fender amp problem

JC@ · 10-30-2009, 08:54 PM

hi
I'm having a problem with a fender clone, but a twin reverb schem is ok to illustrate it.
I get ugly distortion, and I tracked it to pin 2 for a grid of the PI tube. The signal shows distorted and flattened on the lower side of the wave.
I get correct dc on both plates but with .5 AC on them
Feedback lifted and the problems persists
Well, help me please

Lee_ranaldo · 10-30-2009, 09:21 PM

mmm

check every resistor value from the PI with a multimeter
change the electrolitic cathode bypass caps.

check that there is no soldering touching the chassis underneath the board.

i will replace those 0,1 uf poly-film or (chocolate drop) caps (that goes from the PI tube to the output tubes) because they might have DC leakage on them

But go simple first like changing pre-amp tubes, checking voltages according to the schematic.

u might have ultrasonic parasitic oscilation on the pre-amp , that cuts lots of headroom in the output stage making it to sound like a "broken speaker" without having a broken speaker

melvin · 10-31-2009, 08:26 AM

one side of the wave being 'flattened' does not seem terribly unusual. Does the output of the previous stage have the same form?

did you mean .5V on the plates of the PI with no signal input? If so, what does that look like?

And what does the amp sound like?

10-30-2009, 08:54 PM	#1
JC@ Senior Member Join Date: May 2006 Posts: 369	fender amp problem hi I'm having a problem with a fender clone, but a twin reverb schem is ok to illustrate it. I get ugly distortion, and I tracked it to pin 2 for a grid of the PI tube. The signal shows distorted and flattened on the lower side of the wave. I get correct dc on both plates but with .5 AC on them Feedback lifted and the problems persists Well, help me please

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10-30-2009, 09:21 PM	#2
Lee_ranaldo Member Join Date: Nov 2008 Location: Santiago, Chile Posts: 60	mmm check every resistor value from the PI with a multimeter change the electrolitic cathode bypass caps. check that there is no soldering touching the chassis underneath the board. i will replace those 0,1 uf poly-film or (chocolate drop) caps (that goes from the PI tube to the output tubes) because they might have DC leakage on them But go simple first like changing pre-amp tubes, checking voltages according to the schematic. u might have ultrasonic parasitic oscilation on the pre-amp , that cuts lots of headroom in the output stage making it to sound like a "broken speaker" without having a broken speaker

10-31-2009, 08:26 AM	#3
melvin Member Join Date: Aug 2009 Location: Seattle, Washington Posts: 86	one side of the wave being 'flattened' does not seem terribly unusual. Does the output of the previous stage have the same form? did you mean .5V on the plates of the PI with no signal input? If so, what does that look like? And what does the amp sound like?

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