Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: boxcarcut on January 30, 2018, 03:24:45 pm

Title: '75 Marshall 50W MKII ST-1 Bias Help
Post by: boxcarcut on January 30, 2018, 03:24:45 pm
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


Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: shooter on January 30, 2018, 08:33:09 pm
Quote
Not able to get these EL34's in the 35mA range.
what range are you getting?
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: VMS on January 31, 2018, 06:06:15 am
You probably need to lower value in one of the bias circuit resistors, can you measure the resistor values.
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: VMS on January 31, 2018, 06:23:27 am
the gray resistor that looks like 220k is probably not original resistor of the amp
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: boxcarcut on January 31, 2018, 11:39:59 am
Would lowering the 220k resistor to either a 150k or 180k get me there?
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: VMS on January 31, 2018, 11:58:01 am
I will let someone with more experience on these circuits chime in but some schematics show 150k resistor there  :dontknow:
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: sluckey on January 31, 2018, 01:26:04 pm
Quote from: boxcarcut
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.
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: shooter on January 31, 2018, 08:24:30 pm
 :icon_biggrin:
Quote
but we need to know what bias range you're at now
I've been waiting since reply#1
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: boxcarcut on February 01, 2018, 12:01:13 pm
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
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: shooter on February 01, 2018, 12:21:18 pm
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?
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: sluckey on February 01, 2018, 12:33:40 pm
Quote
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...

Quote
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.
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: boxcarcut on February 01, 2018, 01:40:19 pm
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.
Title: Re: '75 Marshall 50W MKII ST-1 Bias Help
Post by: sluckey on February 01, 2018, 01:47:55 pm
Why do you need a power supply and variac to service this amp?