Fender amp Footswitch intrigue.

[Moroccomoose]

I have a Fender Blues Delux reissue amp and I am rather puzzled about how the foot switch works.

It has two switches, one for reverb on/off and one for channel select. Yet the foot switch cable is only a TS 1/4" jack, only one ground and one signal. Unlike my marshall foot switch, also reverb on/off and channel select which has a 1/4" TRS cable which will have one ground and 2 signal.

How on earth does the Fender footswitch work?

Cheers,

Stuart.

[Sam Spoons]

The switch and jack connection is shown in squares B & C, 7 & 8 https://www.thetubestore.com/lib/thetub ... -Rev-A.pdf

Basically, as far as my limited electronics knowledge allows, AC is supplied to the switch which half wave rectifies the AC, the polarity set by the individual switches, and the output determines whether the reverb or channel is switched. I would like to understand how this works if anybody is happy to explain?

[Hugh Robjohns]

Essentially, instead of using three separate wires carrying two DC control voltages plus a ground reference, it uses two control wires (core plus screen) carrying an alternating voltage.

So the switch socket on the amp outputs an AC voltage which is rectified via diodes in the footswitch (and the amp) to create either a positive or negative voltage pulses (or both). These are routed to comparators that detect them and create the required internal switching signals.

[BJG145]

Clever. I’m surprised by the design, but I guess there’s a cost saving. (?)

[Sam Spoons]

Essentially, instead of using three separate wires carrying two DC control voltages plus a ground reference, it uses two control wires (core plus screen) carrying an alternating voltage.

So the switch socket on the amp outputs an AC voltage which is rectified via diodes in the footswitch (and the amp) to create either a positive or negative voltage pulses (or both). These are routed to comparators that detect them and create the required internal switching signals.

Thanks Hugh, I guess I'd already got that far and was curious how the electronic switching works. No signal, +ve, -ve and AC are the possible states, I'll do a bit of reading when I have time to work out the rest.

[Hugh Robjohns]

The output of the reverb comparator (U3A) controls an FET which shunts the reverb return buffer's output to mute it.

The output of the channel comparator (U3B) drives relays to switch the channels.

[siderealxxx]

Not wishing to thread hijack but I was thinking about this the other day on my Fender Twin 65 Deluxe. I've always had problems with that amp (it was B-stock) but it settled down after countless tube replacements.

However, the spring reverb has recently packed up and it's not the phono connectors or anything obvious. I remember a while back I plugged in the Fender foot switch and got an almighty load of noise from it in operation. I think since then it's been defective.

I wonder if anything from the foot switch could have caused anything to backfire down the line? It could be that the tube has gone, though I replaced it with some old spares and that didn't work, need to try a new one.

[Sam Spoons]

If you've tried several different valves it's probably not the valve causing the issue. Which amp do you have, a '65 Twin Reverb (85 watt) or a '65 Deluxe Reverb (22 watt though I suspect they both have the same reverb circuit)?

[Wonks]

No point looking any further than the reverb tank until you’ve put a multimeter on the input and output connections and checked for an open circuit. More likely to be on the output than the input to the tank. Or open it up to check for broken wires.

[siderealxxx]

If you've tried several different valves it's probably not the valve causing the issue. Which amp do you have, a '65 Twin Reverb (85 watt) or a '65 Deluxe Reverb (22 watt though I suspect they both have the same reverb circuit)?

It's the 65 Twin Reverb (85w).

[siderealxxx]

No point looking any further than the reverb tank until you’ve put a multimeter on the input and output connections and checked for an open circuit. More likely to be on the output than the input to the tank. Or open it up to check for broken wires.

I do have a multimeter, but in all honesty I have no idea how to use it or what to look for! I don't really know anything about electronics :/

I can open it up but it's one of the modern ones which is predominantly PCB. The valves I swapped aren't really to be trusted either so a new one won't go amiss.

Not had time to look at it tbh but it's on my list...

[resistorman]

It's almost always the tank

[Wonks]

The reverb tank in your amp is the rectangular-shaped bent metal thing that’s normally in a vinyl ‘bag’ at the bottom of the cabinet, below the speaker.

You’ll probably have to remove the bottom rear panel to access it, and the tank and/or bag will be held in place by four short screws.

Note which way round the tank goes (i.e. which side the RCA connectors are on) and which RCA plug lead from the amp goes to which connector on the tank (you’ll provably find a ‘shielded’ cable on the output and a single cable on the input).

If you physically put the tank back the other way round you’ll often get a lot of hum from the reverb tank as the output (IIRC) transducer is nearer the amps transformers and this weak output signal from the tank picks up hum from them which is then amplified significantly when fed back into the amp circuitry.

Best to take reference photos as you go along.

With the RCA leads pulled off and tank out of the amp (and bag if there is one), you’ll see the two RCA connectors on the side of the tank.

With the multimeter set to resistance/ohms, measure between the inner and outer parts of each RCA connector.

The input RCA should have a DC resistance in the order of 1 ohm between the inner and outer parts and the output RCA a resistance in the order of 200 ohms.

If you haven’t got an autoranging multimeter, you’ll need to set the resistance range accordingly. Probably 200 ohms for the input and 2k ohms for the output (my meters have these ranges but I’m sure others will offer alternative ranges so pick a range that covers the expected value allowing for some variation in actual resistance).

So if you get an open circuit reading from one of the terminals, you know there’s probably a broken wire inside the unit. An open circuit will be indicated by the value displayed when the probes aren’t touching anything. This could be ‘——‘ or ‘ovld’ orc something else depending on your meter.

