief, but I tend to want to squirrel these away. I'm a wuss. I dont think a matched PI does mutch good either. Most PI's are imbalanced anyway so why bother.

 

[Russ]

I guess no one else has had enough experience or cared toeven check their own tubes. Thanks for the response Boogiebabies. I was beginning to think I was an oddball for trying to get the best experience with my tubes. I guess I'll wait to see if anyone else can respond to this to see if there is anyone else that has as much initiative to try to make their tone better.

 

[Boogiebabies]

For 99% of the amp owners here, I could say most of us can only tune our amps by swapping tubes. Back in the day I would run through 25 preamp tubes in every possible position to get the tone I was looking for.
In most cases, it was gone the next day. These tube amps are evil and change depending on many different variables. It todays tube market, most sellers can match triodes and test for gain and it makes it a lot easier. I have wanted a VTV for a while, but I just cannot justify the cost for what I do. It's cheaper to send your stock to a seller who has one and pay to get them tested. I only use Ruby 12AX7C9's in my gain stages and have never been happier with a tube or a company. I dont buy $ 10,000 year, but they treat you like you do. GT has been very good as well. Great customer service and they did a great job on the NVM 6L6, 6CA7 and 12AX7M. I had about a dozen M's go bad and they replaced them asap. Myles Rose tested the ones that were sent to me to be sure they were good for gain and microphonics. Myles is a superb human being.

 

[908ssp]

No tester here except by ear in my amp under the conditions I intend to use it. That said I have a friend who gets and tests old tubes and have been lucky enough to get some good old tubes for short money. I like Telefunkens 12ax7 in my MKIIC+ 60/100 amp for my Santana tones. And mix of Telefunken and Mullards in my MKIIC+ Simul/EQ amp for heavy rock. I have tried the GT M tubes and the ones I tried sounded and worked very well. But the tech I was buying them from tests every tube before selling them and he stopped carrying them because he was returning so many.

 

[Boogiebabies]

I am having a few 3rd generation M's going down. When they work they are great. If I had my choice, the RFT 12AX7 is it for me.

 

[Russ]

I'll have to ask Myles to screen some M's for me. I want to try them as NOS or good OS Mullards are too expensive to run all the time in the Marshall. I think I am going to just make a full change over to GTs though after these tubes die. I just can't see paying the same for the Mesa 12ax7s when you can get better tolerances with the GT stuff. GT tends to have a better selection anyway.

I almost forgot, I have some Mullards on the way........

 

[Shep]

where do you get valve tester's ? or how do you do it?

 

[Boogiebabies]

http://www.vacuumtube.com/toppage31.htm

The pimp daddy of preamp tube testers.

 

[Russ]

Your best bet to find a tube tester is to look online.

As fas as how depends on the unit itself.