Dual Rectifier Gain and Volume Drop on Channels 2 & 3

[casterjones]

Hi folks,

Im new to the forum and also to the MB world. I live in Panama City (Central America) and I recently bought a half working 2000 Dual Rectifier Solo from my bandmate for a reasonable price ($500 with an ATA case), although no store sells Mesa in Panama I thought I could probably get it fixed at some point. I know for a fact my bandmate took it to 3 different amp techs in Panama but none really figured out the problem, I found a promising tech that has fixed other tube amps (no Mesa though) so I decided to give it a shot. As usual, the first thing people blame any problem on tubes amps are the tubes, so I went on and bought all the tubes new (preamp, power tubes, etc., think it was 11 in total), all I could get down here were Russian Electro Harmonix which the tech said would do the job just fine, the guy changed the tubes and gave back the amp saying it was working. I tried the amp back at the studio and at first it was fine but then after 15-20 min the problem came back. So now in detail the problem:

Clean Channel 1 seems to work fine all the time, Channel 2 & 3 worked perfect for the first 15-20min and then it started to drop half the volume and all the gain. The problem first happened when switching from the clean channel 1 to either channel 2 or 3, what it does is that it drops the volume and gain to half and it sounds like a filtered bright slight OD, like when you have a bad cable connection that some signal slightly goes by. At first Channel 2 or 3 would flake out when switching channles only and then come back fine in 2 seconds or if I switched back and forth the channels then it would get fixed 1 out of 3 times. If I left it in Channel 2 or 3 alone it didnt really do the volume and gain drop, although after 5-10 minutes of having this problem it was almost impossible to get the distortion channels to work properly. This is the same problem my bandmate had before with the amp as well, so i guess its not the tubes. My bandmate bought this amp in Miami in the year 2000-2001 and then he moved to Panama in 2004 and in 2005 the amp started having this problem. Back then he called Mesa and got the schematics send to him but it seems the techs find this amp to complicated down here, for them its like a Lamborghini and they are used to Toyotas.

To ship this amp to Mesa directly and get it fixed would be the best solution but an expensive one, my best chance is for someone to point out what the problem could be so that my tech works on it here or worst case scenario I can take the amp to Indianapolis on December when I go visit my brother. I found in the Mesa Boogie website 2 authorized repair shops in Indianapolis and could leave it there and have my bro bring it back at some point. Are those authorized shop as good as the Mesa factory? otherwise I would ask my brother to send it to Mesa in California, I really want to fix the amp and would prefer Mesa to check it directly gladly but if theres a simple solution for this I would prefer to just fix it here and save time and shipping expenses. I read something about LDR but I just play guitar, dont know anything about amp electronics parts.

Anyone has any idea what it could be? maybe someone with a similar problem that got it fixed. Your replies, comments, and any guidance at all would be greatly appreciated.

 

[jbird]

What voltage are you running down there? Transformer? Have you tried cleaning the fx loop jacks?

 

[94Tremoverb]

Russian 12AX7s can cause problems in V3, which is used only on channels 2 and 3. It doesn't seem that likely if it's doing the exact same thing with two different tubes, but it's worth eliminating this first. Any Chinese-made 12AX7 is fine, if you can't find one locally you can easily buy one on Ebay, they're very cheap. It's probably worth buying two since V4 (the FX loop tube) has the same issue, but this would affect all three channels.

If it's not that, it's probably either a bad connection somewhere in the channel 2/3 circuit (a lot of which is common to both channels) or something in the switching system. If your local techs can't find either of these, you may have to send it back to Mesa.

 

[casterjones]

Thanks for the replies!

Jbird: In Panama we dont need any special transformer to run any electronic stuff that works in the US, we got the same voltage and the same electric outlets. The US had military bases and controlled the Canal till year 2000, you would be amazed how mucha northamerican influence we got down here.

94tremoverb: Ill see if any stores has tubes here, i got those EHX from a store that brought some and couldnt sell them so they were on sale, i think no other store sells tubes, not much of a market for expensive music equipment in my country. Im gonna give your advice to the tech, I took the amp yesterday and he said maybe the tubes were making bad contact when they overheated. Ill post back what results he gets soon...

 

[94Tremoverb]

If your tech isn't familiar with the problem of the Russian 12AX7s, it's this - V3 and V4 in the Rectifiers (and some other positions in other amps) are cathode-followers, which means that the plate is connected directly to the high voltage and the output voltage is derived from the cathode, which is the reverse of normal. The problem is that this means the cathode operates at a high DC voltage, sometimes as high as 200V - this should be OK, because 12AX7s are meant to be rated for 250V cathode to filament, but quite a lot of the Russian tubes don't meet the correct spec and leak current or fail outright when used like this. It usually shows up as bad (or oddly distorted) tone or a loss of gain, if the tube doesn't die completely.

EH tubes are of course Russian. While I'm not sure that this is the problem here, it needs checking out before you go any further into the amp, because it *could* be a simple as this. It's not something you would expect if you're an 'old time' tech because all old 12AX7s (and most new-production ones, including Chinese) are perfectly OK used like this, as they should be. It took me quite a while working with old Marshalls (where V2 is a cathode-follower) to notice that a lot of Russian 12AX7s failed in this position, and work out exactly why.