active effect loop question

[Colas LeGrippa]

Hola amigos !!!!  I am back here after touring with my blues band. 
I would like to install in one of my amps ( bogen ) a tube effect loop. I have seen a lot here but I am confused, there are so many.
A simple one that I have seen is the one below. I was wondering if a 12ax7 was suitable for that purpose.
It's got to be simple and easy to install ( I am not a complicated man....).
Thanks !

Colas LeGrippa

[HotBluePlates]

It looks good, but why is the Send a regular gain stage instead of a cathode follower? The follower would probably be better at driving cables and possible-low-impedance inputs.

[SILVERGUN]

This one is from Merlin...designed around a 12AU7

[Colas LeGrippa]

Do I need to put a switch to bypass the whole thing when not in use ? Otherwise, the circuit will alter the tone of the amp no ?

[Willabe]

Wait for Tubenit, he's built several FX loops into his amps over the years and helped many guys build theirs. 

[Colas LeGrippa]

ok thank you

Colas

[2deaf]

That original loop looks pretty good to me -- got the low impedance from the cathode, big coupling capacitor for the low frequency cut-off, and a small pot to keep the impedance low.  BUT, the voltage divider at pin 7 looks like a problem to me.  I don't know what the previous stage is, but you may be able to make its plate resistor a voltage divider which will allow you to reduce or eliminate the resistor going to pin 7.   

[kagliostro]

Ciao Colas

Here one other FXLoop from Merlin that was put (with succes) in a 5E3

Download the image for full resolution


Note that the dual ganged pot is wired in a way that allow the increase of one signal and, at the same time, the decrease of the other when you rotate the shaft

If you prefer you can change the trimmer with a pot as to use the FXLoop as an overdrive

Franco

[Colas LeGrippa]

Ciao Kagliostro ! Grazie mille, fratello !

Colas

[HotBluePlates]

Do I need to put a switch to bypass the whole thing when not in use ? Otherwise, the circuit will alter the tone of the amp no ?

Look again at Silvergun's schematic. If nothing is plugged in to the Return jack, the signal is "normalled" to pass right through to the Return gain stage. If you needed, you could switch a feedback resistor (possibly with a blocking cap) to drop that triode's gain to unity when the loop is not in use.

[Fresh_Start]

Question:  Is the gain of the triode V2a changed by "seeing" the 1 meg resistance to ground of the trimmer vs. the 10 meg +/- it "sees" when the loop is bypassed and the following impedance is the "bootstrapped" grid return resistor? (10 meg was ballpark PRR mentioned in another thread IIRC) 

I'm a bit confused by the "Trimmer" before a cathode follower stage.  Particularly since there is a 10K effects loop send pot right after V3a.   

I assume that you can compensate for any loss of gain with the effects loop recovery pot(s).  Just trying to understand the impact of the impedance following a gain stage.

Question 2: Would a 12DW7 improve the performance of this loop?  I'm thinking about the 12AU7 half for the cathode follower maybe being a better driver for the effects loop.  I understand that 12AX7 are easily obtained and preferred unless there's an overwhelming need for a different preamp tube.

Thanks,
Chip

[Jack_Hester]

Here's one that I modeled after Silvergun's drawing.  But, it's been so long since I made this drawing, that I don't remember if it's the same.  The one obvious difference is that I added a passive loop around the active.  Just something to compare with the others. 

Jack

[arjepsen]

Here's yet another one of Merlins loops. I've tried the one listed above, from his second book. Unfortunately that one gave me some phase cancellation problems. This one works wonders though.

[2deaf]

It looks good, but why is the Send a regular gain stage instead of a cathode follower? The follower would probably be better at driving cables and possible-low-impedance inputs.

The more I look at it, the more I agree with you.  We are talking about a Merlin loop here, so I can't question its merit.  I can, however, point out that is has to be used in whatever context he had it in.  I would guess that it was intended for small signal applications only.

Kagliostro's schematic is a three-stage preamp instead of the usual two-stage 5E3.  Had the FX send come off of the V2A cathode, that may have been what Merlin intended.

The problem with taking a FX send from the cathode of a stage that is being overdriven is that the cathode signal is not the same as the anode signal.  If you like what is coming off the anode, you probably won't like what is coming off the cathode.

[PRR]

> Is the gain of the triode V2a changed

A wee bit.

So?

The at-the-ear "gain" is the product of that sag, the non-unity gain cathode follower, the FX return mixing, and another gain stage. Plus whatever dingus you put in the loop. And how you set the return pots.

[Fresh_Start]

> Is the gain of the triode V2a changed

A wee bit.

So?

The at-the-ear "gain" is the product of that sag, the non-unity gain cathode follower, the FX return mixing, and another gain stage. Plus whatever dingus you put in the loop. And how you set the return pots.

Understood, but then why is the Trimmer needed at all?

Respectfully,
Chip

[Colas LeGrippa]

ok now why not putting the effect loop circuitry  BEFORE the first preamp stage , instead of inserting an effect in a signal already overdriven or amplified to a certain point ?

By example we would have: 1 st half cathode follower ( send ) 2nd  half return,  preamp12AX7, cathode follower,  then PI  and power tubes. Just like pedals are put between the guitar and the amplifier


Colas

[HotBluePlates]

ok now why not putting the effect loop circuitry  BEFORE the first preamp stage , instead of inserting an effect in a signal already overdriven or amplified to a certain point ?

