Could A Pedal Cause An Amp To Cut Out?

[Hexjibber]

As the title says really, is this possible?

I recently built an OCD/Fuzz War dual pedal for a friend which works fine, however when he tried it at rehearsal whenever the pedal was in his chain it seemed to make the amp head cut out after a few minutes. The amp (a Blackstar HT-50 tube head) would come back on again and work fine after a few minutes of being powered off and once the pedal was no longer in the chain, we also tried it through a JCM900 and the same symptoms happened.

The pedal worked fine on my practice amp when I was testing it and also worked fine on my mate's practice amp at home, this problem only seemed to occur at rehearsal through a tube head.

I'm not sure if something else could be the culprit, perhaps the pedal power source (one of those cheap wall adapter daisy chain things) or even dodgy wiring in the room but I wanted to I suppose eliminate or confirm the possibility of the pedal being the cause, the signs certainly seem to point towards it!

Any ideas?

Cheers,
Graham

[Johan]

It could be a leaky output cap on your build, causing charge up on the grid, turning the tube off...?...a solid state amp would probably have a cap on it's input wich is why it didnt happen at home (assuming your practice amp is solid state....)
J

[R.G.]

It could be a leaky output cap on your build, causing charge up on the grid, turning the tube off...?

It could be a leaky output cap on your build, causing charge up on the grid, turning the tube off.

(note difference in punctuation    )

[Hexjibber]

Thanks for the replies guys, I will have to get the pedal from him to check the caps, is it something I could test for using a DMM or should I just swap the relevant caps out on the OCD and Fuzz War boards?

I ended up having to wire the pedal a bit unusually and am also wondering whether this could be a cause, below is how I did it, hope it makes sense! The black wire from 'OCD In' to ground on 'Switch B' was the only way I could get the pedal to work without the OCD producing a high pitch oscillation when in bypass/Fuzz War was selected, I tried many other things and wiring configurations believe me! Could this also perhaps be a cause of charge up on the grid? Apologies as my understanding of these things is limited, this pedal has caused me no end of bother!

[GibsonGM]

I don't have time to look at the diagram (I'm wicked tired from work, LOL!) but I am suspicious of how you have that LED wired...take your DMM and measure for DC at your pedal's output.  Plug a cord into input AND output, and measure for DC at the far end of the output patch cord, if that makes sense....you should have right about ZERO DC there if your output cap is ok.   
Anything else is not good, remove LED and if still present, put in a new cap...

[Hexjibber]

I don't have time to look at the diagram (I'm wicked tired from work, LOL!) but I am suspicious of how you have that LED wired...take your DMM and measure for DC at your pedal's output.  Plug a cord into input AND output, and measure for DC at the far end of the output patch cord, if that makes sense....you should have right about ZERO DC there if your output cap is ok.   
Anything else is not good, remove LED and if still present, put in a new cap...

Thanks a lot for the reply, really appreciate it! I will give that a try when I get the pedal back from my mate.

The bi-colour LED wiring is a little odd in that it's basically always on and the switch position just changes the colour, this was the only way I could wire it up and still ground the OCD's input on bypass to kill the oscillation at the same time. Is there any chance that the oscillation I was battling against initially with this build could have been the fault of a leaky output cap which in turn is causing this DC issue?

[GibsonGM]

Actually, now that I've had a minute to check the switches, it looks ok to me.   I doubt your output cap could have much to do with an oscillation problem.   Dunno the OCD personally, but it's a high gain pedal then you need to look at in/out shielding and power supply decoupling. Often, they'll do that if you use a wall wart but not if you use battery - you could try a batt. to see. 
Grounding an input shouldn't do anything to a following stage, tho...

Before I call any pedal *DONE*, I check for DC on the output as a cautionary last step.   Usually if any is found, it will be slight due to leakage from a crappy/cheap leaky cap, .5V or so.  Even that is enough to annoy me tho not too 'worrisome' or anything.  If it's running to a tube amp, it *could* affect the bias of the input stage, so I will change it.

