Tube/valve amp fuse problem

  • Thread starter Thread starter rayc
  • Start date Start date
rayc

rayc

retroreprobate
I had been playing my guitar for about an half hour through my Marshall Super Bass MkII when the amp blew a fuse. I replaced the fuse & turned the beats on & it blew straight away.
Slow blow 1 amp as recommended on the head.
I used an 8ohm 200w eton single cone cab
a Tubecube power sink set at 8ohms
The guitar - a semi acoustic Bruno Royal Artist from about 1970 into a Big Fluff Muff clone.
Any ideas as to the likely problem?
I don't want to take it to a tech to find it was something I should've taken care of...
 
That's an oldie...right?

What's the tube layout in those?
 
rayc said:
Any ideas as to the likely problem?
I don't want to take it to a tech to find it was something I should've taken care of...
Click to expand...
Ideas? No. Probably something toasted. You could let it cool down and try again but ultimately I think it's going to the tech regardless - unless you do your own work which I'm thinking you don't. I don't either - they scare me.


lou
 
arcaxis said:
If the schematic I found is similar to your amp the fuse directly supplies B+ to the output transformer, power output tubes, and a filter choke that through some resistors feeds the power output tube grids, and preamp section. Possibilities of failure that might cause enough current for the fuse to blow.... 1) Failed tube, 2) Output transformer has winding shorted to ground, 3) Filter choke has winding shorted to ground. Try pulling the power tubes and see if the fuse still blows. If it still blows I'll take a guess it's the output transformer. If it doesn't blow you might be lucky and just have to replace the tubes.

http://www.webphix.com/schematic he...m/marshallamps/jcm800_superbass_100w_1992.pdf
Click to expand...




Arcaxis, isn't it a bit risky for the untrained to go poking about inside valve amps, even when switched off? I assume there are some big capacitors in there that might still be holding charge even when the power's turned off.
chin.gif
 
arcaxis said:
Pulling the tubes is relatively simple doesn't entail going under the chassis where the caps are located. If the fuse blows with the tubes pulled, it's time for the Amp Doctor to see it with no question. I wouldn't even suggest the OP stick his fingers under the chassis unless he really has some grasp of what he's doing.
Click to expand...

Ah, right - fair enough. :)
 
My grasp is summed up in my 1st post.
I can do a very limited range of electrical things: turn on, turn off, pres reset, press standby, remove lightbulb, replace lightbulb, make toast, eat toast & the occasional valve removal without getting a stored blast from a massive cap designed to make pork crackling of the unwary.
Thanks fellows. I'll try the valve free test then take it to a tech. It hasn't had a seeing to since about '84 anyway
 
are you SURE it's a 1amp fuse?
That seems a bit small for an amp like that. Could one of the numbers be rebbed off or hard to see?
 
Lt. Bob said:
are you SURE it's a 1amp fuse?
That seems a bit small for an amp like that. Could one of the numbers be rebbed off or hard to see?
Click to expand...

That's odd. The Marshall manuals don't specify a fuse rating. They say it's stamped on the chassis or cap.


lou
 
if the fuse keeps blowing then it's either a tube or something in the power section. If you have extra tubes, then try swapping them out. when was the last time the tubes were changed?
 
1A is the HT or high voltage secondary fuse. The AC in or 'Mains' is likely a 4A or whatever, depending on how many output tubes you have. With a 1A HT, it's likely a 100-watt head, so 4A for the Mains isn't too far off the mark.
Likely, you have a bum tube. You can pull all four EL34's and try another fuse. It may not blow again, and you know one of those four tubes is bad. If you don't have a tube tester, you can spend a few dollars on fuses and try one tube back in at a time. When the fuse blows, you have the bad tube. You can confirm it with the other three tubes in place.
 
Hey Ray, Takes me way back - I haven't had a fuse go for decades. Used to use a tin foil cigarette wrapper to make it work for the rest of the gig.
 
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