The Unknown Spring Reverb

[11-90-an]

From what @Rob said, try moving the speaker wires further...

Or...

Wrap the speaker wires in aluminium foil (especially the ones near the tank!), then ground that foil....

[razabri]

Things have now gone from bad to worse, as I lost power somehow and the amp's not turning on. Guess one of the fuses blew on the power supply board, as I don't have any power at switch no at a cable that runs to the main amp board.



My guess is that it wasn't such a good idea to patch that power-board-to-main-board cable and take the power for the reverb from there, as it might reflect badly on what was happening within the amps circuit - only reason was to use the same switch and get the effect powered together with the amp and not go directly from the battery. Who knows, maybe that's the reason I got all those oscillations 

[11-90-an]

That also might be a possibility...

So no magic smoke...? (Hopefully not)
Replace the fuse and try powering the reverb tank with a separate PSU..

[Rob Strand]

I've tried many things to solve this, to give it more grounding, shield the wires of send and return, roll the set 180 degrees so that the springs are further away from the speaker (and that only made it worse, maybe it's the closeness of the circuit...),

get all the wires as far away from each other as I could, but noting really helped. Only thing that works and that I don't really like is to mute one of the springs with a piece of felt, so there's less possibility for feedback, but it's kind of still there, lurking...

You have done the right things here.

Separating the wires reduces electronic feedback.   Typically that is very high frequency.   You don't always hear the main oscillation but you hear a lot of weird side effects.

Another type of feedback is acoustic feedback.  The vibrations of the speaker transmit through the cabinet structure and get coupled into the reverb tank.   This type of feedback is similar to microphone feedback.   However you might experience much lower frequencies. 

The fact you can stop the oscillation by touching the springs could imply acoustic feedback.     The way you would prove the point is to remove the Reverb tank from the enclosure the sit it on a pillow outside of the unit.

One way around that is to mount the Reverb tank on dampers.   The simplest form is to have the Reverb tank *loosely* mounted to the base of the amp with rubber grommets under.   Some units have rubber mats or foam pads.   Foam decays after a while so I don't recommend it.

Another solution to the feedback problem is to cut the bass on the reverb.   Say by decreasing the cap in series with the output of the recovery opamp.    Sometimes the feedback comes from very low frequencies outside of the frequencies of the guitar.  In this case you can also decrease the cap in series with the 560k on the input side to say 4.7n.     The two caps together doubly attenuates the vibration frequencies.

Things have now gone from bad to worse, as I lost power somehow and the amp's not turning on. Guess one of the fuses blew on the power supply board, as I don't have any power at switch no at a cable that runs to the main amp board.

You need to find at what point you lose supply.   Check all the fuses.   Also you need to check for shorts on the power.   A short may have developed somewhere.   A new problem completely independent of the mods.   These things happen.
If you wired you reverb board before the fuses and reverb wiring has a short it will kill the power even though the fuses are good, for example where the wires connect to the reverb board.

My guess is that it wasn't such a good idea to patch that power-board-to-main-board cable and take the power for the reverb from there, as it might reflect badly on what was happening within the amps circuit - only reason was to use the same switch and get the effect powered together with the amp and not go directly from the battery.Who knows, maybe that's the reason I got all those oscillations

Normally it doesn't cause big problems like that.   The shared power and audio grounds can cause oscillations.
In the end you need to work out the nature of the oscillations, or if it's an unrelated problem.

Step one would be get the amp up and running.    Perhaps temporarily remove the power going to the Reverb board.

[razabri]

Ok, got it running again - it was one of the pico fuses on the powering board. Didn't have the same one to replace, but temporarily used higher rated one (original was 4A, this one's at 5). I'm not even sure if I'll be able to find the same one, as it seems to me that there's limited offer here at our stores, will see...

Anyway, I'm not sure what caused it - perhaps I kind of exhausted it by switching the amp on and off all the time, as I couldn't really tell if there's a short anywhere and the reverb was connected after these, as the power from it was coming from the cable that sends power to the amp's board.

Separating the wires reduces electronic feedback.   Typically that is very high frequency.   You don't always hear the main oscillation but you hear a lot of weird side effects.

Another type of feedback is acoustic feedback.  The vibrations of the speaker transmit through the cabinet structure and get coupled into the reverb tank.   This type of feedback is similar to microphone feedback.   However you might experience much lower frequencies. 

The fact you can stop the oscillation by touching the springs could imply acoustic feedback.     The way you would prove the point is to remove the Reverb tank from the enclosure the sit it on a pillow outside of the unit.

