Marshall TSL100 head, one valve pair redHOT!

[Raggy]

Hi,

I have a problem with my TSL100: while playing on the clean channel, the sound wen whisper-soft after just a minute or two with the faint smell of something burning. I turned the amp off quickly and after a few hours opened the rear grid to have a look.....nothing looked burnt, so i turned it on again.

On standby, the amp seemed fine. After 10 minutes of standby, I tried to play it again on the clean channel, and within a minute the left pair of tubes started glowing really hot, the plates became orange-red like a lightbulb! The right pair of tubes looked OK.

Shut down the amp again, once it was cooled I turned it on and checked the bias: the Left bias was 92mV and the Right was 60mV. (please note, Left valves were problem-hot, but Left pin bias was almost as per Marshall-spec-90mV, whereas Right bias seemed off).

So I did the only thing I could,which was adjust the bias, got both sides to read 90, and tsted the amp, it sounded fine even after 5 minutes of playing, the tubs seem to run normal and sound is good.

But I really don't know if just this one-side-bias discrepancy is the actual source of the problem, (and so I seem to have fixed it) or could it be something else I should be looking for......I'm afraid to run the amp and ruin it.

Question1: Could any of you experienced chaps care to comment please? I'd be very grateful for help,cos I live in South-east Asia where specialized tech help is extremely difficult

Question2: A related question: I understand that the bias is a -ve voltage, so in effect a 60mV reading instead of 90mV means it is running the valves extremely hot, yes? However on many internet sites there are techhies saying they'll set the bias at 80 instead of 90, cos this makes the amp run cold.......could this be right, if so how in god's name???

Thanks for your time,

Raggy
My website explains what I do with my amp!

[Enzo]

Bias voltage to the tube is a negative voltage yes. But the test point is measuring cathode current through a 1 ohm resistor, so the 60mv represents 60ma of cathode current. 80ma is more than 60ma in terms of current. Increasing the negative voltage at the grid will decrease the current through the cathode.

SInce the test point readings are for two tubes each, a reading of90mv means 90ma is flowing through two tubes - 45ma per tube. If you reduce the flow to 80ma, then each tube conducts only 40ma. That is 5ma colder, but still plenty warm enough for tone in my book.

Please be aware that when you look at the row of four tubes and the two bias controls, the control on hte left adjusts the tubes on the right. And the control on the right adjusts the two tubes on the left.

You could have a failing power tube. Trade places between the left and righ ttubes. If the trouble recurs, note if the same two tubes in their new position are glowing red hot, OR is it the tubes now in the same sockets glowing? In other words did the symptom travel with the tubes or is it assiciated with those two sockets.

[Raggy]

ah, THANKS for the explanation, Enzo.....so the test is showing the cathode current, not the grid voltage, OK, that clears the mystery of hot/cold settings.

Left control adjusts Right pair of tubes....someone at Marshall has a nice sense of humor,eh?

so then in my situation...even with a cold setting (60mV) on the right control somehow allowed the left pair of tubes to draw too much current. So looks like my 5 minutes of normal-running after I set the bias to 90mV on each side was probably just a fluke of good luck.....your idea of switching the left-right tube pairs is excellent, this way i'll know if the problem follows the tubes or the sockets.....OK I'll do that and see what happens. Thanks again for the advice!

The tubes I have on the amp are as when I bought the amp brand-new just a year ago...the left pair have red print and the right pair have black print.....I suppose that's just to denote their pairing so one doesn't end up with red-black pair on each side? Or do I also have to make sure I swap tubes 1 with 3 and 2 with 4?

If I have a failing tube, is there any way I can test to know which one, or do I have to change a pair (if the problem proves to travel with the tubes)?

Thanks for your help.

cheers,
Raggy

[Mars Amp Repair]

TSL-100 with bias issues

Raggy,
These models tend to have issues in the VPL & Mute switching ckt. If you can turn the amp on & quickly try pushing the mute & VPL buttons (before the tubes turn red), notice if you hear an unusual 'POP'. You could also monitor the idle current test points that correspond with the pair that are glowing red...as Enzo indicated, the respective positions of the outer test points vs tube positions are reversed.

If you hear a pop as well as see the idle current xnge when you push the switches, then you may have issues in these ckts. Actually it is 2 coupling caps that I believe send signal to the XLR drive ckt. Because of how these caps tap off from the grid drive of the power tubes the bad caps tend to be drawing the negative bias voltage down hence causing the tubes to draw too much current.
They are C37 & C36. They are mounted on the front ckt pcb near the mute & VPL switches. As I recall they are small yellow rectangular caps.

This ususally occurs when the power tubes have shorted.
Anywhich way you look at it, it's quite a job to get all the necessary boards out of the unit to replace the caps, so it behooves you to be certain that is where the issue lies.
It still of course could be other issues.

glen

[Peter Clothier]

Raggy,
I don't wish to sound alarmist but....... There may be other underlying reasons why your amp got a sudden 'glow-on' (technical term) lol.

