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Line in on S/F Twin

1/7/2000 10:34 AM
James Line in on S/F Twin
Frustration with the sound of my Twin has made me want to use just the power stage with some other preamp. Can I just disconnect the two channels after the 220k series resistors and insert the preamp signal directly into the PI 0.001uF cap?  
Thanks for any advice.  
1/7/2000 6:42 PM
Jim S.

Chances are, the preamp won't have enough output signal strength to drive the Twin's power stage to full power output. All rack-mount guitar tube power amps that I'm familiar with use a pre-driver stage that brings the signal up to enough voltage to properly drive the phase inverter.  
Here's my suggestion for a line-in. I've done this before and it works well:  
Rewire the input jacks on the normal channel, such that when a cable is plugged into jack #2 (which will be the LINE IN jack) the signal from the cable feeds the top of the normal channel volume pot. (If no cable is plugged into jack #2, the jack's shorting connector should connect the output of the treble pot to the input of the volume pot, allowing normal operation with a guitar plugged into jack #1.) With this arrangement, a signal going into jack #2 will be "trimmed" by the volume control and then feed gain stage V1B (acting as the pre-drive stage) which then goes to the phase inverter via the 220K mix resistor.  
Of course, none of this might be necessary if you can get your Twin's preamp section (and/or power section) to sound better. If it has not already done, maybe recapping and blackfacing might be in order.
1/7/2000 10:49 PM
John Stokes
James, what is the amp not doing?  
1/10/2000 9:01 AM
It has been blackfaced already, and it's ok with my Strat, but anything with a humbucker just makes it sound nasty at any volume. It has NOS Phillips tubes throughout and some Eminence legend speakers (I know they're not too great, but they were cheap) I guess Twin's and humbuckers just aren't meant for each other! I have a Boogie Studio pre kicking about and I'd like to run that direct into the power stage.  
1/10/2000 11:33 PM
Jim S.

If the speakers are crappy and are coloring your tone ina way you don't like, that particular situation is not going to change if you use an outboard preamp!  
In general, I would say that blackface Fenders, fitted with good-sounding speakers, sound anything but "nasty" with humbucker-equipped guitars at low to medium volumes. If anything, that Fendery scooped midrange and thin brightness often can sweeten up the sound of, say, a Les Paul or 335.  
My suggestion is to see if the "nastiness" disappears when you plug into an external speaker cabinet containing know good-quality speakers. Also, try substituting some preamp tubes -- some NOS tubes, as expensive as they are, can be real dogs, right out of the box.
1/11/2000 9:17 AM
I don't think it's the speakers that are causing me a problem, they aren't the best sounding unit's I've ever heard, but they sound ok to me. I'd just rather use my Boogie pre for h/b equipped guitars, hence my question about the line in.  
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