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**Enzo** · 03-11-2016, 08:30 PM

SOunds to me like your pickups are feeding back. Does it act the same with several guitars? Does it do this if you plug something like a keyboard or even a CD player into it instead of something with pickups?

**loudthud** · 03-11-2016, 10:03 PM

You have left out several important details.

You mentioned 5F6A. Is the amp built into the traditional 4x10 tweed cabinet? Are you using both channels?

Are the input and speaker jacks firmly grounded to the chassis?

Type of guitar and pickups?

Type of speakers, impedance and how are they connected together?

Output transformer, secondary impedance and feedback resistor?

**sleipnir** · 03-11-2016, 10:07 PM

Thanks Enzo, you might be right about that. I tried connecting an iPhone as a music player, and I did not seem to get any squealing. I made a little video showing what happens with a guitar plugged in and also if I just have a cable plugged in. I don't know if the squeals in those two situations are related, but any insight would be appreciated. Does this seem like normal behavior?

http://vid270.photobucket.com/albums...squeal%202.mp4

**sleipnir** · 03-11-2016, 10:17 PM

loudthud - the amp was in a typical 4x10 tweed cab wired to 2 Ohms with 4x 8 Ohm Weber speakers (10A125s and 10F150s) but I have pulled the chassis out to try and figure out what's going on. In the video in the link above, I have the chassis connected to another 4x10 2 Ohm speaker cab several feet away. It is definitely not being caused by interaction with the speaker themselves. Either channel causes the squeal, but right now I only have channel one connected inside the amp as I was trying to figure out if there were any problems with my wiring and grounding of the input jacks (in fact, right now, I have only one input jack and it is sitting outside the chassis, but is grounded - I have not found any change in behavior from wiring up a new jack). The speaker jack is grounded at the chassis. At one point, I wired in another OT (from a Super Reverb) sitting on the workbench and had the speaker jack outside the chassis and tried grounding it to the main power ground or the preamp ground with no change in behavior. I have tried two strats (single coils) and one PRS (humbuckers). The OT is a Mercury Magnetics FTBO-M multitap OT (2/4/8) which should be ok for this amp as it was listed under Tweed Bassman on their web site). I also tried a Super Reverb OT as mentioned above.

**Malcolm Irving** · 03-11-2016, 10:32 PM

Originally posted by sleipnir View Post

...Although it might be a bad conclusion, I am thinking that somehow the signal at or beyond pin 6 of the phase inverter, which is in phase with the input of the amp, is getting fed back to the input and thus causing positive feedback. The reason I say this is twofold. The squeal seems to present itself as I get close to the PI tube. Further, just out out curiosity, I soldered a long wire to pin 6 of the PI tube, and with the guitar plugged in and controls on max, when I moved the open end of this wire near the input grid of V1, I got a similar squeal (somewhere in the 7-10KHz range). I then unplugged the guitar and cable, and tried moving that wire to the grid of V1 again, and was able to get squeal again, albeit much fainter and higher in pitch (14KHz or so). Presumably, there is enough signal (i.e. noise) at the PI with nothing plugged in and controls on max, that this signal causes positive feedback when brought near the input grid of V1. Further, it seems like this noise, when the guitar is plugged in and brought near the PI area, is being picked up by the guitar pickup and thus is sent to the grid of V1 causing a pronounced positive feedback. ...

How about taking a wire back from the other PI output and putting it close to the V1 grid? If the in-phase feedback encourages the oscillation, maybe out-of-phase feedback might help to supress it? Or on the other hand the instability might just shift to a new frequency. Might be worth a try, if you haven't already.
I remember reading somewhere about some Mesa amp which had a wire going back from a later stage and just wrapping around the input grid wire. The tech removed the wire thinking it was a piece of nonsense, but found it was necessary to stabilise the amp!

EDIT: OK. My suggestion is more for a case where the amp squeals with nothing plugged in but controls maxed - so maybe not so applicable here.

**Enzo** · 03-12-2016, 01:48 AM

Another test of my theory. Does stepping back from the speakers with your guitar reduce the oscillation? And when it squeals, does turning side to side - aiming the guitar different directions - affect it?

**pdf64** · 03-12-2016, 08:45 AM

I suspect that humbucker pickups are being used?
I think that if you tried a single coil type, it may help to bring home how strong, relatively, the EM fields are so close to the amp's iron.
The field strength is decreases with distance according to the inverse square law, so moving a bit further away may mitigate the problem.
Otherwise it may need a mumetal screen around the iron etc.

