Marshall TSL 2000 bias adjusting

I had found and have now lost the DIY link for a tube change / re-bias. If someone is aware of where this information is available I would appreciate it if you would post the link, thanks.

What exactly are you looking for, a how-to, or bias info? Do you usally do your own biasing? if so , what method do you use?

The TSL has 'leads' in the back that you can connect a meter to and adjust accordingly. The post I saw from some forum somewhere that I lost (thinking I'd easily find it again) had specific step by step instructions as well as the proper settings. I've never done it myself before but I'm pretty handy and have read up on all the danger/methods etc. (things I should have known a long time ago, I guess I've been lucky so far in the tinkering I've done with other amps) Since this amp provides a simple avenue I am planning to do this myself once my new tubes arrive-

http://pub31.ezboard.com/fmarshallampsforumfrm1.showMessage?topicID=442.topic

In case others would like to know.



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I put tesla tubes in my DSL401. They are not as clean as the Ei's but sure do sound better in my opinion. As for biasing, any tech could do it. Note you only need to bias if you are replacing the output tubes. I actually biased my amp by myself. It is super easy, but you have to be extremely careful so you don't accidentally kill yourself by touching any high voltage connections. DON'T DO THIS IF YOU DO NOT KNOW HOW TO WORK SAFELY WITH HIGH VOLTAGE ELECTRONICS. Dana123 and SGstrat gave the sequence a while back and it goes something like this.

Instructions:

1) unplug the amp and remove the head unit from the cabinet.
2) locate the 3 pin molex connector labelled (CON 5).
3) securely attach the red probe of the voltmeter to one of the outer pins (use aligator clips).
4) securely attach the black probe of the voltmeter to the center pin (again use aligator clips).
5) plug the amp in (make sure the head is connected to the speaker) and let it warm up for a couple of minutes.
6) then turn on the standby and let the circuit stabilize for a few minutes.
7) adjust the mini pot (PR1) until the voltmeter reads

DSL201 0.675 V
DSL401 1.375 V
DSL50 45 mV
DSL100 90 mV
TSL60 80 mV
TSL100/122 90 mv

I bought some nonconducting pot screwdrivers
from Radio Shack for this.
turn off the amp, unplug the amp, and place the head unit back.

(optional)

9) plug in your guitar.
10) turn the amp on channel 3 making sure all knobs and the master volume are cranked.
11) play til you wake the dead.

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service bulletin given by Dana123
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If the tubes you replaced are exactly the same as your previous set then the bias may be close enough. That being said it is always best to check. I don't like to encourage people to bias their own amps unless they are confident in what they are doing. Below you will find the exact technical bulletin for your amp. Again please be careful. Also they recomend using a multi meter to read from the pin out biasing section of your amp. Good luck.

Dana.



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. 4. Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector CON2 and positive lead to pin1 on CON2.
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 1.375V
TSL100/122 90mV DSL201 .675V

Some of those voltage readings seem very high to me. If the measurement is actualy taken across a 1 Ohm resistor, then the idle current through the output tubes in the DSL201 is 675 milli-amps, and the DSL401 is passing almost 1.4 [u]Amps[/u], even before any signal is applied! :eek:

For the sake of comparison, I bias the 7027A's in the output sections of my Ampegs at 35 to 40 ma per tube, measured at the Anode. Measurements taken at the Cathode-Ground connection will also include the bias currents from the grids, but these are typically less than 1 ma each.

If the DSL201 and DSL401 use Cathode self-bias, then the resistor would be much higher than 1 Ohm, and this would account for the higher voltage readings. But there's no mention of that in the service bulletin!

I may be over-reacting here, but setting the bias incorrectly can trash an amp very quickly.

Hey Ceej, I saw your post on the weberforum too, I agree with Lordathestrings, these are some strange numbers, I was figuring a EL34 at somewhere around 500v on the plates at 28.5 ma , Marshall must be using something different than the 1 ohm resistor method, I have heard that on some Marshalls that even the tranformer shunt method doesn't give an accurate reading, Strange, always something new to learn, Well if the amp sounds good and the tubes aren't glowing red it must be in the right ballpark and thanks for the Link.................

Gee a light bulb just came on above my head, the factory settings Ceej posted are the total for all tubes in the amp, not one tube or one pair but (TSL100) 4 tubes, 4-EL34's at 90mv cathode& 1 ohm resistor method, = 22.5 ma plate current per tube, a typically conservative factory setting that makes for long tube life, I'm a Dufuss, I'm a Dufuss, I'm a Dufuss.................................

Take another look at the first paragraph of my post. I have no idea what the DSL401 has in its output stage, but to cause a 1.375 Volt drop across a 1 Ohm resistor, 1.375 AMPS are flowing. Even at the heavy Plate Dissipation my 7027A's endure at 35 ma, it would take 40 of 'em to draw that much current! :eek:

(Edit:)...and [i]furthermore... The service bulletin says that there is one 1 Ohm resistor [u]for each side[/u] of the push-pull output stage. So [i]each side[/u] would be drawing 90 ma in your example, or 45 ma through each tube!

[Edited by Lordathestrings on 08-09-2002 at 10:54 PM]

After I posted that last post, and got off the net I had one of those "Hey wait a minute, that doesn't work" moments, the 1.375v drop, just doesn't work out to anything easily calculated and the 90mv for the TSL 100 was per side, 45ma on a EL34 is not going to last to long with 500-600v on the plate, I'd like to get my hands on a TSL100, then maybe this would make some sense......

pstring in a music shop, talking to salesman: "I don't want to buy the damn thing, I just want to pull it apart!... :D

So,... I poke around and find some interesting things. Type in EL34, and click "Query".

