FX loops coming to Metro.

[flemingmras]

For the record the loop works fine in the JCA20 amp.

Dave...seriously now...are you fucking kidding me? Please tell me it ain't so..

Seriously...I've been fucking scratching my head trying to figure out how to install it into a JCA20 amp...can you explain to me how in the hell it's even possible?

[polifemo]

A question about the variable gain in the loop:
Does the trimpot adjust the gain before the rack/pedal effects or after them?


Ps. The loop itself is buffered, right?

I´m going to install it in a Ceriatone Hiwatt Clone based on this schematic:



This loop is designed to be installed after a (buffered) chatode driven tone stack (Marshall) right, while the Hiwatt design has a tone stack placed after the anode.

There is a triode unused (V2) that could be used for buffering, but I´m not sure what to do.
All input/help is much appreciated! Ds

[SDM]

Yes the loop is buffered. The trim pot serves to set the loop's over all gain to unity. It effects the send side signal, but shouldn't be thought of as a "send level control" in the traditional sense though, especially as there is no return adjustment. Setting the trim pot too far one way will lead to a volume boost when the loop is engaged (or excess overdrive if the amp is overdriving the PI already), but that also means a boost in the signal level sent out by the loop too, which could/would lead to overdriven effects = not good. Opposite applies of course, send level set too low, you'd get a volume cut (or overdrive reduction if PI is already clipping), when the loop is engaged, but a less than intended/ideal signal level now through the loop, perhaps leading to excess noise from non optimized signal to noise ratios. So, the loop trimmer should be set to unity to avoid the issues above, comes set up that way from Metro.

That said, how would the loop fit into the schematic you posted above? Well the loop can go in amps with anode fed tone stacks, so it could go in after the treble pot, before the 22K preceding the master. But 2 potential problems arise with this particular circuit. One is that in this case the signal will likely be considerably weaker than it would be in a Marshall at this point. Thus the loop will have a lower than ideal signal through it. Still should be close to to a weak pedal level though, so it may work out in that respect. Adding a cathode follower in there to feed the tone stack instead would help some in that regard, but may have good/bad/or non issue tonal effects depending on your ears/perspective. Second issue is that ideally we want to place the loop directly before the PI to capture all preamp overdrive/distortion to feed to the loop. You've got two extra stages here following the tone stack/master in this circuit so ideally we'd like to put the loop after those. Unfortunately you cannot do so in this case without modifying the amp (due to the PI setup and the cathode follower directly feeding the PI input grid (signal and most importantly DC voltage).

So in the end here, if you didn't want to mess with the circuit you posted above, keep things easy, then inserting the loop directly after the treble pot would work, but again is not quite the most ideal location. It is still better to have the loop following at least some of the preamp overdrive and the tone stack IMO though, but you may also pay a slight noise penalty here too (really gonna depend on your particular effects there). A cathode follower would help some in the latter respect, that or tweaking the loop a bit to compensate for the weaker signal, but tweaking the loop is not something we can recommend or officially support.

Overall the amp just isn't the most "loop friendly" design. Can be done, but perhaps with some compromises involved or some mods to make it more "loop friendly".

[polifemo]

Many thanks for your reply

Maybe it´s a bad idea to try to install the loop in this particular amp then...
Great loop though!!!!

[SDM]

Many thanks for your reply

Maybe it´s a bad idea to try to install the loop in this particular amp then...
Great loop though!!!!

Yeah there are some amps out there that just are not well suited to this loop, not well suited to any loop, unfortunately. This particular example is kind of in a "grey area" so to speak, so just wanted you to know the situation.

Want people to be happy with their loops, first and foremost, so when the loop is not a good fit for an amp, or in that "grey area", I'd rather lose a sale than have a potentially disappointed customer.

[polifemo]

What about inserting the loop after the master volume? (I run the amp clean with the master on full)

[SDM]

What about inserting the loop after the master volume? (I run the amp clean with the master on full)

Signal is a bit weaker yet there after the master, so would leave you no better off (a bit worse off ). That, plus the loop would only be getting close to the desired signal level only when the master is fully cranked. Ever turn the master down, the loop would get less and less signal, throwing the signal to noise ratios further and further off (for the worse).

