Marshall TSL60 blowing heater circuit fuse

[Force3fan]

Please help
Bought amp used 4mnth ago -TSL60
sound was dull/muddy especially on OD channels using neck humbucker p/u
Had orig tubes and bias was 75mv
Replaced tubes and (bias to 80mv per marshall) did not change tone
a month and half ago powered up but no sound
Took to a tech-found blown heater fuse(replaced w/ 125v 8a) incase a nuisance problem he said,should be 125v 6.3a
Its been 1 and 1/2 month fuse blew again
I replaced fuse w/ 250v 6.3a can't find a 125v 6.3a fuse anywhere
Another strange thing (been there since I got amp) there is a dim glow in the Power switch when amp is off
may or may not be related
Please help

[Jazz P Bass]

Get a new switch, please.

[g1]

Yes, get the switch fixed first, for safety. The fuse voltage doesn't really matter, it's a rating for arc over once the fuse is already blown, so 125V shouldn't be a problem. The heater fuse feeds AC to the filaments for some of the tubes, but also the DC supply for some of the preamp tube filaments. So if there is a problem in that DC supply it could be causing the fuse failure.
Can you be more specific about the bias procedure? If you are talking about each tube I would think that is way too hot. The 80mV is maybe for the 100W versions?

Edit: I see from the schematic that the bias is measuring both tubes so the 80mV is correct. Unfortunately, the single pot setup (unlike the other 2000's with the twin pots) means you have no matching capabilities. So you could have a situation where one tube isn't doing much and one tube ends up biased real hot. So you would really have to try a new matched pair or use a bias probe to find out if they are both doing their job. Also you could have a bad screen resistor to one tube and end up biasing the other tube real hot.

[Force3fan]

Thank you very much for yor reply.
I am looking for the switch locally but may have to order from Parts is Parts or US Music Corp.
The Power tubes are a matched set from Euro Tubes. I hope they really are because that is what i paid for.
Again thank you.

[Force3fan]

Thank you
I am lookig for the switch now locally.
May have to order from US Music Corp or Parts is Parts
Again Thank You

[R.G.]

Quoting my response to much the same question in "guitar amps":

I think you may have a heater wiring problem, possibly a socket wiring issue.

If the tubes were getting low heater voltage caused by something sucking too much current out of the heater winding, it could account for the fuse blow and also for the dull sound. Tubes with too-low heater voltage do change tone a bit - not enough electrons going. But the tubes last a long time.

The fact that a 6.3A fuse blew, then an 8A fuse blew is a red flag. Something is eating too much current. A useful diagnostic would be to measure the heater voltage on each tube and to measure the current actually flowing in the heater winding of the power transformer. These are skilled-tech-only jobs because of the electrical hazards of working inside a live amplifier.

In addition, a dim glow in an AC power switch is a Big Deal, as you've heard. The only condition where this is OK is when there is a built-in indicator light inside the switch, and even then it should go out when the switch is off. I second the advice you have: get a new switch.

Even more, unless you're skilled at working in a live chassis, get a competent tech to solve these issues. We don't want you toasted and unable to play any more.

[Force3fan]

Thank you
Getting the switch.
Will look into the heater circuit further after.

[Force3fan]

Hi All
I got and installed the switch.
That did not change the Dim Glow in the switch with the Power Off.
I did check the original switch with an ohm meter after removing it ( should have done that before buying a new switch )
With the switch in the off position there is no continuity -all terminals test open -so in my opinion the switch is good.
The new switch tested the same.
I should explain again this is a lighted power switch ( red and lights up in the on position ) .
In a dark room with the power off you can see a dim light in the OFF position. I don't know if that is normal but I don't think so.
Looking at the schematic it looks like the only thing between the power in cord and the switch is a Fuse, W2 (i don't know what that is) and Mains Filter.
I don't know how the mains Filter works but is there a posability of a problem there?
Again I don't think so but is there any chance this is a normal condition?
Thanks
Force3fan

[g1]

I'm assuming when the amp is unplugged, the light is off. Then when you plug the amp in with the power switch turned off, there is a glow from the light?
Is it possibly not the correct switch for the TSL60? Are there unused pins on the switch when it is connected? If there are unused pins I'm wondering if it could be installed incorrectly?

[Force3fan]

Hello again
Yes when the power cord is unpluged the dim light goes out. When pluged in the switch is lit very dim. When you turn the switch on The switch lights up normally.
I bought the switch from US Music Corp by the Marshall part number for the TSL60 amp. The new switch is the same as the original switch.
Ther are only 4 pins on the switch and only 4 wires and I think it is install corectly.
This amp had so much dust in it when I got it that looks like it has never been touched inside since it was built.

[J M Fahey]

Nice switch discussion

Now, what about the heater fuse blowing problem?
Just curious

[Force3fan]

The fuse has not blown again since I replaced it but haven't really done but replace the switch.
The fuse condition seems to be very intermittent.
Any thoughts on the Mains Filter between the POWER IN(cord) and the switch.
Sorry I got back to the switch again but somehow I think the two may be related.

[Force3fan]

Hello All
I found and fixed the light in the switch when the power was off.
I discovered when i pulled the mains fuse with the power off the dim light in the switch went out.
After looking at the schematic some more and testing the power at the switch here is what I found.
The hot wire from W1 on the circuit board to the switch and the switched hot back to the circuit board W9
were reversed at the switch.
Both wires are brown and must have been put on the wrong pins on the switch from the factory.
After changing the wires at the switch it works normal.
Lights up bright when power is on and out when off.
Thanks All.
Still working on the fuse problem but it is very intermittent and has not blown again yet.

[g1]

Thanks for the update, I had hoped it would be a wiring issue with the switch, I've only ever heard of one other person having that issue, and they returned the amp for an exchange unit.
Forgot to ask if the heater fuse you are using is slow-blow type?

[Force3fan]

Hello
Yeah the switch really was a simple issue. Good thing cause I got the amp used and it is 12 years old.
I can't believe the original owner never exchanged it.
As for the fuse -- it is a slow-blow.
The fuse has only blown 2 times and it was after changing the tubes and about a month and a half apart each time.
(When I got the amp the tubes were original)
When looking at the switch problem I noticed the fuse is a little discolored now like it is getting too much current.
The House current is reading 124vAC --maybe a little high but I don't think that is a problem.
One question I have is how does the Tube Bias affect the current on the tube heater circuit?
The bias is at 80mv -- Marshall says that is correct for this amp.
I am going to try changing the tubes back to the original ones and then bias the amp again.
Then put a new fuse in and see if it blows or discolors.
It may take a while to know since the condition is so intermittent.