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Post subject: Re: Fender Super Reverb silver face Question
Posted: Mon Oct 07, 2013 4:08 pm
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That's not what you seemed to indicate here......

jimitrader wrote:
I am following the Gerald Weber video tutorial and he says to put the mm to MA and put pos red lead on center tap and black lead on pin 3 of 6v6 tubes and compare readings. I am getting 2.2? seems the goal is 25 what am i doing wrong?



Arjay

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Post subject: Re: Fender Super Reverb silver face Question
Posted: Mon Oct 07, 2013 5:01 pm
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sorry..my bad

i am thinking that the 22k ohm resistor on the bias pot might be changed to 27k ohm to get the readings to go lower like around 24 25 would that be right?


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Post subject: Re: Fender Super Reverb silver face Question
Posted: Mon Oct 07, 2013 5:52 pm
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That resistor should in fact be a 27kΩ as per the AB763 revision. Could be the source of the issue.

Arjay

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Post subject: Re: Fender Super Reverb silver face Question
Posted: Tue Oct 08, 2013 1:01 am
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Retroverbial wrote:
That resistor should in fact be a 27kΩ as per the AB763 revision. Could be the source of the issue.

Arjay


Not in my opinion, this resistor if too low will give hot bias and with 25 ma we have very cold bias. If no mistake in reading . 22K or 27K do same job on this circuit.

---------------------------------------------
Jimitrader , what is the plate voltage ? You must know it to do a bias.

If 6L6 are too old and worn out you may have a colder bias , without any range on bias pot.

Use this calculator;

http://www.webervst.com/tubes1/calcbias.htm


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Post subject: Re: Fender Super Reverb silver face Question
Posted: Tue Oct 08, 2013 2:24 am
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Stratele, actually if you replace the line-to-ground 22k-ohm resistor with a 27k-ohm resistor, in the bias circuit --- you will get a more negative bias voltage applied to the grid. Making the bias more cold.

The greater the resistance in this resistor, the less voltage in sent to ground. The greater the negative voltage. Or something like that.

:lol:


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Post subject: Re: Fender Super Reverb silver face Question
Posted: Tue Oct 08, 2013 3:06 am
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BMW2002Ti

I know if your are working on the 22K / 27K Or the resistor feeding the diode you will change bias negative voltage . That where jimitrader must work .

But what I mean is going from a 22K to 27K is not enough to have the proper bias IMO. Need to much negative voltage.

Tubes may be worn out


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Post subject: Re: Fender Super Reverb silver face Question
Posted: Tue Oct 08, 2013 8:34 am
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BMW2002Ti wrote:
Stratele, actually if you replace the line-to-ground 22k-ohm resistor with a 27k-ohm resistor, in the bias circuit --- you will get a more negative bias voltage applied to the grid. Making the bias more cold.


The fixed-value resistor determines the useful range of the variable bias pot which itself regulates the bias voltage.

Arjay

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Post subject: Re: Fender Super Reverb silver face Question
Posted: Wed Oct 09, 2013 12:45 am
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I like to use a voltage divide between say 1/2 the OEM fixed value for the pot plus a bit more than half for the new fixed in series. That way, if the pot fails or is cranked to full hot --- there will still be negative voltage in the range. To keep the tubes from going ballistics.


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