Effects Look in an AX84 October

[Bill Durham]

Guys,

I'm thinking about adding an effects loop to my October. I'm thinking that the loop would be located between the tone stack and the Pre PI MV. I am planning on using a Yamaha REX50 for chorus, phase, delay type effects in this loop. The REX50 has an input gain control which should allow me to match levels with what ever I get out of the tone stack.

Am in the ballpark here?

thanks,
Bill Durham

[Ray Ivers]

Bill,

If you take the effects send from the MV pot wiper and run the effect return back into the PI input (don't forget the coupling cap!), the loop may work okay at low MV settings, but as you turn it up you'll probably overload the REX50 pretty quick. If you then turn down the REX50 input gain control to compensate, you'll probably not have enough signal coming out of it to fully drive the power amp. If you come directly out of the tone stack into the REX50, you'll probably hard-clip its input stage unless you turn the gain control almost off - and again, I doubt the REX50 has the output needed to drive the power amp.

The only way to be sure it will work well is to either:

a) take the loop send from the cathode of V2b - using a voltage divider to bring the level down - then add a recovery/tone-stack-driver stage, leaving the MV as-is

or

b) buffer the tone stack output with a self-biased cathode-follower stage or similar, take an attenuated loop send from that, then add another loop-return recovery stage before the MV.

There are other ways of doing this, too - these are just two that immediately come to mind. Also, I have no idea what the specs of the REX50 unit are - if it has a high input impedance along with a very high output signal capability, it may work acceptably from the MV wiper.

Ray

[Bill Durham]

Effects loop

Ray,
thanks very much for the reply. I was thinking that the level would be a little hot for the input of the REX50. I really don't want to add any tube stages to the amp since its pretty much a finished product. Could I add a transfomer (low z to high z) on the return side to get some voltage gain that way, assuming that I attenuate the signal to the send to the REX50?

thanks again
BD

[Ray Ivers]

Bill,

Could I add a transfomer (low z to high z) on the return side to get some voltage gain that way, assuming that I attenuate the signal to the send to the REX50?

Yes, you could, but I would highly recommend against it - I won't go into all the reasons why it's not a good idea (IMO), but you could end up spending a bunch of time and money and then wondering, "what have I done ?!" A better solution might be use the MV-wiper loop with a line amplifier on the REX50's output, to boost its signal to a level equal to the tone-stack's output (there may be a pedal available that does this, I'm not really up on the latest FX units out there), or you could take that one step further and use what some Dumble amps do; a complete outboard effects-loop unit (the Dumbleator) that interfaces outboard effects units to the high signal levels present at the amp's preamp out/power amp in jacks. I sure wouldn't want to go to that much trouble, though.

I wish I had a super-simple, good-performing solution for you, but I really don't. One of the most common mod requests I've gotten was to put a loop in a Marshall, and I always needed to either steal the CF to use as a loop return stage - which detracted from the overdrive sound IMO - or add an FET loop recovery stage (by far the better solution IME).

You could just try the MV-wiper loop out; I don't think you'll be ecstatic with the results, but it might be worth a try just for grins. At the very least, it will give you a firsthand look at the problems involved in inserting an unbuffered, high-level loop at this point in this circuit.

I realize you're not looking for the best FX loop there is, just something that will allow you to patch in your REX50 so it will work, and that leaves your amp's sound intact when not in use.

Ray

[Bill Durham]

Effects loop

Ray,

How bout this....

What if I come off of the top of the preamp volume control, ac couple with a big film so I get some bottom, as the send. It would be a pre-fade send so the level would be constant... in theory. The input impedance of the REX50 is high..it can take a guitar as a source, so there wouldn't be excessive load on the plate of V1, would it?
Then return as a 3rd summing resistor before the grid of V2? Current summers are 470k, maybe adjust the value some to keep the impedance on the grid of V3 high.

As you can tell I'm not a big theory guy, just a working knowledge of whats going on. thanks for your help

BD

[Ray Ivers]

Bill,

If you want to put the effects loop in front of the overdrive - as it would be if placed between V1 and V2 - IMO you might as well put your FX unit(s) between the guitar and amp, and save yourself a ton of trouble. The full output of V1 (from the top of the volume control) will most probably slam-clip the REX50 input.

In a nutshell... tube guitar amps run at very high signal levels, and practically all effects units operate at low to medium signal levels. To interface the two, the tube-level signal must be attenuated, passed through the effect, then boosted back to its previous level (there are other considerations as well, of course, this is just the most basic one). While the first requirement (attenuation) can sometimes be met by creative loop-send placement, loop-return signal boost is practically impossible without circuit redesign or adding devices (gain stages, transformers, etc.).

Ray

[Bill Durham]

effects loop

Ray,

OK, I guess I'm kicking a dead horse!

I think that I will take a look at adding a solid state gain stage for the return. The big issue there will be creating a power supply for it. Maybe I could do something from the bias supply.

BD