DSL biasing question

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SkyhighRocks

SkyhighRocks

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Hey guys...just received my new tubes from Laura (Valve Queen) for my DSL100 and I'm ready to bias it. I haven't done it in awhile and can't remember what it needs to be set at. Right now when testing both sides of the bias points, they are both near 60. That's pretty low isn't it? Or do you add both sides together to get the 90 or so that's recommended?
 
That number is per pair so right now that is 30mA per tube.

Plate voltage on a DSL is around 460-470 and I would bias it anywhere from 68-76mA depending on what sounds best. I know Marshall recommends 90mA per side but that is a bit high, IMO. Sounds kinda mushy that high in my experience with those amps. I usually bias them at around 70mA per side and they sound great there.
 
So that's 90 per side? Not 45 and 45? for some reason I keep thinking I've always kept my DSLs around 45 on both pins...
 
I'd like to know this as well even though I have the 50 not the 100. I think I am currently running both tubes at 38.5, maybe that's cold and I should up it a bit?...
 
The number you get when reading the pins is definitely per pair.

Not sure about the 50 watter, I would imagine there is only one pin to read from and that it would be the summed bias for both tubes. If that is the case, then 38.5 is cold. It should be around 70 (35mA per tube) or so for the pair.
 
So each side would probably be better suited around 80-90?
 
In my experience, biasing the amps that high does not make them sound very good. They get mushy. I would go for around 70-75 per side.
 
killertone":2ambamif said:
In my experience, biasing the amps that high does not make them sound very good. They get mushy. I would go for around 70-75 per side.
Click to expand...


Thanks...I'll give it a try.
 
Here is the bulletin I wrote up when I worked at Marshall/KorgUSA

MARSHALL SERVICE BULLETIN

SUBJECT: Bias Info/Procedure for JCM2000 series Marshall Amplifiers

THEORY OF OPERATION: We are setting the bias using the current method instead of cross over distortion method. The Bias PCB assembly is as follows:
PR1: Bias mini-pot adjustment for pin1 which is one side of the push pull amp (two output tubes in a 100watt amp and one output tube in a 50watt amp).
Pin1: Connected to the cathode of the output tube(s) which then goes through a 1 ohm resistor then to ground.
Pin2: ground reference.
Pin3: Same as pin1 but for the other side of the push/pull amp.
PR2: Bias mini-pot adjustment for pin3, the other side of the push/pull amp.


PROCEDURE:
1. Make sure amplifier is connected to a load with the proper impedance selected.
2. Power up amplifier on STANDBY and let the circuit stabilize for a couple of minutes.
3. Locate the male three pin molex connector CON2 with the two mini-pots (PR1 and PR2) on both ends found on the bottom of the tube bay. NOTE: This will be CON5 for the DSL201 and 401 and there is only one mini-pot PR1
4. On the appropriate connector(CON2 or 5) connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector and positive lead to pin1
5. Take amplifier off of STANDBY with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to the mV voltage that is listed in the chart below.
6. Repeat steps 4 & 5 for pin3 and until both pin1 and pin3 mV are the same.

BIAS CHART mV SETTINGS
(Pin1 and pin3)

DSL50 45mV
TSL60/1/2 80mV
DSL100 90mV
DSL401 550mV*
TSL100/122 90mV
DSL201 .675V

* Change R31 to 2700 ohms and R36 to 3300 ohms

Cheers,
Mike
 
tweed":s6eu8x59 said:
I'd like to know this as well even though I have the 50 not the 100. I think I am currently running both tubes at 38.5, maybe that's cold and I should up it a bit?...
Click to expand...
I keep my 50 around 38mA/tube. If I remember correctly Trace at Voodoo told me 35-39/tube is good.
 
Marshall's silly arbitrary specs for the DSL's are plain stupid. ALWAYS check your plate voltage (power tube pins 3), then calculate from there. My DSL 50 runs 495v at the plates, which at 70% discipation calculates to just 35mv per side on a 50 watter. That 40-42 spec most people go by from Marshall has probably caused lots of premature tube failures in these amps as well as some pretty crappy tones. :doh:
 
killertone":3enq79of said:
Everyone is saying the same thing I said in the very first response! :doh: :lol: :LOL:
Click to expand...

:lol: :LOL:

I agree with you on the bias being a bit too hot, especially if you are using it at high volumes. For bedroom playing the hotter bias works well.

