Footswitch pedal cable induced volume reduction

[WhopperPlate]

I have an amp that sounds and works beautifully. It’s basically a two channel SSS#5 with overdrive and preamp boost. Relays switch channels, engage boost, and engage overdrive . Everything works as designed …

Problem is whenever I plug in the foot switch pedal into the jack on the rear panel I get a slight headroom and volume drop and I have struggled to discern the source of the issue. It’s like the difference between a full and half power mode , feel changes and headroom reduces .

After breaking things apart I have found that as long as current is flowing in the ground return wire on the cable then the amp becomes choked . All it takes is a connection to complete the loop. When the pedal is connected there are always LEDS receiving power , so disconnecting the ground wire eliminates the problem. Grounding the shield of the cable dies not help.

This only occurs with the footswitch . The amp has no issues switching from the toggles in any way. Fwiw The cable length I am using isn’t any different than my previous builds ;, 15ft.

Has anyone else experienced such an issue before?

The chassis is a fender ab763 layout, similar to the MHartman SSS layout .

63FBFC76-6357-4C4E-95C1-D0DDB5A13688.jpeg

the relay wiring LED and return wiring is zip tied tightly and running by themselves along the rear corner of the chassis. Basically ala Dumble with the limiting resistors mounted within the amp. Relay is powered from extra 6.3 tap on pT.

I have never had an issue doing it this way , but after having this issue and sleuthing for answers I am assuming there is some lead dress, oscillation issue within this long path across the rear and this particular circuit.

One reason for this consideration being that I did happen to notice this sss layout seen here that has the relay boards relocated centrally and the wiring physically far away from the back panel besides the path to the foot switch jack .

EBA70D32-7348-49D6-A515-BE4FF90563DA.jpeg

Is there a rationale behind this beyond a convenient and available mounting location for the relay power supply ? Obviously I am just trying to find anywhere to proceed searching to figure this out . It’s such a weird issue in an otherwise quiet and clean and amplifier .

Appreciate any bones thrown , thanks y’all

[bepone]

I dont understand the question,

But in general, switching relays need to have isolated transformer winding , and running to the output fsw jack.
grounded like every other supply node, to chassis in only one point.

If you have 6.3V heater winding , rectified for relay powering, and you put that on the output jack which is not isolated, then every time when you engage the fsw, you are creating short circuit on the 6.3V heater line, but after rectification. so not good.

[WhopperPlate]

I dont understand the question,

But in general, switching relays need to have isolated transformer winding , and running to the output fsw jack.
grounded like every other supply node, to chassis in only one point.

If you have 6.3V heater winding , rectified for relay powering, and you put that on the output jack which is not isolated, then every time when you engage the fsw, you are creating short circuit on the 6.3V heater line, but after rectification. so not good.

I am sorry for being verbose and confusing.

Let me spell it out better:

1) relays are powered by separate 6.3 vac winding tap off the transformer . Heater filaments have their own separate winding.

2) the whole relay system is grounded together at the chassis near the relay power supply board mounted near the pilot light; typical relay switching arrangement used with extra voltage taps on the power transformer

3) ground return and LED wires are ran along the rear of the rear of the chassis zip tied together to relay ps board.

Question: why would a volume / headroom drop occur when plugging in the footswitch?

4) this only occurs with footswitch cable inserted into jack . The jack is isolated from ground. Fwiw Attaching the cables shield to ground does nothing, not that I expected it would , I am just sleuthing

[martin manning]

Question: why would a volume / headroom drop occur when plugging in the footswitch?
...
4) this only occurs with footswitch cable inserted into jack . The jack is isolated from ground. Fwiw Attaching the cables shield to ground does nothing, not that I expected it would , I am just sleuthing

Not much to go on. Can you measure any change in supply node or plate voltages when the issue occurs? This might give a clue as to what area is being affected.

[bepone]

hmm ther should be some error, fsw cable has some capacitance but is only for relay coil, not signal.. signal is on the moving contacts , not on the cable..

[WhopperPlate]

Not much to go on. Can you measure any change in supply node or plate voltages when the issue occurs? This might give a clue as to what area is being affected.

I know I know…I hesitated inquiring here because I can’t measure any differences anywhere and I don’t have much info to provide . All supply and plate nodes retain their idle voltages . I scoped a guitar signal and couldn’t discern any differences , but I will revisit that this afternoon to confirm those observations with an actual tone generator . My phone headphone adaptor broke right as I was going to test the thing on my way to get replacement now .

hmm ther should be some error, fsw cable has some capacitance but is only for relay coil, not signal.. signal is on the moving contacts , not on the cable..

