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Posted: Fri May 23, 2008 6:28 pm
by Brewski
Hey Blakkspider, I hope that you got your amp back up and running. 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.

Possibly same problem with my Stiletto Duece?

Posted: Mon Jul 07, 2008 1:52 pm
by gumbymage
The other day, I noticed I had to crank the Output and Master way beyond 12 O'clock to hear anything from my Stiletto Deuce. Plus, the low E string was almost non-existant. No bass at all, with a crackling sound thrown in. Very thin and tinny. I was baffled. :?

The next day I plugged it up and played some and it seemed better. I was getting louder volume and crunch with Output and Master between 10:00 and 11:00. However, after 20 minutes of playing, I switched up to Bold and nearly blew my eardrums out! :shock: I went to Spongy and it was lower volume and seemed to change on it's own! I flipped back and forth and finally I was getting nothing but ear splitting volume and put both Output and Master down below 9:00 and it was still incredibly loud. In fact, I don't think the Output knob even affected the volume, but the Master (all channel 2) could barely go to 9:00.

Sheesh, I was baffled. So I searched and found this thread. Is this the same issue all of you have had? Tube change for v5 then?

Thanks
Brewski wrote:Has anyone else heard about this one? My Road King was working fine, and then the volume started to drop and then go into volume swells, sort of like a slow tremolo. Then, at times, it would give an ugly static buzz. Finally, the volume totally dropped. I first thought that it was a pre-amp tube, however, when I flipped the solo switch, on the back of the amp to bypass, I got volume on all channels, but no effects or reverb.

Well, I decided to just send it in to Mesa for Repair, so dropped it off at an excellent local music shop, to send in for me. I was thinking that it must be the circuit surrounding the switch.

I got a call from, Doug at Guitar Showcase, who gave me the following info. After trying to get an RMA from Mesa. The Mesa service rep told Doug that the problem sounded exactly like an issue that they had sent a memo out on. On Road Kings and some other amps, the effects circuit is partial to the Mesa Chinese tubes, rather than the Russian made tubes. So he recommended that I try to put a Chinese tube in and see if the problem clears up. The v5 slot is the one closest to the fan and furthest from the transformer. That actually solved the problem and the amp is back up and running. I'm not sure what it is that makes the Chinese made tubes special, but a $16 pre-amp tube is a much less costly resolution than sending the amp in for repair. I just thought that I'd put the information out there. If this is redundant to another thread, sorry, but I tried searching for it first.

I tried searching the Mesa site for this memo, and couldn't find it. And tried searching the BB for it with no luck. I did get a verification that Mesa Techs got this memo. Anyone else hear about this?

Posted: Mon Jul 07, 2008 2:15 pm
by CudBucket
Interesting stuff. I wonder if my LS doesn't suffer from the same thing. It's about 4 years old and I'm the 2nd owner. I get noise (hissy, sizzly popping) in the clean channel only. I changed V2 with a new Mesa tube and it went away for about 2 weeks only to start again a couple of days ago.

Posted: Mon Jul 07, 2008 2:17 pm
by gumbymage
OK, the loop switch in back was down so the output and solo knobs were bypassed. :oops:

But still, I can't explain the strangeness from the other day. Crackling, no bass, thin/tinny sound, low volume even past noon.

Yesterday when I messed with it I removed each tube and put 'em back one by one (not replacing them). Maybe that did something... :?

Posted: Mon Jul 07, 2008 2:21 pm
by CudBucket
gumbymage wrote:OK, the loop switch in back was down so the output and solo knobs were bypassed. :oops:
Looks like you figured that one out yourself. :D

Naw

Posted: Mon Jul 07, 2008 2:39 pm
by gumbymage
CudBucket wrote: Looks like you figured that one out yourself. :D

Well, yes and no. I still want to know why the other day I had crackling sound, no bass, thin/tinny sound with BOTH the Masters and Output dialed to the max. The Loop switch flipped down in the back doesn't cause any of those symptoms and I cant reproduce it. Even if it's working fine now, I still want to know what caused it. :?

Posted: Mon Jul 07, 2008 8:46 pm
by Brewski
I had bought the spare preamp tubes for my RK, but haven't had to install them. It's still up and running. Knock on wood! :lol:

I know that it's not a cure all remedy, but I hope that it helped someone out there, who might have been having problems.

