red plating on a fender blues junior

Curranproducer · Post by **Curranproducer** » Sun Oct 18, 2015 5:06 pm

friend brings me a fender blues junior............ he re-tubed it a few weeks ago with JJ's................. he says its sounding weird. I pop it open and find one of the EL84's is burned up from red plating. I discharged the caps, check OHMS across all of the resistors and everything looks good. pop in a new JJ EL84 and it starts red plating again right away. nothing smells burned up inside, and there is nothing burned on the PCB for the tube sockets. any ideas on where to look next? I have not pulled any voltages yet since I am worried about damaging any components with the red plating going on. can I pull the tubes and check pin voltages with out damaging anything?

YouTube · Post by **xtian** » Sun Oct 18, 2015 5:17 pm

Check bias.

martin manning · Post by **martin manning** » Sun Oct 18, 2015 7:21 pm

Curranproducer wrote:Can I pull the tubes and check pin voltages with out damaging anything?

Sure, no problem; check The voltage at Pin 2's. If they don't match, suspect 0.022uF cap C15 or C16 is leaking. IIRC the bias is a bit hot on those, so you might consider installing a bias trimmer.

Leo_Gnardo · Post by **Leo_Gnardo** » Sun Oct 18, 2015 7:37 pm

Check the land side of the PC board. Short of pulling the whole thing out, peek back there with a dental mirror & flashlight. Juniors like to develop arcs right there where the 1K5 stoppers & plate flyback rectifiers & screen grid resistors are crammed side by each. (What. Was. Fender. "Thinking" ????? DUH!!!!!) Sometimes the arc chars the board then you have to dremel away the conductive burnt bits before reassembling the circuit.

FWIW I ordinarily put a 10K trimpot in series with their bias setting resistor & adjust to 26-29 mA which IMHO is plenty for these amps. Also I've had better luck with JJ than other EL84 in Fender's Blues & Pro Jr. If you feel like spending a bit extra for 7189A or 6P14P-EV, up to you.

Stevem · Post by **Stevem** » Sun Oct 18, 2015 7:53 pm

X2 with what the F was Fender thinking with those circuit traces!

gui_tarzan · Post by **gui_tarzan** » Sun Oct 18, 2015 9:49 pm

I fixed one of those recently and changed R31 from 22K to 33K and it dropped the bias voltage enough to stop the redplating. They're biased very hot from the factory. Also check the cement resistors, they have a tendency to heat up the solder joints and create bad connections.

sixstringer · Post by **sixstringer** » Mon Oct 19, 2015 3:27 am

Bill Machrone (Billm) as lots of information on his Billm Blues Junior mod sit, including info on bias issues.

gui_tarzan · Post by **gui_tarzan** » Tue Oct 20, 2015 12:30 am

Yep, that's where I got my info.

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red plating on a fender blues junior

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