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.

stockcable

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.

pushcable1

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.

pushedcable

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.


dressedcable

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.

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.

Another Update

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.

Click for a larger image.

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.

59 Comments

  1. steve says:

    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.

  2. Jason says:

    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.

    • bill says:

      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.

      • pasquale says:

        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.

        • bill says:

          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.

  3. Andrew says:

    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?

    • bill says:

      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.

  4. Mark Sproull says:

    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

    • bill says:

      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.

  5. Rodrigo Hofmann says:

    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

    • bill says:

      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.

      • Rodrigo Hofmann says:

        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

  6. john says:

    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.

    • bill says:

      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.

  7. Roland Shaak says:

    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

  8. Roland Shaak says:

    i meant the free end resistor lead….

  9. Charlie T says:

    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!

  10. Nick says:

    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.

  11. billy joe says:

    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

  12. Chris says:

    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.

  13. bruce says:

    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!

    • bill says:

      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.

  14. Chuck Leavitt says:

    Bill – great idea regarding the hot-melt glue.
    What about using a bead of Aquarium clear silicone seal – it would be stiff but also a little flexible and (somewhat) easy to peel off if ever need be? – OR – will the silicone sealant, for some reason, affect any electronics circuits or properties?

    • bill says:

      The gases from silicone sealant can corrode electronics, but it’s usually not a problem in well ventilated areas. In my business, silicone takes too long to set up. I can’t wait that long.

  15. Kevin says:

    I had Phase Inverter Oscillation that just would not go away. I tried adjusting the ribbons and still had issues. The 100pF cap did the trick. That 20 cent cap saved me from buying another amp. Thank you Bill and Pete for sharing the info.

  16. Kerry says:

    Thanks for the tip. I dressed the phase invertor ribbon and reintalled the back cover. The problem was considerably worse, sounded almost like a blown speaker on bass notes. This told me I was in the right area. Removed the back and confirmed the ribbon was actually touching the back cover. Redressed this and the other ribbon cables away from thr blue wire and retried, noise was gone. This saved me from a lot of wasted time and guesswork. Thanks again.

  17. yan says:

    Hi Bill,

    thanks for your fabulous website (both old & new) !
    I’m writing from France so I apologize about my english ;)

    I own a 3rd hand cream board BJr since a few years and changed the stock power tubes once when I got it (they were burned).

    the amp has been very reliable since, maybe 100+ gigs without any problem.

    after one year or two got some stranges noises in the preamp section and fixed the tubes connectors from your previous site precious information.

    Recently did the “twin” & tone stack mods (bass cap only) and bias resistor to 27k (checked with probe / -12.5V).
    the amp is cooler and sounds way better than before.

    there is no more hum than before BUT I got two strange things occuring like something “resonating” when I use some settings on the amp.

    when I set volume to max (any master setting, even low) and use a bridge pickup on a particular strat, I have a kind of feedback instead of the signal (high frequency and loud). very strange !

    2nd thing when I use a neck pickup the bass notes kind of make the speaker “buzz” in the attack like a broken speaker or we use to say in french a bee flying in a bottle ;)

    when I turn on the FAT switch or increase the volume this buzzing noise becomes louder.
    it seems the preamp saturates low notes ??

    could it be related to PI oscillation ?

    Thanks again !
    Yan

  18. prof.sherrill says:

    Have you found that replacing the output transformer makes the BJ more likely to have oscillation problems?

    • bill says:

      The oscillations are there; a larger output transformer makes them stronger. But I’ve seldom seen a cream board that didn’t oscillate, even with the stock OT.

  19. brightblack says:

    Hi Bill- I noticed that in your comment regarding the the “theramin video” you stated that you added the 100pF cap across R28 I thought it was the R30….thanks for all the advice you’ve given as well!!

  20. brightblack says:

    Hello again Bill- Just a couple more questions regarding the the 100pF across the R30:

    1)My caps are 50 volts 100pF but you recommended a 1 kilovolt 100pF cap, would 50 volts be suitable?

    2)There’s no need to remove the circuit board when doing this procedure correct?

    Thanks again,
    Tim

    • bill says:

      If you use a ceramic cap, it must be 1KV or more. If you use a film cap, it should be 250V or more.

