TSL100 bias again

[BrotherJ]

I had my TSL100 main board swapped over for the new one. Since then it has been operating fine with no bias drift or anything odd.

Unfortunately my friends DSL100 is now showing the same problems my TSL had which is the bias drift on one side.

Although i'm happy with the fix, I am really annoyed at the Marshall Distributer in Australia for charging so much for this component (AU$600)! when they are definitely cheaper overseas.

[Mars Amp Repair]

Well,
My take on this issue (if I ever get one that drifts that way) would be to heat the board (with hair drier) while monitoring the bias voltage.

Once I see it drifting, I'd get my freeze spray out isolate an area of the pcb...then heat & then drip freeze on each individual component until I found the offending one(s).

I'd hang in the new part using a different brand/variety that the cheap components Marshall uses (ierange drop or better metalic resistors) and do the same to see if the drifting of the component under cool/heat is resolved.

Seems like someone must have tried this at this point...any success? glen

[Alex R]

Well,
My take on this issue (if I ever get one that drifts that way) would be to heat the board (with hair drier) while monitoring the bias voltage.

Once I see it drifting, I'd get my freeze spray out isolate an area of the pcb...then heat & then drip freeze on each individual component until I found the offending one(s).

I'd hang in the new part using a different brand/variety that the cheap components Marshall uses (ierange drop or better metalic resistors) and do the same to see if the drifting of the component under cool/heat is resolved.

Seems like someone must have tried this at this point...any success? glen

Yep. Did exactly that, changed every bias part, caps, resistors, all related parts including coupling caps, feed caps, trimmers, PSU parts on the back board... Still the drift with the hairdryer. New board = no drift = nobrainer!

[Mars Amp Repair]

Well,
I guess I'm just thinking too much like a logical techy type...I guess 'whatever make it go faster'. g

[Alex R]

Well,
I guess I'm just thinking too much like a logical techy type...I guess 'whatever make it go faster'. g

no I know where you're coming from. It's taken me years to get to this point, but now I'm here I'm here. I guess I have returned quite a few TSLs to customers, who have them found that they overheated again, and maybe I lost the customer. This is a problem that can take a couple of hours to develop, that's the problem. But the hairdryer on the board shows the bias voltage drop straight away, and where I've finally got to is, new board if that happens.

[catstrat]

Another Marshall TSL bias issue

I have one of these in my shop and the two O/P tubes on the right, looking at the back of the amp, had no bias voltage at all on the grids. The tubes were , of course, red plating like mad. It looks like the new board has been installed already. I found a bunch of cracked solder joints at the O/P tube sockets. I changed the PI coupling caps just for sure. I put in a new set of JJs and let the amp warm up for about 5 to 10 minutes. Checked the millivolts at the back test points and adjusted. Considering that the trim pots seem to work against each other, the best i can get is about 150 mv to about 160 mv on each side. My internet search shows so many different voltage specs i'm not sure which is right. Some say 90 mv per side someone even said 28mv per side but i think that's per tube. Are we getting millivolts mixed up with milliamps? The plate voltage is 407 volts. Isn't that a bit low? I would think maybe 440volts. If some one can help me i would be gratefull. I've worked on TSLs in the past and never found them that hard. Is this the one everyone has warned me about? By the way Glen, Did you find the right transistors to use in the Peavey biasing circuit. I started keeping better records of stuff like that and i hope i didn't lead you wrong on that stuff. I was in Austin Texas here not long ago and talked to the guys at Austin Vintage guitar about the problems they encounter and it really is the same that we all run into. I think some times that my problems are unique to me alone, but i guess not. Thanks everyone. Terry

[Enzo]

Bias controls work against one another? MAKE SURE the Mute button is OFF. It mutes by shorting the grids together. If you try to adjust the bias with mute on, the two controls are wired together by this.

We are not really confusing miilivolts and miliamps since across the 1 ohm sensing resistor, they will come out the same.

Each test point measures two tubes together, so 160mv = 160ma = 80ma PER TUBE. That is awful high. No wonder your B+ is dragged down to 400v. Those tubes are dissipating 32 watts each at idle.

