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Post subject: Vintage Fender Deluxe Reverb Silver Face bias issue
Posted: Tue Apr 02, 2013 5:23 pm
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Greetings,

I'm having some trouble with biasing this amp, I'm fairly experienced with amp repair, just looking for some tips where to isolate the problem.

My bias pot will only turn to get -27 V to the output tubes. Disconnecting the wire from the bias board gives me a total of -45 V, which is high enough voltage to turn down to have proper current draw.

Something in the amp is loading the bias supply voltage down, and I cannot seem to find it, could anyone offer any assistance?


Thanks in advance


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Post subject: Re: Vintage Fender Deluxe Reverb Silver Face bias issue
Posted: Tue Apr 02, 2013 6:00 pm
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" Disconnecting the wire from the bias board gives me a total of -45 V. "


Which wire are we talking about? If it's the main bias supply wire, then you seem to have a short issue either with the grid-to-ground resistors or the tubes, themselves.

Or a poor contact between the soket and the tubes. Or short or poor contact in the bias pot.

Good luck! Keep us informed.


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Post subject: Re: Vintage Fender Deluxe Reverb Silver Face bias issue
Posted: Tue Apr 02, 2013 6:17 pm
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Right, if I read correctly, the bias board is feeding the right voltage. But the reading at the center lug of the 10K pot is -27vdc?...Oh Yeah, Welcome to forum! Art

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Post subject: Re: Vintage Fender Deluxe Reverb Silver Face bias issue
Posted: Tue Apr 02, 2013 9:25 pm
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Did you check the 10K resistor that connects from the bias pot to ground? With the wire disconnected and no curent draw, you would expect to get -45 VDC I should think. But with things connected ... maybe not.

http://ampwares.com/schematics/deluxe_reverb_boost.pdf

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Post subject: Re: Vintage Fender Deluxe Reverb Silver Face bias issue
Posted: Tue Apr 02, 2013 10:49 pm
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Yeah Bill, the reading needs to be clarified. IMHO That 10K resistor is the fail-safe for the bias supply, should the pot fail. But yeah, every component must be checked in a linear fashion. Just trying to ge a handle on the flow from point "A"...... or is that "-B". Art

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Post subject: Re: Vintage Fender Deluxe Reverb Silver Face bias issue
Posted: Wed Apr 03, 2013 10:31 am
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I'm currently getting 45mA of Bias current to the 6v6 tubes, (Don't worry, they are the old set and haven't been running with the tubes in, only to check bias) I get the same results with a new set as well, only had them in to check bias, I know the dangers of red plating!


Anyways, the highest negative voltage I can get from the bias pot is -27 v. Spin it the other direction, and it goes less negative. Thus making current way too high. The schematic reccomends -35

If I disconnect the wire from the bias board to the pot, I get -45V which is perfect, just to ensure bias board is sending correct voltage. But connected, gets down to -27.

I replaced the 220k resistors near the coupling caps which appeared to be a little burnt, and checked the coupling caps as well.

The resistor on the bias board and capacitor are within specs as well.

Also, the 10k resistor on bias pot to ground has not gone high either, and the voltage is relative with turning the shaft of the pot (Which ruled out a bad pot to me, but I could be mistaken)


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Post subject: Re: Vintage Fender Deluxe Reverb Silver Face bias issue
Posted: Thu Apr 04, 2013 4:22 pm
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Sounds like the pot is shot. You have another 10K-ohm-LINEAR one to try?


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