# strange reverb issue that has stumped several techs (magnatone content)



## jeancoltrane (Feb 20, 2015)

so i thought id post this here in the off chance any one has encountered this before. it seems like there are a lot of good techs here and i havent had any luck fixing this issue despite a lot of money and time invested. i am not a tech myself, so my skills are limited to tube swapping and some trouble shooting abilities. i am basically hoping for advice i might be able to bring to a future tech. 

i have a magnatone m20 http://www.magnatoneamps.com/M20.html (schematic available here) that has a reverb feedback issue. with the reverb on, randomly it will feedback on a low B or Bflat note. it will even happen with with nothing plugged into the amp (no input signal). once it starts happening, you can turn the reverb off and it will stop, but if you turn it back up again it will slowly start self oscillating into the low b flat even without playing anything. its a "head" so no speakers near the tank, even with the speakers 15 ft away and no input signal it will happen. ive even taken the tank out of the amp and moved it as far away as possible to double check, and with no input it will still start self oscillating. id say it is more likely to happen once the amp has been warmed up for a while, but it is seemingly completely random and i cant induce it at will. sometimes it will happen everytime i turn on the amp, other times it wont happen for months (its been several months since it has gone full blown self oscillation but you can still hear it teetering on the edge of doing so) all tubes have been swapped multiple times, and reverb tubes swapped multiple multiple multiple times. the reverb is also extremely noisy, with lots of hiss and low hum. im also pretty sure i have a bad ground on one of the input jacks that connects to a tube right before the reverb circuit - in the schematic, it is the bass input jack. if i turn the bass input jack channel up, i get a very loud hum until i insert a patch cord halfway in. could this have something to do with the reverb issues? 

any thoughts at all? thanks..


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## dtsaudio (Apr 15, 2009)

Wow, that's a hugely complicated guitar amp. 
My first thought is capacitor C1 (possibly C2, 3) in the power supply. The 12AU7 driver for the reverb my not be properly decoupled from the power supply and you're getting some motorboating. The other thing that strikes me as odd is the size of the cap coming off the reverb driver tube. 1ufd is huge for this type of circuit. Must be a really low impedance tank.
The hum from the bass input is probably a bad jack. However it may lead to something else. If that jack is corroded around the ground, what other grounds are corroded. Check all the ground connections.
Has anybody re-capped this amp?


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## nonreverb (Sep 19, 2006)

Which begs the question: Is the tank original to the amp?



dtsaudio said:


> Wow, that's a hugely complicated guitar amp.
> My first thought is capacitor C1 (possibly C2, 3) in the power supply. The 12AU7 driver for the reverb my not be properly decoupled from the power supply and you're getting some motorboating. The other thing that strikes me as odd is the size of the cap coming off the reverb driver tube. 1ufd is huge for this type of circuit. Must be a really low impedance tank.
> The hum from the bass input is probably a bad jack. However it may lead to something else. If that jack is corroded around the ground, what other grounds are corroded. Check all the ground connections.
> Has anybody re-capped this amp?


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## jeancoltrane (Feb 20, 2015)

thanks for the replies! you're right, it is an insanely complex beast and I have had several techs not want to take it on (i dont blame them)
the power section was recapped about a year ago, as well as replacing the molex connector and a few preamp caps/resistors originally to solve this problem as well as some others - but the reverb issue, hum hiss & feedback still persisted afterwards. i believe the tank is original, as it looks quite old and the surround insulation is basically disintegrated. if i remember my troubleshooting steps correctly (it was a while ago i last tried to sort this), the feedback even happened without the tank plugged in. the reverb tone is quite unique, kind of dull and more like a plate reverb than any ive heard, i dont know if that has to do with the large reverb cap. i believe it is a hammond style tank. 

my thought, since the reverb feedback note is the same note as the hum from the badly grounded jack, that perhaps the hum from that bad ground is working its way into the circuit somehow and self oscillating? but i dont know if that makes any sense. 

thanks again for the replies.


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## StevieMac (Mar 4, 2006)

I'm NOT a tech so if this comes off sounding moronic then...so be it. Are you absolutely certain the tube in the 12AU7 slot is indeed just that? Again, probably sounds dumb but a tube with considerably more gain in that position e.g. 12AX7 could conceivably cause problems. If it's the right tube in the right spot then...as you were. Good luck with it either way.


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## jb welder (Sep 14, 2010)

Here's the direct link to the schematic: http://www.magnatoneamps.com/schematics/magnatone_m20.pdf

You will need to verify that it is the right tank. Someone could have replaced it with something different 40yrs ago and it would look old :smile-new:. If there was something you did that made the problem occur without the tank, you will have to repeat that and verify 100%.
This problem has eluded solving so there is no room for guessing or speculation, you must verify.
With the tank disconnected, what do you measure for resistance at tank input and output connections?


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## mhammer (Nov 30, 2007)

Reverb tanks are electro-mechanical devices. The pĥysical structure of the reverb pan has its own resonances. One can easily imagine that a small amount of residual hum - not enough that it would bug you as a player - provides a tiny physical shaking of the pan, through the cab, which then eventually begins to self-oscillate.

