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.
Above 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, ringing tone.
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 you turned it off while the tubes were warm, so that you won’t be shocked. (See info on discharging the caps here.)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.
Above 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 oscillation.
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 may have provided too much capacitance, which rolled off the highs. He pulled off the shielding, did the 90-degree bend, and the ringing went away.
Here’s a better solution, on a BJr that just wouldn’t stop oscillating at 44KHz. At its worst, the oscillations were consuming half of the amp’s power. It sounded awful. I bent both output tube ribbons down almost all the way, but left a little space for heat that rises off the sockets to escape. Positioning the V4 and V5 ribbons out of alignment with the V3 ribbon breaks apparent capacitive coupling. It also gets the V4 and V5 ribbons far away from the V4 plate lead (blue wire).
Note that I’ve applied a bead of hot melt glue to each ribbon where it joins the board. This prevents flexing and bending of the wires at a sharp angle, which eventually breaks them. Fender does this on all its new designs. I recommend it highly.
A Completely Different Approach
Pete Cage, an amp tech in Australia, had the oscillation problem on one of his customers’ amps and even the most radical ribbon cable bending didn’t cure it. He resorted to an electronic cure: Bypass the phase inverter plate resistor that feeds V4 with a 100pF capacitor. That’s R30 on the cream board Blues Junior, and here’s what it looks like:
Here’s what happens: The combination of capacitances and resistances in the V3/V4 portion of the circuit forms a resonant tank that causes oscillation at ultrasonic frequencies, typically between 40 and 50KHz. The 100K plate resistor on V3 looks like a brick wall to high frequencies and they stay in the circuit. The 100pF cap passes only very high frequencies, in this case 16KHz and up. It passes them to the power supply, which is running at 300+ VDC. But a healthy power supply has very low AC resistance, only a couple of ohms. So the very high frequencies jump over the resistor and get absorbed by the power supply.
The bypass cap should be a 1 kilovolt 100pF ceramic disc cap. Despite the bad reputation of ceramic caps in audio applications, caps built for 1KV and up are constructed differently and do not have the nonlinear/distortion issues common to lower voltage ceramic caps. A 500 volt dipped silver mica cap would work too, but there’s no advantage, since the cap is not actually in the audio path. Instead, it merely leads bad frequencies away from the audio path.
A customer in Portugal had an amp that just wouldn’t quit oscillating. He even separated the grid wires in the phase inverter ribbon cable with a razor blade, wrapped them in copper foil, and grounded the foil. In frustration, he sent me the amp. I heard more of a spitting or hissing sound than the typical oscillation, but the notes were fizzy and distorted. I noticed that if I tapped on the ribbon cable, it would send loud pops or snaps through the amp.
I decided that there must be an internal break in the ribbon cable or a microscopic crack in one of the solder pads where it attached to the tube board. It seemed easier to just bypass the ribbon cable entirely. You can see from the photo below that I installed new plate and cathode wires directly to the tube socket and used shielded wire for the grid connections. The grid shields are grounded at the circuit board. The double shielding makes the amp sound exceptionally nice… it makes me want to replace all the ribbon cables with shorter wire and shielded grids.
Here’s a video example of a cream board amp with severe PI oscillation. This isn’t just on the trailing edges of notes; it’s all the time. I’m playing the amp like a theremin, using the capacitance of my hands and fingers to couple stages together and modify the coupling that’s causing the oscillation:
I added the 100pF cap across R30 and the oscillation disappeared completely.