Smoking Wire in Relay Circuit :-(

[Gtr0]

Hello Folks.

I am in need of some heeeelp. I have a two channel amp I am building where I took the relay circuit from a Marshall Silver Jubilee... cropped section of the Marshall schematic here...





I have used this circuit a few times in different amps with success and though I am sure I can figure out something better with the help of Merlin's books and other various resources, this amp was lying around for a few years unfinished... and this was the circuit I installed in 2019. At this point, the main part of the switching circuit is in place, relays all wired etc, but I did not yet wire up an LED, a switch, or jack for footswitching. In other words at this moment there was no way to change the channels but EVERYTHING else is wired through the amp.

I decided to first power up and check the amp before going through the relay circuit. I powered up with a lightbulb limiter connected to a variac. Powered up slowly, checked voltages, everything looking good I did the same with tubes and a speaker load. At this point everything looks great (voltages etc) and sound works etc.

Powered down, bled caps etc... installed the two wires to the foot switch jack... started up again. Smoke stared to emanate from the wired connected to the SLEEVE of the jack. Here is a schematic I whipped up as it sits right now (including the voltages)...

Relay Circuit.pdf

This is exactly as it is laid out on the board as well.

I just unsoldered the wires to the jack in order to get the voltages.

I have gone over it literally a dozen times, unsoldering caps to check them with ESR meter and checking the polarity of the wiring etc to make sure it was right... I can not find the fault, so it leaves me to assume I have something wired backwards or wrong...

Can anyone spot my issue?

OH yea, also important - the power for the circuit comes from a dedicated winding on the transformer... 3.15 - 0 - 3.15 3A tap. In previous iterations, I used the heaters.

Thank you! AS always!!!!!!

[Dave H]

Remove the ground connection to the centre tap of the winding.

[Gtr0]

Okay, will do. I thought, and wondered about that because if I understand correctly, this circuit should be floating... but in other amps, tapping from the heaters, there was always a center tap on that winding, so I left this CT grounded as well.

Thanks!

[catalin gramada]

If so, please check if you jack did not touch the chassis. You can close a loop through the ground wire of power cord as well. Disconect the CT and trace with an ohmeter any shunt to the ground.

[Gtr0]

Remove the ground connection to the centre tap of the winding.

Done and works perfectly!!! Thank you!!

If so, please check if you jack did not touch the chassis. You can close a loop through the ground wire of power cord as well. Disconect the CT and trace with an ohmeter any shunt to the ground.

Yes, I made sure of that after disconnecting the CT. everything is absolutely perfect.

I love learning new things! Thank you both! Immensely!!!!

[g1]

Remove the ground connection to the centre tap of the winding.

I don't see why this would be required when he had the footswitch jack insulated from ground? The 2550 has the heater CT grounded.

Gtr0 , was the footswitch insulated from ground when the wire burnt, or did you insulate it later?

[Dave H]

I don't see why this would be required when he had the footswitch jack insulated from ground? The 2550 has the heater CT grounded.

As the wire was smoking I assumed the jack socket sleeve must be grounded.
I don't think it's a good idea to have the centre tap grounded and the jack socket floating as it puts voltage on the sleeve which will short half the winding if the sleeve contacts ground but Marshall did it as the didn't use a separate winding.

[Enzo]

Marshall used CLiff jacks, so the plastic body would not easily short to ground.

[catalin gramada]

Marshall used CLiff jacks, so the plastic body would not easily short to ground.

It is a DIY project. And more we don't know where is plugged. If had a jack mounted on a pedal board grounded supplied from the wall a short is possible through earthing wire of the power cord.
The issue comes after OP installed the jack to chassis and the wires to the footswitch (the circuit works fine without it as he said)...so a elementary reasoning direct the judgement to a grounding problem and Dave H spot it right.

[Enzo]

I am aware of that, just defending Marshall.

[Helmholtz]

I agree that grounding the jack instead of the winding is the better way.
But even with a separate transformer, the relay circuit should not be left floating to avoid possible radiated interference.
When the heater winding is used as in the Marshall, grounding one point is even mandatory.

[Gtr0]

Sorry about the late response... I didn't receive an email notification... seems it does this on the fist replies only.

At the time the wire was smoking, I didn't even have the foot switch plugged in. I am using an Orange single foot switch I had lying around.

It is certainly possible that the jack is ground via the chassis... I used those switchcraft isolation shoulder/washers but it is very possible I slightly undersized the hole and forced a contact. It wouldn't be the first time I have done that.

[Enzo]

Folks love their Switchcraft jacks, but I don't trust the fiber washers at all. You can get one cocked out of place, and sure as shit someone eventually will leave the washers out when reassembling it. Back to my Cliff jacks. I always use a Cliff-style jack when something ought not be grounded for that very reason.

[catalin gramada]

Folks love their Switchcraft jacks, but I don't trust the fiber washers at all. You can get one cocked out of place, and sure as shit someone eventually will leave the washers out when reassembling it. Back to my Cliff jacks. I always use a Cliff-style jack when something ought not be grounded for that very reason.

Put two spacers fiber shoulder washers on both side of the hole helps to sleep well. Usually problems comes when are used in mix: shoulder spacer and flat.