Jet City FX Loop puzzle

Haych

So, I know about the whole line level and instrument level thing with the Jet City FX loop but I came across a weird thing the other day when messing around with various pedals in the loop of my JCA50H.

Most pedals I have seem to work ok in the loop - to be fair they're all quite modern digital pedals so I'd expect that they would, however, one - a Carl Martin Surf Trem - doesn't work at all in the loop, and I mean it causes the amp to mute whether or not it's turned on or off.

Not that it's a real problem as it works fine in front of the amp, but I just wondered why this might be the case.  AFIAK it's a true bypass pedal so even if it doesn't work when switched on I'd have expected it to be ok while it's switched off?  It's a serial loop right(?) so should just send the signal back into the return when it's off?

Just curious, that's all.

TIA

ICBM

If it's really true bypass then that's impossible unless there's a fault with one of the cables or jacks - even connecting the pedal backwards won't stop the signal in bypass.

So I would guess it's not true bypass. Even then the only thing I can think of that would actually mute the signal with the effect on is if you connected it backwards.

Haych

It gets weirder!

So, the pedal IS true bypass - connected in front of the amp and turned off the power, signal still passes through. 

Still wasnt working in the FX loop so checked that I had it connected up right and it was indeed ok. Just for the hell of it I deliberately connected it backwards and it started working (in bypassed mode) for a couple of seconds before cutting out and going mute on me. 

Then I reconnected everything back the right way and it’s now working perfectly in the loop! Go figure!

I’ve checked all the cables and can’t find a thing wrong or any indication as to why it wasn’t working or why it now is. 

Odd or what!

ICBM

Haych said:

So, the pedal IS true bypass - connected in front of the amp and turned off the power, signal still passes through.

That doesn’t prove it’s true bypass, just that it’s not buffered - it could be ‘half bypass’. But for this test it would still work.

It is all very strange though!

Haych

Half bypass? That’s a new one on me! But yes, I agree, really very strange. 

Thanks for your interest and input, though, @ICBM. ;

ICBM

Haych said:

Half bypass? That’s a new one on me!

'Half bypass' - sometimes known as "half-arsed bypass" - is the old method where only the output side of the circuit is switched, leaving the input connected to the effect circuit and relying on the relatively high input impedance of the circuit not to suck tone... or not too much. Not ideal, and even worse if you have several pedals which are all turned off, since the impedances are effectively in parallel and you *will* get substantial tone suck.

This is the normal way vintage wahs and other pedals with SPDT switches, and later pedals with LEDs and DPDT switches are wired. (But not Rats, which use a clever true-bypass circuit with a DPDT.) It is, fairly obviously, the worst of all types of bypass. The 'passes signal in bypass test' has unfortunately become a popular way to tell if a pedal is true bypass, but a half-bypass pedal will do so too, so the only thing it tells you is if the circuit is not buffered.