One way around that is to mount the Reverb tank on dampers.   The simplest form is to have the Reverb tank *loosely* mounted to the base of the amp with rubber grommets under.   Some units have rubber mats or foam pads.   Foam decays after a while so I don't recommend it.

Another solution to the feedback problem is to cut the bass on the reverb.   Say by decreasing the cap in series with the output of the recovery opamp.    Sometimes the feedback comes from very low frequencies outside of the frequencies of the guitar.  In this case you can also decrease the cap in series with the 560k on the input side to say 4.7n.     The two caps together doubly attenuates the vibration frequencies.

Normally it doesn't cause big problems like that.   The shared power and audio grounds can cause oscillations.
In the end you need to work out the nature of the oscillations, or if it's an unrelated problem.

Step one would be get the amp up and running.    Perhaps temporarily remove the power going to the Reverb board.

Next, I'll try and use these details to see how to deal with this feedback issue. Guess I'll try and power it differently and see what happens, as it sounds most probable to me that that's the cause. Perhaps I can patch the unit directly to the battery and use a small switch to turn the effect on and off...

[razabri]

Got it all running up again - tried setting the powering differently, directly from the battery, but the feedback was still present, starting right behind the reverb splash. Also, it got sort of noisier, as maybe the unit needs certain amount of mA set for it to run correctly and it's maybe getting a "better" power via that patch cable between power and amp boards. I have then reversed it all as before, making the wiring even more tidy than earlier and I've managed to set it running pretty good - the feedback would only appear if the trimpot would be at highest value, but I don't even need it to run that high, at half I can get a decent amount of effect.

I was also thinking to place a switch for the effect, and have tried a simple on/off one, but I'm getting huge pop with it. Guess the simple switch cutting the effect's power won't do and I'd need a DPDT one to avoid popping.

[11-90-an]

If you want to use a switch, it *would* make more sense to use it to turn the output of the reverb on and off...

[razabri]

Thanks Nathan, I didn't even think of that one, since I got myself all worked up around powering... 

[Rob Strand]

To me it seems the oscillation is acoustic.

It's possible your reverb spring hasn't been engineered as well as the Accutronics or OC electronics tanks and it is more sensitive to vibrations.   If you look at the unit the internal part floats in springs.  Even those springs can affect things.

The power-ups might sometimes trigger the oscillation but that doesn't mean it's the cause.  A strong pluck might start things oscillating.

It's fine to knock the reverb level down to prevent oscillation.

Adding damping to the mounting points has to work but it's more a question if the practicalities of that are enough.   Decreasing the cap in series with   560k input resistor is also worth trying.  It's all about shaving off the problems.

You aren't the only one to have these problems,
https://www.amplifiedparts.com/products/grommet-rubber-reverb-tank-vibration

Driving the springs harder and knock the recovery gain down helps.   However, I'm pretty sure you are close to the drive limit on your unit.   You might be able to squeeze a small factor out of it by dropping the 560k to 470k or 390k.   However, when you push things this close you need to check the reverb spring isn't being overdriven with an oscilloscope.

[razabri]

To me it seems the oscillation is acoustic.

It's possible your reverb spring hasn't been engineered as well as the Accutronics or OC electronics tanks and it is more sensitive to vibrations.   If you look at the unit the internal part floats in springs.  Even those springs can affect things.

The power-ups might sometimes trigger the oscillation but that doesn't mean it's the cause.  A strong pluck might start things oscillating.

It's fine to knock the reverb level down to prevent oscillation.

Adding damping to the mounting points has to work but it's more a question if the practicalities of that are enough.   Decreasing the cap in series with   560k input resistor is also worth trying.  It's all about shaving off the problems.

You aren't the only one to have these problems,
https://www.amplifiedparts.com/products/grommet-rubber-reverb-tank-vibration

Driving the springs harder and knock the recovery gain down helps.   However, I'm pretty sure you are close to the drive limit on your unit.   You might be able to squeeze a small factor out of it by dropping the 560k to 470k or 390k.   However, when you push things this close you need to check the reverb spring isn't being overdriven with an oscilloscope.

Did the thing - went for 470k input resistor and the cap in series is now at 47n - this is a great combo! I do drive the springs more this way, but it seems like it's just enough so I can get more effect and reduce trimpot, thus canceling some more noise.

The feedback is no longer an issue and I think so too - that it was acoustic problem rather than electrical - I can get it with all the parts out and if I turn the tank in a certain position, but it doesn't appear by itself anymore. Now I only have something like tape saturation noise and it's probably just the way this was designed. I did place a switch cutting the output on the effect, so that I can have a clean tone without opening the amp, but it does still have a huge pop on the first hit. 

