NFB switch in 50w Marshall mutes signal

[Randy Lahey]

I've installed a NFB on-off-on switch in my 50w Marshall clone as per Rob Robinette's amp mods.  Both the "on" sides function as advertised, however, when I switch to the middle "off" position (which I assume applies no NFB to the PI), there is no signal out of the speaker.  The signal is muted which is not the desired effect.

Would anyone mind explaining what could be the issue and possible solutions to this problem?

Thanks very much!

[Williamblake]

Well, no you didnt. And what is your circuit?

[sluckey]

Wiring error.

[PRR]

> Well, no you didnt. And what is your circuit?

13 seconds on a crummy wifi in a crummy motel at the NH shore:

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#3-Way_Negative_Feedback_Switch

> Wiring error.

+1

[acheld]

Post a picture of your wiring, and we may be able to assist.   

This is likely a wiring error.   We've all made them.   

[sluckey]

I've installed a NFB on-off-on switch in my 50w Marshall clone as per Rob Robinette's amp mods.  Both the "on" sides function as advertised, however, when I switch to the middle "off" position (which I assume applies no NFB to the PI), there is no signal out of the speaker.  The signal is muted which is not the desired effect.

I don't see any Marshall specific NFB switch mods on Rob's site. However, this is a very simple mod and is easily adaptable to any amp. I can't imagine any wiring scenario that would work normally in either "ON" position but mute the amp when in the "OFF" position. You should provide more info and some hi-rez pics of your mod.

[PRR]

> I don't see any Marshall specific NFB switch mods

Am I out of town here? In the crummy motel I saw:

3-Way Negative Feedback Switch

This is my favorite and most useful amp mod, a three-way SPDT ON/OFF/ON mini-switch gives you: Normal feedback / No Feedback / Heavy feedback. The 'No Feedback' position makes the amp break up early kind of like a 5E3 Deluxe. The Marshall JTM45 was a direct copy of the 5F6A Bassman but the feedback signal was tapped from the 16 ohm output transformer secondary instead of the 5F6A's 2 ohm secondary resulting in the JTM45 getting 2.8 times more feedback. The extra feedback of the Heavy position makes the cleans cleaner and tightens up the boundary between clean and overdrive which can make it easier to control breakup with finger technique. It also makes the amp's overdrive tone cleaner, tighter and more Marshallesque.

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#3-Way_Negative_Feedback_Switch

Yes it is a Fender but clearly traced to the Marshalls.

[mresistor]

Never mind  sorrY.

[Randy Lahey]

Thanks everyone for their input.  PRR, that’s exactly the link and schematic I was going to reference.  I’ve wired the NFB switch (or attempted to) as in that schematic.  Would different component values or presence control wiring affect the intended operation of the NFB switch?  It has a standard long-tail phase inverter. That’s kind of where I was going with the question originally.  The amp itself is at the rehearsal space at the moment.  I will review my wiring and take photos as soon as I can.

Thanks again for everyone’s input.

[PRR]

> Would different component values or presence control wiring affect the intended operation of the NFB switch?

Why speculate? Much better to show what you actually built (may not be what you thought you built).

Silence on center may be a major mis-wire. But there's many way to do that.