Try this:-
1. Remove 3 of the EL34's. Leave the one on the left in (rear view).

2. Connect DVM to the bias point (centre and right pin)

3. Switch on and warm up.

4. Apply HT and record the meter reading (40-45 mV nominal).

5. Power down and move this valve to the far right socket and adjust the
bias (centre and left pin) to give the same reading as previous.

6. Test each valve in turn using socket 8. A duff valve(s) may show a bias
value that is significantly different to the others.

7. Use one valve and measure its bias value in each socket sequencially.
The recorded values should be identical. Any difference suggests a
fault / intolerance with the components supporting that valve base.

With reference to the schematic the valves are numbered:- 1,2,3,4,5,7,6,8. working left to right (rear view).

Hope this helps.

Pete.

[Enzo]

Yes, that is methodical and tells you the condition of both the tubes and the amp circuits.

I thought of earlier but didn't mention another possibility - an intermittent tube/socket. If the bias is adjusted when only one of the pair is running, your adjustment to get the 80-90ma will be all in one tube, but when the tube is working you can adjust it properly. On the other hand he reported two tubes red hot, so it is unlikely one was not running...

Never mind.

[Raggy]

Thanks for all the helpful advice, guys,
OK i'll test the amp and report on how it goes.
can't do it today cos I've got to help a friend take his dog to the vet. Then tomorrow he helps me check out the amp based on your tips!
I was planning to complete my album's recordings by end-march (just an indie solo effort, nothing of any commercial value) but now everything hinges on the TSL...if i can't get it working, it'll take me forever to be able to buy anything else in the same league, and after the TSL how can I ever go back to using stompbox-effects and floorboards........life sux when you've tasted heaven, who was it who said blessed truly are the retards for only they can ever be truly happy for never being able to imagine anything better.......

[Raggy]

JUst one more quick question: my left pair of power tubes have the marshall logo and wordings printed in red, the right pair printed in black........I assume this is just a stylistic thing, and the colors don't mean anything, i.e. I wasn't given a head with swapped tubes when i bought it ?

[Peter Clothier]

It probably means something to Marshall but dont be concerned about it. To be honest, the stock tubes supplied with the amp SUCK. Thankfully there are a few very reputable suppliers of tubes on either side of the pond. (Remember to buy your PA valves as a 'quad matched set')
All are made on this side but retail for half the price on the other side.
What a pisser!!!
If it wasn't for American investment though we would probably have none.

Long live Rock!!!!!

[Enzo]

A lot of tubes these days are labelled in thermal paint. It starts out as red or white or blue, but when it gets too warm it turns dark. Possibly yours?

Of course plenty are indeed printed in plain old black ink

Also, if this amp has this problem, maybe someone replaced just the two bad ones, so they are different from the other remaining two.

SOme tubes the color of the print indicated the "grade" of tube - cool, medium, hot.

[Raggy]

Well, here's how things went over the weekend:
first off, we checked the biasing again, set it to 86mV on each side...then played the amp for 30 minutes without any problems at all, but when we rechecked the bias it was now 100mV on the left and 88mV on the right......maybe we didn't wait long enough for the amp to warm up when biasing, or after playing it got too hot and we needed to wait longer for it to cool back a bit?.....anyway we re-biased at 84mV each side, and played the amp and it sounded fine for about 30 minutes, and we werent brav enough to go switching tubes etc,figuring if the amp worked OK what the heck but we'll keep checking the bias and see if it runs stable.

here's a site with a really good video on biasing this amp (for newbies like me)
Biasing Amps videos

[Amp Kat]

You mean 84 ma's per side which is 44ma's per tube which is pretty hot. If the bias moves it could be because the tubes aren't warmed up as ne tubes should warm up for twenty minutes. If the bias current changes then the bias voltage is changing also getting less negative which would then lead me to believe that the coupling cap is giving it up and DC getting thru it. If you monitor your bias voltage on pin 5 after a minute or so and see the negative voltage start to drop towards zero or less negative then that is a sign it's leaking.

[Mars Amp Repair]

remember my prior post about the caps to the XLR drive ckt...I've seen this issue many times on this amp...to reitterate, if you get a pop when you push either the mute or VPL switches (can't recall which one right now) , there could be trouble there affecting the bias voltage. You can monitor the bias voltage as you switch these switches to be certain...g

[Enzo]

Either or both, maybe. The mute switch shorts the two sides together. If there is any voltage difference between them at the time it can pop. And regardless of how balanced the tube currents are, their grid voltages can differ. SO it may or may not represent a problem.

The VPL shunts a large resistance in the signal path - without looking, is it 680k? - and is less likely to pop unless there is a voltage dropping across that resistor. And there shouldn't be.

[ahamay79]

Marshall JCM2000 DSL... no bias voltage at test point

I have no bias voltage on one test point on a Marshall JCM2000 DSL. The other side reads around 50. The tubes test good. I can't find any bad resistors. Any ideas?

Thanks,