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SF Super Reverb low freq. really muddy when turned up

4/15/2003 3:55 PM
SF Super Reverb low freq. really muddy when turned up
I tried 4 new Jensens in place of the CTS's to see if it was the speakers, retubed n biased, still ugly on both channels when turned up over 4. I considered trying one of my many 12AU7's in the PI position as a quick experiment to see if it was an issue with PI gain. Would this be possible/safe to try? Any suggestions on where else to look?  
4/15/2003 4:01 PM

Forgot to mention, I also replaced the filter caps. I have 2 Angela 18uf Fast caps in parallel ( in place of the 2 Sprague 35uf electrolytic's in series, balance resistors removed of course, and the next 3 filter stages are all Angela 18uf fast caps as well.)  
Thanx again,  
4/15/2003 6:40 PM

Bill, I have a possibility based on my limited experience. I like Fender circuits for clean playing. I have an old Seymour Duncan KTG-1 preamp, have owned an Allen Old Flame head and have two AB165 Bassmans. The Seymour Duncan preamp has the exact same circuit as the classic Fender for its clean channel. I used to plug this into the return jack of my Soldano HR50 to play clean. It actually sounded pretty good. Not great, but pretty good.  
The point to this is that the KTG-1/Soldano and the Allen Old Flame did exactly what you're describing. My old Bassmans with their original 'lytics do not. The Soldano has a lot of uF for the input cap. The Old Flame had a 47uF for the input cap, which is still much more than traditional Fender. There was no other difference in the preamp circuitry that was used. All coupling caps were the same values and here's the kicker - they were all played through the same cabinet.  
This is what has me mostly convinced that a lot of the old "magic" to the Fender sound is the crappy 'lytic arrangement on the first filter node. 35uF and doubling the ESR.  
There's someone on this board that uses those fast caps (I think it might be Matthew Springer?) and he has reported that to get the same basic response, tone, etc. you actually need to use a lower value than the regular aluminum 'lytics. By taking the two 18uF Solens and putting them in parallel, you're improving the ESR which is already way ahead of the typical atom type. So you sort of have a Super Duper Solen, if you will, at 38uF.  
Just to prove I'm full of crap and because it's so damn easy, why don't you pull one of the two in parallel and see what that does? It won't do any harm and it sure will tell you right away if over filtering is some of or all of your problem.  
4/15/2003 4:42 PM
Chris ( CMW amps )

Hi Bill  
Seems like the famous Fender farting .  
1st Blackface the circuit , if you're still have problems after that you can do a couple of things :  
- lower the coupling caps between PI and 6L6s ( let's say 0.047uF or lower )  
- lower the coupling caps from normal-channel 2nd stage ( try 0.022uF ) and vibrato-channel 3rd stage ( try 0.047uF )  
- lower the cathodebypass-caps to 4u7 or lower  
Hope this helps ,  
4/15/2003 7:29 PM
e.z. rider Re: SF Super Reverb low freq. really muddy when turned u
I find the .02 ceramic midrange cap and the .1 bass cap (usually that big brown crappy cap) contribute a lot to the bass fart syndrome. Put good caps in there. The Mallory 150's are good.  
Check out Steve Ahola's site, he pdf'd a whole thread about the bass fart thing.
4/23/2003 6:10 PM
Jack Koochin

Sorry for the late response, Bill, did you get this sorted out ? What was the cause ?  
If not sorted out, perhaps the following might help narrow things down:  
- try replacing the coupling cap between V4 and the PI with a 500pF cap or so.  
If this gets rid of the mud, you know the problem's in the preamp stages.  
If not, the problem's likely in the PI or output stages. This of course still leaves the power supply as well.... but it's beyond my experience that a little too much uF in the PS will cause major mud.  
- if the problem's in the preamp, you might try moving the 500pF cap to different locations to see if you can narrow down the culprit(s). I've read that excessive cathode capacitance could cause mud, as well as bad coupling caps.  
- if the problem's not in the preamp, try lifting the feedback loop wire to see what effect this has.  
- because the problem only occurs above 4, I wonder about some sort of weird oscillation.... do you have access to a 'scope?  
Hope this is of some use,  
4/26/2003 1:59 PM
Paul M.

Another possibility is worn out speakers...  

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