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?

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

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".

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.

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.

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