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SFPR Boost Circuit

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4/19/2000 4:34 AM

SFPR Boost Circuit

I have a Silver Face Princeton Reverb with the pull-switch boost on the volume control. The boost sounds terrible, and now that I've seen the schematic for this thing, it's obvious why: The boost switch adds one more gain stage - the one that's driving the reverb pan!

I'm wondering if anyone has modified this circuit so that it sounds like an amplifier and not a cat with his tail in a grinder? I'd really like to be able to use the boost feature, but if it must sound like this, well.....

Or perhaps mine sounds bad and others sound good?

Comments/help/suggestions are welcome!

Steve Cavanaugh

4/19/2000 6:23 AM

Steve:

It's not just your amp... that is one of the screwiest boost circuits I've ever seen. I'd suggest that you "restore" your amp to BF specs and then use the p-p pot switch to increase the 6k8 mid resistor in the tone stack to something like 56k or 68k (thanks to Bruce Collins and John Stokes for that idea!) Only drawback with that plan is that your p-p pot is probably SPST so you'd pull the knob out to defeat the boost... (that does bother some people because it is backwards).

Another idea would be to use the p-p pot to switch in a larger treble cap for a a mid-boost (like on the older Mesa Boogies). If you are using a stock 250pF treble cap I'd try something like a 750pF cap in parallel to that for a net capacitance of 1000pF. Or go crazy and add in a 1200pF cap. (Be sure to add a 56k-220k resistor across the switch contacts to reduce or eliminate any switching noise.)

Steve Ahola

4/19/2000 12:54 PM
BentTop	Steve, thanks for the suggestions. I have been considering removing the boost and making it a BF but I wanted to see if anyone else thought this boost was weird.

4/19/2000 8:31 PM

I concur with Steve and probably all the other owners of this PR version that the boost feature totally sucks. This feature, even when not engaged, sucks some tone (so I was told) so I wound up completely dismantling it. In this process, I did add a 25/50 cathode bypass cap and change the cathode resistor from the stock 680 ohm to BF/early SF 2200 ohm value on the reverb driver (12AT7). Quite frankly, I didn't notice any difference in tone after this, however the reverb did sound better. I still wasn't satisfied with the overall tone so I then reverted everything to early SF specs (AB1270) and the tone is now almost where I want it (just have some ghost notes to contend with now). I elected not to revert to BF specs (AA1164) because the coupling cap values in the preamp stage are .08's which many feel are better at around .02 as in the early SF circuit. Regarding Steve's post for lifting the tone stack some, I've been meaning to try that mod but it would be better to use a DPDT push/pull switched pot instead of the stock.

4/20/2000 12:45 AM
BentTop	Although I haven't decided yet, I may pull this boost circuit completely out. I is nice to be able to boost, but not if it sounds terrible. If I could get the amp to have a good tone at all I'd be pretty pleased I think.

4/20/2000 1:35 AM

Steve,

Just looking at the schematic real quick, I have a boost suggestion. Remove both wires from the switch (I'd completely remove the wires going to the reverb pot and reverb transformer - they may be accidents waiting to happen as far as feedback and noise are concerned). Ground one side of the switch, and connect the other end to the top of the 47 ohm feedback resistor, which is connected to the 1.5K resistor connected to pin 8 of V3A (other half of reverb recovery tube).

Pulling the boost switch will disable the power amp's negative feedback loop, giving you more gain (how much more I'm not sure - maybe 6 to 10 dB or more) and a slightly different 'feel' to the output stage distortion, along with perhaps a bit more bass, especially with a closed-back extension cabinet. You might want to try jumpering this 47 ohm resistor with a clip lead first to see if you like the mod - it will have to sound better than the current pulses coming off the reverb transformer! This mod takes place at a very low-impedance point in the amp's circuitry so feedback shouldn't be a problem, but you might want to use shielded wire just in case.

Of course there are a bunch more things you could do, but this one shouldn't affect anything else in the amp (reverb, tremolo, tone controls, etc.).

Ray Ivers
R.A.G.E. Electronics

4/21/2000 3:00 AM
andy fuchs	I'd replace the pot with a dual 500K and put in a post MI master volume. Remove all the old antennas (er wiring) from the old master setup and BF the rest of the circuit. God that boost truly SUCKED ! A fine example of engineers what were likely not musicians designing something for use by musicians ! Did anyone listen to this thing ?

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