90mv means 45ma per tube. At 450v B+ that would be 20 watts, which is pretty hot for an EL34 to idle in my book.

Keep in mind that on the Marshalls, the left control and test point are for the right pair of tubes, and the right control and test point are for the left pair of tubes. I have no idea why they are "backwards" like this.

As to what is on the internet, i am sure you found all manner of advice. Problem is asking what current setting to use is like asking how far down you should push your gas pedal to go 70... it all depends.

The standard rule of thumb in tube amps is that we set the idle current for tube idle dissipation at 70% of rated power. SO you have to consider both current and B+. 40ma at 400v is 16 watts. 35ma at 500v is 17.5 watts. SO lower current can still be a hotter tube when B+ is considered. SO find out what your plate voltage is when the tubes are close to bias, then adjust the current to achieve your target dissipation.

You can look up the data sheet for the tubes you use for dissipation. A typical EL34 might have a 25 watt rating, so 70% of that is 17.5 watts. SO with that example, if your B+ were 500v, then my 35ma per tube looks good. If the B+ is more like 450v, then your 17.5 watts would mean more like 39ma.

Of course you double those readings for two tubes.

There is nothing set in stone abuot 70%, bias is not a critical setting, but it is a common favorite level.

[Luthin You]

Regarding the bias in TSL100 , I have also spent days chasing this and ended up removing the the tube bases and drilling holes so that the grids arent in contact with the cct board then re-installing and running the bias and signal through wires suspended away from the board.This fixed the problem, so in my book its simply conductive circuit board but only when hot!About 12 mths later the amp came back with a similar problem in the preamp , but after looking it over , and going crazy with the multiple diagrams , I handed it back and said take it to someone else.Len

[killertone]

Parts replaced:

output tubes
C6, C7 (post-PI coupling)
R67, R69 (220k bias feed)
C36, C37 (bias smoothing)
R68 (bias supply to ground)
C42 lifted - symptoms unchanged
bias trim pots
C43, C44 (bias feed caps)
C36, C37 (VPL/mute coupling)
C46 (plate-to-screen)
D7-D10 all check ok (bias supply bridge)

Hi -

I have a TSL 100 that is humming and I have traced it to a blown C46. The cap is totally smoked and I have no idea what the value is, can you tell me what I should replace it with? Thanks!

[Enzo]

22pf. Make sure it is 1000v type.

If the solder pads are destroyed, just tack solder a new part between pins 3 and 4 of the octal socket next to it.

[Mars Amp Repair]

Say,
i have found my 1KV typical tan disc caps blowing after a few months, so I've moved to using 2KV (2000volt) caps. They're pretty cheap from Mouser if you have a big enough order to make.
The ones that marshall is using now are blue & physically larger than the 2kv ones I'm getting. Unfortunately, I didn't make note of their voltage rating. glen

[killertone]

22pf. Make sure it is 1000v type.

If the solder pads are destroyed, just tack solder a new part between pins 3 and 4 of the octal socket next to it.

Thanks a lot. The solder pads are destroyed. So I can just solder in the new part across pins 3 and 4 of the tube socket right next to it? I just want to make sure I understand correctly. Thanks again!

[Enzo]

Right. The solder pads are often destroyed. Look at the traces from those pads - they lead directly to pins 3 and 4. I find it easier to tack tight to the socket pins than to fight the old holes.

I have not had a problem with 1000v parts coming back, but Glen's advice sounds good.

I mentioned 1000v caps because there are plenty of 50v or even 500v caps on the market, and I didn;t want you grabbing one without thinking.

[52 Bill]

And didn't we find that he cause of these cap failures was the grounding scheme of the output jacks?

So be sure to check the solder connections and the closed circuit contacts of the speaker jacks.

[catstrat]

Marshall tsl

Looks like you've already got your answer. I haven't worked on enough of the TSLs to be that familiar with them. Just enough to know there seems to be a lot improper info floating around about them. This site is probably the best place to get the right information about something. These guys are pretty knowledgeable.