'75 Marshall 50W MKII ST-1 Bias Help

[boxcarcut]

Recently serviced a '75 Marshall 50W amp. Can't find the exact schematic for this model.

Replaced filter caps, bias caps, coupling caps and gave it a general cleanup.

With no tubes in the amp, I'm getting 420V on the plates and a negative bias range of -26v to -39v.

Not able to get these EL34's in the 35mA range.

Any help would be appreciated.

https://flic.kr/s/aHsksktfTT

[shooter]

Not able to get these EL34's in the 35mA range.

what range are you getting?

[VMS]

You probably need to lower value in one of the bias circuit resistors, can you measure the resistor values.

[VMS]

the gray resistor that looks like 220k is probably not original resistor of the amp

[boxcarcut]

Would lowering the 220k resistor to either a 150k or 180k get me there?

[VMS]

I will let someone with more experience on these circuits chime in but some schematics show 150k resistor there 

[sluckey]

Not able to get these EL34's in the 35mA range... Would lowering the 220k resistor to either a 150k or 180k get me there?

Maybe, but we need to know what bias range you're at now. You would decrease the 220K to decrease the mA. You would increase the 220K to increase the mA. You can also juggle the value of that 47K just below the bias pot. It has the opposite effect as the 220K does.

[shooter]

 

but we need to know what bias range you're at now

I've been waiting since reply#1

[boxcarcut]

The range I'm getting is:

6ma/6ma @ 366v (negative bias -35V)
27ma/28ma @ 280v (negative bias -21V)

Seems like i need to get the negative bias up around -50V

Tom

[shooter]

I'm looking at this chart as a reference;

http://el34world.com/charts/Schematics/files/marshall/Marshall_ampchart.gif

It shows 380-ish plate and -30 bias

your #'s look to me to indicate something is sucking voltage down more than it should.
has new/other tubes been tried?

[sluckey]

Seems like i need to get the negative bias up around -50V

No. That's thinking opposite. Bias voltage needs to decrease in order to increase cathode/plate current. But, read on...

your #'s look to me to indicate something is sucking voltage down more than it should.

I agree. Your plate voltage should not change nearly as much as you say. And in a healthy amp when you turn the bias voltage down to -21V, those EL34s would probably be screaming and glowing bright red.

[boxcarcut]

I figured out the problem.

My power supply and variac is acting up.

Plugged straight into the wall and poof....everything seems fine.

Time for a new regulated power supply.

[sluckey]

Why do you need a power supply and variac to service this amp?