Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ToneQwest on February 17, 2020, 09:07:26 am
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Does anyone here know how to add a passive FX loop to the RR JCM-800/22 watt circuit?
There is s very simple and effective mod that Rob Robinette posted for the much modded 40watt 5E3.
Can this additional feature be easily added to the 22 watt JCM800?
I emailed Rob, but he doesn't seem to be answering emails as of late. Everyone is busy these days.
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fwiw
I've put this FX in 2 amps, works great, no hard thinking required :icon_biggrin:
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Just install the send and return jacks between the treble wiper and the .022 coupling cap (input to LTP PI). Kinda like this...
http://sluckeyamps.com/lightning/matchless_lightning.pdf
...or this...
http://sluckeyamps.com/november/november.pdf
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Passive fx loop is kind of useless in this type of amp, IMHO. Either, the send signal will be so hot it will distort any effect in the loop (or even damage it), or you have to pad the signal way down, leaving the amp unable to reach full power.
At minimum, some kind of active return stage is necessary for a working loop.
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I think there are advantages of an active FX and currently, my amps only have an active FX. However, I have run the following pedals in a passive FX (with both Dumble inspired and Marshall inspired topologies) and I thought this worked out just fine:
Wampler Faux Echo, Boss Digital Delay, Digitech reverb, Moen Pretty Dolly Delay, MXR Carbon Copy & old Ibanez Flanger.
With respect, Tubenit
I've also read on more then one occasion that someone preferred the passive FX loop on their Dumble amp or Dumble clone instead of using the Dumbelator active FX loop.
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Rob Robinette replied to my email suggested this:
"Yes, we can insert a passive FX loop.
I haven't tried it but I would put
the loop between the cold clipper's output coupling cap (.022uF) and the
470k Attenuator resistor. Rob"
The only candidate 470K resistor on the JCM800/22 diagram is the 470K treble bleed that is wired in parallel with a 470pF capacitor.
Could this be the "attenuator" Rob is referencing?
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Yes, that's the one.
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I would go with Sluckey's idea in reply #2 instead of Rob's suggestion.
With respect, Tubenit
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Tubenit, with all due respect to you and everyone on the forum helping me on this; what is your preference based on? I am purely curious.
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My suggestion is based on a proven circuit. I bet tubenit's preference is based on the same. I doubt Rob or anyone else has actually tried Rob's suggestion, which may work way better, but who can say. It's very easy to try it as Rob suggested and then try it as I suggested. Then you'll be the passive loop in the RR JCM-800/22 watt circuit expert. :wink:
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By the way, what are the values for the metal foil resistors on the Pre-Amp Out and between top and sweeper lug?
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100K .5 watt & 220K .5 watt.
Thank you all!
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By the way, what are the values for the metal foil resistors on the Pre-Amp Out and between top and sweeper lug?
Talking about my Lightning? Color code is on the layout. Written values are on the schematic. You don't need either of these for rob's amp.
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Yes, got it! Thanks again!
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My suggestion is based on a proven circuit. I bet tubenit's preference is based on the same
And the reason that is my preference is I have tried a passive effects loop earlier in the signal path and did NOT like it as well. I am usually using delay. I don't want the delay to have gain added to it, so further down the signal path is my preference.
Sluckey's suggestion is a proven circuit and I've used it successfully numerous times.
With respect, Tubenit
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Thank you for the sound reasoning! That is what I will do, as the effects I have in mind are modulation and delay.
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Based on the original post, I reached out to Mr. Sluckey for some help with my Robinette 800Micro. Mine is the second one shown on the website, and I was lucky enough to buy it from the original owner/builder. One of the best amp buys yet! Mr. Sluckey helped me with my questions, and educated me to the 'magic' that I never really understood, until now. As a result of his generosity, I now have an improved amp with a fully functional FX loop (for my needs!), and have now taken the next step to add an FX loop to my Dabeck 18W next. Preliminary results were fruitful - awaiting my Swithcraft shorting jacks now to finish off both amps properly! Thank you so much for the info here, and to the help received from Mr. Sluckey!
https://hearditontheweb.com/gear
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Lastly! Are there Hammond PT and OT alternatives to the Allen Amps PT & OT listed on Rob Robinette's website for the JCM 800 6V6 22 watt amplifier?