debugging FX loop problem

[fuzzfreek]

Hi - I'm a new Mesa owner. I just bought a used 10/50/100w Lonestar head that is in mint condition a couple of months ago. I know it has two channels, but the clean channel is so awesome I just don't get to much else! I play it on the 100w tweed setting almost all the time. I have some pedals in front, and a delay in the loop. I always have the loop engaged, although not always the delay pedal.

About a week ago, after playing for an hour or so, the volume dropped in a major way, the tone turned to mud (no treble), and the sound became distorted. In trying to figure it out I noticed that if I switch off the FX loop the problem goes away. And while the problem is happening, if I adjust the FX loop level knob, the volume cuts in and out, as does the distortion. Apparently after it sits overnight and cools off, it's good for another hour of playing until it happens again. And I switch my exact same pedal setup between amps, and the problem doesn't appear on my other amps.

So I put a new SPAX7-A in V3. The problem went away briefly, but returned. I saw in searching this board that maybe I need a Chinese tube in V3. I didn't have a new one, but there was one in V2. So I put the SPAX7-A in V2 and took the Chinese tube from V2 and put it in V3. Again, it worked OK for a little bit, but the problem is back.

I haven't noticed any problems with the other tubes (extra glow, arcs, etc) when the problem is happening. When it happens, it continues for some time and doesn't get better. At least not in the amount of time I've kept going with the horrible sound.

I have a new Chinese Groove Tube I can try and I'm waiting for some balanced Chinese tubes I ordered. I'll try those when they arrive, but I'm not optimistic that another tube change will matter.

I saw there is apparently a bit of trouble with rectifier tubes in the Lonestar, but since I'm in 100w mode all the time, I wouldn't think that applies. I do have a spare rectifier tube and spare power tubes, but haven't tried those since I'm kinda stuck on thinking it is related to the FX loop.

Any other suggestions for what to try? The amp is still under warranty, but I'd rather not take it in and be without it if I can resolve it myself.

Clean the FX loop jacks? Clean the V3 socket? Just don't play for more than about 45 minutes at a time?

Thanks!

 

[1028paul]

Hi Fuzzfreek, Have you tried jumping the loop with a short patch cord to see if the problem dissapears. the loop inputs can cause problems if they are oxidised or dirty. plug a patch cord in and out a few times then run the amp with the loop engaged jumping send & return with a good patch cord to see if that is your problem.

Paul.

 

[fuzzfreek]

Thanks, Paul. I tried your suggestion this evening, but I need to verify my results because I goofed up the experiment. Yes, I know, it was a simple task...

I used a pedal cable to clean the jacks. I sprayed some contact cleaner on the end of the cable and plugged it in/out several times in both loop jacks, leaving that cable as a jumper between the loop jacks. My new Chinese tubes arrived today, so I also took the used Chinese tube out of V3 and put one of the new ones in there. I played for quite a while and wasn't having any problems.

But I got a little carried away and started messing around with a new pedal I received today, too. (Whizzer mod Russian Big Muff). Of course while I was doing that, I was changing the tone and volume from the pedal, trying to dial it in. After a while, I decided to add my delay to the new pedal. So I took out the short jumper cable on the loop and plugged my normal loop in. I think that as soon as I plugged them in, my volume dropped.

But since I had been messing with the pedal volume/tone, I couldn't tell if it was the same problem, or just some normal volume drop related to going through a longer signal path, or maybe just confused ears after all the other goofing around with the pedal. I believe the problem is still there, but I think the volume dropped less, the tone doesn't get as bad (still missing some treble), and the distortion is much less. It would seem kinda weird to make it better, but not fix it. To try to reproduce the problem again, I have to wait a couple of hours and then play for another hour or so. And this time I won't be messing with new pedals. I'll have to wait until tomorrow for that.

I also have another idea to try. I'm going to re-cable my loop setup. I see I have some tension on my loop cable connectors from the weight of the cable hanging on them, maybe messing with the connector. And my cables run among some power cables. Not likely the problem, but it is easy to change.

 

[1028paul]

Hi again Fuzzfreek,

Other obvious checks... but ones that have been missed by others, including me, so its worth going through it all.

firstly, loop volume (send level) on the back of the amp should be at around 12 - 1 oclock. your problem is intermitant, so I doubt this is it.. Im sure you know this already but Its easy for the knob to be accidentally turned down.

I presume your effects have already been checked out with the same cables on another amp ? .. I only mention this because I had a faulty patch cable once that had me changing V3 then scratching my head for hours before I realised.

