Series vs Parallel FX loop

[hesamadman]

I have been wondering about different fx loops. In a blues jr, the reverb is installed via a parallel fx loop. I was curious how this might change the reverb if it were a series loop. Or if the 220k resistor was a pot. Would that make the signal a little more wet or dry?


http://el34world.com/charts/Schematics/files/fender/Fender_blues_jr.pdf

[SILVERGUN]

I have been wondering about different fx loops. In a blues jr, the reverb is installed via a parallel fx loop. I was curious how this might change the reverb if it were a series loop. Or if the 220k resistor was a pot. Would that make the signal a little more wet or dry?

Theoretically, it would be ALL wet because the whole signal is going into the reverb circuit in series and then off to the power stage. So 100% of the signal is going through the reverb..but it's an unclear approach because it doesn't address the rest of the circuit and how that reverb circuit was designed into that amp.
 
Think of it like this:
Series loop isn't actually a loop as much as it's a break in the dry signal chain where the entire signal goes through whatever effect you apply and there is no mixing of the wet and dry signal other than what the effect offers. Once the signal goes through the series loop it is as wet as the effect made it.
 
Parallel loop is where a portion of the dry signal goes through a parallel signal loop while the other portion stays dry and unaffected, and then the two signal paths are summed back together (typical Fender reverb application)

[hesamadman]

I have heard of some FX not acting the way they should in a parallel circuit. That's part of why I asked. Also a reason I ask is I installed a solid state reverb from a blues Jr. in a 2204 preamp with an ac30 power amp. I like the reverb. It's ok. But I can't help but wonder what differences it would make if I broke that loop and made it series. Or if it would change anything. Maybe I should just try it. 

[jjasilli]

I was curious how this might change the reverb if it were a series loop.
+1 to SG.  Also, a typical reverb pedal is an example of a series FX circuit.  And, in a parallel circuit the design should be that wet signal should be in phase with the dry when re-mixed.




Or if the 220k resistor was a pot.
Maybe that resistor, R42,  needs to be in balance with the surrounding circuitry.  It looks like a 1M (1000K) pot wired as a voltage divider (See, TP 21) could replace R43 & 44.  It looks like such a pot might function as a dwell control.  Don't know how much useful range it would have.

[2deaf]

A typical reverb pedal can be inserted into the amp's series loop, but the pedal itself is an example of a parallel FX circuit.  They always have a mix control for the two parallel signals whether it is labeled as such or not.

[SILVERGUN]

I have heard of some FX not acting the way they should in a parallel circuit.

Here's an extreme example...
Let's say you want to run a pitch shift pedal so that you come up with a 100% pitch shifted tone.
 
If you run it through a series loop and set the pitch shifter for 100% wet mix, then you will wind up with a 100% pitch shifted sound.
 
If you run it through a parallel loop, then you will wind up with a 100% effected signal mixed with the dry signal at whatever ratio the parallel loop has been set up to provide...so it might come across as sounding 30% dry and 70% effected
 
Mesa Boogie (and others) use a MIX control to alter that ratio.

[jjasilli]

A typical reverb pedal can be inserted into the amp's series loop, but the pedal itself is an example of a parallel FX circuit.  They always have a mix control for the two parallel signals whether it is labeled as such or not.

Good point.

[Fresh_Start]

As I understand it, the primary problem with parallel effects loops is that some pedal designers are not careful about ensuring that the return signal is in phase with the "dry" signal.  IIRC there are some amps which have a phase reversal switch for their parallel effects loop so that the wet and dry signals don't cancel each other out.

Respectfully,

Chip