Mark III not powering up right.

[SonicProvocateur]

So a couple weeks ago my Mark III lost power suddenly across all the channels and I replaced V4 with a known good tube. Turned out to be the culprit then. Today I fired it up and it sounded the same way - on normal (pretty loud) volumes, it was whisper quiet like a preamp tube was dead, but there was full gain on lead channel and the clean was clean. I replaced V1 and it was the same. Checked all cables and connections, still the same. After a couple more tries I eventually flipped it from standby and pow it was there at full power and tone, with no adverse sound or tone...everything was dead normal, Sylvania 6L6's roaring away. I played it for a good long while, checked all the pulls, ran through the volume and gain range, and everything was normal. Any ideas, guys?? This kinda worries me...alot.

 

[Rob Lockwood]

To start with something easy, it could be the old effects loop jack problem if you are running with nothing plugged into the loop. The contacts that make the signal path between send and return when nothing is plugged in can corrode and make intermittent connection. If it does it again you could try connecting send and return with a patch lead. If it comes back then the sockets need cleaning. You could try spraying some contact cleaner onto a jack plug and working the plug in and out of both sockets if you dont want to take the chassis out. Another thing worth checking would be that your speaker connections are all sound - seems less likely but a fault here can seriously damage the amp. Sounds like there is a loose connection somewhere. Did you try moving the master volume pot when the sound was not there? Pots can suffer from bad contact too.

 

[SonicProvocateur]

Update: I've fired her up twice for intense rocking sessions now twice with no flaws. Once straight guitar -> amp, and once with my new SKB Stage Five running the full 4 cable method, neither with any ill effects. I noticed some slight tone dampening/(distortion?) on the effects loop with the Carbon Copy, which is the cleanest analog delay. I had switched my DD-3 from that position (which was distorting there, I thought it was clipping the input gain) with the CC to find out it was changing the tone slightly and distorting slightly. I figure this could be caused by:

1) That new V4 tube; it's a Marshall labeled balanced JJ.
2) The SKB's buffered loop, or internal power supply/cables - however on the other loops i'm running, it's totally clean.
3) A bad cable running to or from the EFX loop - however I know one cable has a slightly jacked end and it works fine.

I'm thinking it may be that V4 tube, but i'll try and track it down more tommorow, just too much to trace tonight if I have to pull the chassis. My mark III is a giant mess of flux. I actually thought it was rust at first sight when Victor sold it to me, so sometimes pulling to chassis makes it hard to find problems.

 

[MrMarkIII]

If the Carbon Copy or DD-3 is causing distortion/clipping/signal degradation and the distortion/clipping/signal degradation stops when the pedal is no longer in the signal path, it's the pedal.
V4 is the reverb tube. Doubtful it's the culprit.