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Post subject: Amp is producing unpleasant bass distortion.
Posted: Mon Dec 17, 2012 4:38 am
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Recently, I decided it was about time to change the Electrolytic filter caps on my Quad Reverb (basically a Twin Reverb with 4 speakers) as I noticed they were stock from the 1970's and had begun to bulge and leak a bit.
After changing out the stock Mallory caps with new F&T Germany caps, I noticed the amp would produce out of tune low-frequency distortion when I cranked the volume past about 4, getting worse the closer I got to 10. I then switched the new caps back with the old Mallorys, to see if I had just got some bum caps. That seemingly fixed the problem in the Normal channel, but the Vibrato channel still produces the awful low-freq distortion which wasn't there before I swapped those original capacitors.
Any suggestions on how to fix this problem? :cry:


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Mon Dec 17, 2012 7:15 am
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Do you check bias with new capacitor ? Voltage could be higher. Do you use same value


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Mon Dec 17, 2012 8:08 am
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stratele52 wrote:
Do you check bias with new capacitor ? Voltage could be higher. Do you use same value


+1

As well, did you check the power-rail resistors under the doghouse for component drift when you replaced the filter caps? Power-supply issues could be the source of your IM distortion.

And check to see if a bias-balance adjustment is needed.

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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Mon Dec 17, 2012 8:06 pm
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Also, did you change the electrolytic bypass caps? An increase in mfd here may lead to bass heavy and even speaker "farting" tones. I usually stick with OEM mfd value cap with incrazse in voltage spec (for instance, 25VDC rating to 50VDC).

Good hunting!


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Mon Dec 17, 2012 9:35 pm
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Why would any of these problems come up only after I changed the caps, and persist when I changed back to the old original caps?


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Mon Dec 17, 2012 9:43 pm
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Also, any explanation on why it would be only effecting the Vibrato channel?


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Mon Dec 17, 2012 10:27 pm
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Notniko wrote:
Why would any of these problems come up only after I changed the caps, and persist when I changed back to the old original caps?


Notniko wrote:
Also, any explanation on why it would be only effecting the Vibrato channel?


Simple answer: You did something wrong the first time you changed the caps. The second time you changed them you apparently corrected the problem in one channel but not the other.

Now would be a good time to sit down with the schematic and go through each circuit component by component to find your mistake. If you don't have a schematic, there are three listed here for the Quad Reverb. Pick the correct one and start tracing.

http://www.webphix.com/schematic%20heav ... ender.html

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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Mon Dec 17, 2012 10:33 pm
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bluesky636 wrote:
Notniko wrote:
Why would any of these problems come up only after I changed the caps, and persist when I changed back to the old original caps?


Notniko wrote:
Also, any explanation on why it would be only effecting the Vibrato channel?


Simple answer: You did something wrong the first time you changed the caps. The second time you changed them you apparently corrected the problem in one channel but not the other.

Now would be a good time to sit down with the schematic and go through each circuit component by component to find your mistake. If you don't have a schematic, there are three listed here for the Quad Reverb. Pick the correct one and start tracing.

http://www.webphix.com/schematic%20heav ... ender.html

Thanks much, I'll see if any of this fixes the problem.


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Tue Dec 18, 2012 3:38 am
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Notniko wrote:
Also, any explanation on why it would be only effecting the Vibrato channel?


Bias voltage effecting the vibrato circuit.


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Tue Dec 18, 2012 7:21 am
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stratele52 wrote:
Notniko wrote:
Also, any explanation on why it would be only effecting the Vibrato channel?


Bias voltage effecting the vibrato circuit.


The three schematics in the link I provided show that the trem uses the opto-bug type circuit, not bias modulation.

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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Tue Dec 18, 2012 7:35 am
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You are right , my mistake is tremolo circuit is connected to wiper of the balance ( bias) pot. But this go to ground, nothing to do with bias.

IMO this part of shematic is poorly drawn, poorly represented. That's CBS area


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Tue Dec 18, 2012 3:31 pm
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" Also, any explanation on why it would be only effecting the Vibrato channel? "

OK... gotta really study the schemo. But, if you look below point "B" which is below the Master Volume, you'll see the last two electrolytic caps in you PS rail (20mfd/500VDC). Point "B" feeds the vibrato circuit. So, it's possible a bad cap (yes, new caps sometimes are bad) or a mis-wiring has lead to funky vibrato as well as the Input stages.

I'd love to know WHO drew this schematic. One of the the most serpentine ever.

http://www.webphix.com/schematic%20heav ... strvol.pdf


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Sun Dec 23, 2012 3:50 am
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Is it possible I damaged one of the resistors under the doghouse? Would a damaged resistor cause that sort of bad distortion?


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Sun Dec 23, 2012 4:18 am
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Forgot to specify this as well: The model I have is the version with a Master Volume, and the Master Volume has no effect on distortion, the distortion only gets better or worse depending on how high I have the Pre-amp volume on the vibrato channel itself, which initially had me thinking I just had a tube go microphonic or something.


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Post subject: Re: Amp is producing unpleasant bass distortion.
Posted: Sun Dec 23, 2012 4:48 am
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You can try rotating 12AX7 from socket to socket. Or insert a new one into each socket, one-at-a-time... to rule out microphony.

Yes, damaging a resistor (really anywhere) can lead to poor tonal response. Better to rule-out the tubes, first.

Good luck!


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