amp techs,why did this fuse burn?

[metalspider]

well my marshall has acted up again but after a bit of pocking around i found the problem.the fuse for the tube heaters current burned out so the tubes didnt light up at all and i got no sound even though the power was on and the ht fuse was fine.
anyway did a quick fix with some spare wire since it is a 6.3 amps fuse and sure enough it works now,ill get a proper fuse tomorrow.

so my question is why did this fuse burn out anyway?what should i watch out for?ive had the ht fuse blow because of old tubes but this is a different fuse all together and although the tubes are getting old i havent been using the amp much for a while(been using a modeler and studying).

thanks

edit:amp is a marshall tsl601 btw

 

[muttonchopsrule]

Re: amp techs,why did this fuse burn?

if you have a meter, measure the voltage on the two heater wires...you should get 6.3V AC. if it's higher, then you have a larger problem that you'll have to look deeper into. if the voltage is right, then perhaps you might have a bad tube that's drawing too much heater current and blowing the fuse.

 

[WhoFan]

Re: amp techs,why did this fuse burn?

i'm no amp tech but i'd be carefull running the amp with a jumper cable like that.. fuses blow and sometimes, most times it protects the amp from damage..

 

[metalspider]

Re: amp techs,why did this fuse burn?

thanks ill check the voltage later.
i only ran the amp for a few minutes with the jumper to see if that was indeed the problem,besides i figured 6.3 amps is one hell of a big fuse rating.

what i thout was really strange is that the fuse read as ok when in its housing so i kept looking for a while before i went back and took it out and saw it was bad with my meter.
the two terminals it sits in read as a short even with no fuse in there.

also the is the first time this fuse ever blew and ive had the amp for 6 years now.
stuff ive done to it recently is reattach one of the ac in's to the pcb and i replaced the speaker cable with a new one with thicker gauge wire.

 

[metalspider]

Re: amp techs,why did this fuse burn?

ok put in a new fuse and the amp is working,however i checked the voltage to see if i get 6.3 ac and what i got was 2.6v ac on both power tubes and v4 while v1 v2 and v3 read as 6v ac.
mains is 230v were i live,not 110 like in the usa this make any difference?

 

[crusty philtrum]

Re: amp techs,why did this fuse burn?

How are you measuring the heater voltages? i.e. using one probe contacting ground and the other probe to a heater pin of a valve (tube) socket, or putting the 2 probes on the 2 heater pins of the socket being tested?

Measuring a fuse IN the holder will not give any indication of the fuse's condition, It has to be removed to be tested.

I don't have the schematic for this particular amp in front of me but i would guess that the fuse in the filament(heater) wiring would need to be a Slow-Blow type, not a fast-acting one.

 

[metalspider]

Re: amp techs,why did this fuse burn?

i attached one probe to the chassis and used the other one to measure the voltages.
i did put in a slow blow fuse now with the right rating so thats ok now.
is the fact the the filament current is ac cause hum? could i convert it to dc with some diods and caps or even some voltage regulator easily?
this amp has a constant low pitched hum when its on,i replaced the mains filter caps a long time ago but that had no effect.

 

[crusty philtrum]

Re: amp techs,why did this fuse burn?

OK...with one probe to the chasis, you will get about half the voltage readings on each heater wire. Therefore, the readings of 6V may indicate a second winding that is rectified to run the preamp tubes at 12VDC. I will have to find a schematic and have a look.

Simply rectifying the AC winding to DC will probably provide the wrong voltage.

The AC filament current can induce some humm, but if the amp is factory original and the wires have not been re-routed it shouldn't be prominent.

A source of hum after the filter caps have been replaced can be the flter cap(s) in the bias supply. They usually get overlooked. If replacing them, it's vital to note their polarity orientation....usually reversed from normal usage (i.e. the cap's positive lead is grounded and it's negative lead attaches to the bias voltage, assuming it's a negative supply, which i am sure it would be

 

[crusty philtrum]

Re: amp techs,why did this fuse burn?

Looks like V4 and the power tubes are running AC and the others are already running DC filaments. Quite a few caps in the DC filament filtering, too. There's not much point in V4 and power tubes running DC heaters, hum issues from filaments are usually at the front end, and it's not a good idea to do that without knowing the current ratings of the transformer windings, as you need about 40% more current handling capability when you go DC., and i doubt the power tranny would have that much in reserve.

I'd look into updating the filament filtering caps if your HT filter caps are good.

 

[metalspider]

Re: amp techs,why did this fuse burn?

thanks ill look for em and see about replacing them.getting under the pcb is a real hassle though with this amp.