**Chuck H** · 03-12-2016, 01:30 PM

Guitar amps are very high gain. And you have (had?) additional gain boosting circuits (means more wire leads running back and forth across other circuitry). Since you're probably not going to play the amp with the guitar a foot from the tubes or input I don't think your demonstration shows a problem. I've had amps do that sort of thing before and didn't sweat it because the extra gain and very close proximity can and will cause what you have in your video. Also, a wire from the PI input moved to intentionally interact with other internal wiring inducing an oscillation doesn't demonstrate a problem either. Only an induced problem outside of normal operating conditions.

Does the amp become unstable with no guitar plugged in and no extra lead added to the PI circuit as you fiddle with the controls? ie: high treble, presence and volume settings. If not then plug in the guitar and sit a normal distance from the amp (say four or five feet) with the amp chassis relative to the guitar as it would be in the amp (as in not with the pickups aimed at the crowns of the tubes). Does the whole circuit become unstable now when you fiddle with the controls?

I'm trying to determine if you actually have an instability "problem" or if you are just unnaturally forcing it to happen. Which should be entirely possible with most amps the way you are testing.

**sleipnir** · 03-12-2016, 02:15 PM

Guys - thanks for all the replies!

Malcolm - your idea was interesting, so I had a chance to try it out this morning. I ran a wire from pin 1 of the PI tube (V3) and wrapped the other end (which is insulated) around the grid wire of V1 pin 2. It totally killed the oscillation even with the guitar right up against the chassis. However, it also increased hum and muffled the tone. I will have some more time later today to play with it some more - perhaps the number of wraps around the wire, proximity to other wires, etc. can improve it. I guess this is similar to a cap across the V1 plate and grid in that your creating a negative feedback loop to the input of V1?

I also heard back from a nice guy on youtube who has a couple of vintage tweed bassmans, and he was not able to reproduce the squeal by maxing out the controls and bringing the guitar to within a foot of the chassis, so it seems like it should not happen if the amp is working 100% right.

I can confirm that this problem does not seem to eb affected by which guitar I use as I have tried two strats with different single coil pickups as well as a PRS humbucker guitar. I can also confirm that the problem has nothing to do with proximity to the speakers, but does indeed depend on how close I am to the chassis of the amp.

I will play around with how the squeal is impacted by the orientation of the pickups relative to tubes and iron later today when I have some time, and I will report back.

Chuck - I first discovered the squeal when I was playing with the amp in its cabinet several feet away and had a fuzz pedal on as well. As I was trying to find what was causing it, I unplugged the fuzz pedal, played with the various Marshall gain mods I have, and ultimately realized that although the squeal was more prominent the more gain I had (i.e. I would not have to be as close to the amp to achieve it), the fact that I could still achieve it in stock 5F6-A Bassman mode (with controls dimed) made me think there is something not 100% correct, even though I had to get what might be considered unreasonably close. The higher gain mods are not connected now, which is why you see me in the video having to get relatively close to the chassis to obtain the squeal, but if I wired the amp back up with the gain mods, it would happen at a farther distance. If this is normal, I don't understand why it happens in some amps but not in others, but perhaps it is so on the fringe that it only takes minor differences in component values, wire placement, etc?

One more thing - when I had another OT connected outside the chassis, there was definitely a different behavior depending on how the OT was oriented relative to the guitar (or perhaps it was the chassis), but I don't recall ever being able to eliminate it with anything other than distance.

I'll do some more experimenting later today and report back!

**R.G.** · 03-12-2016, 02:25 PM

I have not slogged through the write ups in detail, but it's worth noting that ***everything*** will oscillate if it has enough gain.

Oscillation requires the Nyquist Criteria be met: a phase change from input of the amp to the output of the amp that is an integral multiple of 360 degrees and a product of gain and feedback attenuation greater than unity.

In layman's terms, if there is enough gain so that any signal getting back to the input from the output can be enough to make it oscillate, it will. This is because all devices have a phase shift as the frequency through them gets higher and higher, and at some point it becomes a multiple of 360 degrees. All that's necessary for this to cause oscillation is enough gain. The phase shift means that it doesn't matter whether you were trying to make negative or positive feedback, there is some point where it becomes positive.

Making things NOT oscillate requires that you make the gain from input to output low enough that all those high frequency places where the phase has shifted to make feedback positive be below the amount of gain that is lost in the feedback path. You can work on either lowering the forward gain of the amp, or modifying the phase characteristic, or increasing the attenuation of the feedback path, or all three.

