SVC20C and JHS little black amp box

Plexi4Ever

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Hi all, I'm new here. If i'm posting in the wrong group then my apologies. I have some questions about the SV20C and using a JHS little black amp box as a volume control in the FX loop. I've played Marshall amps most of my life and i have the SVC20C since a week now. I want to use the JHS pedal as a sort of master volume to control the output of the FX loop into the Output Tube stage. This works generally fine, i can control volume much better and the sound is still great with the normal and treble channel dials cranked up. I've noticed a strange thing though. Occasionally the output volume spontaneously drops to a much lower level and sometimes to almost off. I then have to turn up and down the volume control on the JHS box to restore the volume. It appears to happen randomly and i can't really explain. It almost looks like there is some kind of signal limiting circuit active that randomly tunes the output down. With the FX loop off i have no problems. All cables are brand new and checked. The Questions i have : Did someone else also notice this or similar behaviour ? I this a amp thing or a JHS thing ?. The JHS is a passive box (most likely just an expensive potentiometer in a pretty enclosure) but it seems to behave like there is some kind of an active circuit inside. Anyway i thought i post here to see if i'm doing anything wrong before i call the service desk of the shop where i bought the JHS box. Thanks !
 
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Michael Roe

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Sounds like it could be a bad cable or a loose connection in the JHS. Try using both cables ( one at a time ) in the loop. In other words, just plug one of the cables from the send to the return of the amp. If it does it with just the cable in the loop then it is the cable or possibly the amps FX loop. Also, if you have another amp with an FX loop then try the same cables and JHS in it and see if it does the same thing.
 

Plexi4Ever

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Thanks Michael. I see what you mean. I will try it later to see if i can rule out the cables from the equation.
 

purpleplexi

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The JHS box pot is really sensitive. Even if you move it a fraction it can affect the volume dramatically. If you're sure that's not happening could be a cable or maybe a tube.
 

SkyMonkey

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I don't think there is anything fancy going on inside the LBAB. I run one in the loop of a non-MV VS65R and have experienced none of the problems you describe. But mine is firmly stuck down on a pedalboard.
 

paul-e-mann

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Hi all, I'm new here. If i'm posting in the wrong group then my apologies. I have some questions about the SV20C and using a JHS little black amp box as a volume control in the FX loop. I've played Marshall amps most of my life and i have the SVC20C since a week now. I want to use the JHS pedal as a sort of master volume to control the output of the FX loop into the Output Tube stage. This works generally fine, i can control volume much better and the sound is still great with the normal and channel dials cranked up. I've noticed a strange thing though. Occasionally the output volume spontaneously drops to a much lower level and sometimes to almost off. I then have to turn up and down the volume control on the JHS box to restore the volume. It appears to happen randomly and i can't really explain. It almost looks like there is some kind of signal limiting circuit active that randomly tunes the output down. With the FX loop off i have no problems. All cables are brand new and checked. The Questions i have : Did someone else also notice this or similar behaviour ? I this a amp thing or a JHS thing ?. The JHS is a passive box (most likely just an expensive potentiometer in a pretty enclosure) but it seems to behave like there is some kind of an active circuit inside. Anyway i thought i post here to see if i'm doing anything wrong before i call the service desk of the shop where i bought the JHS box. Thanks !
You could just send the JHS back for another one easy enough to rule that out, if the problem continues its easy enough to replace the cables, if it continues then you know its the loop and send the amp back.
 

Benny2112

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I am using the same setup with the SV20H and JHS LBAB in the effects loop. I have not experienced any volume drops whatsoever. It's the only thing I have in the loop, so I have the LBAB attached with velcro on top of the head towards the back with a couple short cables. Sounds like you definitely have a problem somewhere. I would do as @pedecamp suggested and try to narrow down where the issue is coming from.
 

scozz

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Have you played the amp for a while without the LBAB, to be sure it’s the box and not the amp? I would think that’s the first thing to determine.


