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.
Update:
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).
This turned out to be a very effective cable dress for a persistent oscillation.
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.
I appreciate your efforts the blues jr is one hell of an
amp and this ribbon cable dressing tip helped the
tone noticeably. I just added a celestion greenback
and I had ho electronics build an attenuator for it.
with the amp cranked and the output partially attenuated
the tone is really world class, sustains with a
rich harmonic overtone. I run a 7-band eq in front
and then its just the amp cranked.
I just wanted to emphasize that your work is appreciated.
I have a cream board blues jr. Thanks for the filter cap article, one of the three in mine were leaking so I replaced them all. Ever since I got this amp used, it has had a very irritating buzz not loud but irritating. This is the same sound as if you were to touch the the center pin of the guitar cable connected to the amp, but much quieter.
I have checked the tubes and even changed them out with another set and it still buzzes. I have removed v1 and v2 and left v3 and the power tubes in place and it still buzzes. When I remove v3 the buzz goes away, of course I knew that it would since this disconnects the power tubes from the pre amp. This buzz is heard with both volumes at zero and all they way up. The buzz is still there with the guitar cable disconnected from the amp. I have checked the tube sockets and ribbon cables, still buzzing. Sometimes the buzzing is accompanied by crackling like a burning piece of wood, and then sometimes it will be accompanied by a loud hiss as if I have tuned the radio off the station and then the hiss will disappear, but I still have the buzz. The buzz seems to be centered around the Phase Inverter.
Please help if you can.
If shielding the ribbon cable doesn’t help, it needs to be gone through with an oscilloscope to see what the buzz is (is it 120Hz power supply buzz or subharmonics of ultrasonic oscillation? Or something else?) Oh, and just a random thought–does the buzz go away when you turn down the reverb? If so, it may be plugged backwards into the tank.
Hi bill, hope you can help me.
My tweed BJ is almost perfect but it buzzes when reverb is on and completely quiets when reverb is set to 0.
Reverb works well anyway, it’s just the buzz…
How can I do?!!?!? thanks in advance for your precious support for all of us.
If it’s an older tweed with the green board, you need to do the reverb mod. http://billmaudio.com/wp/?page_id=34. If it’s a newer cream board, the problem has to be in the tank or a bad connection to the tank. Make sure that the wires are in the correct location. The red one goes towards the center of the amp, the black one to the side, under V1.
Hi Bill,
I just finished the cathode follower mod on mine (cream board model with your tone stack mod) and it sounded great with much more touch sensitive dynamics. But then I started to notice a slight fizzle on the bass notes as they sustained and then decayed. At first I thought something had gone wrong with the mod or something was loose and rattling like a screw, so I was listening all over for the source and it was coming out of the speaker! So then I thought perhaps it was oscillation, and then I opened the back and was poking around with a chopstick. I then noticed that the ‘fizzle’ was actually caused from the OT blue wire’s proximity to the power tube ribbon cables. As soon as I separated the blue wire from those ribbons, it was gone. Of course I didn’t remember this page until after the fact, but I just thought I’d let you know the blue cables proximity definitely affects the bass notes clarity and tone. Next I want to try the upgraded OT and the filter cap.
How’s the progress going on the upgraded PT to run cooler and run those 5881′s? Ever try increasing the coupling caps values to open up the bass?
I’m glad to hear that the mods are working well for you.
To get the most out of 5881s, the BJr is going to need a whole different power transformer. David Allen and I are discussing the design, but it will be a while. In the meantime, JJ 6V6-S tubes with the upgraded output transformer really sound great, with more clean headroom than the 5881s. I don’t think that the coupling caps need to be any larger; the PI can drive the output tubes into full power output and the frequency range is reasonably flat. You don’t want to overdo the caps or it’ll sound tubby and maybe push the output tubes into cutoff.
Hello,
A green board Blues Junior came in the shop today and it is ocillating at about 40Khz(40 VAC) on the OT blue secondary and power tube plate. The ocillation goes away when I remove the PI tube. The amp burnt up one power tube already and after I raised the bias voltage so the tubes are at about 9 Watts I noticed the ocillation. I am going to try your lead dress ideas and hopefully it solves the problem. Thanks for the helpful information.
Mark
The oscillation is an interaction between the PI and V4. The dress with the output ribbon cables pressed low seems to work best. Keep the V3 ribbon high so the back of the amp provides a ground plane.
Dear Bill,
I tried to install the basic mods this weekend but desisted because I couldn’t take the knobs off and I didn’t want to damage anything in the amp. I guess I’ll try again with a different technique (spoons?).
Since I couldn’t do the mods, I did this phase inverter “mod” even though I don’t have any background noise on my amp.
Weird thing is that when I was playing today, when hitting the 6th string (open or fretted) I could hear that the sound oscillated a lot, it sounded like if I was using a “vibe” effect. This was with the bass knob on 11, when I turned it down, the oscillation was less noticeable but still was there. Do you have any idea about what it could be?
Thanks for your advice!
Rodrigo
If it wasn’t oscillating, you didn’t need to move anything! Maybe you should just move them back. Be gentle where the ribbon cables meet the board. They can break from too much flexing.
I know but, it still has no background noise or hiss, and since the oscillation problem you describe is actually a supersonic sound I shouldn’t be able to hear it but this oscillation I have is on the low frequencies, that’s the odd thing.
Next week I will do the basic mods and see what happens.
Thanks a lot!
Much Regards,
Rodrigo Hofmann
Question:
For those who are capable.
Why not pull the ribbons out entirely, replace each lead with a single conductor w/ braided shield, grounded on one side only?
