ugly distortion in twin reverb

leejon · 01-18-2009, 08:26 PM

The vibrato channel of my twin reverb has an unintential ugly distortion. Espacially on the low notes with the channel volume over 3. (guitar is directly connected to the amp)
Since the normal channel is okay I guess it is not the power output stage.
I swapped all the tubes (no result)
I switched the normal channel and the vibrato channel by switching pin 6 of v1b and v2b (so the normal channel goes through the reverb and vibrato and the vibrato channel not) This time the normal channel distorts.
Reverb and vibrato are turned off (zero)
changed the .022 cap after V2b and the .1 cap after V4b (no results)

At this moment I'm a bit clueless. Where can I look any further.
One thing I noticed was that the voltage on pin 3 and 8 on v4 is a bit low 1.8 V instead of 2.3 V (How can I fix this?

Can somebody help me? You can find the schemantic on:
http://www.ampwares.com/schematics/1...F_Twin_Rev.pdf

Thanks
In advance

Leon

pdf64 · 01-18-2009, 09:13 PM

Does removing V3 reduce the distortion?
Re V4 cathode voltage - you can't just look at that in isolation, what are the grid, plate and supply voltages, what happens if a known good tube is put in?
Peter.

leejon · 01-18-2009, 10:36 PM

Hi Peter,
removing v3 does not help.
about the cathode voltage, turns out i was looking at the wrong schematic, its a 135W twin instead of a 100W.
http://www.ampwares.com/schematics/t..._135_schem.pdf
cathode voltage is okay (switching tudes does not give a better result)
thanks for the answer.
Leon

Alex R · 01-18-2009, 11:33 PM

sounds like you isolated the problem to V2a and the following tone stack. And you already tried a new tube in V2. So are the V2a voltages in the right area? A little high or low is ok.

Maybe swap the cathode bypass resistor - 25uF/25v.

Not an awful lot else I can think of to cause distortion in there - poor jack contacts? - noisy pots? What does it sound like?

Enzo · 01-19-2009, 04:17 AM

Leaky cap upsetting the DC levels in the stage?

01-18-2009, 08:26 PM	#1
leejon Junior Member Join Date: Jan 2009 Posts: 3	ugly distortion in twin reverb The vibrato channel of my twin reverb has an unintential ugly distortion. Espacially on the low notes with the channel volume over 3. (guitar is directly connected to the amp) Since the normal channel is okay I guess it is not the power output stage. I swapped all the tubes (no result) I switched the normal channel and the vibrato channel by switching pin 6 of v1b and v2b (so the normal channel goes through the reverb and vibrato and the vibrato channel not) This time the normal channel distorts. Reverb and vibrato are turned off (zero) changed the .022 cap after V2b and the .1 cap after V4b (no results) At this moment I'm a bit clueless. Where can I look any further. One thing I noticed was that the voltage on pin 3 and 8 on v4 is a bit low 1.8 V instead of 2.3 V (How can I fix this? Can somebody help me? You can find the schemantic on: http://www.ampwares.com/schematics/1...F_Twin_Rev.pdf Thanks In advance Leon

01-19-2009, 04:17 AM	#5
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,366	Leaky cap upsetting the DC levels in the stage? __________________ Education is what you're left with after you have forgotten what you have learned.

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01-18-2009, 09:13 PM	#2
pdf64 Senior Member Join Date: May 2006 Posts: 611	Does removing V3 reduce the distortion? Re V4 cathode voltage - you can't just look at that in isolation, what are the grid, plate and supply voltages, what happens if a known good tube is put in? Peter.

01-18-2009, 10:36 PM	#3
leejon Junior Member Join Date: Jan 2009 Posts: 3	Hi Peter, removing v3 does not help. about the cathode voltage, turns out i was looking at the wrong schematic, its a 135W twin instead of a 100W. http://www.ampwares.com/schematics/t..._135_schem.pdf cathode voltage is okay (switching tudes does not give a better result) thanks for the answer. Leon

01-18-2009, 11:33 PM	#4
Alex R Supporting Member Join Date: Mar 2007 Location: UK Posts: 677	sounds like you isolated the problem to V2a and the following tone stack. And you already tried a new tube in V2. So are the V2a voltages in the right area? A little high or low is ok. Maybe swap the cathode bypass resistor - 25uF/25v. Not an awful lot else I can think of to cause distortion in there - poor jack contacts? - noisy pots? What does it sound like?

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