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Post subject: How to bias a 1983 Fender Concert II
Posted: Mon Jul 26, 2010 11:56 am
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Hello,

I need to find out how to bias the Concert amp, install a variable pot to adjust on the fly and what the specs are.

Many thanks in advance.


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Posted: Sun Aug 01, 2010 7:07 am
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Anyone?


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Posted: Sun Aug 01, 2010 12:47 pm
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Looking at the schematic, it appears that Fender MAY have place a sort of auto-bias circuit into this amp (if I have the correct schematic). There are diodes placed around the bias power supply circuit (see the taps left of the 6L6GC's?). I believe there are extra circuits to control the amount of negative voltage MAYBE in sort of a feedback circuit from the cathode current draw.

Do you have any owner's manual with this amp wich may clarify this issue. Looking at the schemo & knowing the pain in working with printed circuit boards, I'd be very careful about putting in variable pots without really knowing how this bias power supply works. Just my 2¢ worth.

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


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Posted: Sun Aug 01, 2010 1:23 pm
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Forgot to mention in is a Rivera 83 Concert II. And thanks for the lead.


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Posted: Sun Aug 01, 2010 6:54 pm
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Okee dokee. Here's a schemo I was able to find on the Net. Looks like the amp comes with a bias balance pot, stock. This is to help balance non-matched power tubes. But, no provision for changing the negative voltage generated by the bias power supply. You'd prolly need to replace that 1.5k-ohm/5watt set resistor after the 1A/800VDC diode with say, a 1k-ohm/5 watt linear pot and a 500-1000 ohm/5 watt resistor in series. Check the wiring on the bias circuit of the SF Super Reverb (AB868), as an example. Note the voltage divide tap where the negative voltage is tapped from on the SF Super Reverb.

If you use matched tubes in the power stage, maybe simply copying the Super Reverb bias power supply circuit. Removing the current balance pot circuit and replacing wiring and this opening for the bias pot. Using all the values of the SF Super Reverb resistors, caps, and pot. Would be the easiest way to add idle bias adjustment to your present amp. But, the power MUST be matched in that case (cathode draws, say, within
10-15% of each other).

In any event the negative voltage, around minus 51 volts VDC applied to the grids is what you NEED to have after all the circuit mods. If you add the BIAS pot to the BALANCE pot, keep this is mind. That is the ultimate goal. Then, additional bias pot adjustment added should be able to vary the negative voltage for this FIXED BIAS amp a few volts +/-.


http://www.stratopastor.org.uk/strato/a ... iagram.pdf

http://www.ampwares.com/schematics/supe ... _ab568.pdf


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Posted: Sun Aug 01, 2010 8:02 pm
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Or you can change the 33k resistor.


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Posted: Mon Aug 02, 2010 8:35 pm
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I like to keep a set resistor inline with the newly applied pot. Then, if the pot fails (shorts out) or the adjustment is accidentally cranked to zero ohms, you still have negative voltage inline to the grids. A voltage divider in the fixed resistor to ground is a nice place to put the pot.

Calculate the total resistance of the pot and set resistor to equal about 1.5-2X the fixed resistor you are replacing. So that the "ideal" bias point is around midpoint of the new pot. Be sure to use a linear potentiometer.


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Posted: Mon Aug 30, 2010 4:41 pm
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Ok what value to replace the 33k resistor with?
Anyone help please.


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Posted: Mon Aug 30, 2010 5:47 pm
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27K with a 10K tweek pot in series.


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Posted: Mon Aug 30, 2010 6:01 pm
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Series?
I'm not sure what you mean ...
THe Concert II has a balancing pot that can be altered to bias the amp also and is mentioned by Dan Torres and others. If you modded the balance pot, what do you do with the 10k pot with the 33k on it?

Thanks for the help.


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Posted: Mon Aug 30, 2010 6:04 pm
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It's just a balance. The actual voltage that is going to the balance pot can be adjusted.


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Posted: Tue Aug 31, 2010 10:55 am
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Don't forget first : there are LETHAL voltage in tubes amp can kill you.

For proper bias, you should mesure aprox 38 milliamp at the cathode of each of your 6L6GC. This is for 475 volts, plate voltage as it write on shematics.
But you better to mesure plate voltage make shure. Lower volt = higher milliamp, higher volt= lower milliamp.

To obtain this 38ma you have to vary you negative bias voltage, the -51 volts that BMW talk about.
You have to know for the same (for example) -51 volts some 6L6GC could give you 55 ma or 25 ma, or anything between, that why you should mesure cathode current.

Don't know what I'm talking about : see a good tech


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