Fender 75 effects loop

[JMMP]

Does anyone know why any pedal, chorus and/or delay for instance, put through this amps single TRS loop is completely distorted? Sounds like an impedance mismatch or the out level is too hot. I've heard of some people having issues with this amps loop, is there a fix or? Funny thing is I can plug the Y cord in and short the tips, the signal cleans up and sounds normal, but when any pedal is put in the loop the level drops and it sounds distorted. Using standard Boss chorus and delay pedals that work perfectly into the front of the amp. Poked around the circuit a bit and compared some components measurements to the schematic and nothing seems out of spec. Is there something wrong with this loop by design or am I missing something? Any help appreciated.

[pdf64]

The design is bad, the signal level is ridiculously high at any normal control setting; regular pedals in the loop will tend to clip the signal.
There's nothing much that can be done about it without a total re-design of the pre-amp.
http://schems.com/schematicheaven.ne...r_75_schem.pdf

[Leo_Gnardo]

Does anyone know why any pedal, chorus and/or delay for instance, put through this amps single TRS loop is completely distorted? Sounds like an impedance mismatch or the out level is too hot. I've heard of some people having issues with this amps loop, is there a fix or? Funny thing is I can plug the Y cord in and short the tips, the signal cleans up and sounds normal, but when any pedal is put in the loop the level drops and it sounds distorted. Using standard Boss chorus and delay pedals that work perfectly into the front of the amp. Poked around the circuit a bit and compared some components measurements to the schematic and nothing seems out of spec. Is there something wrong with this loop by design or am I missing something? Any help appreciated.

That loop was intended for use with studio level devices, not guitar pedals. Easy solution? Use guitar pedals with guitar inputs. If you have studio effects, generally rack mount, handle nominal +4 dB signals, they belong in that 1970's era Fender loop. "How do I know?" Read the owner's manuals. Just about everything's on the interwebs these days, so "I lost it" and "Didn't get one when I bought it" is no excuse.

Since then some amps have been released with optional +4/-20 switches, or level controls that make it easier for the loop to work with pedals but often at the cost of increased noise because it typically takes a large signal on the return side to drive the amp's power amp input. And mixing guitar pedals with line level effects always makes for a mess, you have to have a collection of attenuators and buffer/booster preamps. Those who have a lot of $$$ can spend it on Bradshaw, Cornish or similar FX switchers that can handle anything.

[pdf64]

As the loop is taken without attenuation from the CF buffered output of a CC plate 3 stages in, I think that the signal level at the fx loop insert point can reach >60Vrms; so more like +40dB than +4!

[JMMP]

Yea, that would explain it. Thank you guys for explaining it, I don't want to re-engineer this guys amp. What a useless feature, no?

[Leo_Gnardo]

As the loop is taken without attenuation from the CF buffered output of a CC plate 3 stages in, I think that the signal level at the fx loop insert point can reach >60Vrms; so more like +40dB than +4!

Unless there's something magic about that cathode follower, it can't deliver every volt the plate can. When I was doing a lot of adding FX loops, I found the CF would start clipping hard, furthermore one side of the waveform, at a fairly low level still safe to run +4 effects. If you hear distortion, that's it, and back off the drive from the FX loop output. OTOH it often took a good +11 signal to drive an amp's output to clip. Most +4 rated effects can actually deliver well in excess of that nominal rating. +20 is common and some run to +26. It was still tricky to get FX loops to work well without some attenuators & boosters so it was a good thing to see some 80's-90's manufacturers build in loops that are friendly to pedal fx. My rule of all-thumbs is, use guitar pedals with guitars and run 'em into the amp input as intended, and use (most) effects loops with studio level effects.

[Jazz P Bass]

" Funny thing is I can plug the Y cord in and short the tips, the signal cleans up and sounds normal,"

This is confusing.

Have you set up a test of V4A?
A: do you have +62Vdc on the cathode test point?
B: do you measure 600mv ac signal at the Grid test point?
C: do you measure 500mv ac at Ring test point?

fender_75 FX Jack.pdf

fender_75.zip

[Randall]

"What a useless feature, no?"

Actually no. There was a time when rack effects were all the rage and many units had +4/-10 switches. Pedals are designed to see input from a guitar pick up which is tiny compared to a pre-amplified line level signal that is coming out of a FX out jack. And guitar pedal outputs are designed to feed the input of a preamp tube, which again is tiny compared to the line level signal that FX return jack is looking for. So really, this is apples and oranges.

[pdf64]

I think that the max output signal from the CF would get limited, asymmetrically clipped if something with a medium input impedance was connected, eg 10k - 20k typical of studio gear.
Whereas if only loaded by a 500k - 1M input impedance of a typical fx pedal, the max available signal swing should be pretty close to that at the preceding stage's plate.

[Rocket Roll]

Would something like Klein-ulator/Dumblelator work with Fender 75? Would it help with the distortion and signal levels in/out?

[pdf64]

Well, such a device wouldn't 'not work', but in regard of resolving the issues of the stock design, it might not be beneficial.
https://el34world.com/charts/Schemat...umble/Loop.pdf
A key issue being that the loop insert point really needs moving to after the final preamp clipping stage V4B, immediately prior to the master volume.https://el34world.com/charts/Schemat.../Fender_75.pdf
An fx loop send and return circuit with a low output send impedance and all round flat response would be theoretically technically 'better' than the Dumble style. eg consider a Metroamp solid state type.

[Rocket Roll]

Thank you for the Metroamp suggestion, and I agree that it would be the easiest to move the loop insert point, but I'm not into deviating from the original schematic, because I simply do not have enough knowledge to deal with the possible problems and I certainly wouldn't like to leave the mess for the next person who will own the amp (presumably, after I'm gone, but then you never know when a nice trade might come up, and it's better for both scenarios that the amp is stock).

So, dealing with whatever situation Ed Jahns has gotten us into with the effects loop, we're discussing here the possibility of having an external device that would:
1) attenuate the hot signal from the preamp as it gets out (nominally thought to be +4dB, but possible to be even +40dB)
2) loop it through some guitar pedals
3) bring the level back up to a pretty hot level
4) while inserting as little noise as possible.

As far as I know, and it's second-hand knowledge, Klein-ulator/Dumblelator solutions can deal with 3) and 4), and some even insist on 3) overdriving the amp further, but I'm concerned with 1), because I simply don't see a simple tube/JFET as is drawn on Klein-ulator/Dumblelator schematic being able to bring the signal down without extra distortion, caused by them being hit with a large signal from the preamp.

I hope I'm making sense.

[Rocket Roll]

(Klein-ulator PCB - high voltage, 33V in this case, means high headroom and should prevent clipping?)

[pdf64]

The signal from the amp's send would really need to be reduced before it gets to Q1.
All it would need is a simple pot adding, maybe a trimmer eg something in the range 50k to 250k, inside the Klein-ulator box.
Even simpler, reduce the 1M input resistor in the bottom left corner to 4k7, and add a 100k resistor in series between the Klein-ulator input and the board.

[Rocket Roll]

Well! The good news is that this layout is working and that Klein-ulator does what it says it does on the tin.

Thank you again, pdf64.

However, with the values you suggested (100K before input, 4K7 to ground), the signal does not reach unity gain.