71' Marshall Bias Problem

[ptrickamp222]

My bias pot is bad in my new 71 marshall!!!! Where can I find one!!!!!The bias is way to low also. Do I increase or decrease to increase the mA. It was at 25mA max. I need at least 35 to 40mA. Thanks!!!

 

[PaoloJM]

My bias pot is bad in my new 71 marshall!!!! Where can I find one!!!!!The bias is way to low also. Do I increase or decrease to increase the mA. It was at 25mA max. I need at least 35 to 40mA. Thanks!!!

How do you know the bias pot is bad? Does it still increase/decrease the idle current?
If so then it's probably ok.
Have you measured it value with a multi-meter?
It's more likely that the amp was modded to take a different valve type and so the bias circuitry was modded for a different bias range.

If I were you I would dig out a schemtic for the amp with whatever valve is used in your amp (EL34 I guess) and measure each componant in the bias circuit against the values in the schematic. Whichever part reads funny is the guy to change.
Look out for things like resistors piggy backed on others in the bias area it's a sure sign of a mod.

 

[Purgasound]

+1.
Doesn't sound like a bad trimpot unless the bias doesn't move at all when you try to adjust it. We need more info. What kind of tubes are in it, are they matched, where are you reading the bias from?
Sounds like you just need to change out or piggyback the final bias resistor. Get a picture of the inside of the chassis on here.

 

[MajorNut1967]

Yes do like Paolo suggested, measure the value of the POT (the two outside terminals) then measure the wipe to each of the outside terminals as you turn the POT use a analog meter if you have one then you can watch how the needle sweeps.

 

[ptrickamp222]

The pot came loose inside messing with the bias, way to cold. The best I could get with my second set of Mullard's was 25mA at 417Vdc!!!! Way to cold. Now the pot is bad messing with it. All stock amp. I ordered all new parts from Metro this morning. Bias pot,33k resistor to change out the 47k to increase current. And screen and bias resistors for the power tubes. Also a diode for the bias just incase. Thanks for all the replys. The amp came with it set at 17mA with 417Vdc!!!!!!!How could someone play it like that for all these years??????? I like around 65% on el-34's using the shunt method.

 

[ptrickamp222]

+1.
Doesn't sound like a bad trimpot unless the bias doesn't move at all when you try to adjust it. We need more info. What kind of tubes are in it, are they matched, where are you reading the bias from?
Sounds like you just need to change out or piggyback the final bias resistor. Get a picture of the inside of the chassis on here.

If you look on the LP forum in amps I have a whole set of Pic's. All original amp, Thanks!!

 

[ptrickamp222]

Fixed the amp today!!! Installed new 25k linear bias pot and a set of NOS Mullard's and biased at 65% with 432 on the plates. The bias pot was shot, that's all I had to change. It biased right in the middle of the sweep of the pot! No resistor's to alter! Also hooked up the bias to the hot side of the standby switch........will add screen and bias resistor's later this week. This thing would eat anything but NOS Mullard's