I can suggest two potential solutions:
1) Try to physically isolate the pan from the cabinet/chassis a little more. Some nice soft silicon rubber washers or spacers should help.
2) While it won't stop the low-frequency vibrations of the cab that might micro-jiggle the pan, blocking some of the low end in the reverb signal might curb the self-oscillation. If you have the schematic (I can't see pics if you posted any), try and identify the first cap in series with the reverb out going to the recovery stage, and cut the value by half.


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## jeancoltrane (Feb 20, 2015)

thanks for the replies. i have definitely verified it is a 12au7 in the reverb slot  i'm not sure how to verify if its the right tank, unless there are specs for it in the schematic that i cant see (or understand). theres not much info on this amp at all as only about 20 were made, even the schematic is a relatively recent development. my friend has an m15 that is similar and i could compare the two tanks, but the reverb controls on this amp are different (and dont know if it would require/use a different tank, the m15 is a solid state reverb return i believe) - 

"There are two reverb controls, one for each "speaker channel" as labeled on the control panel, however both use the same reverb tank (each control adjusts the input level into the tank independently, and those controls are buffered with triode stages -- really pretty slick)" - per website. 

i will try to repeat the problem with no tank, but as said i cant induce the problem willingly so it could take, weeks, months, or minutes for it to occur to verify. i'll be jamming with a friend later who is a bit more handy with this stuff and see if he can measure the resistance of the tank - but i have paid a tech already to check the reverb tank, and he re soldered the reverb connections and couldnt find any issue. i will say the problem used to be way more common , and has somehow started occurring less and less despite no further work being done on it. but the reverb still doesnt seem to decay properly, after its on for a while it just sits at a backround wash/hum level that doesnt drop until you turn it off, teetering on the edge of full blown very loud low hum feedback. compared to my friends magnatone 'verb, mine is VERY noisy/hummy. i can also say this problem did not exist when i got the amp, it used to be very very quiet, the quietest amp ive ever owned, now it is among the noisiest despite a recap. its for this reason i dont think the wrong tank is the issue because im assuming i wouldve had this problem all along if that were the case. i guess it is possible one of the many benches its been across replaced the tank, or messed something up along the way.

edit: i have definitely isolated the reverb from the speakers as much as possible, it is a head and i set the speakers up in a separate room with no guitar attached and it still produced the feedback (even with no playing)


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## mhammer (Nov 30, 2007)

Which might suggest a microphonic tube. It happens.


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## dtsaudio (Apr 15, 2009)

> Which might suggest a microphonic tube. It happens.


Very possible.

On the schematic it shows two footswitch jacks each at the input of the reverb drive tube. I assume for the two channel reverb. Do either or both of the footswitches make a difference. The answer to that may help narrow this down.
Also coming out of V9 there appears to be two 1M pots wired in series on the grid of V10. Are they external or an internal adjustment?

On a different unrelated note. Looking at V6, V8 there is a symbol, looks like a parallelogram with blackened corners and white strip. What is that?


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## jeancoltrane (Feb 20, 2015)

i have about 20 12au7s that i tried, 7 or 8 were completely brand new and made no difference in the noise, hum or feedback, several did sound better though  

that is an interesting point about the foot switches, as i didnt get them with the amp. could a bad jack on the footswitch inputs be causing the issue? hmm.. i guess i can try popping a few patch chords in them to see if anything changes.

thanks again, please forgive my lack of knowledge.


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## dtsaudio (Apr 15, 2009)

The jacks probably aren't the problem as they are an open circuit when using the reverb. The footswitch shorts out the feed to that particular triode thus stopping the reverb. Turning the level down should accomplish the same thing. My point here though is that each half of the drive 12AU7 takes signal from different places in the circuit. If we know which is causing the problem it will help localize it a bit.


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## mhammer (Nov 30, 2007)

I can't see the diagram posted, but the mention of parallelograms suggests the component in question is a varistor, which Magnatone used for their vibrato effect. It is usually drawn as a parallelogram.


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## dtsaudio (Apr 15, 2009)

Doesn't look like a varistor or at least it's a strange place for one (actually 4)


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## nonreverb (Sep 19, 2006)

Looks like some kinda diac on drugs...


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## greco (Jul 15, 2007)

nonreverb said:


> Looks like some kinda diac on drugs...


Now I see what you mean...I had never heard of a "diac" before.


Cheers

Dave

*DIAC*

From Wikipedia, the free encyclopedia


_For other uses, see DIAC (disambiguation)._

DIAC Typical DIAC voltage and current relationships. V[SUB]BO[/SUB] is the _breakover voltage_. TypeActivePin configurationA1, A2Electronic symbol  
The *DIAC* is a diode that conducts electrical current only after its breakover voltage, V[SUB]BO[/SUB], has been reached momentarily. The term is an acronym of_diode for alternating current_.
When breakdown occurs, the diode enters a region ofnegative dynamic resistance, leading to a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode. The diode remains in conduction until the current through it drops below a value characteristic for the device, called the _holding current_, I[SUB]H[/SUB]. Below this value, the diode switches back to its high-resistance, non-conducting state. This behavior is bidirectional, meaning typically the same for both directions of current.

