Blues Jr - cathode bias mod?

[bluesbear]

My drummer and my son both have Blues Jr's. EL84's being what they are, I believe the amps would really benefit from a cathode bias mod. The problem is that the tube sockets are mounted directly to a PC board. Has anyone ever done this and/or come up with an acceptable way to make this mod work with PC mounted sockets?
Thanks,
Dave

[LooseChange]

I've modified many PC boards...
The easy part is connecting new stuff to the solder joints of the tube sockets. You can usually heat them up and slide a wire alongside. Sometimes you can lift a component off the board to break a connection and add the new circuit. Other times you will need to do a more permanent trace cut.

The mod you want to do should be fairly easy.

Do you have the PC board layout?

[HotBluePlates]

Unfortunately, the PC board layout included in Fender's schematic does not show the traces themselves, only part # designators. It is funny that they do that for both top and bottom views, while still omitting the traces.

From what I can tell based on the layout, the bias supply components are tightly placed next the the EL84 grid reference resistors. Cutting the traces and laying in a new wire to run to ground may be a very tight fit (perhaps not impossible though).

[Jack_Hester]

Will this help?

http://home.comcast.net/~machrone/bjr/follower/cathodefoll.htm

Jack

[jojokeo]

Yep, been there done that (and about 6 or 7 others too). Just do some reading and be sure which version you have and which mods that are suggested for which particular board you have to assure no surprises or misadventures.

[bluesbear]

Any suggestions on where to cut in to add the cathode resistor/cap? I've never tried this with an amp with a PC board and... ribbons!! They make me nervous. I don't really like the idea of scraping traces but it seems as though I may have no choice.
Thanks,
Dave

[jojokeo]

Um, which board do you have? You're not messing w/ the cathode resistor/cap. Only adding in the extra triode w/ wiring and new 100K plate resistor. You cut V2A's traces then directly add wires to the three pins for hooking up. My part numbers were pretty much the same as his but there are a few differences if take note of my photos to his. Hope this helps.

Mark's site is easier to follow IMO. http://mhuss.com/BluesJr/index.html
He's got the older green board too but it doesn't matter if you follow the schematic or yours where the part #'s are the same or you figure it out by comparing them. It's really pretty easy once you open things up and get to it. Don't worry about cutting a trace, they can easily be repaired (jumpered).

[John]

I've been following this thread, but now I'm confused. I thought BB was wanting to change the power tubes to cathode bias?

[sluckey]

You're not confused. That's what he asked for.

[jojokeo]

Sorry for the screw up  I followed Jack's link and got mixed up

[John]

Phew, thank goodness.  I thought it was me again!