Posted: Sat Sep 13, 2008 11:27 am
by konstantine RK
what is the full lifespan of the pre amp tube?


I have absolutley no idea how old my tubes are in the RK1, but ....after reading this thread I think Ill lay off the tube change for a bit.

Posted: Sun Sep 21, 2008 9:19 am
by styphon
Should have read this damn article before I was swapping tubes on my RKII.

I was f%&# around, trying all kinds of combinations when I lost the sound of CHI and CHII, I found the problem was V3: Which I had just inserted a Mesa Russian tube. I replaced it out, and it was just fine. I tried to use this tube in other locations, but it destroys what ever it is suppose to power. I thought, hey just a bad tube right?

So then I try a new Tung-sol in V3, hoping to get more variation to my sound, and guess what? CHI and CHII hardly had any sound, CHIII and CHIV had the normal volume, but less distortion. Again, Changed he V3 out, and bamn, Back to normal. And again, I tried the Tung sol in V1 (which had worked with no problem before) and the tube does nothing. Second tube killed in under 30 minutes.

Posted: Fri Oct 17, 2008 2:05 pm
by KoskineN
Where do you get these chinese preamp tubes? Are the dealers supposed to have both the russian and the chinese?

Re: Did You Get The Memo?

Posted: Fri Sep 25, 2009 10:16 pm
by Armando
Well, I maybe resurrecting this post as I may have fallen to the "russian" 12AX7 issue. In my case, it's channel 4's sudden drop in level of my Road King Series I head. Unfortunately, I don't have any spares to try this tried fix and don't believe I can get my hands on a set until this upcoming Wednesday. After reading all previous posts, I'm not sure if this will be the fix but I'm sure hoping it is, as it seems to be the most minimal.

Are there any ways(considering I don't have any spare 12AX7s) to verify/test this? I would appreciate any advice.

Thanks in advance!

Re: Did You Get The Memo?

Posted: Wed Jan 13, 2010 3:49 pm
by Armando
I just came across this old post again and found out through some troubleshooting that the issue was not at all related to the pre amp tube but instead with the speaker A/B selector toggle switch located on the rear of RK head on Ch. 3. I must have bumped it by accident. It caused the exact same issue described by the original poster. It sucks because I went out and purchased new pre amp and power tubes only to discover that the original 6L6 tubes in the RK I heads are no longer sold as Mesa replacement tubes. The newer 6L6s don't have the same bottom end as the older tubes. Hate them!

Re: Did You Get The Memo?

Posted: Thu Jul 07, 2011 9:32 am
by Addictedtokaos
Found this info over on a JVM forum. the 11th post down here http://jvmforum.com/phpBB3/viewtopic.php?f=21&t=4409
Here is important information I´ve got from New Sensor in reply to my latest inquiry:
"The Electro-Harmonix 12AX7EH has a spiral filament and should not be used in cathode follower circuits. The Tung-Sol 12AX7, Mullard 12AX7, and Sovtek 12AX7LPS also have spiral filaments."

That would explain reported failures in V3.

The guy from New Sensor recommends Sovtek 12AX7 WA and WB:
"The most rugged 12AX7 types for cathode follower service are the Sovtek 12AX7WA and WB."

I will give the Svetlana a chance and eventually report its failure, incremental noise or any other symptom.

Very best,
Marcelo

Re: Did You Get The Memo?

Posted: Mon Mar 05, 2012 7:11 pm
by jpdennis
I am thankful for this post as the V4 in my Heartbreaker fits this discussion.

Dennis

Re: Did You Get The Memo?

Posted: Wed Jan 14, 2015 10:31 am
by talltxguy
Sorry to revive an old thread, but I have a question about the cathode follower thing.

Mesa says V3 & V5 are the cathode followers in the Roadster. I checked the manual to see tube functions, and V3A is the channel 3 & 4's 4th gain stage, and V3B is channel 3 & 4's the 5th gain stage.

V3A & V3B cannot be a cathode follower and a gain stage, so what gives? Is the cathode follower only active in channels 1 & 2?

Thanks for anybody who can clarify for me.