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71' Marshall Bias Problem

ptrickamp222

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Aug 22, 2007
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I just got my 71 and the bias is way to cold on all sets of Mullards. Now the bias pot feels bad! Where can I get a bias pot and what do I change the 47K bias resistor to get the right bias???? Plate voltage is 417Vdc. and the bias at max was 25mA. Now it dropped and the pot is bad...........Or do I get a newer pot say 50K and where to get, What to Do!!!!!!:wah
 

Hamilton

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First thing is to always change the bias diode for a new one.

If you can't find a 25K pot right away you can solder in a resistor that will get you close, just like the way JTM45s were.

Pull the power tubes and find a resistor that will get the grids nears -35 vdc. Then reinstall the tubes and check plate current of around 36-38mA.
 

ptrickamp222

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Thanks, I believe Metro has a bias that will fit. I just checked their website. What is puzzling is the same tubes out of my 72 bias real low also. Do I increase the bias resistor to say 68K to get more current??? Right now I had 40 volts bias with 25mA max! But the bias pot is still bad.
 

Hamilton

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First, get the pot out of there before it causes more problems.

Then add in series resistors with jumpers to the 47K until you get close to the desired voltage. Maybe start with the 47K plus a 4K7.

Then add the two resistor values together and find a resistor close to that total and try it. :biggrin:

EDIT :

You need to add resistance, what I said before was wrong and I edited it....sorry. You need more resistance than 47K.
 
Last edited:

ptrickamp222

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The bias pot is bad, I am going to change it first. Good idea! Any thoughts on the standby switch. Swap leads so the bias is hot??? The red tranny leads and the blue bias leads to the bias on the big silver cans. Then one goes to the bias supply. I looked at the amp and schematics and it looks like I should take the 2 top blue leads and swap with the red tranny leads???
 

ptrickamp222

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First, get the pot out of there before it causes more problems.

Then piggyback resistors with jumpers on the 47K until you get close to the desired voltage. Maybe start with the 47K and a 220K.

Then do the math of finding the total resistance of the two paralled resistors, find a resistor close to that total and try it. :biggrin:

So you're saying to lower the bias resistor????Less resistance more current??
 

Hamilton

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Sorry, I had it backwards. You need more resistance. I just edited my post to change my stupidity.

Sorry...
 

Wannatone

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Feb 10, 2003
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The usual Marshall bias circuit works like that:
When the pot is set to its max resistance, you hav the max. negative grid voltage, so there is the min plate current.
When you lower the pot resistance (by rurning it), you get more by more plate current.

If the pot is bad, it may be open circuit, so you'll get much more negative voltage (less plate current) than with a max'd good pot in place.

If, with a new 25k pot, you still get too less plate current, you can replace the 47k resistor (that is in series with the pot) with a lower value. Try 39k first. Less required resistance might indicate other out of spec or bad components.

As an alternative to replacing the 47k resistor with a lower value, you can achieve a lower value by paralleling another resistor (so the original one stays in the amp). 220k parallel to the original 47k would give a total of 39k.
 

Hamilton

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I apologize for getting this backwards, I was going by memory and was thinking the 47K was hooked to the diode and not to the pot.

Man, did I ever mess that up - slap slap slap :rip
 

ptrickamp222

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Thanks I answered in the other post. I ordered all new parts from Metro. And extra for adjusting. The pot would not increase current using the shunt method enough. Then came loose messing withit. The highest I could get was 25mA with my second set. The first set was only 21mA. The bias was set at 17mA when I got it!!!!!!!!!! At 417Vdc on the plates........That's Artic Biasing!!!!!! It's hard to believe that the amp has been like that for all these years!!!!! other than that it's all original and in excellent condition inside and out. Just a Bummer that I could not hook up my 20 watters and R9!!!!!
 

Hamilton

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A couple more things to check :

Make sure the heaters are at 6.3 vac, this assures someone hasn't miswired the voltage selector.

Also check the fuses for the correct ratings.

(I still can't believe how much I messed up the bias question....) :fall :tri
 

ptrickamp222

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Aug 22, 2007
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A couple more things to check :

Make sure the heaters are at 6.3 vac, this assures someone hasn't miswired the voltage selector.

Also check the fuses for the correct ratings.

(I still can't believe how much I messed up the bias question....) :fall :tri

All ready change one of the fuses. Please read other post. I am still confused on which resistor to change and to what, thanks
 

Hamilton

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In a nutshell :

For more plate current either raise the value of the 150K diode resistor, or lower the value of the 47K pot resistor. Personally, I would *up* the 150K diode resistor (I never change the 15K).
 

ptrickamp222

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Aug 22, 2007
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My diode resistor is 220K to diode to 15K to bias feed to 47K to bias pot. I am going to lower the 47K to 33K or 27K which ever works. Bought all new parts for the whole bias supply just in case. I was getting to many mixed answers from to many sources, Thanks for your help!!
 

ptrickamp222

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Aug 22, 2007
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879
A couple more things to check :

Make sure the heaters are at 6.3 vac, this assures someone hasn't miswired the voltage selector.

Also check the fuses for the correct ratings.

(I still can't believe how much I messed up the bias question....) :fall :tri

Also just checked the heater AC without the power tubes is 6Vac. Will test once is fixed, Thanks!!
 
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