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Opinions on modding BF SR

5/4/2006 9:17 PM
Gee Geddy Opinions on modding BF SR
I've been thinking about installing a post-pi master volume on my bf super reverb. Not to get a lot of distortion, but to get that tighter response at lower volumes. I've already tried an attenuator, and I feel it robs too much low end kick. I was playing around with a little Fender super champ and I preferred the sound with the volume around "6" and the master low, than if I cranked the master volume (to simulate no MV) and turned up the regular volume.  
I don't want to drill holes in the amp, I was thinking about using a spot at the tremolo controls, since I don't use the tremolo.  
Am I about to commit sacrilege, or do other people do this?
5/5/2006 6:17 AM
It's your amp, do with it what you will. I've heard stories of people putting a master volume in the ground switch hole, or one of the speaker jack holes. If it's reversable you stand less chance of reducing the value of the amp.  
Regarding the attenuator, which one did you use?  
The thing about attenuators is that you have to use them in moderation. I use the Sholtz Power Soaks, and you are right, you lose low end umph if you dial it down too much. That's gonna happen with any attenuator. The most you want to do is a couple of clicks, about 6 db. The Super Reverb is a loud amp due to those four 10's, a MV might work, I personally hate MV's because you don't get the power tube overdrive.  
I do some rants about this on my Power Soak page:
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5/5/2006 10:41 AM
Steve Dallman

I've done a lot of post PI masters in Fenders and Marshalls, using a dual 1meg audio stacked pot, followed by 1uF/250v caps after. I've not had problems with low end loss. You could also add series resistors (33k or so) after the master, but I've not used the resistors.  
In an amp like yours, I'd probably put it in place of the vibrato intensity, and move the vibrato intensity to the ext. speaker jack hole in the back.  
In the 80's I owned a Scholtz Power Soak, but truthfully, I preferred the sound of the post PI master over the Power Soak, and so did all of the people I did the post PI masters for.  
A large part of power amp distortion comes from the PI breaking up. This often masks the power tube distortion to an extent.  
In the Marshalls I put the masters in, the customer would compare the master to the Power Soak, and found the tone tighter and more pleasant than with the Soak. I put these masters in non-master Marshalls, and replacing the pre-PI master in other Marshalls.  
But distortion is subjective and what you love, I may not and vice versa.  
Whatever you decide, do a neat job and make sure it's reversable should you ever want to sell the amp. I put in a small terminal strip to support the new caps. Since the caps only have to block the bias voltage, the power handling can be less than the other coupling caps, but the uF value should be 10 times the value of the coupling caps preceding the master.  
There are several types of post PI masters. The one I use most is using a stacked pot that just turns down the signal going to each power tube, before the bias resistors.  
Another design replaces the bias resistors with a dual pot, usually 250k or 100k, but bypass the terminals with a 1meg resistor in case the pot opens up, which would break the bias voltage to tha tube, which will then cherry up before dying.  
Another is the cross-line master. This one sounds fine in smaller EL84 and 6V6 amps, but I don't like it as much in a bigger amp. I use a 100k audio taper pot, with the carbon trace scraped off at the high end of the travel. This takes the pot out of the circuit when turned up. The 100k audio taper pot gives the smoothest response to my ear. Some like a 250K audio pot with the same trace mod.  
Radio Shack carries a 1uF metal film cap that works well with the dual 1meg (or 500k) pot. They don't carry the pot though.
5/5/2006 11:21 AM
Gee Geddy
How about in the No. 2 input jack, the one that no one uses? I know the input resistor would have to be changed to 33k or so. Or would it be too far from the PI section? I was thinking about the post-PI MV that uses the ganged pot and 'splits' the coupling caps to the power tubes.
5/5/2006 12:54 PM
You could, but you're just asking for oscillation. The speaker jack or the tremolo hole would be better.  
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