TSL100 HT blows

[Lucifuge]

Hi everyone,

Been a while since I've been on here, but I'm still alive

Got a technical question about my TSL.... Last night we went to practice and I had to use a different cab than what I usually do. No big problem normally, but I didn't take my glasses with me (my age is showing) and apparently misread the drawing on the back and plugged my 8ohm amp into the 4ohm speaker plug. I played for a while and all of a sudden the amp cuts out (light on the switch off as well). I walk to the amp and notice that the left tubes (seen from front) are definitely wamer. Long story short, the HT fuse was blown. Turned everything off (power and standby) Knowing it wouldn't help, I replaced the HT fuse and switched the power on.... Fuse blows...

So, as I haven't done any searching in the amp yet (will get to it tomorrow or in the weekend), I was wondering if anyone has any good advice. Can the mismatch blow the mains transformer? I thought that the OT would get damaged. Or can the mismatch cook the first two tubes to the point that they will take out the mains transformer?

Any help would be appreciated... I'm a a/v electronics tech, just not amp tech and don't have schematics..

Thanks!! Lucifuge

 

[2L man]

Too little loudspeaker load can cause "flyback voltage pulsing" which can destroy power tubes or output transformer or cause sparking in tube socket. Possibly just power tube(s) did broke.

Remove power tubes and test if fuse burn. If not get new set of power tubes.

 

[Lucifuge]

Thanks 2L. Will do the test of course and get back to you, but just thinking.... Why would the fuse still blow if I just switch the power ON and not out of STANDBY if it's the tubes?
As I said, I don't have schematics, but isn't the whole purpose of the STANDBY switch to let the amp warm up before using the tubes?

 

[Spanngitter]

If powertube developed internal short between screen and heater it will cause the fuse to blow.
Honestly spoken, I doubt that it was incorrect impedance, more likely your TSL developed the Bias disease and red plated the tubes until they did internally melt down. So remove power tubes and let the amp warm up whilst measuring voltage at Pin5 on the failed side…I bet I’ll drifting….

 

[2L man]

Thanks 2L. Will do the test of course and get back to you, but just thinking.... Why would the fuse still blow if I just switch the power ON and not out of STANDBY if it's the tubes?
As I said, I don't have schematics, but isn't the whole purpose of the STANDBY switch to let the amp warm up before using the tubes?

Sorry I missed that! I'm sure someone can find the schematic?

Standby function is not standardized! Stby On should connect mains so that filament heat but sometimes Stby On connect the HV.

 

[Lucifuge]

If powertube developed internal short between screen and heater it will cause the fuse to blow.
Honestly spoken, I doubt that it was incorrect impedance, more likely your TSL developed the Bias disease and red plated the tubes until they did internally melt down. So remove power tubes and let the amp warm up whilst measuring voltage at Pin5 on the failed side…I bet I’ll drifting….

I bought the amp new in 2009 and think that the Bias drift was corrected by then. Never had problems with the bias (honestly though haven't checked in about 2 years). But I'll measure though. What should it be?

 

[Dr.Twang!]

Pull the power tubes first and see if the ht fuse still blows.

Tube Amplifier Debugging Page

 

[Pete Farrington]

From the schematics, I can’t see where the HT fuse sits in the circuit

https://www.drtube.com/schematics/marshall/JCM2-63-02%20(2005)%20iss7.pdf

https://www.drtube.com/schematics/marshall/TL10-60-02%20(2003)%20iss7.pdf

 

[Dogs of Doom]

not sure how much this helps, but...



the ht fuse is installed like any typical Marshall up to that date.

The speakers out, are a single 16 ohm out & then a pair, of 4-8 ohm jacks. 4-8 is switchable.

The 16 ohm out is fitted w/ a disconnect jack, so, when you plug into the 16 ohm jack, it removes the ground from the 4/8 ohm jacks, regardless of switch position. The 16 ohm jack can get faulty in this purpose, leaving an open ground when using the 4/8 ohm jacks, even if there's nothing plugged in to the 16 ohm jack.

Many people work around this, by hardwiring the ground to the 4/8 jacks.

The other things that give these amp's problems are R6 & R9 pop very easily. AMS used to say that, it's because they work also as a fuse. If one pops, the bias will run high on the affected side.

Lastly, the bias pot's are cheap & go bad, allowing bias to run wild.

If a tube arc'd & blew, there's a possibility that C-45 also blew. Not always, as they upgraded that cap, in later models.

Do note, that these amp's were problematic up until the end. The latest "fix" mainboard was ISS 20 or 21...

 

[Dogs of Doom]

From the schematics, I can’t see where the HT fuse sits in the circuit

https://www.drtube.com/schematics/marshall/JCM2-63-02%20(2005)%20iss7.pdf

https://www.drtube.com/schematics/marshall/TL10-60-02%20(2003)%20iss7.pdf

from the 1st drawing:

 

[Pete Farrington]

from the 1st drawing:

Indeed, but as drawn there, I can’t determine what current flows through it.
ie W14 and W15 don’t correspond to anything on the main schematic, that I can see.
The HT current flow paths on the main schematic are unbroken, there’s nowhere for the fuse to be inserted.

 

[Dogs of Doom]

Indeed, but as drawn there, I can’t determine what current flows through it.
ie W14 and W15 don’t correspond to anything on the main schematic, that I can see.
The HT current flow paths on the main schematic are unbroken, there’s nowhere for the fuse to be inserted.

one thing, is, there's IIRC 3 drawings...

not sure if this is the missing one?

https://www.drtube.com/schematics/marshall/TL10-62-02%20(1998)%20iss6.pdf

ISS 6, but, I think that is the same.

 

[Jethro Rocker]

The bias can be measured using the 3 pins on the back. Centre is common. Each side shouldn't really be above 40 and should be stable. If yours was after the bias fix, try changjng tubes, or first, as mentioned, try without tubes just to see.
I would try the simplest stuff first.