So, you'd be better off putting the loop directly following the treble pot, as loop signal will stay consistent and a bit stronger too. Still have the compromises described in my first response above of course, and those same compromises would exist just the same if the loop were post master. You'd only add the new issues described here by installing the loop post master unfortunately.

Any way sent you a PM, check your inbox.

[polifemo]

Any way sent you a PM, check your inbox.

Reply sent as requested

[erigm]

Steve,

Is the output of the FX loop board (not the send to the fx, but the actual output of the board that sends the signal back to the PI) low impedance? I'd like to use this loop in a tube pedal on my pedal board, but I need to make sure it can drive a 20' - 50' cable at line level. Thanks.

Also, any development on a "line level" FX loop, or a mod to the existing board, for use with rack effects?

Eric

[SDM]

Is the output of the FX loop board (not the send to the fx, but the actual output of the board that sends the signal back to the PI) low impedance? I'd like to use this loop in a tube pedal on my pedal board, but I need to make sure it can drive a 20' - 50' cable at line level. Thanks.

The return stage of the entire loop is meant to drive the PI directly, won't work out well for that plan unless a buffer were added following the entire loop circuit. A decent tube pedal should have an appropriate output buffer already though, are you sure yours does not (or is it pedal level and you need line)?

Also, any development on a "line level" FX loop, or a mod to the existing board, for use with rack effects?

The current loop boards have had built in provision for easy set up as line or pedal level for a good while now. Hopefully that option will be added in the store directly, but Metro can set them up easily as line level now if you ask when ordering.

So, concerning your original plan (and assuming your tube pedal is not buffered): Since the ZL loop can indeed be set up as line level, if you just need the tube pedal's output to send a low impedance line level signal out, then you could use a line level ZL loop's send jack as the final output of the pedal. That will send out a low impedance line level signal, would just basically ignore the return side of the loop altogether.

Perhaps not the most cost efficient use of the loop, nor cost efficient plan overall, but should work well (assuming the tube pedal's circuit is compatible with the loop in general).

Hope that made sense.

[erigm]

Here's what I'm trying to do; I want to design and build a tube preamp that stays on my pedal board and will work with my pedal effects. I have to have delay and boost after the distortion, and also I"m just not satisfied with distortion pedals running in front of a clean amp. So I plan to make a pedal with a dirt and clean channel, and then an FX loop at pedal level, and then a buffered output at line level to drive poweramp and cab. I ZL loop works so well and takes up so little space, that was my motivation for asking you these questions.

[SDM]

So I plan to make a pedal with a dirt and clean channel, and then an FX loop at pedal level, and then a buffered output at line level to drive poweramp and cab.

In that case a regular (pedal level) ZL loop followed by a simple buffer stage would serve you best. Could use a tube stage or go SS for the final output buffer. Could use a second line level ZL loop send side (as described above) for the output buffer, but if designing/ building the thing yourself, the output buffer need not be anything too fancy here, should just be designed into the preamp pedal in general. Not hard to do.

[erigm]

Yeah, that's what I was thinking of doing. It was going to be like the London Power Standard preamp topology, but the last mixing stage would be a buffer instead of makeup gain, and at that point in the signal path it should be able to accomodate instrument level signals, then the signal would return from my pedals and go to a gain stage and then output buffer stage, probably direct coupled. I was thinking all tube, but then the thought of the ZL fx loop popped into my head and for the reasons mentioned earlier thought it might be a viable choice. Still is, but like you said I would need a buffer after the loop. A simple mosfet buffer should work fine here.

[MaximumGtr]

Steve:

Have you heard of any issues using the loop with a 12xxx kit incorporating the Caswell/Levi SIR mods? I am anticipating installing per psychodave's "far left" technique, maybe with flying on/off switch (Jan 19, 2009), and am hoping to get some additional access to the stages....

Thanks!!!

[SDM]

Should be no problems with the loop in a Caswell 39 or Levi 36 or 34 style amp. It's perfect for circuits like those actually. Not sure what you mean by "far left" technique though, but if you mean installing the extra stage there, to the left of V1, the loop should have no issues with that either.

Can see the pic on my homepage, in the link below my sig. A '69 chassis of course there, but extra stage board and extra tube added to the far left. Circuit incorporated there is not all that different from the alleged #39 apparently, pushpull pot or foot switchable. No problems with that amp.