Mark III lost the gain stage

[carlosasi]

Weird problem with my Mark III (first ever by the way) I plug in the guitar and it sounds really thin and now matter how high I turn up the volume on the guitar it has little affect on the amps volume or gain. The cleans sound ok but no real noticeable increase in volume when I dial up the master volume. I've never had the master past 5 and I can easily turn it over 5 without ripping my face off. When I pull R2 or Lead channel, no gain stage just a slightly thicker clean sound.

Went and changed out the preamp tubes and all were lit except V3 regardless of the tube in the socket.

What could be the problem?

Thanks

 

[stokes]

"Went and changed out the preamp tubes and all were lit except V3 regardless of the tube in the socket."
Sounds like the heater is out to this tube,which would cause the problem you describe.

 

[carlosasi]

I was hoping I didn't have to pull the guts.

Thanks for the tip.

 

[edward]

The first and most obvious thing to do, as you had already pointed out, is replace the preamp tubes, one at a time, with a known good tube.

If it's still out, the you have a dead LDR. These are little "switches" on your circuit board, of which you have a few (5, 6 ?? something around that). These go bad and you get/hear exactly what you described.

My Simul III greenie did the same thing ...perfectly fine, then one day started to go intermittant. Thought it was a bad preamp tube and replaced it. It was "fine" again and thought I was good to go. Bam, happened again (which really sucked because I had thrown away a perfectly good tube that I thought was bad). At that point, I figured something was amiss. Sure nuff, it was LDRs gone south. Addressed three bad ones and all is as good as new.

Edward

 

[scottcrud]

sounds like the ldr, same thing happened to my buddies

 

[stokes]

If the tube isnt lit,as he said,the heater is out.The LDR has no bearing on the heater operation.

 

[acorkos]

before you pull the chassis, flip the amp upside down and try rocking the V3 tube around and press it firmly into the socket to make sure it's completely seated and all pins are making contact. do it with the lights turned off to make sure you can really see if it glows....some tube brands glow fainter than others.

 

[carlosasi]

I re-seated the V3 preamp tube pushing it and pulling it out a few times and the tube lit up and the amp was back to normal operation. However it doesn't sound quite the same. It could be my imagination, it could be because the tube I pulled out of V3 was an awesome NOS JAN Philips 12AX7WA and I thought made the Lead Channel sing.

The amps seems harsher in all three channels if that makes any sense. The cleans are not quite as clean, the R2 not quite a chunky and the lead not quite as gainey. Am I imagining this or could it be the LDR's

 

[acorkos]

before you crack open your amp, you need to rule out general tube related stuff first. get yourself some spray contact cleaner....i would pull the tubes, one at a time, and spray in some contact cleaner. insert/remove/reinsert each tube several times....this insures the tube socket elements and tube pins are clean. if it's still sounding bad, next step would be to replace the tubes....i would replace all the power tubes first, and if it's still sounding bad, the preamp tubes one at a time, listening after each tube change. if after all that it's still not sounding right, put the old tubes back in (keep the new set for backup) and (if you're experienced) start nosing around inside the chassis, or take it to a tech.

 

[stokes]

What did you replace that NOS JAN Phillips with?If it is a current production tube,I would say it is a safe bet that that is causing the harshness,to me all the current production tubes sound harsh,compared to NOS tubes.Cleaning and re-tensioning the tube sockets is a good idea,but that usually doesnt cause harshness as much as noises and intermittent tube function,like the heaters not working as it was doing.

 

[edward]

Ignore tone for just one minute: If your Lead Drive and Lead Master are working as they should, and you get the righteousness of volume and OD back, then your LDRs are working ...in other words, it is switching properly into your Lead channel mode.

Now as for the diff in tone you're now hearing, I'll let others more knowledgeable help you with that

Edward

 

[carlosasi]

acorkos - that was the answer. I turned the amp over, pulled all the tubes noting there position even brought the NOS Phillips into the mix. I used contact cleaner in each socket and pulled/pushed the tubes into each socket. (that is a lot harder to do then you think) Ideally I should have pulled the whole amp out and I may do that when I have more time, the point is this process completely cleaned out the sockets and all the tubes including the Phillips worked great. The amp is right back to where it was before God bless the Mark III. All is well. Thanks to everyone who contributed.

Best,
Carlo

 

[colorsoundkid]

I just picked up a Mark III on the cheap because it has issues similar to this. The first two channels sound like a broken radio, really low volume. The third channel, the lead channel, is totally kaput. No sound at all. Could this be an LDR? What exactly is an LDR? I'm pretty comfy inside an amp, and will be swapping out one of the preamp valve sockets because it's all screwy. Are LDRs expensive? Do you get them from Mesa direct?

 

[acorkos]

LDR's perform the switching operations

I would swap in some new tubes before you do anything else.

 

[edward]

I just picked up a Mark III on the cheap because it has issues similar to this. The first two channels sound like a broken radio, really low volume. The third channel, the lead channel, is totally kaput. No sound at all. Could this be an LDR? What exactly is an LDR? I'm pretty comfy inside an amp, and will be swapping out one of the preamp valve sockets because it's all screwy. Are LDRs expensive? Do you get them from Mesa direct?

As said, LDRs are switches. So if yours are bad, one of the channels should be working

Try known good tubes first ...could be V1, V2, or PI; could be pwr tubes. Go sytematically and eliminate this obvious variable. My guess is you'll get the R1 back. At which case you can evaluate from there. And yes, LDRs can be ordered from Mesa; about $13 IIRC, and with the PCB out of the amp it's not a difficult operation (if you already know what yer doing ).

Edward