Marshall TSL: 3 pins connector for bias setting.

Mick90 · #1 10-24-2003, 05:29 AM

Hello everybody !

I have a unique JCM2000 TSL602 "cutted Head" (the combo was cool, but I prefer to transform it like a head and plug it into a 4x12"... that's why I say it is a unique head !)

I have seen on the rear pannel, next to a trim pot (for bias settings I suppose), that there is a 3 pins connector

Are these pins here to connect a bias control unit ? Does anybody know if it is possible to check and set the bias with a simple multimeter connected to these pins ?

If yes, how should I connect the multimeter ?

I will change the EL34s for 6L6s, so I will have to set the bias correctly. Maybe that this connector will save me...

Thanks a lot.

Gresh · #2 10-25-2003, 01:59 PM

You're right, the three pin connector is for measuring the bias. The middle pin is neutral/ground, the outside pins are for the red leads of a multimeter.

Just connect the neutral lead from your multimeter to the center pin, connect the red lead to one of the outside pins and read for millivolts. The left trim pot is used for adjusting the bias of the left side of the power amp which you will measure with your leads reading from the left pin. Adjust to the desired setting. Then just move the red lead over to the opposite pin and do it again.

You will need your speaker load hooked up and the standby switch must be in ready to play mode to properly read bias. Different wattage marshalls require different bias settings so don't use the settings from someones TSL 100.

larsmars · #3 10-26-2003, 05:20 AM

just send a mail to the service department of marshall(you can find the address on their website) and they will help you with the correct settings.

Lars Mars

Mick90 · #4 10-27-2003, 03:53 AM

Hi, and thanks for your answers.

I will ask the Marshall's tech for the correct settings.

I have searched on the web for more informations about tubes and tubes amps:

Does the trim pot affect only the balance between the 2 tubes (and the currents are controled by an automatic bias), or really the current of each EL34s ?

Am I right ?

Drew · #5 10-27-2003, 06:41 PM

Can you even bias the TSL series to take 6l6's? I've REALLY wanted to try that for some time, i think id prefer the amp with 6l6's (i'm not a huge EL-34 fan), but i didn't think it was possible.

-D

JerryP · #6 10-27-2003, 08:54 PM

Here's the bias specs for the Marshall DSL/TSL amps:

Bias Settings:
DSL50 45 mV
DSL100 90 mV
TSL100/122 90mV
TSL 601&602 80 mV
DSL 401 1.375V
DSL201 .675V

These specs came from Marshall and were posted at the Harmony Central forum.

Jerry

Mick90 · #7 10-28-2003, 04:00 AM

Hi,

Thanks for the specs datasheet Jerry !

To Drew: You are right, it's impossible to replace the EL34 by 6L6 without heavy mods...
Some custom shop can modify an amp to put 6L6, but it's a very expensive mod... (about 600€=$)

Not sure to do this mod now...

JerryP · #8 10-28-2003, 08:43 AM

Generally all that's required to run 6L6's in most amps that have EL34's is a change to the bias circuit to get the correct bias voltage.
Jerry

Mick90 · #9 10-28-2003, 11:20 AM

For the experimented ones:

I have searched on different web sites informations about tubes technologies...

So, on my TSL, I will measure a tension (in mV, Direct Current). This tension, correspond to what exactly ?

-to the cathod intensity, in mA (because of an 1 ohm resisor: T=RxI, so if R=1, T=I)

- to the plate or screen tension (I don't think, because these ones seems to be generaly high tensions)

- to another tension, far from my knowledge...

I know, my questions are often stupid !!!

JerryP · #10 10-28-2003, 12:01 PM

Your questions aren't stupid at all. When you measure the bias point on the back of a TSL/DSL your measuring the voltage drop across 1 ohm resistors on the cathodes of the power tubes. The mv reading you get is equal to the current draw (because of ohms law) of each tube or pair of tubes. On a TSL 100 a reading of 90 mv is equal to 90 ma or 45 ma per tube.
Jerry

Mick90 · #11 10-28-2003, 12:37 PM

Ok,

I've just controled the bias. The measure was about 76mV, so I put it to 80 mV. :P

Not sure that it will affect the sound significantly...

Thanks for all !

Drew · #12 11-01-2003, 02:02 PM

you can safely go up about another 10 mv's there...

the TSL's are notoriously cold, out of the factory. I really ought to grab a multimeter and check mine out- it's been retubed, and the guy seemed pretty damned knowledgeable about tube amp design (i talked with him for about an hour about amp circutry and design when i picked the head up- this is one of the reasons I've gotten inspired to design my own head), so i'm assuming it's higher than factory now, but i'd like to check it out anyway- you know, that constant urge to tinker all us gearheads get.

-D

Mick90 · #13 11-07-2003, 05:19 AM

Quote:

Originally Posted by Drew

you can safely go up about another 10 mv's there...

, but i'd like to check it out anyway

-D

Hey Drew,

Have you check the value of your bias ?

I'm searching any informations to optimise the sound of my TSL...

... because I'm pretty sure that I will throw it through a window...

Gresh · #14 11-07-2003, 12:59 PM

I believe that bias setting are more related to tube life than any real significant effect on tone. When I swapped tube brands in my TSL, I adjusted the bias as it was pretty cold compared to the recommended voltage. The fattening of the tone I got was probably more due to the tube swap than the bias correction.

I think there's more to gain by swapping out your preamp tubes than tweaking bias on power amp tubes, not to say that getting your bias right isn't important, I just feel it's more about the efficiency of the electrical circuitry than it is about tone. Just speculation.

Drew · #15 11-07-2003, 02:23 PM

Yeah, there SHOULD be a change between tubes running cold and tubes running hot (i guess a lot of the EVH sound is from 6551's running near-meltdown...?), but that would only become really pronounced at ear-shattering volumes where the poweramp takes on a more pronounced effect in the nature of the distortion.

Definately swap out your preamp tubes. It's cheap (probably no more than $12 a peice), and has a HUGe effect on the sound. i haven't really checked out the whole market, but i love what JJ's did to my TSL- compared to a stock amp, it hardly sounds the same. Much smoother, without that papery edge to the gain.

-Z