woodbutcher65
Well-known member
- Joined
- May 4, 2018
- Messages
- 590
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So my Mark III blue stripe long head (My favorite amp EVER!!!) went weird some time ago, but not TOO weird, I was able to live with it and still get my tone out of it, or at least MOST of that tone. I figured I'd put it on the bench and check it out when I got good and ready to.
Symptoms:
V1 only works properly over a narrow range, between 3 and 7. Out of range, volume would drop off drastically.
Treble behaved in a similar manner.
Turning up the bass muffled the amp rapidly and then above about 4 on the bass control, it acted like a master volume in reverse. The higher it goes, the quieter the amp gets.
Midrange behaved in a similar manner. I like to run the mids fairly high to accentuate pick scrape and pick harmonics. You don't hear a lot of midrange in the post EQ, shaped tone, but it does make a difference when you want to bring out those pick scrapes and harmonics.
The entire tone stack and first volume were basically all working badly if at all.
The problem as it was found: A .1 uF 400 volt Orange Drop, located close to and behind the treble and bass controls, had shorted.
This allowed DC to get into the tube on the grid, biasing it to cutoff at higher control settings. And this also made the controls very noisy.
Replace cap, in this case with a Panasonic polyester film cap, .1 with a 630 volt rating.
All problems solved. Amp works properly, and its aggression is back.
I also checked out all other orange drops for shorts. They're OK.
I had previously replaced the driver stage .1 orange drop coupling caps with what I had available, just to get the amp running,
and replaced those again, with correct value and rated components.
It's such an aggressive, rich sounding amp again, that even my Mark V 90 has a tough time keeping up. And I love the Mark V, too.
Symptoms:
V1 only works properly over a narrow range, between 3 and 7. Out of range, volume would drop off drastically.
Treble behaved in a similar manner.
Turning up the bass muffled the amp rapidly and then above about 4 on the bass control, it acted like a master volume in reverse. The higher it goes, the quieter the amp gets.
Midrange behaved in a similar manner. I like to run the mids fairly high to accentuate pick scrape and pick harmonics. You don't hear a lot of midrange in the post EQ, shaped tone, but it does make a difference when you want to bring out those pick scrapes and harmonics.
The entire tone stack and first volume were basically all working badly if at all.
The problem as it was found: A .1 uF 400 volt Orange Drop, located close to and behind the treble and bass controls, had shorted.
This allowed DC to get into the tube on the grid, biasing it to cutoff at higher control settings. And this also made the controls very noisy.
Replace cap, in this case with a Panasonic polyester film cap, .1 with a 630 volt rating.
All problems solved. Amp works properly, and its aggression is back.
I also checked out all other orange drops for shorts. They're OK.
I had previously replaced the driver stage .1 orange drop coupling caps with what I had available, just to get the amp running,
and replaced those again, with correct value and rated components.
It's such an aggressive, rich sounding amp again, that even my Mark V 90 has a tough time keeping up. And I love the Mark V, too.
