Rampage
Well-known member
Hey guys, I grabbed a pretty hardwood/wicker Mark IV combo and received it yesterday; however, I am having some issues on the lead channel. I know this is a long post, but I want to completely share all of my findings/actions to see if anyone can think of anything, or recommend any other steps to me before I have to consider taking it to a tech.
Here's what I did:
1 - Cleaned the potentiometers, jacks, switches, sliders and sockets with contact cleaner.
2 - Completely swapped the preamp tubes.
3 - Completely disassembled the cabinet, cleaned away any debris, and reassembled it.
4 - Removed the reverb tank (in case it was a source of noise).
5 - Tested the speaker and combo cabinet with a different amp at high volume/bass setting, with both clean and overdriven tones, and was not able to replicate any buzzing after going through the cabinet cleaning/reassembly.
6 - Put the chassis back into the shell for troubleshooting.
Now, for the troubleshooting process, here's what I tried:
1 - Full power settings: Simul-class, pentode, harmonics and full power with and without the foot switch. The clean sounds perfect (tells me that it can't be the speaker), the second rhythm sounds good and the lead channel sounds very woofy and muffled, almost like there is a fuzz or octave pedal turned on. Definitely nowhere close to the typical MKIV lead tone. I also tried using the recording and preamp outs into my interface (separately) and still noted the octave/fuzz tone.
2 - Low power settings: Class A, triode, midgain and tweed power with and without the foot switch. Again, the clean and Rhythm 2 tones sound good, but the lead channel still suffers from the fuzzy, octave-like tone. I tested it again with the preamp out and recording outs with the same results.
So here's what I am thinking:
1 - It is not cab rattle. If it was cab rattle, I would have been able to replicate the problem with the other amp after I went through it. It also would rattle on the clean and second rhythm channels.
2 - It is not the speaker. Again, the speaker would have been farting out with the other amp and also on the clean channel and second rhythm channel.
3 - It is not likely to be a preamp tube problem. I completely swapped out the preamp tubes that were in the amp with known-good preamp tubes which did not alleviate the issue. I then went back through the preamp replacing each new tube with another known-good replacement without fixing it.
4 - It is not likely to be a power tube problem. Any time there is a weird noise, I figure it is a tube; however, I don't think that the power tubes would be affecting the tone only on the lead channel and also while the amp is in the silent recording mode or whenever the preamp signal is being sent out to my interface.
Based on all of this, does anybody have any ideas as to what it might be that is causing this issue with the lead channel? Thanks in advance!
>>>>>EDIT FOR RESOLUTION<<<<<
I just wanted to update this thread with the resolution: A shorted potentiometer for the Bass control on the Lead channel. As we have all experienced, the Mark IV with the Bass dimed in a farty, mushy mess and this is essentially what I was experiencing. It was a simple, affordable repair by my tech.
Here's what I did:
1 - Cleaned the potentiometers, jacks, switches, sliders and sockets with contact cleaner.
2 - Completely swapped the preamp tubes.
3 - Completely disassembled the cabinet, cleaned away any debris, and reassembled it.
4 - Removed the reverb tank (in case it was a source of noise).
5 - Tested the speaker and combo cabinet with a different amp at high volume/bass setting, with both clean and overdriven tones, and was not able to replicate any buzzing after going through the cabinet cleaning/reassembly.
6 - Put the chassis back into the shell for troubleshooting.
Now, for the troubleshooting process, here's what I tried:
1 - Full power settings: Simul-class, pentode, harmonics and full power with and without the foot switch. The clean sounds perfect (tells me that it can't be the speaker), the second rhythm sounds good and the lead channel sounds very woofy and muffled, almost like there is a fuzz or octave pedal turned on. Definitely nowhere close to the typical MKIV lead tone. I also tried using the recording and preamp outs into my interface (separately) and still noted the octave/fuzz tone.
2 - Low power settings: Class A, triode, midgain and tweed power with and without the foot switch. Again, the clean and Rhythm 2 tones sound good, but the lead channel still suffers from the fuzzy, octave-like tone. I tested it again with the preamp out and recording outs with the same results.
So here's what I am thinking:
1 - It is not cab rattle. If it was cab rattle, I would have been able to replicate the problem with the other amp after I went through it. It also would rattle on the clean and second rhythm channels.
2 - It is not the speaker. Again, the speaker would have been farting out with the other amp and also on the clean channel and second rhythm channel.
3 - It is not likely to be a preamp tube problem. I completely swapped out the preamp tubes that were in the amp with known-good preamp tubes which did not alleviate the issue. I then went back through the preamp replacing each new tube with another known-good replacement without fixing it.
4 - It is not likely to be a power tube problem. Any time there is a weird noise, I figure it is a tube; however, I don't think that the power tubes would be affecting the tone only on the lead channel and also while the amp is in the silent recording mode or whenever the preamp signal is being sent out to my interface.
Based on all of this, does anybody have any ideas as to what it might be that is causing this issue with the lead channel? Thanks in advance!
>>>>>EDIT FOR RESOLUTION<<<<<
I just wanted to update this thread with the resolution: A shorted potentiometer for the Bass control on the Lead channel. As we have all experienced, the Mark IV with the Bass dimed in a farty, mushy mess and this is essentially what I was experiencing. It was a simple, affordable repair by my tech.