      • brightblack says:

        Thanks Bill- What’s the best way to to do this procedure?…My soldering skills are limited to guitar pots and pick-up wiring…I have a quality temp controlled iron and plan on doing your basic mods to my cream board series II.. but my experience with working on printed circuit boards is lacking…

        Happy New Year
        Tim

        • bill says:

          Just slip the leads of the capacitor through the leads of the resistor, and half-bend the leads around the resistor leads so you have good contact. Tin the iron, then apply it and some solder to the joints. Remove heat as soon as the solder flows and coats the leads, so you don’t loosen the joint on the other side of the board. Clip off the excess capacitor leads.

  21. erlowther says:

    Bill, I’m wanting to bypass the ribbon cables in my cream board Blues Jr. entirely for V1 – V5, but I have no clue which wires go where. I see above your V5 ribbon bypass. Do you have pictures of a completed cream board ribbon bypass that shows the correct V1 – V5 wire conversion? If so, can you post it here? Sincerely, Ernie, Lexington, KY…

    • bill says:

      I don’t have a picture of an amp in which all of the ribbons were replaced. Is there any reason to do this? If the ribbons are not damaged, I’d leave them alone. The amp won’t sound any different.

      • erlowther says:

        I hear you. Seems I just keep taking out the circuit board a lot to work on your mods. The ribbon cables (even though I hot glued them) don’t seem to like being bent in the middle in order to remove the board to work on the soldier points. I keep envisioning the eventually breakage of the tiny ribbon wires from my repetitive tinkering with the board.

        There’s an old Henny Youngman joke that goes well with this scenario…

        The patient says, “Doctor, it hurts when I do this.” “Then don’t do that!”

        Thanks Bill for the reply and advice… ernie..

  22. bluesjunior says:

    I have an older made in USA green board BJr’to which I applied the basic mods pack to as well as the recap kit about a year ago. The only thing I didn’t install was the bias adjuster, not because I couldn’t but with my limited knowledge of using my multimeter (ie;testing p/ups and guitar leads) I just didn’t fancy sticking those probes into a live amp so decided to just leave that mod for another day. Anyway the mods all worked above and beyond expectations and I was a very happy camper.

    A couple of months back or so I noticed that I was getting a sort of distortion/scratchy type sounds when I was playing. It seemed to be more affected on certain notes and I at first thought it might be a sign that the power valves were on their way out. I changed out the EL84’s with a known good pair in my toolbox(the original Groove Tubes) which I pulled out a few weeks after buying my BJr’ in 2001 when I put in JJ’s instead. This didn’t help and after browsing this site and Google learned that it might be the pre-amp tubes but especially V1, I tried different combinations of swapping them back and fore and also with 3 known good ones I have but nothing helped so I came back here and constituted that it could also be a cold solder joint on the power tubes. I resoldered all the points on both and switched the amp on to find that I had no sound at all now and worse, V5 was “redplating”. I switched it off and after a bit of trial and error found that I had lifted a couple of the traces on V5. I read up on the repairing circuit board damage section and had a go but it didn’t work and decided enough was enough and contacted Cameron Electronics just up the road in Sunderland (a highly recommended pro) After explaining the problem to him on the telephone he told me to bring the amp up to him which I did that same day and received the now repaired and tested amp back four days later at a very reasonable cost I believe of £35.

    At home I plugged the amp in and hurra hurra, fire in the hole. I was also amazed that V5 which had been the several times roasted valve was still working. It was the next day before I got a chance to really test the amp out and after about an hour or so playing along to backing tracks I noticed that I still had the initial problem of the scratchy/distorted sound. At first I thought valves and went through the same rigamarole again of testing albeit without any resoldering this time. The next day I went and bought a complete new set of 5 valves put them in, found the problem still there so decided to keep strumming the guitar with one hand while adjusting all the knobs, to see if I could pinpoint the problem or perhaps clear it if it possibly might be a dirty/faulty pot. This only resulted in the noise becoming much worse and I switched the amp off deciding to take it back to Cameron Electronics. Before I did this I decided to put the valves back in that were in it when he had it first. I thought it was best as with the new valves his first thought was bound to be that was the problem.

