Mod to reduce bass in 5f6a tweed bassman?

[ragamuffin]

With some help from you all last year I successfully built a 5f6a tweed bassman clone in head format. I'm pairing it with a 2x12 closed back cab with two Celestion Cream alnicos and it sounds excellent, my dream amp really! The only hitch at this point is that its a little bass heavy; I generally leave the bass knob in the 0 to 1 range. Is there any simple mod that I could do to reduce the bass response just a little? Thanks guys!

 

[JRapp]

Change the tone stack to 250pF/.02/.02/56K slope. Like the original schematic.

 

[mountainhick]

Trimming bypass and coupling cap values can work wonders. The 250uf on V1 for example can easily be dropped to 22uf, or even 10uf.

 

[Michael Smith]

Have you thought about installing a port or 2 in the cabinet? I have found closed back cabinets can be a bit boomy.

 

[Lowerleftcoast]

There are two schools of thought for reducing bass in the circuit. (Use one or the other, not both.)

1. To reduce lows, Marshall would reduce the cathode capacitor in the preamp section. In the photo below it is marked in green. The original value is 250uF. If you choose this method to reduce lows, try several cap values to have a listen before you commit. Use alligator clips for the tests, then solder in the *best value*.
22uF essentially allows all frequencies through and it shouldn't be a noticeable difference from the 250uF cap. Marshall often used 0.68uF which cuts a lot of lows. Imo, I bet you will land somewhere between 2.2uF and 4.7uF but 10uF is popular as well. (Cap values add when in parallel so a 2.2uF parallel with a 3.3uF = 5.5uF.) (There are only a couple of volts at this location. Typically we see 25v rated caps here, but 5v would be enough. Price may be cheaper for certain voltage ratings.)

2. Fender would reduce lows by reducing the cap values pre and/or post the phase inverter. In the photo below they are marked in white and blue. You might try 500pF to .001uF or so to replace the .022uF (marked with white). You might try 0.022uF to 0.047uF to replace 0.1uF caps (marked in blue). FYI the Showman used 500pF and the Tremolux used .001uF before the phase inverter (cap marked in white) yet maintained the 0.1uF (caps marked in blue) after the PI. Use the same or higher voltage rating as the original caps.

For the bass reduction you have described, I would probably choose #1. OMMV

 

[ragamuffin]

There are two schools of thought for reducing bass in the circuit. (Use one or the other, not both.)

1. To reduce lows, Marshall would reduce the cathode capacitor in the preamp section. In the photo below it is marked in green. The original value is 250uF. If you choose this method to reduce lows, try several cap values to have a listen before you commit. Use alligator clips for the tests, then solder in the *best value*.
22uF essentially allows all frequencies through and it shouldn't be a noticeable difference from the 250uF cap. Marshall often used 0.68uF which cuts a lot of lows. Imo, I bet you will land somewhere between 2.2uF and 4.7uF but 10uF is popular as well. (Cap values add when in parallel so a 2.2uF parallel with a 3.3uF = 5.5uF.) (There are only a couple of volts at this location. Typically we see 25v rated caps here, but 5v would be enough. Price may be cheaper for certain voltage ratings.)

2. Fender would reduce lows by reducing the cap values pre and/or post the phase inverter. In the photo below they are marked in white and blue. You might try 500pF to .001uF or so to replace the .022uF (marked with white). You might try 0.022uF to 0.047uF to replace 0.1uF caps (marked in blue). FYI the Showman used 500pF and the Tremolux used .001uF before the phase inverter (cap marked in white) yet maintained the 0.1uF (caps marked in blue) after the PI. Use the same or higher voltage rating as the original caps.

For the bass reduction you have described, I would probably choose #1. OMMV

View attachment 1213561

Thank you, this is extremely helpful! I think I will try reducing cathode capacitor. I will order some caps and report back.

 

[mountainhick]

Thank you, this is extremely helpful! I think I will try reducing cathode capacitor. I will order some caps and report back.

Another thing to consider going that route: You can separate the V1 cathodes into two individually voiced channels. Instead of 820R and 250uf to both cathodes, one side might be 1.5K and 10 or 22uf (Fender-ish), and the other 2.7K and 0.68 (marshall-like, tighter, more bass/flab cut early which tightens overdrive response). I do this often in various amps.

 

[Pete Farrington]

Regarding the coupling cap value between the input stage anodes and the channel volume controls. 22nF into 1M forms a high pass filter that only rolls off the deep sub bass.
If the goal is to tighten up the low end a smidge, 2n2F might be more appropriate.

 

[Lowerleftcoast]

If the goal is to tighten up the low end a smidge, 2n2F might be more appropriate.

Good call Pete.

If you are purchasing caps... as Pete said .0022uF would be a good value to choose. You could also try .001uF for a little more bass reduction.

The coupling caps are shown in yellow below.

Another thing to consider going that route: You can separate the V1 cathodes into two individually voiced channels. Instead of 820R and 250uf to both cathodes, one side might be 1.5K and 10 or 22uf (Fender-ish), and the other 2.7K and 0.68 (marshall-like, tighter, more bass/flab cut early which tightens overdrive response). I do this often in various amps.