With a manual range meter it’s worth stepping up the resistance ranges a few steps just in case you get a reading, but anything substantially over the nominal values will indicate a problem.

Anything significantly below the nominal values could also indicate a potential fault, though a dead short is unlikely unless there’s a solder error from the factory.

[Moroccomoose]

Aha I see, clever use of AC. It sounds like a very complicated way to make a switch. It has been suggested further up the thread, it might be motiviated by cost saving but are there any sonic advantages? quietness of the switching or less suscetpable to radio interference or something like that? I suppose one advantage is if you are a gigging musician, and the cable is faulty, you only need guitar lead, it is easier to replace or you are more likely to have a spare than if you needed a TRS cable.

[Folderol]

It's almost always the tank

... and almost certainly one of the very thin wires from the coils to the RCA sockets come adrift at the socket. That's a well known stress point where the most flexing takes place.

[Hugh Robjohns]

... are there any sonic advantages?

There are no sonic or technical advantages. If anything, it's a more expensive and complex way of doing it.

But it does have the advantage of only needing a standard guitar cable, rather than a TRS cable.

[Wonks]

It's almost always the tank

... and almost certainly one of the very thin wires from the coils to the RCA sockets come adrift at the socket. That's a well known stress point where the most flexing takes place.

The Accutronics tanks take the thin wires from the transducers directly out to the RCA sockets, making them very vulnerable. The Belton equivalents use dedicated wires to the transducer so are more robust.

[Folderol]

It's almost always the tank

... and almost certainly one of the very thin wires from the coils to the RCA sockets come adrift at the socket. That's a well known stress point where the most flexing takes place.

The Accutronics tanks take the thin wires from the transducers directly out to the RCA sockets, making them very vulnerable. The Belton equivalents use dedicated wires to the transducer so are more robust.

Interesting. I don't think I've seen the Belton ones - mind you, it's been a 'little' while

[Moroccomoose]

So I recently 'won' a G System on ebay - so far very pleased with it. It has relays built in for amp channel/reverb switching which will obviously not work with the Blues Deluxe Reissue.

However, an internet search shows some people are suggesting a custom cable to connect the FBDR to a conventional foot switch using a TRS at the foot switch end and and TS a the Amp end. There is a diode in the tip-tip line and another diode in the other direction going ring to tip. and the shield to shield connected as normal.

Can the electronics bods shed light on whether I should trust this solution and go and buy the parts to make a custom cable?

Cheers

[Wonks]

This needs a different solution to the one in this recent thread...

https://www.soundonsound.com/forum/view ... hp?t=94182

...as the amp uses a different switching circuit.

https://irationaudio.com/wp-content/upl ... ev-a-1.jpg

The footswitch circuit is shown on the bottom left.

The amp will be sending out an AC signal on a TS jack, and depending on the footswitches selected, it will see either no signal, just the positive half, just the negative half, or the full signal (though slightly modified by the voltage drop across the diode).

So yes, you can do it with diodes. You 'should' only need a single diode as you won't need the LED indication. But the footswitch wouldn't use two diodes if all it needed was the 'rectification' function so it may need the extra volt drop that two diodes in series give to work reliably.

The plain diode is a 1N4448 (as it's not labeled otherwise on the schematic so comes under 'all diodes are'). The indication LEDs are red ones but no make is specified (not a more modern high light output version though). Current limiting resistor for the LEDs is back in the amp.

It looks like a single diode per channel works, but one with a higher forward voltage drop than the 1N4448 (which is between 0.6 and 0.7v) might be prudent in order to mimic the footswitch circuit. The whole switching circuit is fed from 48v ac, though is dropped down from that by various resistors, but still best to stick with a higher voltage rating (meaning a bulkier diode) to be sure of dealing with any voltage that may present itself. The 1N4448 is rated at 100v.

Which means it may be easier to stick the diodes in an external box rather than try and fit them in a jack plug housing.

[Moroccomoose]

So eventually, I made a sniffer cable to rectify the switching signal.

I bought a cheap balanced extension cable with a female 6 35 TRS on one end and a male TRS at the other.

I bought a 6 35 TS, and the diodes. soldered the 2 diodes into the tip, one the opposite way round. Cut the male connector from the cable and soldered in my moded TS in its place.

So now I can use an ordinary foot switch for the channel and reverb switching by connecting it to my sniffer cable, then connecting that to the amp. More importantly, I can now use my TC Electronics G System to do the switching.

The only gripe is that it is the opposite way around to my Marshall switching, so that means either you have the channel switching the reverb and reverb switching the channel .... or.... have it switching the correct things but off is on and on is off!

Perhaps if I put the diodes the the sleeve instead of tip, it might all be the correct way around... not sure... and not gonna bother trying! A quick reconfig of the relay settings in the G system and all is golden!

I know for many of you, this is little more than a quick routine soldering job... for me, this is a huge achievement!

[Martin Walker]

Bravo Moroccomoose - glad you got something that works for you (quirks aside)

[ef37a]

Yes, as Hugh says that Fender switcher is more complex and expensive to implement than a TRS cable and trigger some FETS, transistors or even relays and yes, they might have done it to enable the owner to just use a standard TS lead. More likely though IMHO is that they were avoiding a patent fight? It's not unknown for a firm to be contacted by a lawyer to the effect...|"Oi mate! You can't do that, my client holds the patent"

There was a multifunction wired remote for a VHS VCR years ago which I always thought very clever. The remote was the size of a thick credit card and a single screened cable terminated in a 3.5mm mono jack. The system worked from a resistive "ladder" whereby pressing different buttons sent a different voltage back to the machine and a wee chip decoded that and sent commands to the transport.

Dave.