Welcome to the Marshall Forum @Plexi4Ever.
 

Plexi4Ever

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Thanks for your support everyone ! i appreciate it. From the answers i get it's not something structural with either the JHS or the amp. I will try with different cables and will also try a straight through connection between the send and return first with the FX loop on. Will let you know about my findings. it might take a while because as i mentioned the issue happened randomly. We'll see what happens. i also got an attenuator on order that i can fit between speaker and the amp output. Wanted to see what works best.
 

marshallmellowed

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Thanks for your support everyone ! i appreciate it. From the answers i get it's not something structural with either the JHS or the amp. I will try with different cables and will also try a straight through connection between the send and return first with the FX loop on. Will let you know about my findings. it might take a while because as i mentioned the issue happened randomly. We'll see what happens. i also got an attenuator on order that i can fit between speaker and the amp output. Wanted to see what works best.
I agree that it sounds like a bad cable connection or faulty/dirty volume pot on the box. The best way to troubleshoot this type of issue is to try things one at a time. The key, is to catch it when you get the volume drop. If it happens again, try this...
  • Very carefully rotate/wiggle the cable going into the "Send" jack on the amp, taking care not to disturb the other connections.
  • Very carefully rotate/wiggle the cable going into the "Return" jack on the amp, taking care not to disturb the other connections.
  • Very carefully rotate/wiggle each cable going into the volume box, taking care not to disturb the other connections.
If the problem persists, repeat above steps, only this time un-plug and re-insert each cable.

If this doesn't reveal a bad cable connection, slowly turn the knob on the volume box and see if the volume is restored.

This step by step approach will help isolate the problem, rather than randomly trying things.
 
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Michael Roe

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Thanks for your support everyone ! i appreciate it. From the answers i get it's not something structural with either the JHS or the amp. I will try with different cables and will also try a straight through connection between the send and return first with the FX loop on. Will let you know about my findings. it might take a while because as i mentioned the issue happened randomly. We'll see what happens. i also got an attenuator on order that i can fit between speaker and the amp output. Wanted to see what works best.
Your post got me wondering so, I took my black box and opened it up. There is a pcb board with plastic jacks and a pot soldered directly to the board. I noticed on one of the solder joints on mine that it did not look really well flowed. I haven't had any issues but if I did, I would definitely re-flow that solder joint first and see if that cured the problem.
 

Plexi4Ever

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Thanks Michael, Meanwhile i think i found something. All cables are allright, no loose or bad contacts. No issues with the jacks on the amp too. no bad contacts, no noisy connectors or jacks when you move or turn connectors. I verified them all. All is brand new. @purpleplexi mentioned that the volume control on the lBAB is very sensitive. He is right, the volume control is indeed very sensitive particular at low volumes. It only takes a minor touch to reduce/increase the volume significantly. When i was playing i had the LBAB sitting on top of the AMP vent roster. And while i was playing the amp was slowly warming up and also warming up the LBAB a little bit. Nothing extreme, just warm to the touch. I moved the LBAB away from the amp to a location where it would not be affected by a change in temperature. Played for 45 minutes and the problem seems to be gone. So what might have happened is that the increasing temperature caused the potmeter's resistance to drift a little. And perhaps only a little change in that current setting was enough to change the volume quite a bit. That's theory #1. The second theory is that vibrations caused by the amp sound might have affected the potmeter too. It was really in a position that a small change would have a big effect. I will experiment a little more to see if i can find some more proof for this. That solder joint you are showing indeed doesn't look too good. Probably it's good enough to not cause an issue now but a joint like this could be sensitive to cracks in the future imo. Thanks for opening your LBAB up to have a look. I already suspected it was just a Potmeter and nothing else inside. On another note : The LBAB seems to have a clearly defined input and output. It doesn't become clear by looking at the enclosure and as a matter of fact you can switch inputs and outputs and the LBAB does work in both configurations. the 1 page manual with the LBAB actually says that the input is on the side where the pictogram is. Learned a thing or 2 today ;-) Many thanks all, Great forum !
 

