Any thoughts on how to "soften" attack on a Fender Bassman ?

[Chevy]

Hi, subjective question here...
Have a 1972 Fender Bassman 50, got it sounding pretty much like I want, but was hoping someone may have an idea or two on how I might "soften" the attack of higher frequency notes... not sure it's even possible... and I know this may fall very much into the realm of speakers, but was wondering if there is something that can be done on the amp side of things. I'm using the typical 27K / 4.7K feedback resistors. Amp has diode rectifiers. New JJ 6L6GC output tubes. Biased at around 35 mA. B+ around 470V. Decent old stock preamp tubes.
Is there some generic thing that may cause an amp to seem like it has too much "attack" (on the high end) ? Or is this perhaps just a frequency response (EQ) type of thing ? And again, I'm just referring to the amp... I know some speakers sound much spankier than others.
Any ideas would be most welcome !

[Jazz P Bass]

Just a thought, but have tried lowering the bias to Fender standard 25ma's?

[The Dude]

Are you using it for bass or guitar? If bass, you might try running a compressor in line. It'd work for guitar also, but depending on what sound you're looking for, it might be a bit much.

[Tom Phillips]

Just to be sure. Is your amp a stock "Bassman 50" using the circuit I have attached?

Some thoughts...
Since most of the Bassman head amps do not seem to suffer from the problem you describe. I wonder if your amp has been modified over its life or if there is just something wrong. The Bassman 50 attached uses a different feedback circuit and a different gain stage before the PI than most of the various Fender Bassman circuits. Many people decide to convert the circuit to one of the earlier Bassman versions. Some of these conversions require reversing the phase of the feedback because the injection point is different. If someone tried that at some time in the past on your amp then maybe there are remnants of mods left over. Does your amp stay stable if you turn all the volume and tone controls all the way up?

[Chevy]

Just a thought, but have tried lowering the bias to Fender standard 25ma's?

No... just down to 30mA, and no difference there.

[Chevy]

Are you using it for bass or guitar? If bass, you might try running a compressor in line. It'd work for guitar also, but depending on what sound you're looking for, it might be a bit much.

Guitar... yeah, I guess that's an option. But I have run across other vintage Fender guitar amps with the same issue... like a Pro Reverb for example. Never found out what the deal was with that amp, either.

[Chevy]

Just to be sure. Is your amp a stock "Bassman 50" using the circuit I have attached?
[ATTACH=CONFIG]29311[/ATTACH]
Some thoughts...
Since most of the Bassman head amps do not seem to suffer from the problem you describe. I wonder if your amp has been modified over its life or if there is just something wrong. The Bassman 50 attached uses a different feedback circuit and a different gain stage before the PI than most of the various Fender Bassman circuits. Many people decide to convert the circuit to one of the earlier Bassman versions. Some of these conversions require reversing the phase of the feedback because the injection point is different. If someone tried that at some time in the past on your amp then maybe there are remnants of mods left over. Does your amp stay stable if you turn all the volume and tone controls all the way up?

Yes, it has been modified to a tweed 5F6A circuit before the PI. It is very stable at any control setting, and I've gone over the rest of the circuit carefully, and can't find anything out of place or incorrect, or left incomplete. Can't get it to oscillate at any frequency from 60hz to 8khz on the 'scope either, max power, 4 ohm dummy load. The amp plays very well, lots of balls, but with a top end attack that I'd like to soften. Tried some Groove Tube GT-6L6-R tubes as well, and that didn't help. I recalled that a tube rectifier was supposed to give an amp a smoother, spongier feel, wondering if this is what I'm looking for. I tried a series resistor with the B+ (think it was 100 ohms), but that didnt' seem to do it either... maybe there is more to the rectifier tube than just a fixed R.

[Tom Phillips]

Yes, it has been modified to a tweed 5F6A circuit before the PI...

That's quite a different thing than a Bassman 50. Did you include the presence control? Is the the high end currently "icepick" sharp?

[Chuck H]

Almost anything you do to add compression is going to have a more profound effect on the bottom end than the top. That is, softening the attack will also, and more notably, limit bass clarity and dynamics. The amp would need to be revoiced to compensate and the end result would be lower power overall. To stick with simpler tweaks, maybe try lowering the preamp voltages a little. You could also try darker preamp tubes. I find the NOS short plate GE 12ax7 tubes and Sovtek 12ax7wa and wc tubes are short on top end dynamics.

[Leo_Gnardo]

Quick cheap experiment: reduce the voltage for preamp stages. Simply replace the dropping resistors in the power supply with larger values, say about double what's there. If that doesn't help put 'em back to what they were. Should take 15-20 minutes even on a slow day.

[Chevy]

That's quite a different thing than a Bassman 50. Did you include the presence control? Is the the high end currently "icepick" sharp?

Yes, presence control is there; 27K feedback R, 4.7K pot, 0.1uF cap. Hmmm... don't know if I'd call it icepick sharp... unless you roll up the presence control... this is subjective, but I'd say yes there is too much of some top end frequency in the overall balance, even with the presence off completely; but WHICH frequency or frequencies, I just don't know. I don't like playing guitar with the treble turned down much either... I really like the chime and shimmer. What seems to happen is some of the high frequencies jump out ("attack") at you in not a pleasant way... make sense ? I'd just like them to be softer.

[Chevy]

Quick cheap experiment: reduce the voltage for preamp stages. Simply replace the dropping resistors in the power supply with larger values, say about double what's there. If that doesn't help put 'em back to what they were. Should take 15-20 minutes even on a slow day.

I was wondering about this... will do... do higher voltages make the tubes more linear or something ? Less compression ?

[Chevy]

Almost anything you do to add compression is going to have a more profound effect on the bottom end than the top. That is, softening the attack will also, and more notably, limit bass clarity and dynamics. The amp would need to be revoiced to compensate and the end result would be lower power overall. To stick with simpler tweaks, maybe try lowering the preamp voltages a little. You could also try darker preamp tubes. I find the NOS short plate GE 12ax7 tubes and Sovtek 12ax7wa and wc tubes are short on top end dynamics.

Ahhh.... now there's a good one... thanks, for the tube tips... didn't think about the tube compression part... will try get hold of a couple of those tubes. Any other suggestions for tube types will be appreciated... don't have to be 12AX7's; 12AY7, 5751, maybe even 12AT7 would work fine I guess, especially if they compress the top end. Good idea !

[g1]

Tubes compress because they run out of headroom due to limits of the power supply. So yes, decreasing supply level will increase compression.

[Chevy]

Tubes compress because they run out of headroom due to limits of the power supply. So yes, decreasing supply level will increase compression.

Makes sense, but won't all audio frequencies would be compressed the same amount ? It's just the top end I'm concerned with. Also, you'd have to drive the preamp stage close to the power rail limits to take advantage of power supply compression... not something that would be available at any volume setting.