I have had the dirty/oxidised input problem once, but the method you used cured it, its not returned, as I plug and unplug my loop regulary every time the amp is moved.

having changed your valves, and that would normally be the next thing to check.. .. my guess is a faulty patch cable,.. or a dodgy connection on the input on either send or return loop. if there has been tension on either of them.. that could be your problem.

Hope this is of some help.

Paul.

 

[1028paul]

And just in case you do have tube probs...

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

Lone Star & LS Special: V3
Stiletto: V3 & V4
Road King I: V3 & V4
Road King II: V3 & V5
Roadster: V3 & V5
Dual & Triple (2ch or 3ch): V3 & V4
Tremoverb: V3 & V4

 

[fuzzfreek]

Thanks again, Paul! It's definitely worth going through all the "obvious" stuff, since it appears that may have fixed my problem. I played for about 3-4 hours last night and didn't have any problems like I originally had. Since it has been intermittent, I won't declare victory just yet. But it looks promising.

The only changes I made since the last test were:

cleaning the FX loop jacks as I described earlier
rerouting my FX loop cables so that they don't go through the mess of cable behind some of my gear (including several power cables)
making sure that I don't have too much cable weight hanging on my FX loop connectors (they are Monster Rock cables, straight connectors)
made sure my FX loop level was near center (mine's a little less to prevent some clipping on my Memory Man when I boost)

I had also already replaced the V3 FX loop tube with a new balanced Chinese tube (not Mesa). While I was in there, I also replaced the V1 and V2 tubes with Mesa SPAX7 tubes, but I doubt that has any impact on what was going on.

My tone has maybe changed a tiny bit, but that's probably because of the tube changes in V1 and V2 more than anything else I might have done. I'll definitely be playing it this weekend. If the problems don't show up, great! If they do, my next thing to try will be changing FX loop cables. These Monsters have cable just going directly inside the connectors. There's no apparent strain relief. You'd think that on a $50 cable that might be included...

So to summarize so far:

1. put a new Chinese tube in V3
2. clean the FX loop jacks
3. keep your signal and power cables away from each other
4. make sure your cables and connectors are good
5. make sure your cables and pedals don't have the same problem with another amp
6. try putting just a jumper cable across the loop, removing your normal cables
7. if you put a new tube in V3, might want to clean the connector while you're at it (I haven't done this yet, but it is easy to do while you're at it)

 

[ajhops]

And just in case you do have tube probs...

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


GUITAR AMPS

Lone Star & LS Special: V3
Stiletto: V3 & V4
Road King I: V3 & V4
Road King II: V3 & V5
Roadster: V3 & V5
Dual & Triple (2ch or 3ch): V3 & V4
Tremoverb: V3 & V4

Does this apply to the Maverick line? Do you know the location? This sounds similar to a problem I've been experiencing with my lead channel. I once thought it was my effects loop but currently I have nothing in the loop and it's still happening. The volume just suddenly drops but does not fall silent.

 

[DanSRV]

I'm having the exact same problem with my hardwood Lonestar. My amp has about 20hrs on it. I'm getting the same signal loss with the loop engaged. I swapped the V3 tube with another Mesa tube(brand new). Same problem. All me levels are set they way they should be, I'm not sure why this problem is happening either.

 

[fuzzfreek]

The steps in the summary above solved my problem. I suspect it was the tension on my cables all along, not the V3 tube, dirty jacks, or anything else.

I had a similar volume drop problem on one of my Marshall heads. It would only happen after I played for an hour or more, so I suspected tubes or something related to heat. I spent a lot of time trying to figure it out. Eventually it was diagnosed to be the switch that engages the FX loop. That switch was going bad. I suppose that could happen on some of the Mesa heads. I almost always have my loop on to use a pedal or two there. I think I remember something about having to have the loop on to be able to use the solo volume, but maybe I'm mis-remembering. If you have the loop on for that reason, then something in the loop circuit (like the switch) could cause a problem, I suppose. Seems like a bit of a long shot, but it happened to me...

Over the past 6 months, I've had an unusually high number of amp problems. Except for the FX loop switch problem, I think all of my problems have been tubes. I normally catch those, but I've had a couple of times where multiple tubes were failing and swapping only a single tube at a time didn't fix my problems. And once I even tried swapping multiple tubes with new tubes, but wasn't able to find the problem. Turned out a couple of my new tubes (JJ's no less) weren't good. So if you have some extra *known good* tubes, try swapping multiple tubes. Or see if swapping tubes from Channel 1 to Channel 2 causes the problem to move.

Good luck!