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Post subject: Fender "Twin Amp" 02-03 (Non Verb) No Bias Pot Red Plating
Posted: Tue Aug 26, 2014 9:57 am
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Hey Guys - Any of you have a schematic you would share for this amp?

I had to replace the (2) 47uf 500V Caps because they were leaking and the amp was squealing (not a microphonic tube).

After the replaced caps, no more squeal but now its RED PLATING.

I've tested all the other caps and they are in spec.

I've checked out and even raised the value of the screen grid resistors but still cant get it to stop red plating.

All tubes are perfectly new (and even pulled other sets) so its not defective tubes.

Any help would be appreciated.

Ryan


Last edited by Ryan41018 on Tue Aug 26, 2014 12:49 pm, edited 1 time in total.

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Post subject: Re: Fender "Twin Amp" 02-03 (Non Verb) No Bias Pot Red Plati
Posted: Tue Aug 26, 2014 12:41 pm
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When you changed the caps the B+ onto the plate prolly went up a bit. Like you said there doesn't seem to be any easy way to adjust the idle bias. Except playing with resistor values in the bias supply or hunting for tubes that can handle the increase idle dissipation. Maybe, drop a line to Fender?

http://support.fender.com/schematics/gu ... ematic.pdf


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Post subject: Re: Fender "Twin Amp" 02-03 (Non Verb) No Bias Pot Red Plati
Posted: Tue Aug 26, 2014 4:01 pm
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No reason that the B+ would be any higher than it normally was before the cap change. You will have to measure the Plate current (or Cathode current), in order to know what you are dealing with as far as red plating. Not much point in discussing further until you know the current. Different output tubes can draw different current than the previous set, you must check the current when changing output tubes.

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Post subject: Re: Fender "Twin Amp" 02-03 (Non Verb) No Bias Pot Red Plati
Posted: Tue Aug 26, 2014 6:09 pm
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UPDATE - The amp is up and running.

What I did not know (I was blind today) is the bias pot is at the rear of the chassis not on the board. Before realizing this I changed out the ceramic resistors to 1K 5wts
and the 22ohm 1 watters to 1K ohms (for the sake of stopping red plating) but again now its up and running no red plating.

However, V11 & V12 are drifting in bias. Amp calls for 60MV but its drifting to 74.

No problems on V9 & V10, they are stable......any clues why only 2 tube are drifting?


Thanks

Ryan


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Post subject: Re: Fender "Twin Amp" 02-03 (Non Verb) No Bias Pot Red Plati
Posted: Tue Aug 26, 2014 8:09 pm
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My guess is that V11 and V12 make up one push-pull pair. And one or both of the two tubes may have poor contact with the socket. Or is going bad. Try cleaning the sockets and tube pins with contact cleaner.


Good luck! Post your results.


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Post subject: Re: Fender "Twin Amp" 02-03 (Non Verb) No Bias Pot Red Plati
Posted: Tue Aug 26, 2014 8:53 pm
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S O L V E D - there's also a small balance pot on the rear of the chassis, it was turned all the way to one side (the side that was drifting) one I balanced it - everything was stable.

THANKS GUYS!!

Ryan


Last edited by Ryan41018 on Tue Aug 26, 2014 10:33 pm, edited 1 time in total.

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Post subject: Re: Fender "Twin Amp" 02-03 (Non Verb) No Bias Pot Red Plati
Posted: Tue Aug 26, 2014 10:24 pm
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Check for a bias pot on the rear of the chassis, not on the board. Also check the balance pot, make sure that it isn't turned all of the way to one side, the side that is drifting.

The perfect ending to a bizarre thread. :lol:

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Fender "Twin Amp" 02-03 (Non Verb) No Bias Pot Red Plati
Posted: Tue Aug 26, 2014 10:34 pm
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Shim..............Smarty pants! LOL!! :lol:


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