Three-layer DIAC​
Most DIACs have a three-layer structure with breakover voltage of approximately 30 V. Their behavior is similar to that of a neon lamp, but it can be more precisely controlled and takes place at a lower voltage.
DIACs have no gate electrode, unlike some other thyristorsthat they are commonly used to trigger, such as TRIACs. Some TRIACs, likeQuadrac, contain a built-in DIAC in series with the TRIAC's gate terminal for this purpose.
DIACs are also called _symmetrical trigger diodes_ due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or _main teminal_ MT1 and MT2.


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## epis (Feb 25, 2012)

These are varistors.

http://www.magnatoneamps.com/FMvibrato.html


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## dtsaudio (Apr 15, 2009)

Odd place for a diac as well. 

Since I'm the one that de-railed this thread, I'll try and get it back on track.

Looking at the two reverb signal circuits, the right hand drive tube takes it's signal from a much different point in the circuit, up near those unknown parts. It also looks like the is feedback around the cathodes of those tubes. I'm wondering if there is a bad coupling cap, or open resistor and instead of getting the correct signal, it is out of phase and is turning the reverb amp into an oscillator. Might also account for some of the noise too.
A scope would be needed to track it down though.
Long shot guess.


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## greco (Jul 15, 2007)

mhammer said:


> *I can't see the diagram posted*, but *the mention of parallelograms suggests the component in question is a varistor*, which Magnatone used for their vibrato effect. It is usually drawn as a parallelogram.


You never cease to amaze me!

Cheers

Dave


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## mhammer (Nov 30, 2007)

Nothing particularly amazing. We've had a great many discussions about vibrato in amps over at the stompbox forum, so I already knew they were in there.


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## Church-Audio (Sep 27, 2014)

I think its time to take it to a tech, I have worked on lots of maggies. Including the m15 very similar to your model. They are complex and need a ton of tlc to get working right. It could be anything as some have mentioned caps are always suspect on these amps. I have had some of these where almost all of the caps inside were leaky. So its really just going through it point by point until you find the issue. That can also include replacing the reverb cables. As I have seen some that had issues with them. Oxidation on the rca on the tanks jacks is quite common as well. Give me a shout if you want to get it fixed up correctly.


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## jeancoltrane (Feb 20, 2015)

thanks for the discussion everyone, trying to keep up  the problem occurs with either reverb knob/channel. also switching between mono/stereo doesnt seem to have any effect on it. and funny enough when i did a search a few years back for "magnatone" repair in ontario chris churchs name was the first that popped up from fixing that m15, i really wanted to bring it to someone that was already familiar with them but unfortunately i was living several hours away at the time and went with a few local options. 

and i know im in way over my head and need a tech in the near future, but its nice to hear suggestions and heck maybe ill even learn something  2 blown speakers and fixing my reel to reel in the past 2 weeks has depleted my funds, but hopefully the magnatone will be appearing on your bench this summer chris. would be cool to update this thread with the repair notes then!


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## jeancoltrane (Feb 20, 2015)

Well it looks like I may have more serious issues. I've blown 2 speakers recently and chalked it up to bad luck, until I just put in the new one and it sounded blown instantly as well- very distorted, low output etc. I would normally think pre amp or power tubes but I'm not sure if that would actually blow speakers. The weird thing is on channel one, set mono or stereo both speakers sound fine. Channel 2, set mono or stereo both speakers sound bad. The other strange thing is both blown speakers show no rubs, signs of voice coil overheating etc but test bad on a voltmeter. I've read how parasitic oscillation can cause speakers to blow and bad/thin/harsh sound in general. I've also read bad power tubes, phase inverter can cause this, probably among many other things. No fuses have blown etc so I'm kinda leaning towards something else, and maybe in ties into the reverb problem. I know I need a tech ASAP but any thoughts as to what could be blowing speakers etc is appreciated.


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## jb welder (Sep 14, 2010)

What kind of voltmeter test are you doing that makes you call the speakers "blown"?
If you can set up the amp in any manner where the speakers sound ok, then they are ok. The problem is elsewhere.
Have you tried the speakers with some other amp?


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## jeancoltrane (Feb 20, 2015)

it was a friend who tested them for me, apparently they were reading very low or no ohms, plugged in they sound awful and even with the amp volume cranked they produced the tone & volume of a battery radio. i shouldve been more clear, i have 2 blown speakers, and 2 that are good - the 2 good ones produce a sound that i would call blown when plugged into channel 2/output 2, but sound fine on channel 1. just trying to figure out what could cause 2 speakers to blow within 1 week at normal volumes. obviously there is a problem with channel 2, but i wasnt aware that amp problems could potentially take out speakers until now.


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