    When I put the old valves back in I decided to see if the amp was working. I left the back off when I did this and was surprised to find the amp was playing fine. I played it like this for around an hour without any odd sounds at all. The next day I swapped out the valves again, this time putting in the new set of 5 I had bought and again left the back off. Switching the amp on I again played it and found no problems at all. This time I really tried to force the issue by turning everything to max but the amp sounded just fine, no sonic problems at all. I came here again and browsed to this page/thread and after listeng to the sounds in the video above came to the conclusion that is more or less the same sounds I was getting before. Now it seems clear to me that the problem seems to be something to do with the screened back plate. I have moved the flat grey plastic cables as described above and would like to know if I can apply the same fix as described above for the cream board amp, or in fact any other ideas or possible solutions to this problem?.

    Sorry for getting a bit wordy in this post but thought it best to give as much info as possible. Thanks in advance George Thompson.

    • bill says:

      Yes, you can apply a 100pF capacitor across the leads of R19. That’s the equivalent resistor on the green board.

      • bluesjunior says:

        Thanks very much for the very quick reply Bill. Along with the drill bit and the bias adjust control I have a small capacitor left over from your upgrade pack in my toolbox. This cap states,(y5p 101k 1KV)on itand as far as I can determine on Google it is a 100pF capacitor. Is this correct and can I use it?. thanks again for your help.G.T.

  23. bluesjunior says:

    I applied the fix to R19 but it hasn’t helped. The problem is apparently when I put the back plate back on. Without the backplate = no noise but within 10-15mins of putting the back plate on it starts to oscillate. The back plate on mine is a very tight fit so as an experiment I removed the 6 retaining bolts and squeezed the backplate just into place so that it was just on and about 4-5mm raised from the bolted home position. I played the guitar like that for an hour or so without any noise so I plan to use some locking nuts on the underside of the bolt as spacers to stop the backplate going so deep. Do you know what gauge the threads are on the back plate bolts I tried some nuts from my toolbox but they are either too big or too small which suggests that it isn’t a metric size. I measured the bolts with a caliper and the threaded part has a diameter of 4.75mm but I can’t remember if you use Whitworth or Imperial threads in the USA.

    I have bent the ribbons and applied the 100pF cap as talked about here but the problem definitely seems to be caused by the backplate. Is there any other reason why that could be?.

    • bill says:

      You clearly have a capacitive coupling issue. Does the back of your cabinet have the aluminum sheet in place? Is the chassis bare where it contacts the back? Some early Blues Juniors had rubber gaskets there to prevent vibration, but they insulated the back from the chassis and caused problems. Remove gasket material if it’s present.

      Maybe you should move your V3 cable away from the back of the amp. Take care not to bend it right at the circuit board. Use a pencil or something similar held against the back of the cable so that it cannot stress the short, exposed wires. Flexing right at the board will cause breakage.

      The screws are 10-32.

  24. SteveC says:

    Since I put in the presence control in my greenboard BJ, I’ve been having high-pitched oscillation issues, especially with the fat switch engaged or the master volume above about 9. B and C notes would bring out a very high pitched sympathetic rattle, which would come and go in the background even without the guitar, and which turning up the treble and the presence would turn into very a high pitched feedback-like howl.

    I tried all new tubes, the lead dress above, putting the 100 pF cap across R19 (the greenboard version of R30), getting the OT leads further away from the PI leads ribbon, but none of it helped. Then, just now, chopsticking everything, I noticed the oscillation sound was sensitive to the position of V2’s lead ribbon. So I bent it down the same way Bill recommends for the power tube ribbons above, and the oscillation disappeared — and the amp’s noise floor fell dramatically, as well.

  25. Frankli says:

    Hi, I bought a kit and loved the modifications!
    I damaged my fault 47pF 1kv capacitor. Only against 47pF 500v to buy. I use this?

    Thanks!

  26. Jagmaster says:

    Of the 5 grey ribbon wires on my BJ, I have noticed that if I tap on the center one of the 5, that it is somewhat microphonic . Is this normal for the center one ?

  27. CleanMAC says:

    You said ; “The grid shields are grounded at the circuit board.”
    where it is grounded?
    Thank’s

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