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Help! My LJ4 has a lisp!

tubeampking

tubeampking

Chronic dabbler
Hello again. Just moments ago my Grommes LJ4 developed...can you believe it...a lisp!!?? Yup...a lisp. Seriously, words starting or ending in "s" have a hiss-like sound to them. I noticed earlier that with no signal and the volume up I was getting some significant wash. Coupling cap or maybe a plate resistor on an output tube? I changed out all but the rectifier, so it's not a tube. Any help is greatly appreciated...:tears:
 
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Well the only thing that's changed is the tubes so may not want to rule them out yet. I think I'd try swapping the originals back in stage by stage. It would be a quick test anyway even if it is something else.
 
I'll give that a try. It seemed to happen after it got pretty warmed up, so I'm hoping it's not the output tranny.
 
Manifestation

I tried replacing tubes stage by stage in both directions and this manifestation still persists. On a live recording, applause had an overlay of...for lack of a better description..percussive sound. Like someone clapping with a microphone in their hand, or wind across an open mike. :wtf:

" Curiouser and curiouser " Pooh Bear
 
Sibilance can be the result of a high frequency oscillation riding on the signal. It could be hard to track down without some test equipment. Has the amp been upgraded or rebuilt? It may be time...
 
I did recap the power supply recently and doubled the capacitance at the plates and screens of the outputs. Could that be a factor?
 
If you didn't do the coupling caps i would suspect on of them is leaky changing the bias of the output tubes , have heard similar in tube radios when the coupling cap to the output tube was leaky .
 
That was going to be my next course...to replace the coupling caps. There's not many in there and I have the proper values here, so....away we go !! :banana:
 
Success is Sweet

Topeka....I have done it ! :banana::banana: Changed out the coupling caps and the lisp (sibilance ?) is gone. Thanks to everyone who gave me input on this...great to have my LJs back together. Next I'll do the same to the other just to have them on level ground. Peace !!! :scratch2: + :music: = :D
 
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