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SF Twin Reverb mod ?'s

5/3/1999 10:00 PM
Michael Nocera
SF Twin Reverb mod ?'s
I need some advice on having my Twin modded to better suit my piezo/magnetic stereo guitar.  
If I plug one pickup system into the reverb channel, and the other into the normal channel, they sound out of phase when played simultaneously. Is this because of a negative feedback loop? If so could this be disconnected and how would the sound change? Is is bad for the amp to have two inputs feeding it simulataneously?  
Is it possible to have independent reverb controls for each channel? I plan to disconnect the trem circuit, so I'll have 2 free control spots.  
Any thoughts or other suggestion for making this amp more acoustic friendly are appreciated.  
always grateful,  
5/3/1999 10:10 PM
Michael Nocera

I forgot to say its a '71, no master volume, model AA270.  
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5/4/1999 12:00 AM

The reverb channel has a stage of gain more than the other channel causing it to be out of phase. The Ultimate Tone (a book) has schematics for allowing reverb for both channels, among other useful things..
5/4/1999 7:06 AM

After you've made the simple wiring change to allow reverb on both channels, and both channels in phase, you'll need to change the input (grid leak) resistor of one channel to something 5-10 times larger than 1megohm, to accomodate the high impedance of the piezo pickup. This piezo pickup input mod is in one of the Ultimate Tone series of books, but I'm not sure if it's in the same volume as the reverb mod.
5/4/1999 12:42 PM
Jim S.

Add'l info: the grid leak resistor Doc is referring to is soldered between the two outside terminals of the #1 input jack of each channel. The stock value is 1 meg (brown, black, green). For piezo pickup use, try replacing that resistor with a new one that is either 5.1 meg or 10 meg.
5/4/1999 11:20 AM

If you pull the tremolo completely, you'll need to either drop gain somewhere in the pre-amp or the PI. The 50K intensity pot pds the pre-amp a lot, and removing it will give you too much gain for clean acoustic stuff.  
ideas for acoustics: play with the tone control cap values to find perhaps better choices for acoustic tone.  
Replace the speakers with something loud and efficient like EV's. Heavy, but you'll get a huge increase in headroom.  
Try a lower gain 1st pre-amp tube, like a 5751. This will help your clean headroom as well.
5/4/1999 12:55 PM
Jim S.

I disagree about removing the tremelo circuit from the audio path (which, by the way, is one of my favorite Fender mods). On the contrary, removing the tremelo circuit will give BOTH some additional potential gain (along with better tone IMO) AND slightly more headroom, at the expense of a bit more background hiss.  
Here's why: the 50K intensity pot is located in the circuit between the preamp and the phase inverter. By removing the pot from the audio circuit, you can obtain the same speaker output level at a lower volume control setting (all else being equal). At this lower volume pot setting, the phase inverter is seeing the same signal level as before, but the 2nd and 3rd preamp gain stages (which come after the volume pot) are seeing a lower signal level and are operating just a tad more linearly, with less saturation.  
Don't get me wrong -- for certain applications I love tremelo, but that 50K intensity pot is a real tone sucker!
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