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(This one is also well and truly cooked and had not been converted… looks like it might need the valve board replacing. Whether these things are related I can't say.)
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The EL84 is actually a well designed valve from the point of view of ensuring that the high voltage pins are not adjacent to other pins reducing the chance of arcing.
However, Fender in their infinite wisdom run the track from pin 3 (cathode) of the left hand valve from the back as you look at between pins 6 and 7, ie next to the anode, and exactly what the designers of the EL84 were trying to avoid.
Heat breaks down the insulation on the PCB and the anode arcs over to the cathode.
They could easily have routed the track away from pin 7 and this would have avoided most of the instances of tracking I've seen in the BJ.
Wondering if this might be a good candidate for a 6V6 cathode bias conversion!
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
In my experience it always the left valve that shorts out.
HT fuse might help here too!
Not done the 6V6 conversion, although I image that would make the amp sound much better.
The board is so burned on this one that I think replacement in one way or another is the only option.
That indicates the valves are running a lot less hot. I haven't always noticed it being much reduced but it can't be a bad thing if it is.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
That's almost certainly due to the mains transformer not running into saturation due to lower mains voltage,
It's the same on the other side too. It didn't actually arc between the valve socket pins, the fire was at the ribbon cable solder pads. Even given all that, the fuse didn't blow either.
Luckily there's a chap on Ebay selling complete board assemblies from Mk3 BJs - presumably he's doing hand-wired conversions or something, as he had several of them - which the owner of this amp bought for less than it would have cost me to fix the mess, so all I had to do was drop the new board in and he gets an upgrade as well as a repair…
(Aha - turns out it's the Rat Mods guy.)
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
If you've got the PCB out it's worth chaning C14 from 47pF to 120pF or 220 pf 100V to kill any changes of oscillation.
It tested fine, no oscillation. I think the Mk3 layout is less prone to it - one of the things I noticed is that the power valve ribbon cables are shorter than on the old one.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein