SWR Bass 750 no bass, cap values?

[glebert]

I've got this Bass 750 that I bought as a project. Replacing the preamp tube got it working, but the low end is really weak. If I put test tones into it the voltage amplitude (at speaker out and effects send) rolls off considerably starting at about 1k. Since it happens already at effects send I know it is a preamp problem. I can get full swing power out of the power amp FYI. Going into the passive/active input the voltage amplitude at 2k is about 3x the ampilitude at 100Hz. Changing to a different preamp tube does not change this.

Probing through the circuit I couldn't find any one point where the amplitude difference was as big as I see for the whole preamp, it is more like it happens in stages. I then looked closer at the coupling caps and it looks like C15 and C23 are 0.022uF caps on the board, instead of the 0.1uF caps that are on the schematic. BUT, if I look at the BOM in the service manual it calls out C15 as 0.022, but does not call out C23 at all that I can find.

My preamp board is dated March 1999, the schematic is May 1999, so there is some possibility there were changes. This amp came out in 1999 so this may be a very early one. Looks like some work may have been done on the board, but the caps don't look like they have been noticably reworked. Note, the amplitude difference does not seem as bad going into the active input, which makes sense since does not go through C15 then.

Has anyone seen anything like this before? Can't imagine that someone bought this amp with such weak low end and played it for 20 years. The guy I got it from made it sound like he had played it regularly before putting into storage.

SWR_Bass 750 Bass Amplifier_SM.zip

[glebert]

Just realized it probaby can't be a problem with C15, as the response is flat at the tuner output when going into the passive/active input. I should also note that the voltage measurements I took were with the "aural enhancement" full off CCW. Reading about this tone control it supposedly brings up the fundamental frequencies and brings down harmonics, so I did try adjusting it but there were no positions I found that gave anything close to a flat response, usually the high frequencies amplitude increased as well.

If this is just because I don't know how to use the knobs on this amp I am going to feel dumb.

[glebert]

OK, pretty sure I figure it out, and I am just ignorant. I think what I was seeing wasn't so much a bass rolloff as it was a treble boost, which might be part of the "SWR sound". That combined with a pretty weak gain front end that seems like it really wants hot bass pickups makes it seem very treble heavy when you don't push the front end. I plugged in a passive Yamaha 5 string, had the gain maxed, and had to mash a power chord really hard to get the front end to clip.

[Jon Snell]

I would change C4 and C8 to ensure full low frequency response. C15 forms part of the Zobel network to aid HF stability.

[glebert]

I would change C4 and C8 to ensure full low frequency response. C15 forms part of the Zobel network to aid HF stability.

I think we are talking about different things, but you helped me figure out the BOM issue. The BOM is only for the power amp module. The C15 that I am talking about is in the preamp, which is the last page of the service manual. Still doesn't explain why the cap values on my preamp are different than the schematic, other than sometimes schematics are wrong.

[Enzo]

Not wrong, just different age. Many amp circuits change over time, and they don't issue a new schematic every time a part changes. Some companies do update now and then, and you see a revision A or B down in the corner. many schematics if not most have a small note somewhere that says specifications can change at any time.

[nevetslab]

SWR 750 Rev D 2002 Schematics attached

I think we are talking about different things, but you helped me figure out the BOM issue. The BOM is only for the power amp module. The C15 that I am talking about is in the preamp, which is the last page of the service manual. Still doesn't explain why the cap values on my preamp are different than the schematic, other than sometimes schematics are wrong.

I've attached my file copy of the SWR 750X Preamp and Power Amp schematics, just in case these offer any additional details, being Rev D 2002 Preamp and Rev E Power Amp 2002.

750x Preamp Rev D Sch.pdf
750x Power Amp Rev E Sch.pdf