Dr Tony Balls wrote: Mon May 24, 2021 8:47 am
If it turns on but makes no sound at all its very likely that you blew a
fuse on the high voltage supply. The amp turns on because the mains fuse is intact and the filaments of the tubes work and light up. On that amp, however, the secondary fusing is all internal (and possibly hard-wired direct to the board), so as previously directed get a tech to check it out.
The question is what caused the fuse to blow, and if your drummer was correct and one tube was not lit-up, then it seems likely to me that you just killed a tube which is bound to happen from time to time. Fortunately the amp should be properly protected so that when that happens it doesnt damage anything important.
It's almost as if this fuse business was already covered...
There's a mains fuse (accessible from the outside) and a slew of different fuses that will be on the inside. It's likely the HT fuse which is blown, which should be fuse #FS103, and is a 1A slo-blow fuse.
oZZma wrote: Sat Jun 26, 2021 6:42 am
- I am more scared by removing the amp from the chassis than working inside of it. I mean, how should I grab that heavy thing without touching any capacitor of death? Sorry for the stupid question but I don't even know for sure where is it safe to touch.
- Do I really need to discharge capacitors? What's the point, if the amp needs to be powered and switched on for the biasing?
- Do I need to calculate the bias value? I have found a video for cold and hot biasing on my amp, but I don't know if the voltage values are dependent from the tubes you are putting inside, or they are always the same for the same amp.
Not sure I'm gonna go this route but I have found the tubes for 165€ so I'm not keen to spend 300€ if I can do this myself *. Thanks for your help.
- Another question: some say you can safely run the amp at 50watts by removing two tubes and divide by two the impedance to connect your cab. Is that TRULY safe? I have read different opinions on that.
*ETA: Are Electro Harmonics KT88's decent? I have found they are a lot cheaper than other brands so I dunno.
ETA: Another question: I know my amp has "bias terminals" which should make biasing a little easier. I haven't found photos for my specific amp but on another Marshall amp, I saw this "terminal" has three pins. But the bias trim pots are only two. How do I take my measurement exactly? Someone can explain how it works?
- Nothing on the outside of the amp will shock you. You can pull the screw out of the bottom and slide the chassis out or grab it by the transformers and lift and pull.
- YES you REALLY need to discharge capacitors before you stick anything in the underside of a tube amp. This includes changing the fuse.
- You need to measure the plate voltage to determine the right bias setting and then adjust the bias as needed.
- Yes you can remove two of the tubes and run the amp at 50W and its totally safe
- You sure about those bias terminals? I dont think it has them.
and finally but most importantly...
- You should really bring this to a tech.