You cannot make the feedback zero. This is because even the capacitance of several inches of air (or vacuum!) between the output wires and input wires is enough to let signal through at some frequency. That's why there are all those little metal walls in high gain RF stuff - they're trying to shunt signal in that mystery capacitance to ground.

It takes three time constants to make enough phase shift to oscillate. A triode or pentode stage all by itself has one time constant (that is, lowered gain and phase shift at some frequency) and maybe two, depending on how it's used. A transformer has at least two time constants and sometimes more if it's poorly designed. So even one gain stage and a transformer will oscillate if the gain is high enough. The resistance of a ground wire will cause oscillation if the gain is enough to let those millivolts get back into the inputs.

So the comments you're getting are pointing you in the right direction. There is something about getting the guitar near the amp that is letting signal get back into the input wiring and make it squeal. Could be speakers (acoustic feedback); could be bad cable shielding (electrical field feedback); or it could be magnetic field feedback from the output transformer.

**R.G.** · 03-12-2016, 02:32 PM

You posted while I was typing.

Originally posted by sleipnir View Post

I ran a wire from pin 1 of the PI tube (V3) and wrapped the other end (which is insulated) around the grid wire of V1 pin 2. It totally killed the oscillation even with the guitar right up against the chassis. However, it also increased hum and muffled the tone. I will have some more time later today to play with it some more - perhaps the number of wraps around the wire, proximity to other wires, etc. can improve it. I guess this is similar to a cap across the V1 plate and grid in that your creating a negative feedback loop to the input of V1?

It's killing the high frequency response of that tube. The key words there are "muffled the tone", which translates to "cut the high frequency response a lot, lowering the high frequency gain".

I can also confirm that the problem has nothing to do with proximity to the speakers, but does indeed depend on how close I am to the chassis of the amp.

Eliminates acoustic feedback.

I first discovered the squeal when I was playing with the amp in its cabinet several feet away and had a fuzz pedal on as well. As I was trying to find what was causing it, I unplugged the fuzz pedal, played with the various Marshall gain mods I have, and ultimately realized that although the squeal was more prominent the more gain I had (i.e. I would not have to be as close to the amp to achieve it), the fact that I could still achieve it in stock 5F6-A Bassman mode (with controls dimed) made me think there is something not 100% correct, even though I had to get what might be considered unreasonably close. The higher gain mods are not connected now, which is why you see me in the video having to get relatively close to the chassis to obtain the squeal, but if I wired the amp back up with the gain mods, it would happen at a farther distance. If this is normal, I don't understand why it happens in some amps but not in others, but perhaps it is so on the fringe that it only takes minor differences in component values, wire placement, etc?

That is correct - it is on the edge, all it takes is a little change in how much signal is lost in the through-the-air feedback path.

One more thing - when I had another OT connected outside the chassis, there was definitely a different behavior depending on how the OT was oriented relative to the guitar (or perhaps it was the chassis), but I don't recall ever being able to eliminate it with anything other than distance.

That argues for it being either a magnetic leakage path from the OT, or perhaps a shielding issue with the input. Did you try a different guitar cord on these iterations? Probably, but some guitarists use their favorite cord for everything. I had to check.

**Malcolm Irving** · 03-12-2016, 02:48 PM

Originally posted by sleipnir View Post

... I ran a wire from pin 1 of the PI tube (V3) and wrapped the other end (which is insulated) around the grid wire of V1 pin 2. It totally killed the oscillation even with the guitar right up against the chassis. However, it also increased hum and muffled the tone. I will have some more time later today to play with it some more - perhaps the number of wraps around the wire, proximity to other wires, etc. can improve it. I guess this is similar to a cap across the V1 plate and grid in that your creating a negative feedback loop to the input of V1?

Thanks for giving the feedback-wire idea a shot. The results are interesting. The feedback wire is coupling back to the V1 grid via the (very small) capacitance between the feedback wire and the V1 grid wire. I guess that will cause a greater amount of negative feedback for higher frequencies – hence the muffling of the tone. A shorter coupling length between the two wires would reduce that capacitance and may give you some stabilisation without too much treble loss. Coupling back via a resistance (it would have to be a high value – something like the impedance of that wire-to-wire capacitance at say 1kHz (which could be guesstimated, I suppose) might avoid the treble loss. However, we are just introducing global negative feedback from the PI output all the way back to the V1 input. The effect of that negative feedback would be to reduce the overall gain (which could be done in various other ways, and might actually be defeating your tonal objectives).
I think the hum is probably coming from the feedback wire picking up interference on its way back through the amp – that could be addressed by using a screened cable perhaps.