I know I know … it makes no sense to me…

[WhopperPlate]

So I scoped the amplifier. There’s no discernible difference on voltages or waveform whatsoever.

What I am hearing is very subtle , so it’s hard to hear when it’s getting better or worse, but and on closer listening I find grounding the shield of the footswitch cable does help. I have gone through and added a few more zip ties on the bundle to keep them tighter together, and I can hear a slight improvement here as well.

I am thinking that the AB763 platform is so snug with the socket proximity to the rear of the chassis that this spacing is problematic , that or maybe the grid wires to the power tubes are possibly more sensitive with the cathode follower grid driver . I may shield and shorten them as much as possible to further limit any possible interactions .

Having never had this issue previously , I am wondering is this why the relay wiring is laid out as so in other SSS with switching, as I posted above . Maybe I should post this in Dumble thread and ask whether or not anyone who’s built a SSS with relays had any such issues and how they ran their wiring to the jack.

[Stevem]

Give more detail if you can.

Are you more hearing a slight change in tone or high end then a overall change in volume?

If so then I would say your issue is more due to interaction in the wiring .

[Reeltarded]

It seems to be induction. You can detect where like this.

Ground a spatula to the preamp ground (smaller is better) (I use one made for clay from an art store) and cover it in electrical tape. Now all you do is hover close to the suspected problem area in both conditions for a clue about where the field is generated. You can then make a simple shield or attempt a different routing.

I have frequently created multi-layered shield with heavy foil and black tape to cover the shielded multipair wiring in several devices. (and used horrible holiday print duct tape at least twice)

[WhopperPlate]

It seems to be induction. You can detect where like this.

Ground a spatula to the preamp ground (smaller is better) (I use one made for clay from an art store) and cover it in electrical tape. Now all you do is hover close to the suspected problem area in both conditions for a clue about where the field is generated. You can then make a simple shield or attempt a different routing.

I have frequently created multi-layered shield with heavy foil and black tape to cover the shielded multipair wiring in several devices. (and used horrible holiday print duct tape at least twice)

Don’t you know it! I have essentially been doing this exactly yesterday after my last post . This is how I have been sniffing out trouble areas , which is leading me to further conclude that you are right and it’s induction picked up at the output grids . V1 and v2 input tubes are surprisingly insensitive , though I do hear small improvements overall when shielding there as well.

What I am thinking I will do is remove the current LED and return ground wires and replace them with some shielded category 5/6 type cable. Practically easier than building makeshift shielding throughout the amplifier, and will look cleaner , but it sounds like you have even further shielded those in your builds with positive results … whatever it takes I suppose lol

Give more detail if you can.

Are you more hearing a slight change in tone or high end then a overall change in volume?

If so then I would say your issue is more due to interaction in the wiring .

Again, I think you are right . At first it was a dramatic volume difference . We are talking the dynamic difference between 50 and 100 watt. With the adjustments made it has mellowed into a slight compression and attenuation of highs and lows. Hard to hear . I will upload some YouTube clips later for reference so everyone can hear what I am hearing .

[WhopperPlate]

I made these before the adjustments. It’s better now.

No pedal:

https://youtu.be/Q7jWZGpE2JE

Pedal:

https://youtu.be/UVeN4P9FVak

[Reeltarded]

Hah! It cleaned it up in a LNFB sort of way.

[WhopperPlate]

Hah! It cleaned it up in a LNFB sort of way.

You said it better than I could with all my extra words . I just pulled out the old bundle and installed a cat 5 shielded cable . Will post update when I get the chance to test.

[Reeltarded]

That tells the story though. Think about it for a minute.

I think I would fly that wire high above the board all the way against the headbox shield.

[WhopperPlate]

That tells the story though. Think about it for a minute.

I think I would fly that wire high above the board all the way against the headbox shield.

Might have to try that . This is essentially the point if my initial inquiry concerning the SSS layout pictured. Most leads that would come into close proximity in my layout never have the chance in that layout . Deductive reasoning left me considering this fact in relativity.

IMG_0805.jpeg

The Shielded CAT5 cable from relay PS to footswitch jack , albeit looks nice and clean when installed, did not provide the results I am seeking.

Pretty tight fit as is so I am unsure how to relocate the cable per se, but at this point even just relocating the cable outside for diagnosis purposes is the next step as far as I can see .

Will post update then.