A good cap does not pass DC.  You should just get a 'fantasy garbage reading' on your DMM, like "122mV   50mv   12mv .776 mv "  telling you whatever static/remaining charge is quickly bleeding off.  That would = a spurious reading that means nothing.

[Hexjibber]

Dunno the OCD personally, but it's a high gain pedal then you need to look at in/out shielding and power supply decoupling. Often, they'll do that if you use a wall wart but not if you use battery - you could try a batt. to see. 

Hi Mike, thanks for taking a look at this, I'm at a bit of a loss with it myself!

Regarding the oscillation it only ever occurred when the OCD and Fuzz War were linked via the selector switch (switch B), when wired independently both ran quiet as a mouse. The jumper wire I added to switch B was the only thing that worked out of the many things I tried.

I will have to wait until I get the pedal back to the DC on output but will report back my findings when I do

Cheers,
Graham

[Hexjibber]

Hi Mike,

Finally got this pedal back from my mate and measured for DC on output but got a zero reading so it doesn't appear to be that.

I'm really stuck with this build and can't really give it back to the guy knowing it could damage his tube head. Is there anything else I can try to find out why it would be making an amp cut out?

It's very frustrating as on the face of it the pedal works fine.

Feeling pretty despondent if I'm honest!

[Will.mendil]

Maybe you could post a complete schematic with the voltage values of every components. Mate you will find an odd one out.

[GibsonGM]

Sorry if i didn't read it somewhere above, but how are you powering the pedal - battery or adapter?   Very odd problem, esp. where you say it's done this to TWO tube heads?   

Is the pedal in the middle of the chain?  The very end?  ...    Still need more info, like Will said above this.

[Hexjibber]

Sorry, I was feeling a bit grumpy about the whole thing yesterday! Am trying to re-approach with a more positive attitude!

Maybe you could post a complete schematic with the voltage values of every components. Mate you will find an odd one out.

Ok I will give that a go, might take me a while though!

Sorry if i didn't read it somewhere above, but how are you powering the pedal - battery or adapter?   Very odd problem, esp. where you say it's done this to TWO tube heads?   

Is the pedal in the middle of the chain?  The very end?  ...    Still need more info, like Will said above this.

The pedal was being powered with an adapter, one of those daisy chain ones so perhaps there is an issue there? All his other pedals work fine with it though. As I say, we tried the same set up through two different tube heads (a Marshall JCM900 and a Blackstar HT50) and they both cut out, only coming back on again after a few minutes of having been turned off. The issue did not re-occur once the pedal was removed from the chain. It was a few months ago now, apologies I can't remember whereabouts in the chain it was.

Unfortunately I don't have a tube amp set up at home to test it on but I am planning to take it to another studio with a different set of amps to see if I can replicate the issue, just hoping I don't end up wrecking a studio amp in the process! :S

Combined with this problem and the oscillation issue I had to begin with this pedal has given me sleepless nights! Just want to see the back of it now!

Thanks for your help though guys!

[Hexjibber]

Had another thought, could any of this be related to power supply decoupling (or lack of)? I've been doing some reading about it and I didn't really pay much attention to the power side of things when I hooked the circuits up, both +9V connections from the boards are just going straight to the + DC jack, could this be a problem? I did have some resistors between the +9V DC connection and board +9V at one point in an attempt to combat the oscillation but it didn't make any difference.

Here are the schems for the two circuits, the Fuzz War has a 100uf going to ground before the +9V pad so I presume would be ok but I'm not sure about the OCD?

http://pedalparts.co.uk/wp-content/uploads/2013/04/DisorderDrive.pdf

http://pedalparts.co.uk/wp-content/uploads/2012/12/rawfuzz-v1.pdf

[GibsonGM]

The schematics on both look ok, at a casual glance.   We could assume that others are building these, and having success, so....our problem is with OUR build or wiring.        Tip: If you put a resistor (say, 100R to 680R) between 9V and board to combat oscillation, follow it with a cap between + and -, of maybe 470 to 1000u. 