Because you already have your pedals between the guitar & amp to do that. Or you do the same thing with less space, weight and heat by using a guitar or pedalboard buffer/line-driver.

I'm not a big effects guy (though I have been getting some effects here & there and may change how I typically operate). That said, some effects are best placed before the amp. Buffers, boosts, volume pedals, overdrive/distortion, fuzz, wah, and some others (maybe compression, flanging, chorusing, phasing, maybe EQ). Here you're changing the guitar signal and/or driving the amp input harder than the guitar can on its own. You're either wanting the amp tone controls to shape the resulting sound or you're relying on the amp for a clean sound only and using pedals for alternative sounds.

Some effects sound better after the preamp, especially if the amp's preamp will contribute significant distortion and/or tone shaping. Time-based effects (delays, reverbs) might be better employed to add reverb, slapback or delay to your already-complete guitar/pedal/preamp-shaped sound, right before it gets amplified (probably cleanly) by the amp's power section. For instance, if you're adding reverb and the preamp contributes significant distortion, do you really want a distorted reverb sound? Or did you want to add space/depth by adding reverb to the distorted sound? The latter (if the output section amplifies cleanly) will sound more like a studio recording where an overdriven amp track is sent to reverb in mixing.

Some folks will put modulation effects (flanging, phasing, chorus, rotary speaker sims) in the loop after the preamp as well. If you're trying to simulate the original tape flanging/chorusing that old studio engineers did using a pair of tape decks, that might be best done after any tone contribution from the preamp. Then again, some rock & grunge sounds were done with stomp box pedals where a chorus was placed before a distortion to add some swirling nastiness...

It's really up to you to figure out what effects you will be using, what sounds they should shape (or what things will follow afterwards to shape the sound provided by your effect), and determine where they should sit in the total signal chain between guitar and speaker.

[Colas LeGrippa]

i'll go to bed first , and read back your post tomorrow...

thanks, bro

Colas

[Colas LeGrippa]

HBP,  would'nt there be a difference between guitar pedals inserted between the guitar and the amplifier input jack and pedals inserted AFTER the cathode follower, if the CF was first in the amplifier, before the preamp ?

Colas

[kagliostro]

Ciao Colas

I think you must read one other time what HBP told

Franco

[HotBluePlates]

HBP,  would'nt there be a difference between guitar pedals inserted between the guitar and the amplifier input jack and pedals inserted AFTER the cathode follower, if the CF was first in the amplifier, before the preamp?

Not usually.

You're asking "would there be a difference" in the pedals/effects themselves. However, I'm telling you what people already do & how. I'm guessing that like me, you're not a big effects-user.

Regarding the cathode follower, it's just being used to make sure cable capacitance out to your pedalboard/rack doesn't degrade the signal going into the effects.

You're probably thinking, "but aren't pedals low-signal level, and need boosting?" Yep, and that's why there is a make-up gain stage at the loop return.

[Colas LeGrippa]

I got it now, thanks, HBP. No, I am not a pedal user, I am a guitar user !    

Kagliostro, I know I have to read many times b4 understanding something ! I speak 3 languages but english is my worst !  When I go to Italy, you'l teach me Italian, if you invite me

Colas LeGrippa

[kagliostro]

 

[Colas LeGrippa]

Today I have performed Merlin's active effect loop to my Bogen CHB-20A. I added a 12AU7 ( there's plenty of room in there ! ) . I took the HT from the 6EU7 supply and added a 10K / 5W  and a 20uF  / 350V to it, to get rid of the oscillation. The PT is strong enough and the heaters read 6,6 V with the extra tube. . I took the signal from the treble pot    and the new signal goes back to where the treble wiper was connected, to the ECL86. This amplifier was awesome befre,  once slightly ( very slightly ) modified. Now, with the 12AU7, it's got a KILLER TONE, and IMHO, is way better than many amplifiers designed for guitar, including Marshall and Fender. It's got aggressive tone when cranked, with rich and warm overtones. It will be even better with an ALNICO 25W speaker. Now it is plugged in a Fender speaker from a twin, freshly reconed.

Many thanx to all of you who helped me with this small project.

Colas LeGrippa
 

[SILVERGUN]

 
 
Glad to hear it Colas, thanks for the update!

[Colas LeGrippa]

I 'll post pics once layout redone cleanly. ( I don't know if you're like me, but when trying a new circuit, I always do it fast, too anxious to see if it'll work ! After, calmly, I redo almost all. I take my time with the layout and lead dress, good solders, heat shrink where needed, terminal strips, good grounding, and so on).

Colas

[Colas LeGrippa]

....and after, I plug in my Jazzmaster 1962, I play my blues licks, stopping once in a while to drink a shot of the Rhum I brought back from Cuba ( Sorry, I do not smoke, cigarrettes, cigars or other stuff , then my lungs will live longer than my liver hey hey !! ), and I relax, I play love songs as well, thinking of the beautiful girl I met on the beach in Varadero, and suddenly my wife rushes in MY OWN practice room, shouting that the volume is too loud and that she needs milk  for the supper.............

OMG, life is tough here......I gotta get back there

Coca Colas