Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #115064  by claytushaywood
 Sun Jun 17, 2012 3:27 pm
So I am thinking about finally tapping the preamp on my fender pro reverb... for a few reasons other than the SS power amp. On waldo's site the post alembic mod is listed as having a nice amount of other uses. Those include- using the normal channel with the internal power amp and also being able to use the vibrato channel (or vice versa) with an external power amp simultaneously... HALF REVERB HALF CLEAN! that sounds like serious fun... and the tonal differences of the channels, especially with the tone stack differences... it also says you can use the jacks with a stereo Y cable as an effects loop! that's something I personally would love to have... I love running delay after the preamp, even if its a clean preamp it really does add some clarity to teh repeats that allows you to turn the mix down and keep everything from getting muddy. but how awesome does it sound to be able to use a tube power amp (fender stock) and an SS power amp, with reverb on one channel and slightly different tone on both? different cabs too! that's waht jerry did in the early 70's right? had 3 twins and on one he used both channels for 1 channel with no reverb and 3 channels with... then at another point he used half and half?

So... my concerns are the fact that its not buffered. Wouldnt that be a problem if I used a longer cable for the fx loop, or even a long cable to my external power amp? Has anyone done these mods? I'd love to hear some experiences!

Weber has a kit (and schematic) for a tube buffered effects loop that looks pretty easy to me to install... it also includes the ability to put pots to control the send and return level's seperately... and the ability to run a lineout with adjustable send level without interupting flow to the power amp. plus being tube buffered you dont have to worry about cable length.

Anyone at all used the jerry post alembic preamp taps as an effects loop or used both preamp channels- one to external power amp and one to internal power amp?

Any knowledge would be greatly appreciated before I just do the mod and find out about a quirk i wasnt planning on!
 #115102  by gpilcher2001
 Mon Jun 18, 2012 10:54 pm
From what i understand you would probably need to look at it like an old serial effe ts loop. Impeadences would be your biggest enemy. Think of it like the first effects loops that came on a marshall or boogie. Theree wasn't a whole lotta things that worked in there. I have had a conversation on here about the how hot the signal is when you tap it like that. I can't remember, it was a calling on brad or waldo thing. If you search that you might find it. Brad talks about the line level signal. You can also google effects loops, there are a couple of pages that describe effcts loop design. It also pays to get a grasp on send impeadence and return impeadence. Again a google search will also get you answers on that. As far as will it work like that, if you can find rack effects or pedals that play nice with it it will work just fine. I wish i could repeat verbatim what i found on my google searches but i can't. Hopefully that will stear you in the right direction. Greg...
 #115103  by gpilcher2001
 Mon Jun 18, 2012 10:58 pm
Oh by the way the only thing the buffer does is dial the signal level to a low enough impeadence to play nice with virtually any kind of effect. Greg....
 #115131  by claytushaywood
 Tue Jun 19, 2012 7:02 pm
gpilcher2001 wrote:Oh by the way the only thing the buffer does is dial the signal level to a low enough impeadence to play nice with virtually any kind of effect. Greg....
yeh... that's a pretty huge "only thing" considering the dilemmas of non buffered fx loops you described in the above post.
 #115136  by gpilcher2001
 Tue Jun 19, 2012 8:35 pm
If you go the weber route let me know how it works out. I've been lookin at that buffer for years. Fuchs audio makes a pedal/box version of a tube buffer, exspensive though. Greg...