Marshall TSL60 - heater circuit blows

[Force3fan]

Hello all
I am kind of new here and may have posted this question in the wrong place so I will try again.
Sorry in advance for a long post but I am going to try to give as much info as I can.
Bought the amp used about 4 months ago. The owner said the tubes were original and amp never been worked on or modified.
I believe him due to all the dust inside. Looked like nothing has been disturbed for 12 years, thats how old the amp is.
The tone has always been dull/muddy on the OD channels using the Neck P/U on my Les Paul.
The Bridge P/U not too bad. The clean channel sounds okay with both P/U.
I thought it may be due to old tubes. Had Svetlana and Marshall in it and bias was 75mv.
I also noticed that the Power Switch glows dim all the time even when the amp is off.
I replaced all the tubes with JJ E34L and ECC83s(bought from Euro Tubes) and set bias to 80mv(per Marshall).
That did not change the tone very much. I am also not shure what is normal for these amps but I don't think it should sound this dull/muddy.
Now to the more urgent problem.
A month after replacing the tubes turned amp on on day and no sound any channel.
Both fuses tested good w/ohm meter.
Took amp to a Tech - he found a blown Heater Circuit Fuse inside (125v 6.3AL - Slow Blow I think) and said he did not see any other problem.
He replaced that fuse with a 125v 8A fuse (slightly higher rating) just in case it is a nuisance condition.
I was not too happy with that but he only charged me $31.00
Its bee a month and a half and the fuse blew again.
I replaced the fuse myself with a 250v 6.3A slow blow -- could not find a 125v 6.3a fuse anywhere.
I would be greatful for any information on the Fuse condition and the Tone condition.
AGAIN SORRY FOR THE LONG POST. I wanted to give as much info. as I could.
Thanks for any help I can get

[R.G.]

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.

[Force3fan]

Thank You very much.
I have ordered the switch
I also talked to the Tech that worked on the amp again and he said the only thing in the heater circuit are the Tube Heaters.
I don't beleive that for a minuite and now really don't trust him.
Will get the switch in first thenstart looking for other trouble in the heater circuit.
Again Thanks and my need some further help.

[J M Fahey]

I also talked to the Tech that worked on the amp again and he said the only thing in the heater circuit are the Tube Heaters.
I don't beleive that for a minuite and now really don't trust him.

Me neither.
I´ve answered in your other post about the same issue.
I suggest you do not ask the same question in different sections because it becomes impossible to keep track.

[Force3fan]

Thank you
Sorry about posting the question in 2 diffrent places.
I am new here and trying to learn this forum.
I wasn't shure I posted the first time in the right place.
I won't be doing that again.
Thanks for all your help

[Jazz P Bass]

Yeah, sometimes when you post something it may take awhile for it to appear.
So, if you do not see your post, the first inclination is that it did not take.
So I'll post again.....
If you really cannot find it, try the "search" function.

[Force3fan]

Hello All
I found and fixed the switch problem.
The Hot and Switch Hot wires were reversed on the switch causing it to light dim even with power off.
Thanks all.