**Chuck H** · 03-12-2016, 06:15 PM

It was good of that other guy to test with his 5f6a for you. You can take what's been said so far and boil it all down to a parasitic oscillation due to the layout, lead dress, ground scheme or some combination there of. I build some hot rodded circuits. Not the modern uber gain preamp type amps but amps that develop high gain and incorporate the power amp in the process. Much more than a vintage amp cranked up. Layout, lead dress and grounding need to be "right" to pull this off. There are multiple threads you can search here that cover these subjects well. But since it's a whole system ideology it can be frightfully hard to spot the offending error and even then it's not always obvious.

I think it would help at this point to have some gut shots of the amp and a willingness to take more pics if there is something specific (angle or detail) that a responder wants to see. Trust me when I say that we here are all aware that you've made a lot of changes and efforts inside and we know what to expect. No one is going to judge your solder work or how smoothly bent your wires are or if there's some solder burns on some insulation.

**sleipnir** · 03-13-2016, 03:06 AM

Thanks again everyone. Chuck, I am not at all worried about you guys making fun of my wiring, soldering skills, and various melted insulation, so I am happy to post some pics and also take additional ones as I am sure not everything is clear in these shots.

First, please see my layout as it might be helpful. Note that I have now pretty much disconnected all of the wiring related to the mod switches located on the front panel. There are some switches which I have not disconnected: The switch between the rectifier tube V6 and power tube V5 allows me to switch in 5 zener diodes to lower my B+ (my PT is 380-0-380 and when I switch in the zeners, I get down to the voltages found on the original 5F6-A schematic). The amp squeals in both high and low power modes and I have been running it primarily in low voltage mode lately while debugging. The two switches located between the power tubes are for pentode/triode mode and to change between 1K and 470 Ohm screen resistors (the latter might have been somewhat pointless to include as a switchable option). Here's the layout:
http://i270.photobucket.com/albums/j...0mods%20v8.jpg

Here's what the amp looked like wired as per the layout above:
http://i270.photobucket.com/albums/j...202015%206.jpg

And after replacing capacitors, adding shielded wires to grids and presence pot, twisting OT primaries and secondaries, disconnecting the front panel switches, unsoldering and re-soldering various wires and components too many times while splattering solder and flux all over, this is what it looks like now:
http://i270.photobucket.com/albums/j...02/Chassis.jpg

You can see a few more close-up photos here:
Tweed Bassman 2 Slideshow by osingiusa | Photobucket

The bias circuit will look a little odd with its 3 50k pots - this is what I came up with in order to allow me to bias the amp in tube rectifier and low voltage (with zeners) mode, and also maintain proper bias in high voltage (no zener diodes) tube rectifier and SS rectifier modes. I have not touched this while debugging but don't how this could affect the squeal (maybe I am wrong?)

I was able to place the chassis upright so that it resembled how it would be relative to the guitar while mounted inside the amp cabinet. The squeal occurs when I approach the chassis from either the back or front with the guitar positioned as it would be strapped around my neck. From the front, when I get too near the OT, a much louder and more obnoxious squeal takes over, which I assume is normal and due to magnetic coupling taking place. I can record and post a video if this would be helpful, but the squeal I am trying to remove is the one shown in the video with the link pasted in an earlier post.

RG - I have tried a couple of different cables, and today I connected two 20ft cables using a simple junction box and found that it improved the situation a little bit. Presumably, the added cable capacitance reduced the highs enough to force me to move even closer to the amp to get the squeal.

RG - I think I can rule out acoustic feedback as the speakers are several feet away. I am curious about electrical field feedback and magnetic field feedback. The problem still exists when I shield the open chassis with a piece of sheet metal and aluminum tape (as in the video) and also have tube shields on the preamp and PI tubes, so I am not sure where the electrical field feedback is coming from. However, I experienced squeal with the current OT as well as with one wired to the amp but located outside the chassis on the bench and at one point I concluded that the feedback was coming from some sort of coupling between the speaker wire and the guitar pickups, which I guess would be electrical field feedback? Is there anything definite I can do to determine whether this is electrical or magnetic feedback?

I had made a video when I had the second OT connected and positioned outside the chassis, but I was testing with just and instrument cable connected and no guitar, so I don't know if this is relevant:
http://vid270.photobucket.com/albums...A%20squeal.mp4

I truly appreciate you guys taking the time to look at some of this and would love any further suggestions as what to do next. As mentioned, I would be happy to take photos from other angles as needed as I realize it is difficult to see everything given the messy wiring right now.

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Squealing/positive feedback - explanation of what is happening

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