Cream Board Phase Inverter Oscillation
Did you ever hear a kind of hissy resonance in the notes when you play your cream board Blues Junior? Maybe a blurry, ringing sound that sounds like a bad tube or a bad speaker? It's possible that the problem is oscillation. The phase inverter (V3) is prone to oscillation, and I often see it when I'm working on cream board Blues Juniors with the back removed. The back has a sheet of aluminum foil on it, and it grounds to the chassis at the top and bottom. Its job is to keep electrical noise out of the amp, but it also provides a ground plane that helps reduce oscillation in the PI.
The PI has a 47pF capacitor across the plates to help control oscillation, but lead dress--where you run and bend the wires--has always been part of amp mojo. The ribbon cables used in modern amps don't let you separate the wires leading to the tubes, so traditional dressing is not possible. The ribbon cable can be rerouted, however. I do it on every Blues Junior that I mod, as a matter of course.
A couple of people have told me that they've encountered the same problem with green board Blues Juniors, but I haven't experienced it personally. They've told me that this fix works for them, too.
|This is how the ribbon cables are dressed on a
typical Blues Junior. The phase inverter is the wide cable at the right
of the picture; the two to the left lead to the output tubes. The blue
wire runs from the output transformer to V4, one of the output tubes.
Note that it's laying against the output tube ribbon cables. When the
back is off, coming near the PI ribbon cable or the blue wire with a
finger (don't touch anything!) or a meter probe will send the PI into
oscillation. It typically oscillates at 48KHz, well above the audible
range. But the oscillation mixes with the notes you play, making a hissy,
The oscillation also steals power: anything that goes into sounds that you can't hear steals energy from the sounds you can hear.
The oscillation occurs freely when the back is off, and occasionally when it's on, but not on every Blues Junior. This simple mod reduces or eliminates the oscillation.
|Before you touch anything inside, make sure your
amp is unplugged and has been off for a while. The bleeder resistors
will discharge the filter capacitors so that you won't be shocked. You
can use an insulated jumper wire to connect ground (the chassis) to the
+ side of the large gray capacitors. Just touch the wire to to the +
side briefly, and the amp will be safely discharged.
First, straighten the upper bend in the cable. Grasp it on both sides and push the cable down into the gap between the chassis and the tube daughterboard. I've marked this cable to show you how far you can typically push it, usually between 3/8 and 1/2 inch.
|Here is the cable, pushed as far as it will go smoothly, without forcing. Don't overdo it; the cable will form a natural radius that follows the chassis and then curves around to the point where the cable is soldered to the tube daughterboard.|
|This is how the dressed cable should
look. Straight up from the circuit board, a smooth curve, and then
straight to the chassis. When you replace the back, the ribbon cable
will be close to the back. The wires in the cable and the back form a
very small capacitor, but it's enough to reduce or eliminate the
Note that I've also moved the blue plate wire and the output tube ribbon cables away from each other. I'm not sure how much this helps, but I think it helps to control oscillation.
Obviously, if your Blues Junior doesn't oscillate, this isn't necessary. But if it does, this simple fix can put an end to a seemingly endless chase of replacing tubes, speakers, capacitors, and who knows what else.
Why not just shield the entire ribbon with aluminum or copper foil? One BJr owner who tried that said that the tone became noticeably duller. Full shielding evidently provided too much capacitance, which rolled off the highs. He pulled off the shielding, did the 90-degree bend, and the ringing went away.
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