The coupling caps mentioned above can also be used to separate the channels, with or without the mod offered by mountainhick. Iow, these two caps do not have to be of equal value. Further reading... Robinette's 5F6A Mods site has similar info titled *Voice a "Lead" Channel*.

5F6A Mods

5F6A Bassman Amplifier Modifications

robrobinette.com

 

[SoK66]

Change the tone stack to 250pF/.02/.02/56K slope. Like the original schematic.

THIS! ^

 

[JRapp]

THIS! ^

I've done that mod to a few of the RI Bassmans and they sound a lot better to me and the folks I did it for, not so boomy and more mids. Add in a 5R4 to drop some B+ and it sounds pretty good IMHO. Flame away...

 

[Pete Farrington]

Change the tone stack to 250pF/.02/.02/56K slope. Like the original schematic.

I’m confused
As you note, those component values are as per the schematic of the amp stated in post 1 that was built.
Did the amp design not comply with the I-EG 5F6A https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a_schem.pdf

I successfully built a 5f6a

 

[ragamuffin]

Regarding the coupling cap value between the input stage anodes and the channel volume controls. 22nF into 1M forms a high pass filter that only rolls off the deep sub bass.
If the goal is to tighten up the low end a smidge, 2n2F might be more appropriate.

Good call Pete.

If you are purchasing caps... as Pete said .0022uF would be a good value to choose. You could also try .001uF for a little more bass reduction.

The coupling caps are shown in yellow below.

View attachment 1213807


The coupling caps mentioned above can also be used to separate the channels, with or without the mod offered by mountainhick. Iow, these two caps do not have to be of equal value. Further reading... Robinette's 5F6A Mods site has similar info titled *Voice a "Lead" Channel*.

5F6A Mods

5F6A Bassman Amplifier Modifications

robrobinette.com

Ok thanks guys so what would be the difference between changing the cathode cap vs coupling caps

 

[2L man]

Ok thanks guys so what would be the difference between changing the cathode cap vs coupling caps

I can't say exactly what changes effect how.

Shared cathode bypass change will change all inputs. There are two coupling capacitors and you can tune more difference between Normal and Bright changing only other coupling capacitor.

But then separating cathodes to individual circuits allow even more possibility to tune channels without much more complexity. I always do that in my builds initially. My memory say that 470nF cathode bypass capacitor has highest range in guitar fretboard frequencys attenuating more 82Hz vs. 1kHz. But when there come lots of harmonics typical 680nF is used often in bright channel.

 

[Jasonpatrick]

I’m confused
As you note, those component values are as per the schematic of the amp stated in post 1 that was built.
Did the amp design not comply with the I-EG 5F6A https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a_schem.pdf

There are updated schematics that show the true as found in the wild values of real 1958-59 amps (100k slope and .1 bass cap and I think there is another resistor change along with different pots. Maybe they used one of those?

 

[drew1d]

Have you thought about installing a port or 2 in the cabinet? I have found closed back cabinets can be a bit boomy.

Pairing the amp to a speaker you think compliments the sound, or, change the amp to match the cables. I would think that changing the speaker would be easier (but more expensive) but changing the amp would be more fun. The comprise would be a pedal that equalizes what sound you’re looking for before the amp. But what’s the fun in that?

 

[Jasonpatrick]

But, if you want to reduce bass, I’d use what people above has suggested. Lower that cathode cap to like 4.7uf. I bet that will get you where you want to be. The closed back cab is causing your problem to be honest. I’d also try what @Lowerleftcoast said about changing the .02 to .001 or 500pf.

 

[Jasonpatrick]

Ok thanks guys so what would be the difference between changing the cathode cap vs coupling caps

My hot take, Changing the cathode caps takes care of the low frequency’s you want it cut early on. But then that’s all the palate you have to work with. Changing the coupling caps helps you sculpt and fine tune the full spectrum of frequency’s in the amp and what each component adds and subtracts in that equation of the final sound wanted to be achieved. I think. Do not quote me on this lol

 

[2L man]

This link has the graph I search when I ponder pre amp cathode bypassing but there are other as well.

The Valve Wizard -Cathode Follower

 

[Lowerleftcoast]

Ok thanks guys so what would be the difference between changing the cathode cap vs coupling caps

The cathode cap will only cut about maybe 6dB of bass before it hits the lower shelf frequency. It is dropping a little less than 2dB per octave.
Marshall's trick was keeping bass due to that shelf and mostly distorting the frequencies above it. The leftover bass would be amplified making the *thick* sound Marshall, and now others, are known for. (Distorting bass frequencies makes *mud*.)
Here is a Cathode cap calc. It is an interactive version of the Valve Wizard's diagram:

Cathode Bypass Capacitor Calculator

www.ampbooks.com

The Coupling Cap works with Resistance forming a First Order High Pass RC Filter. A First Order RC Filter drops 6dB per octave. (There is not a lower shelf.)
There are RC filter calculators available online. (In circuit, RC filters can be tricky to decipher.)