Benny2112

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Hopefully that is all it was. I definitely wouldn't keep it on top of the vent. I have mine attached with velcro to the back left corner. It's far enough away from the vent that it doesn't get hot and the velcro keeps it from moving around.
 

WellBurnTheSky

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On another note : The LBAB seems to have a clearly defined input and output. It doesn't become clear by looking at the enclosure and as a matter of fact you can switch inputs and outputs and the LBAB does work in both configurations. the 1 page manual with the LBAB actually says that the input is on the side where the pictogram is.
Well, that's because the LBAB literally is just a pot and 2 jacks inside a 1590A box. And depending on how you wire it, a pot is just a variable resistor (you could alternatively wire it as a voltage divider, wiring lug 3 of the pot to ground, in which case switching the input/output jacks would reverse the way the pot works, ie turning the pot clockwise lowers volume).
Mine usually sit on my pedalboard (I've actually built 2, plus a "dual volume box" that allows me to switch between 2 settings), so I've never had the problem, but glad it seems to be fixed now.
 

Old Punker

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Thanks Michael, Meanwhile i think i found something. All cables are allright, no loose or bad contacts. No issues with the jacks on the amp too. no bad contacts, no noisy connectors or jacks when you move or turn connectors. I verified them all. All is brand new. @purpleplexi mentioned that the volume control on the lBAB is very sensitive. He is right, the volume control is indeed very sensitive particular at low volumes. It only takes a minor touch to reduce/increase the volume significantly. When i was playing i had the LBAB sitting on top of the AMP vent roster. And while i was playing the amp was slowly warming up and also warming up the LBAB a little bit. Nothing extreme, just warm to the touch. I moved the LBAB away from the amp to a location where it would not be affected by a change in temperature. Played for 45 minutes and the problem seems to be gone. So what might have happened is that the increasing temperature caused the potmeter's resistance to drift a little. And perhaps only a little change in that current setting was enough to change the volume quite a bit. That's theory #1. The second theory is that vibrations caused by the amp sound might have affected the potmeter too. It was really in a position that a small change would have a big effect. I will experiment a little more to see if i can find some more proof for this. That solder joint you are showing indeed doesn't look too good. Probably it's good enough to not cause an issue now but a joint like this could be sensitive to cracks in the future imo. Thanks for opening your LBAB up to have a look. I already suspected it was just a Potmeter and nothing else inside. On another note : The LBAB seems to have a clearly defined input and output. It doesn't become clear by looking at the enclosure and as a matter of fact you can switch inputs and outputs and the LBAB does work in both configurations. the 1 page manual with the LBAB actually says that the input is on the side where the pictogram is. Learned a thing or 2 today ;-) Many thanks all, Great forum !

@Plexi4Ever - I have one of these JHS pots too and I had experienced the same issue in the past with two different amps. In fact, I knew what your 'problem' was before I got half way through your post. Like you said, any small vibration or induced movement can cause the LBAB's setting to shift a very small amount, so small that it's not even discernable to the naked eye. The result is that you can go from normal playing volume to almost zero volume instantly.

I keep mine on a 'pedal board', actually just a wooden board, on the floor. In my case, just brushing up against the LBAB while setting up or maybe dragging an instrument cable across the board can cause this slight movement. Now when I hit the first chord on my guitar and nothing comes out of the amp, I move the JHS pot dial slightly higher and it's usually the culprit.

I doubt there is anything wrong with your amp or cables. I don't know what the taper is on the LBAB but it seems that for those of us playing at low volumes at home, the setting we use is probably right on the border of the 'no sound' zone. Mine generally sits around 8:00-8:30.

Other than that I think this cheap little box is pretty handy. I can get pretty decent tones while using it and I don't need to run my attenuator as much.

:cheers:
 

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