Loss of crunch on the Mark IV...

[alphadog808]

Hi guys,
I was using my Mark IV out the other night and had it cranked pretty loud. I noticed that on the lead channel I lost alot of crunch. This was with both gains cranked pretty high. It almost sounded like the lead channel just had a volume gain rather than crunch.

I thought it might have been volume related, but when I turned it on at home last night(much lower volume), the crunch wasn't there. Again, it was just like a boosted clean on the lead channel. It had some grit, but nothing like how a distorted boogie should sound...

At this point, I changed out V5 as it didn't light up(I later found out it shouldn't due to spiral filaments) and put in another tube. I don't know if it was coincidence, but the crunch came back...so I put the original tube back in...and the crunch was there...so it most likely wasn't V5...

That being said, I'm wondering if maybe the tube/s are messed up and needed to be warmed up a bit longer than before? Or could I be experiencing a hardware problem? If it's a tube issue, is there a certain tube I should start looking at? I heard maybe check V1?

 

[Buddy]

Could be some crud in the tube socket(s) .... dirty pins ... mung in the pots .
I'll always check out the simple stuff first , when working on someone 's amp .
Run a signal thru the amp and start tapping the aforementioned components and see if you can isolate the offender .

You can do this without opening it up .

 

[shredding]

This may not be your problem but once I accidentally turned down the treble on the lead channel to 5. All my gain was gone. I freaked out and changed the tubes but nothing happens until i turn my treble back to 8.

 

[alphadog808]

Ok guys, an update....so last night I tried swapping each preamp tube one by one to see if I could find the culprit. No big crunch change till I got to V1. That being said, it *seems* that V1 is bad...

But, would a bad preamp tube do that? Also, would a bad preamp tube allow me to sometimes have crunch and sometimes not? I've also seen my crunch slowly build up over time(as if it was warming up). Just wondering if maybe V1 isnt' the problem, maybe some other hardware tied to V1 might be it. Note, I had the shielding around V1...

 

[Paul Secondino]

Ok guys, an update....so last night I tried swapping each preamp tube one by one to see if I could find the culprit. No big crunch change till I got to V1. That being said, it *seems* that V1 is bad...

But, would a bad preamp tube do that? Also, would a bad preamp tube allow me to sometimes have crunch and sometimes not? I've also seen my crunch slowly build up over time(as if it was warming up). Just wondering if maybe V1 isnt' the problem, maybe some other hardware tied to V1 might be it. Note, I had the shielding around V1...

This isn't very typical in my experience but it is possible. V1 is the tube where it all starts and they can be over worked and wear out fast. Also, the Mark 4 lead channel needs the treble control to be pushed a bit. I turned on my Mark 4 for the first time and almost wanted to sell it. Until I turned the treble up and it all came together. I'd recommend running the lead channel treble at at least 6.5 or 7

 

[alphadog808]

Well, the tube/s are new, I just got them from Dougstubes. It hasn't had more than...10 hours or so. I know what you mean by the treble control. When the amp is in a bad state, even the treble and both gains on 10 doesn't give it any type of major crunch...If I strum/pick really hard I could get some growl, but a light to medium strum/pick would result in a gritty clean...

 

[Paul Secondino]

IF changing the preamp tubes solves it, then you are golden. V1 and V5 are the biggest offenders in my experience. unfortunately, if it isn't the tubes , it's almost certainly a bad resistor or cap in the preamp section. I hope it turns out jsut ot be the tube.