Cheers,
Mike
 
PeteLaramee":2sitxh72 said:
tweed":2sitxh72 said:
I'd like to know this as well even though I have the 50 not the 100. I think I am currently running both tubes at 38.5, maybe that's cold and I should up it a bit?...
Click to expand...
I keep my 50 around 38mA/tube. If I remember correctly Trace at Voodoo told me 35-39/tube is good.
Click to expand...

Thanks Pete! Last time I biased it, I had a friend help me figure out the plate voltage and used that to calculate the bias. Seems like I am in the correct ballpark. :rock:
 
tweed":2udltrkx said:
PeteLaramee":2udltrkx said:
tweed":2udltrkx said:
I'd like to know this as well even though I have the 50 not the 100. I think I am currently running both tubes at 38.5, maybe that's cold and I should up it a bit?...
Click to expand...
I keep my 50 around 38mA/tube. If I remember correctly Trace at Voodoo told me 35-39/tube is good.
Click to expand...

Thanks Pete! Last time I biased it, I had a friend help me figure out the plate voltage and used that to calculate the bias. Seems like I am in the correct ballpark. :rock:
Click to expand...
I'm not going to figure out shit. I just went to the source...since he modded it he must know how it's supposed to be set. :lol: :LOL:
 
PeteLaramee":1cnqny38 said:
tweed":1cnqny38 said:
PeteLaramee":1cnqny38 said:
tweed":1cnqny38 said:
I'd like to know this as well even though I have the 50 not the 100. I think I am currently running both tubes at 38.5, maybe that's cold and I should up it a bit?...
Click to expand...
I keep my 50 around 38mA/tube. If I remember correctly Trace at Voodoo told me 35-39/tube is good.
Click to expand...

Thanks Pete! Last time I biased it, I had a friend help me figure out the plate voltage and used that to calculate the bias. Seems like I am in the correct ballpark. :rock:
Click to expand...
I'm not going to figure out shit. I just went to the source...since he modded it he must know how it's supposed to be set. :lol: :LOL:
Click to expand...

LOL!! :lol: :LOL:
 
FortinAmps said:
Here is the bulletin I wrote up when I worked at Marshall/KorgUSA

MARSHALL SERVICE BULLETIN

SUBJECT: Bias Info/Procedure for JCM2000 series Marshall Amplifiers

THEORY OF OPERATION: We are setting the bias using the current method instead of cross over distortion method. The Bias PCB assembly is as follows:
PR1: Bias mini-pot adjustment for pin1 which is one side of the push pull amp (two output tubes in a 100watt amp and one output tube in a 50watt amp).
Pin1: Connected to the cathode of the output tube(s) which then goes through a 1 ohm resistor then to ground.
Pin2: ground reference.
Pin3: Same as pin1 but for the other side of the push/pull amp.
PR2: Bias mini-pot adjustment for pin3, the other side of the push/pull amp.


PROCEDURE:
1. Make sure amplifier is connected to a load with the proper impedance selected.
2. Power up amplifier on STANDBY and let the circuit stabilize for a couple of minutes.
3. Locate the male three pin molex connector CON2 with the two mini-pots (PR1 and PR2) on both ends found on the bottom of the tube bay. NOTE: This will be CON5 for the DSL201 and 401 and there is only one mini-pot PR1
4. On the appropriate connector(CON2 or 5) connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector and positive lead to pin1
5. Take amplifier off of STANDBY with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to the mV voltage that is listed in the chart below.
6. Repeat steps 4 & 5 for pin3 and until both pin1 and pin3 mV are the same.

BIAS CHART mV SETTINGS
(Pin1 and pin3)

DSL50 45mV
TSL60/1/2 80mV
DSL100 90mV
DSL401 550mV*
TSL100/122 90mV
DSL201 .675VGreat thread, gents
Click to expand...
FortinAmps said:
* Change R31 to 2700 ohms and R36 to 3300 ohms

Cheers,
Mike
Click to expand...
Great thread, Gents!

I am a little confuzzled though...

I am seeing the bulletin measurements in millivolts, but the conversation is in milliamps. Where is the current measured (and how, exactly - is the cat interrupted and a milliampere ,ever in there? Also, why id the DSL 401 saying at Asterix "R31 to 2.7 k, etc.?)

Thanks,

FNG
 
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