Road King Tube problem

[doctadre]

Hey guys,


Was giggin with my new Road King Series I head last week. During a song, I noticed my gain had dropped immensely in all channels. Luckily it was close to the break, so I had some time to figure out what was happening.

The volume and gain was dropped on all channels. Channel 3 and 4 sounded like they were clean channels, and raising the master volume brought these volumes up a bit. Channel 1 and 2 were barely noticeable. I suspect it was a preamp tube, so during break I tried to swap one at a time with my spare tube and see if it fixed the problem.


I went through all 5, and eventually it fixed the problem. I was not quite sure which tube it was (I kinda confused some of the tubes since I was rushing). I narrowed it down to 2 preamp tubes that it could be, and troubleshooted yesterday to find out which one.


The amp worked with every tube I tried! Now I cannot tell which one was the culprit, and what coulda happened! Anyone have any tips?


I am pretty sure it was preamp since it seemed like the early stage of gain was affected last week. Could these tubes be intermittently bad and then work again? I think I need to swap out all of the tubes ( I bought it used recently) anyways, but I wanna know what could have caused the problem.


Thanks!



Dre

 

[domct203]

Could be a tube in the Cathode Follower positions. Below is the "meat & potatos" from this post.

Here's the info from the Mesa Memo...


SOUND “DROP-OUTS” / INTERMITTENT SIGNAL

One Possible Cause



Most (but not all) Mesa/Boogie amplifiers have one or two “cathode follower” tube
stages in their preamps. Tube selection is CRITICAL in these stages.



Specifically, in a 12AX7 tube used as a “cathode follower”, the voltage difference
between that present at the cathode, as compared with the heater voltage, can be
withstood or tolerated by certain types of tubes, whereas other tubes will fail. The
failure of a “cathode follower“ tube will cause sound dropouts or signal loss.



For the past few years, Mesa has been using two types of 12AX7 tubes: ones
originating in Russia (Sovtek EH), and ones originating in China. The Russian (Sovtek)
tube is NOT reliable as a cathode follower. Of the tubes we are using today (March
2008), ONLY THE CHINESE 12AX7 IS RELIABLE AS A CATHODE FOLLOWER.



In conclusion, if you are troubleshooting for signal dropout in a Mesa/Boogie amp,
suspect a cathode follower tube, and try replacing it with a Mesa 12AX7 that is marked
as “CHINESE” (silk-screened on the tube itself).



Below is a partial list of Mesa amps and cathode follower tube locations:

GUITAR AMPS BASS AMPS

Lone Star & LS Special: V3
M-Pulse: V2
Stiletto: V3 & V4
Venture: V2
Road King I: V3 & V4
Big Block 750: V4
Road King II: V3 & V5
Titan: V4
Roadster: V3 & V5
M2000: V2
Dual & Triple (2ch or 3ch): V3 & V4
Bass 400+: V2
Tremoverb: V3 & V4

ANOTHER POSSIBLE SCENARIO may occur in the “SPONGY” (or on some models,
“TWEED”) power setting: the reduced filament voltage may cause very low output from
a RUSSIAN preamp tube. Again, the recommended fix is to replace the “sagging” tube
with the CHINESE type of Mesa 12AX7 - which are more immune to this type of failure.

Hope this helps,
Dom

 

[doctadre]

Thanks Dom! That was very helpful information. I believe I have russian 12AX7s. Should I just get a new set of Chinese tubes, or just use Chinese tubes for the two cathode follower positions and russian for the rest?



Thanks,


Dre

 

[domct203]

I would just do the cathode follower positions.

BTW- the current new Mesa Russian (NOT Russian-1 or Russian-2) 12AX7 is the JJ ECC83 and has no issues in this position either.

Dom

 

[doctadre]

Hey Dom, thanks for the great tips. I now have chinese tubes in V3 and V4, and things are good.


I am looking for the best tubes to try in the other positions, V1 V2 and V5.


Right now I have 2 Sovtek 12ax7-WAs, 1 long plate Groove Tubes Russian, and a bunch of Mesa 12AX7-A's.


Couple of questions:


A few of my mesa tubes have 12ax7-A on it, and others have this plus "russian 2" on it. Does that mean that the one without the writing is a russian 1? Whats the difference?



Which tube positions really affect the tone the most? Is it going to be V1? Got any tube recommendations for this position? I like a nice balanced tone, with depth and richness.

When I went from the Mesa tube to the Sovtek in position 1, I noticed more brightness. Not sure if its because the Sovtek is brand new or not.


Any advice?


Thanks,


Dre

 

[doctadre]

BUMP



any tips?


Thanks,


Dre

 

[domct203]

The Russian-1 is a Sovtek 12AX7. It will say "Russian-1" on the tube. The Russian-2 is an Electro-Harmonix 12AX7. It will say "Russian-2". The most current Mesa 12AX7 is a JJ, and will just say "Russian" on the tube.

Yes, V1 will have the most effect, as it is the input stage of the entire amp, affecting all channels.

As far as tube selection, that is really dependent on what YOU are looking for from the amp. IMO speaker choice will have more of an impact on that "full balanced" tone, as well as the guitar & p'ups.

As far as new production 12AX7's, many here like the combination of a Tung-Sol reissue in V1, a "high gain" JJ in V2, a Shuguang (Chinese) in V3, and a Sovtek LPS in the Phase Inverter position ( V5 in the RKI, V6 in the RKII ).

Personally I prefer NOS/VOS tubes, with my favorite for V1 in my Roadster being a Seimens short-plate 12AX7, followed by a good strong Mullard (not reissue) in V2, and a long grey plate GE in V3. My personal fav for the PI ( V6 ) is a Matsushita 12AX7, but it seems such a waste to put a vintage tube there when the Sovtek LPS responds almost the same.

My advise is to try one position at a time, really taking note of what it does for the amp. You may be suprised to find that the changes are minimal to you, or you might find a combination you really like. The Tung-Sol/JJ/Shuguang combo is a great place to start.

Remember, there is alot of opinion here, and what works for one rig setup may not work for another. There are just too many variables with guitar construction, p'up choice, speaker/cab selection etc.

Hope this helps.

Dom

 

[doctadre]

Thanks man!!


Yea, I was going back and forth with a lot of the tubes, and found a few that work well for me. Originally I placed the Sovtek 12ax7WA in position V1, but the sound became more trebly and brittle. I threw in a Groove Tubes Long Plate that I had, and that added a lot of nice juice and richness to the tone.


Now, I have Sovtek LPS tubes in V1, V2, and V5, and the Groove Tubes Chinese tubes in V3 and V4, and all is pretty nice.


I have a ton of Mesa russian 2s laying around that I can use for spares. I believe I have a Mesa Russian that I would like to try in V1 as well. We'll see how it goes!!!


Thanks for the tips!!


Dre

 

[doctadre]

Quick question, I have a couple of Mesa tubes that don't say Russian on them at all. They just say 12AX7-A. What kind of Mesa tube are these?



Thanks,


Dre