A Class AB1 push-pull looks to be comfortable with a zero-signal Plate current on the order of 125 ma or so, depending on the circuit. At 475 Volts on the plate, typical values can range from 60-250 ma.

Now, 90 ma for two tubes seems a bit low! :rolleyes:

... time for my big red capsules...'n a bit of a lie-down...

I smell smoke!

My interpretation of the piece about the Marshall schematic for my amp (TSL100) is the measurement is 90 Mv through two tubes, or 45 Mv per tube, and then you repeat the process from the other pin to get the measurement for the second pair. Does this seem reasonable to those that understand this sort of thing?


(oops I was on page 1 of the thread and didn't see the other responses when I posted this seems you reached the same conclusion)

And if anyone wants to come to my house and poke around a JCM2000/TSL100 please let me know, I'd love a amp lesson :) I'm in Central Illinois.

Also, I just got my tubes, hopefully I'll know within a few days if this works!

After taking the back off I found that the one tube that was having intermittant problems appears to have taken some sort of direct impact shot to it and it's a little loose from the base to the tube. Any chance this can be salvaged or should I just deep 6 the entire set? I guess I could throw the three remaining ones ON Ebay as a matched pair with 1 backup?? I'm guessing they're not worth much.

[Edited by Ceej on 08-13-2002 at 10:27 PM]

Originally posted by Ceej
... Any chance this can be salvaged or should I just deep 6 the entire set? I guess I could throw the three remaining ones ON Ebay as a matched pair with 1 backup??...

The tube with mechanical damage isn't even any good as a paperweight (it keeps rolling off the papers). The remaining three should probably get tucked away as backup for your new set.

The more I study tubes, the less convinced I am of the necessity of matching tubes beyond getting the same idle current at the same Plate and Grid voltages. My Ampeg VT-40, with the original pair of Magnavox 7027-A tubes, sounds every bit as good as my Ampeg VT-22, with a fresh quad of Groove Tubes 7027's in it.

While I was playing around with the VT-22 this weekend, (I just had to look into this bias thing), I pulled the 7027's and stuffed in a set of Sovtek 5881's. The difference was dramatic! Power dropped from 125 Watts @ 5% distortion, to just under 75 Watts. And the sound wasn't anywhere near as crisp or punchy. (The VT-22 is a good platform for this kind of experiment, because it has a 5 Ohm resistor on each Plate, which permits easy, though dangerous, Plate Current measurement). I spent some time tweaking the bias, and bashing away with my guitar, to find optimal settings. The 5881's seemed to be happiest with an idle current of about 45 ma with 586 Volts on the Plate. That implies about 26 Watts of Plate Dissipation. My data sheets list 23 Watts as the maximum acceptable value, so they might not have lasted very long in my amp. Ampegs apply voltage levels meant for seriously robust tubes (7027), so 'lesser' types tend to take a beating. For comparison, when I re-installed the 7027's, I biased them at 65 ma of idle current, with 586 Volts on the Plates. That's 38 Watts of Plate Dissipation, which would fry 6L6's, or EL34's, but I ran the amp at 70 Watts rms at 165 Hz, (E at the second fret of the D string), into a resistive load bank for an hour, and the plates didn't even change colour, much less glow red and threaten to melt! Balls aplenty!

So, back the Marshall's EL34 bias. The data sheets tend to agree that 25 Watts of Plate Dissipation is all they should have to deal with. Push-pull circuits allow you to use a large portion of this for the biased idle condition, because one set of tube(s) only conducts the audio signal for half of the time. So, let's assume 45 ma of Plate Current, and 500 Volts on the Plate. That implies 22.5 Watts of Plate dissipation at zero-signal (idle), with a little room left for when its actually doing something. If the Plate voltage is less than 500 Volts, the Plate Dissipation will be lower as well.

45ma@586v, on a 5881, I've heard of them being ran at 50ma, but not at that voltage, I personally don't much care for the 5881's either, rather muddy sounding to my ears. As far as the Marshall goes, Ceej I hope you get a quality set of tubes, Marshalls do have a reputation for eating up cheap tubes, less expensive to buy quality once, than cheap often..........

I got matched groove tubes and I hope these are what you consider quality, they seemed costly! The ones I pulled were J/J and the sound difference is big, but the GTs I got were 'hardness 3' and the sound is still very distortion oriented I think, I can't imagine the 9 or 10 rating. So I think the J/J tubes are worthless to me as backups, I was going to see how I liked the '3' rated groove tubes and then order a full backup set.

The J/J tubes didn't appear to be biased properly either, hard to say for sure since one was damaged but it did still work just had occasional noise with vibrations that occasionally occur when playing through a Marshall :o

Anyway, i think I've successfully biased the groove tubes and will be playing cautiously for the next few weeks with the back of the amp facing me to keep an eye on them.

Wouldn't it just be great if Groove Tubes would give us a set of each hardness scale, so we could figure out which rating we liked the best! Nothing wrong with Groove Tubes Ceej, in fact the GT EL34B, is made by Tesla

I agree that a 3-rated set of Groove Tubes is likely to distort early and often. The set of GT7027's in my Ampeg are rated '4', and I find them a bit dirty, (but in the nicest kind of way :) ). I'd like to get my hands on a set rated at 7 or 8 and see how many rows of the audience I can melt before I get a nice, tangy, crunch! :D