Super Reverb tech question ???

[freecrowder]

Forgive me if this is the wrong forum to look for repair help but here goes.
I am working on a Fender SR circa 1969, original condition to begin with.
I have replaced all the power supply caps. I have replaced the back half(phase splitter on) of the coupling caps. The problem is that the owner said that it breaks up distorts too early. I can hear what he hears. It only does it if I hit the lower note strings on the guitar together( not a guitarist, sorry). It will do it from the normal channel and the vibrato channel. I have looked at all the resistors and coupling caps and everything is in tolerance. This will show up at volume levels in the 4-5 range on the dial. Input connectors,pots,sockets have all been cleaned and checked. Voltages at the different test points on the schematic are in spec. Overall this amp looks to be in very good condition.
I can get rid of it almost completely by changing input tubes to lower gain 12xxx
versions. Then it doesn't distort until 8-9-10 range. Am I looking at a normal amp that just has a high gain frontend?? Frontend tubes are 12ax7. Using 12at7 and 12au7 to get rid of problem. Thanks for any help or direct me to the proper forum.

 

[glassman]

Re: Super Reverb tech question ???

Breakup in the 4-5 volume range would be normal operation. The question I would have is what the waveform looks like and if it comes on quick or is it a smooth transition. Sample the waveform at each stage using a 120 Hz input signal (sine wave). Once you've found the offending stage. PM me for further instructions.

 

[freecrowder]

Re: Super Reverb tech question ???

Thanks for the help. I have all ready tested with signal generator sweeping from 20 hz through 500-600hz. I didn't hear the breakup at all. I heard smooth distortion as the tube saturates. I also noticed that when I pluck the lower strings individually it sounds the same way no breakup. It only distorts when I strum the 2 lower strings pretty aggressively. I think that the amp is working correctly and that the user will have to choose his input gain by the tube he picks. I have not looked at signal generator signal on scope but can tell it is not badly distorted with pure tone. BTW have worked on SS and tube hi end for years but new to guitar amps. Its hard getting used to the purposeful distortion. Not sure what "PM" meant. thanks again.

 

[glassman]

Re: Super Reverb tech question ???

I would say that the operation is normal. Most Fender amps start to soft clip around 3.5 to 4 on the volume control.

I know exactly what you mean about the adjustment to a different tube application. I started with guitar amps and added audio amps and juke boxes later. The basic theory is the same but guitar amps break a lot of rules. Also, there's the issue of linearity; guitar amps deliberately create a non-linear waveform...something that audio equipment doesn't do on purpose. The duty cycle is sometimes modified in guitar amps as well.

A PM is a personal message; you can get to it from my user profile.

By the way, welcome to the forum!

Jerry