I am not sure the impedance difference, might be negligible with such short runs.
Just a thought.
You could replace the cable with individual wires and gain more control over routing and placement, but it’s a fair amount of work. People have a tendency (based on repairs I’ve had to do) to lift the pads where the ribbon cable attaches to the boards.
It’s not necessary to shield every lead, only the grid leads. The amount of added capacitance would be slight. I doubt that it would change the tone.
Hey bill you might put note in you cap discharge text that it is best to solder up a 100K+ resistor to the wire…then discharge to ground b y holding on the +lead for 2-5 seconds…..way safer. having had electolytic all over my face and glasses and stuff from a simple discharge of an ampeg vt22 amp years ago….no more i use a resistor,wire and a clip for ground with all exposed bare wire areas covered by shrink tube except the tip of one of the resistor leads ….haven’t had to take a shower since…..much to my wife’s chagrin….hee hee
i meant the free end resistor lead….
I use a 10K-27K resistor at the end of a clip lead. No sparking, discharges quickly.
Just wanted to warn everyone that the cream board tube socket contacts are *easily* deformed. I had a output tube that was glowing red. As it turns out the socket contact for pin-2 was not making contact with the tube pin which allowed the grid to “float” thus turning the tube on almost full tilt. Took a machinist’s scribe and carefully re-formed the contacts for pin 2 and the problem was solved. Also had bad microphonics on the input triode. Again…bad tube contact…reformed with a machinist’s scribe. So not all microphonics are a faulty tube! Check the *socket* contacts first before tossing the tube in the trash bin.
Another tip…and I have been soldering for 30 years….the cream PC board traces are quite fragile and easily separate from the board material. YES I used a controlled temperature Weller WES50 soldering iron set to 650F! So be VERY careful when removing components from the board.
Lastly…and this one was my fault…. when re-connecting the output transformer blade connectors to the little PC board that contains the speaker and footswitch jacks be sure you DO NOT REVERSE the two. It turns out that the negative feedback to the phase splitter is sensed from this board. Reversing the two blade connectors from the output transformer will turn your amp into a high-powered howling oscilator because the negative feedback becomes positive feedback. DUH!
You know that I have a page devoted to tightening the tube socket springs, right?
Re-Tension Your Tube Sockets
Yes Bill and I thank you for pointing that out. I was really puzzled when one of the EL84′s was glowing cherry red but the tube socket grid pin was reading -10.5 volts. Then I noticed your discussion of re-tightening tube socket contacts. Happy tube afterwards!
I noticed on my amp that the removable back part seems to vibrate against the rest a bit, causing a mild rattle. I had some extra pieces of Dynamat lying around, so I thought I’d put those in between the two pieces. I don’t have rattle now, but it just occurred to me that by doing so, the back plate is no longer in contact with the metal casing, so it doesn’t really act as a proper ground plane (I would assume). Does that negate the usefulness of this mod? If so, do you have any ideas for eliminating the various sources of rattle? I get very obsessive about having a quiet amp.
Put dots of Dynamat or felt or whatever between the screws. That way you have grounding and vibration damping.
my 08. nos jr started making this very loud feedback sound while i was playing at a low volume.i turned it off threw the cheap guitar chord away got my good chord turned it on fine started playing today 5 minutes started same crazy sound turned it off wited a minute turned i on it was gone,now i afraid to play it dont want to rune it .just got it 6 mnths ago. thnks for your time . billy joe
It’s probably a bad tube or bad connection on the tube pins. If it’s under warranty, get it fixed. Otherwise you should get a tech to look at it.
Bill,
Wanted to say that i recently bought a cream board bjr and it had the usual hum issues and fuzzy bottom end issues right out of the box. I haven’t performed any changes to the componentry yet, however – twisted and redressed the cables from the power tranny, wrapped the exterior in copper shielding tape (probably didn’t do anything, but i had plenty laying around and figured it probably wouldn’t hurt), built a bent piece of metal (an old bottom of a hammond box) applied b/w the op tranny and power tranny, bent the power tube ribbons away from the plate lead and also had to slice up the ribbon to the phase inverter to get rid of nasty oscillations that weren’t too noticeable at low volume, but terrible when cranked. The amp is now completely silent…no hum whatsoever, no ringing when cranked, and as a completely unexpected bonus – the bottom end cleared up significantly after splitting up the ribbon to the phase inverter.
For anyone planning to use this little amp in a studio scenario – i highly recommend these “physical” modifications, and as always, use a solid separate cab to minimize the chance of rattles getting into the mic. Thanks for the ideas.
Bill,
My bjr is almost 2 yrs old and has always had a annoying hum/hiss. I read the last comment with interest since he seems to have fixed his, but I dont understand much of what he wrote. Can you translate into the clear instructions you usually use? I am going to buy the mod kits to improve the overall sound (i play jazz) but really want to get rid of the noise to have a quiet amp. I have taken it to the shop once, got new tubes, and had the reverb repaired under warranty. And I “lifted the ground” with a plug adaptor to see if it was a ground loop, but that did not change anything. The hiss is still there. Will the mods work by themselves to quite the amp or do I need to do something like Chris, the last commenter, recommends?
Thanks for all the great work!
Hum and hiss are not typical for a Blues Junior, even stock. Hiss is usually a tube thing, but can also be caused by plate resistors or other noisy components, or even a bad solder joint. Sometimes weird noises go away in the course of doing the mods, but there’s no way to be sure.
A knowledgeable tech who cares about this stuff needs to track it down at the source. If the hiss is coming from one of the above components, the cathode follower mod will not eliminate it.