Okay, so don't kill me, I'm modding my Sceptre. It wasn't 100% original when I got it and it's not in great shape anyway. And hey, let's face it, these amps can certainly be made better for guitar. They were, after all, designed to be bass amps. The only change they made was to add reverb and trem.

http://www.freeinfosociety.com/elect...unnsceptre.pdf

First thing I did was to change out the 1st stage cathode bypass cap (250mF became 2.2mF). Big improvement already. Goodbye woofiness.

Next I got rid of all that mess at the inputs and replaced with the standard 68k/1M. Goodbye brittle crunchiness. Huge improvement. I don't hate the stock Sunn sound, actually I really like it, but it is very much a specialized sound. Super bright treb, super fat bass and no mids (unless the mid boost is engaged and then it just sounds bad). Don't even think of trying to use a distortion pedal in front of a stock Sunn, sounds awful.

I'm tweaking with the second stage cathode bypass cap and the treble cap values at the moment.

Two things that still need to be addressed (two that I can think of anyway): The midboost switch and the .1 caps just before the power tubes. I have questions about these caps ... in a minute.

I haven't seen a schematic with the midboost circuit anywhere online. Anybody got one? There is one in the Aspen Pitman book that I've been studying.

The midboost switches in or out three different changes. One, it switches out the stage two .2mF cathode bypass cap when the boost is switched on. I disabled this so the brightness stays in both positions. I like this better. BUT, I'm not stopping there. I think that boost sounds awful so....

There are two 100K resistors that moderate two 12pF caps when the switch is in the off position. So there is some midboost at all times, regardless of switch position. When the switch is on, the 100Ks are bypassed altogether for a huge freq boost. I plan to make the 100K setting the maximum boost (I may tweak beyond this. TBD) and to eliminate the current maxed out setting, altogether. Once I taste test the sound with the 12pFs out of the circuit I will decide on a plan.

About those .1 caps before the power tubes though, can I change these to, probably, .02s to reduce the bass? Will this require rebiasing? I don't know that much about this end of an amp. It won't require any other components be changed, will it?

Do those Black Cat caps have any monetary value, like with mustard caps for instance?

I'm looking to get the best possible clean sound out of the amp, and it already sounds better than my other Sunn. I realize that you're not going to get great distortion sounds with just one preamp tube, but I don't plan to repurpose the reverb tube at this time. I'm just looking for great clean sounds out of this amp.

Anybody have any suggestions for what I'm doing? Or for other mods for Sunns? Just for curiosities sake.

Lastly, remember opinions about tone are subjective. Your mileage may vary.