Increase gain on Fender Bassman 150

[glebert]

Picked up a Fender Bassman 150 solid state bass combo as a project. The only problem with it is dirty effects jack, so that is good. Jumping the effects jacks together the amp works, but can't get much umph out of the front end. If I dime the preamp volume I can't get the preamp clip indicator to light until I yank an open E as hard as I dare. The gain is "right" according to the schematic, when I put in 100mV input I get the right AC magnitude at the tuner out and the effects send. Basically it seems like it was designed for hotter output basses than mine.

I would like to add some extra gain in one or two op amp stages. It is preferable to reduce a resistance rather than increase, it is easier to experiment with values if you can put something in parallel with the existing one. Thoughts?

Schematic:

[Enzo]

Easiest way to add a couple stages for more gain is...use a pedal.

[glebert]

Forgot to mention that I have verified that the 6dB cut switch does work and it just makes the gain issue worse.

Yes, a pedal would help. That isn't an amp tweak though.

[dmeek]

You could parallel a resistor across R2 which will boost the gain when S1 is closed (out). Try 1K, and parallel a 100uF cap across C2 to maintain low end response.

https://music-electronics-forum.com/...1&d=1582153314

[g1]

Just to be sure, when you had proper level showing at effects send, you also had 10VAC at the 4 ohm load?

[glebert]

Just to be sure, when you had proper level showing at effects send, you also had 10VAC at the 4 ohm load?

No, I didn't check through the power amp. I just like to be able to get full range of the preamp, so think I should be able to get it to clip when played hard and the gain up. I did try putting a standalone mic pre in between the bass and the input and 6-8dB of extra gain really filled it out nicely.

[olddawg]

Have you tried using a bass with active pickups? If it sounds good and you don’t want a pedal... you might want to build an LPB1 on some perf board and add a switch to take it in and out of circuit.

[g1]

No, I didn't check through the power amp. I just like to be able to get full range of the preamp, so think I should be able to get it to clip when played hard and the gain up.

Yeah, I get it now. Also remembered I have a 400 version of that that I parked away because it's so anemic.
Pulled it out and found pretty much the same as you, but with input gain at 8 out of 10 so my pickups are probably a bit hotter than yours. Wide open I can get red light ok.
If you try dmeek's mod, looking forward to hearing about how it works out for you.

[glebert]

Yeah, I get it now. Also remembered I have a 400 version of that that I parked away because it's so anemic.
Pulled it out and found pretty much the same as you, but with input gain at 8 out of 10 so my pickups are probably a bit hotter than yours. Wide open I can get red light ok.
If you try dmeek's mod, looking forward to hearing about how it works out for you.

At least for me the 1k parallel resistor is great. I tried a 4.7k resistor first, thinking 1k would be too big a change, but dmeek was totally right. I did not do the bypass cap he recommended because I thought this amp was too boomy anyway (especially with a single 12" speaker). Seems just about right to me now. Hope to use it at jam tonight.

[glebert]

Finally got a chance to take this amp to a jam. Not sure about it. Setting the preamp gain to be about where I think it should be and it starts to distort when I dig in even a little. The distortion kinda worked OK when I was soloing, but was unexpected. Found the best sound and levels I could get was to just dime the master volume and then set my volume level with the preamp gain. Kinda of like you might with an MV tube amp. Tried working with the compression ratio and contour settings. Jury is out on those. Sound seems pretty scooped in general to me. My E and A strings both gave very fat sound and my D and G were thin, which again worked OK for soloing as it sounded more guitar-like. Thinking I am just going to go back to my Peavey Max 115, maybe put some extra handles on to make it easier to carry and load.

[The Dude]

That sounds like it could be a pickup height issue. Maybe you just need to adjust your pickups. Have you checked string to pickup distance on all strings?

[olddawg]

Use a compressor pedal.. One that has more than unity gain.

[glebert]

That sounds like it could be a pickup height issue. Maybe you just need to adjust your pickups. Have you checked string to pickup distance on all strings?

Haven't touched the setup on this bass in about 20 years and am not going to do it to fit an amp I don't even know I like. Tone is killer with the Peavey Max and plenty of other amps.