71 SFSR w SS rectifier mod...too much low bass.

jcs

Member
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8,069
What are the best ways to reduce the very low bass in this amp?

The amp sounds fine for the most part but I can only use the bass set no higher than 3....usually all the way down.

I've tried the SFSR thru many other cabs at 4 ohms....its very bassy, which sounds good clean but not so good with much gain applied.

AFAIK, the amp is stock with some replaced electrolytics and heavy ceramic speakers....obviously the SS rectifier mod adds some more power and headroom, especially over the 5U4 that came stock.

The i think the PI cap is bigger than the BFSR.......any other suggestions for improving a 71 SFSR are welcomed!
 

robrob

Member
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409
I like to do the low freq filtering at the first coupling cap on only one channel so the other channel remains completely unmodified. This "lead channel" mod for the 5E3 will work great to do what you want, reduce lows and improve the high gain distortion tone. Doing the filtering early allows low freq harmonic distortion to make it through the amp to keep the tone from being too thin. It works great with gain boost pedals and heavy reverb and delay effects too:

https://robrobinette.com/5e3_Modifications.htm#Lead_Channel

 

zenas

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8,769
Are the bass notes farting out ? If so find out what electrolytics have been replaced because that's a symptom of weak filter caps.
 

robrob

Member
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409
I also recommend you compare the actual value of all the coupling caps to the schematic or layout to verify someone didn't "upgrade" them with a larger value for more bass response. Larger coupling caps can increase blocking distortion which can sound "farty."
 

zenas

Member
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8,769
I'm a big fan of making sure the amp is working right before modding or changing things. Tubes, bias, electrolytics are three things I want to know about.

At this point all we know is "stock with some replaced electrolytics". It's a silverface so stock means it has a bias balance pot, (unless it's very early) and it has those crappy late 60s or 70s electrolytics. Or some of them.

But whatever excuse interpretation. :)
 

swiveltung

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14,490
My guess is you are just hearing the effect of 4 x 10. It's huge on the bottom. The speakers are pushing nearly as much air as 2 x 15!
 

jcs

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8,069
My guess is you are just hearing the effect of 4 x 10. It's huge on the bottom. The speakers are pushing nearly as much air as 2 x 15!
Yes, as I stated,it has HUGE bottom end into several cabs including 4 ohm 4x12's....WAY too much...I cant get the bass backed down enough with heavy gain.
 

jcs

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8,069
Are the bass notes farting out ? If so find out what electrolytics have been replaced because that's a symptom of weak filter caps.
Electrolytics are 7 to 8 years old and don't appear to be bulging or leaking...but I may go ahead and replace....the amp doesn't hum.
 

Blue Strat

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30,185
Electrolytics are 7 to 8 years old and don't appear to be bulging or leaking...but I may go ahead and replace....the amp doesn't hum.
7-8 years shouldn't be an issue. Visual indications shouldn't be used as anything other than desperation indicators,. 90% of the bad caps I've replaced showed no visual indications at all.
 

jcs

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8,069
I'm a big fan of making sure the amp is working right before modding or changing things. Tubes, bias, electrolytics are three things I want to know about.

At this point all we know is "stock with some replaced electrolytics". It's a silverface so stock means it has a bias balance pot, (unless it's very early) and it has those crappy late 60s or 70s electrolytics. Or some of them.

But whatever excuse interpretation. :)
The amp has stock dark blue molded caps but the first 25/25 pair are replaced with some Taiwan made caps...I cant see the values....you think they are crap?

Only one brown turd cap....a .047 in front of the 25/25 replaced pair....rest are mostly navy blue stock molded ones.

There are 3 white Mallory 25/25 in the middle of the board.

Transformers are stock Fender.

Remember its a SS RECTIFIER mod.....amp always runs cool.....transformers never get much beyond warm......obviously the PT is always cool with the SS rectifier mod.
 

jcs

Member
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8,069
I bias my own amps with a Weber Bias rite with plate volt option......I have many NOS tubes that I always test in one of my testers and then monitor for stable current draw.

Bias around 55% mostly.

The darn wall volts around here worries me.....I have a variac I use with other amps set around 110 usually as the wall volts commonly gets to 122 to 125.....but I haven't been using the variac with this amp.
 

zenas

Member
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8,769
Sounds like the electrolytics are ok. I wouldn't have left any of the originals like the white Mallorys but lots of guys do.
And it sounds like you have the bias/tube thing nailed down.

Try that cap Blue Strat mentioned.
 
Messages
540
That's pretty common in a Super. I always run the bass 3 or below. I wouldn't mod it unless you can't get the sound you want between 1 and 3. It used to make me nuts that I had to keep it that low, but it sounds great, so I just don't look at the numbers on knobs anymore when I dial in my sound.
 

Steppin' Wolfe

Silver Supporting Member
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5,064
IF one is worried about wall voltages with an amp like that, one wonders why there is a solid state rectifier in the amp??? Unless that is a SS rectifier that mimics a tube rectifier.... A 5U4 as stock in that amp will lower the B+ and the amp's sonics will improve, ime.
JCS, you say that you can't eliminate enough bass when running gain. That is a problem for AB763 types of circuits. Mesa Boogie manuals have stressed this since the MKI's in the 1970's. What are your control settings?
 

jcs

Member
Messages
8,069
IF one is worried about wall voltages with an amp like that, one wonders why there is a solid state rectifier in the amp??? Unless that is a SS rectifier that mimics a tube rectifier.... A 5U4 as stock in that amp will lower the B+ and the amp's sonics will improve, ime.
JCS, you say that you can't eliminate enough bass when running gain. That is a problem for AB763 types of circuits. Mesa Boogie manuals have stressed this since the MKI's in the 1970's. What are your control settings?
Advantage of SS rectifier is the power tranny runs cool....the plate volts and B+ aren't that high at all...no higher than 450 range....nothing like my 62 Bandmaster that is over 500 plate volts.

I vary the control settings but obviously turn the bass way down at higher volumes.

Bass control never ever above 3 anytime......and that's for lower volume clean settings with the mids turned down, to get a very clean low gain/scooped mids tone.

For higher gain tones the bass needs to go down a lot to at least 1 or it sounds like crap.

I probably need a gain pedal or 2 that really lose the lower end....its obvious why the Tube Screamer is popular with Blackface Fenders!

I will hook up the TS9 and try it again....but overall it seems just too much bass.

Think I will try the .001 PI cap....I can always change it back.
 




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