To me, it still remains a mystery of what that 3080 part of this circuit did and I'd kind of wish that there could be a way of finding out about it and using it too, instead of just bypassing it... 

[11-90-an]

To me, it still remains a mystery of what that 3080 part of this circuit did and I'd kind of wish that there could be a way of finding out about it and using it too, instead of just bypassing it... 

sWELLs

i think...

[Rob Strand]

so that I can have a clean tone without opening the amp, but it does still have a huge pop on the first hit. 

Good to see it's all coming together.

You need to add a 1M resistor from the output terminal  of the reverb to ground. (at the PCB connector)  This is permanently connected.  The switch then connects between the reverb output terminal and the amp input.

The reason you get the bang is the output cap on the reverb side isn't charged-up when the switch is off.  When you close the switch it charges up through the amp input and that charging current is the bang you hear.    The addition of the 1M resistor provides a path for the cap to charge when the switch is open.    Additional resistors to stop bangs are common place for audio.

[razabri]

so that I can have a clean tone without opening the amp, but it does still have a huge pop on the first hit. 

Good to see it's all coming together.

You need to add a 1M resistor from the output terminal  of the reverb to ground. (at the PCB connector)  This is permanently connected.  The switch then connects between the reverb output terminal and the amp input.

The reason you get the bang is the output cap on the reverb side isn't charged-up when the switch is off.  When you close the switch it charges up through the amp input and that charging current is the bang you hear.    The addition of the 1M resistor provides a path for the cap to charge when the switch is open.    Additional resistors to stop bangs are common place for audio.

Thanks Rob, but I've tried this and it didn't really work. At the reverb's circuit PCB I've made a connection between the out and the ground, sort of like when we placed that filtering cap of 39n over there, but the effect's intensity got way cut off and the pop was still there. I'm not sure if I understood you correctly... I'm using this simple switch, perhaps a DPDT would be better...

[11-90-an]

If you place that 1M pot and there's a difference, you might be putting the resistor wrongly...

Mind i ask for a pic of how you did it...?

[razabri]

To me, it still remains a mystery of what that 3080 part of this circuit did and I'd kind of wish that there could be a way of finding out about it and using it too, instead of just bypassing it... 

sWELLs

i think...

You're probably right - I suppose, since Rob did say that one of the pins that was related to this part of the circuit is a DC control for the 3080 that depending on the voltage let through that there would be swells, as in playing keyboard there would be some sort of charge released upon pressing the keys, and depending on how long you're holding the pressure, the swells would begin and stop upon releasing... Not sure how this would apply with playing the guitar, if I'm on a right track with this at all 

If you place that 1M pot and there's a difference, you might be putting the resistor wrongly...

Mind i ask for a pic of how you did it...?

There's no pot, I've only did what Rob suggested - tied the 1M resistor at output on the reverb's PCB to the ground at the same connector

This is just a temporary connection, to try it out. You can see the paper 39n cap that's filtering some noise at the back, tied to the same posts.

[Rob Strand]

There's no pot, I've only did what Rob suggested - tied the 1M resistor at output on the reverb's PCB to the ground at the same connector

Maybe it's not connecting properly.    It happens a lot when you poke the legs down, even though it looks good to the eye.

Other than that I can't see why it shouldn't work.   That *is* the way you fix this type of problem.
   

[11-90-an]

Wiggle it around?

[Rob Strand]

Wiggle it around?

Yes!

Unfortunately as soon as you get the cap charged up the pop isn't there anymore.

Maybe this would be more reproducable:
- close switch  (so the cap discharges into the amp's input resistor)
- turn off amp
- wait
- open switch (so the cap is hanging at power-up)
- turn on amp
- close switch.
   The cap would need to charge up.
   If it didn't charge-up through the 1M resistor then you should get a bang when the switch is closed.

[razabri]

I'm embarrassed to say that the issue was that I've read my resistor colors backwards, so I placed the 240Ohm resistor in stead of 1M and it was "sucking" the signal. Once I got the value right the thing actually works! The pop is almost entirely gone, but I do need to wait a bit for the cap to charge before I turn on the effect. This is not a problem at all, as to get rid of the popping was just not to stress the speaker when switching between clean and reverb.

So, got it all sorted! Oh, and the issue about the feedback was indeed due to acoustic issue - I think that it's vibrating was caused by the position of one of those four short springs on which the tank floats, as wiggling it around made it disappear. I assume that those would need to be a bit more loose, a bit longer, for the tank to hang more freely and so that there's no acoustic effect, passing the vibrations from the speaker to it.

Here are some pics...

[11-90-an]

Glad to see it working now. 

Hope you stick around too in this forum...