Fender blues junior mk3 240v conversion

ICBM

Not unless you break the fuse. But I'm not really sure how it will help… just take a firm hold of the top of the push-connector, wiggle it a bit and pull - it should come off fairly easily. I just tried it on the one I have here and it came off with no trouble at all - admittedly this is a MkII but I can't see why it would be very different.

(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.)

jpfamps

ICBM said:

Not unless you break the fuse. But I'm not really sure how it will help… just take a firm hold of the top of the push-connector, wiggle it a bit and pull - it should come off fairly easily. I just tried it on the one I have here and it came off with no trouble at all - admittedly this is a MkII but I can't see why it would be very different.

(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.)

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.

ICBM

I won't be at all surprised if that's the cause with this one either, since the board is heavily charred around there and the left-hand ribbon cable is melted - internally from a hot wire, not just from the fire in the PCB. Something has shorted in a major way…

Wondering if this might be a good candidate for a 6V6 cathode bias conversion!

jpfamps

ICBM said:

I won't be at all surprised if that's the cause with this one either, since the board is heavily charred around there and the left-hand ribbon cable is melted - internally from a hot wire, not just from the fire in the PCB. Something has shorted in a major way…

Wondering if this might be a good candidate for a 6V6 cathode bias conversion!

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.

gfeeney21

Switched the wires amp now working with less noticeable hum . 

ICBM

jpfamps said:

In my experience it always the left valve that shorts out.

Yes, this isn't the first one I've done either - although it's the most seriously damaged - and it was always the same.

The board is so burned on this one that I think replacement in one way or another is the only option.

gfeeney21 said:

Switched the wires amp now working with less noticeable hum . 

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.

jpfamps

gfeeney21 said:

Switched the wires amp now working with less noticeable hum . 

That's almost certainly due to the mains transformer not running into saturation due to lower mains voltage,

ICBM

This is the board that was in that Blues Junior, by the way…



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.)

jpfamps

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.

ICBM

jpfamps said:

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.

Too late - all back together and returned to the owner… I just forgot to upload the pic earlier.

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.

jpfamps

ICBM said:

jpfamps said:

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.

Too late - all back together and returned to the owner… I just forgot to upload the pic earlier.

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.

I've still seen plenty that oscillate.

Alexlotl

Quick thread ressurection - am I right that there's no soldering required to do this 230/240v change, just swapping crimped cables?

There's a cheap (and apparently very lightly used) BJ3 near me which I'm thinking might be good for a lockdown fling.

ICBM

Alexlotl said:

Quick thread ressurection - am I right that there's no soldering required to do this 230/240v change, just swapping crimped cables?

Yes. You may need to cut a cable tie, but it's not strictly necessary even if you can't replace it.

Alexlotl

Cheers! We'll see if the eBay gods smile on me...