Yes, if there were a problem with the power supply, you'd expect the trouble you describe, but if you measure no DC, then either you're not measuring it right, or it's not there!  Sounds like you are very sure it's in the heads, as they take time to 'recover', huh?

Can you hook the chain/double box up again with no amp, play hard, and measure the AC output too?    Could it be that a way high output is driving your tubes into cutoff/grid clipping to such a degree it frazzles 'em?     I've had weird things like this happen, kind of....I've made my fairly limp PT sag because of too much input.   I wonder.....not likely since it's 2 heads, but.....the 'staying screwed up and coming back later' part really has me bugged.

Another thought...all your outlet grounds are ok?  Using the same circuit for the amps as well as the power supply?    Not that there should be any consequences with respect to the power supply.... (?)

[Will.mendil]

I think that if the problem only occurs when the pedal is in the chain on two different heads. The problem must come from the pedal.
Couldn't it be a short somewhere. Maybe you should, when you get the chance to test on an amp, try to use the pedal out of the enclosure. And if the issue still occurs, try puting the pedal on a bread board with new components and test it. I am just saying what I would try out. However, I would be worry that I could damage the amp, well at least the tubes.

[Hexjibber]

I think my first port of call is going to be using the pedal with some different tube amps in a different room to where the problem originally occurred, that way at least I can definitely single out the pedal as being the cause or not as the case may be. My pedal power supply is a Carl Martin Pro Power so I am at slight disadvantage in that if it is a power supply issue it probably won't happen with the Carl Martin as the outputs are all isolated and it's of a much higher quality than the wall wart daisy chain thing my mate uses.

I've also built quite a few pedals since this one and my off board wiring has much improved since then so I am also considering starting from scratch on that front as it's a bit of a mess in there at the moment and there are some places where components could be potentially causing a short due to proximity to the jacks etc so I think I may also re-house it and lay it out a bit better this time.

Mike - I will try measuring for AC output later this evening and let you know my findings.

[Pojo]

Finally got this pedal back from my mate and measured for DC on output but got a zero reading so it doesn't appear to be that.

To be sure, I'd recommend checking the output caps to each circuit individually. Clip the negative lead on your DMM to ground and on the OCD clone, put the positive lead at the junction of C11, R12, and R13 and see if any DC is present there. Next, check the Fuzz War by keeping the negative DMM lead at ground and putting the positive at pin 3 of the volume pot.

Definitely a strange issue and I hope you get it sorted! The only thing worse than having 'phantom' issues in a build is when it happens to a build for someone else! Best of luck!

[Hexjibber]

To be sure, I'd recommend checking the output caps to each circuit individually. Clip the negative lead on your DMM to ground and on the OCD clone, put the positive lead at the junction of C11, R12, and R13 and see if any DC is present there. Next, check the Fuzz War by keeping the negative DMM lead at ground and putting the positive at pin 3 of the volume pot.

Definitely a strange issue and I hope you get it sorted! The only thing worse than having 'phantom' issues in a build is when it happens to a build for someone else! Best of luck!

Thanks man, I will give that a go later when I get home, I only measured for DC on the tip and sleeve of the patch cable coming from output and also the output lug of jack the other day.

[bwanasonic]

Are you 100% sure it's the amp cutting out and not something in the signal chain?

[Hexjibber]

Are you 100% sure it's the amp cutting out and not something in the signal chain?

It's definitely the amp cutting out as we were using the pedal and signal was coming through fine, then after approx. 2 mins, nothing. The first thing we did after it died was plug the guitar directly into the amp, checked various different leads and there was still no signal. We thought the head had gone altogether but after leaving it turned off for a few minutes we turned it back on and all was fine, provided the pedal was not in the chain. I didn't attribute this to the pedal to begin with but it happened with two different heads, same symptoms.

This only happened however when using a tube head, the pedal has been tested for 30+ minutes on two separate solid state amps with no problems whatsoever.