100 watt Valve head blowing mains fuse

Antique_Guitars

I am rubbish at amp stuff...............

I have a 70's valve head that instantly blows the Mains fuse as soon as you switch it on, what could be the most likely cause?

As what is indicated on the chasis, the Mains fuse is 2 amp and the HT fuse is 1 amp

Interestingly I have another one of the same amp that works perfectly, weirdly I just checked the fuses against the recommened printed on the chasis and on this one it has a 500MA anti surge fuse on the HT and a 3 Amp fuse on the mains...............

Any help would be great, I have no idea about any of this stuff the only thing I am good at is giving myself electric shocks. :-)

jpfamps

It's quite helpful to KNOW WHAT THE AMP IS!!!

Assuming there is a standby switch and it switches the HT on:

If the fuse is blowing before you switch on the HT then it's likely to be either the rectifier, the mains filter caps or the mains transformer.

Unless you know what you are doing I wouldn't try trouble shooting this.

If there is NO standby, or the standby operates in a different fashion than turning the HT on and off, try removing the power valves and see if this stops the fuse blowing, if it does then you have a blown valve.

AGAIN WITHOUT KNOW WHAT THE AMP IS IT'S DIFFICULT TO GIVE ACCURATE ADVICE

Antique_Guitars

Lol

Its an old STAK head like a Hiwatt kinda thing

Yeah there is a standby switch and a power switch.

As soon as I switch on the power switch the power light goes on for 1 second then goes out,  when I then check the fuse its blown.

Antique_Guitars

Its exactly like this amp

Antique_Guitars

Antique_Guitars

This is not my one jsut one that is the same in theory..............

ecc83

Well, the chassis in the picture rates a 2amp mains fuse but it also has an internal voltage selector. Might it have been set down to 115V for the US at some point?

Dave.

Antique_Guitars

ecc83 said:

Well, the chassis in the picture rates a 2amp mains fuse but it also has an internal voltage selector. Might it have been set down to 115V for the US at some point?

Dave.

Yeah I did look at that and im pretty sure it was set to 240, I could be wrong though ill check again in the morning (im in bed now) :-).

ICBM

Remove all the power valves and switch it on just once more. If it doesn't blow the fuse it's a valve problem.

If it blows the fuse again, take it to a tech, it looks like it needs a complete overhaul - at least, it probably needs the filter caps replacing since they look to be original and are at around double their normal life expectancy now.

If you don't have any other means of assessing it, don't keep putting fuses in - you're stressing the amp and in particular the mains transformer every time it blows one. Once is enough to tell you there's a fault.

100W amps can take either a 2A or 3A fuse depending on how they're designed and whether that's intended for both 240V and 120V, or not. It's not the fuse value that's the problem.

Antique_Guitars

ICBM said:

Remove all the power valves and switch it on just once more. If it doesn't blow the fuse it's a valve problem.

If it blows the fuse again, take it to a tech, it looks like it needs a complete overhaul - at least, it probably needs the filter caps replacing since they look to be original and are at around double their normal life expectancy now.

If you don't have any other means of assessing it, don't keep putting fuses in - you're stressing the amp and in particular the mains transformer every time it blows one. Once is enough to tell you there's a fault.

100W amps can take either a 2A or 3A fuse depending on how they're designed and whether that's intended for both 240V and 120V, or not. It's not the fuse value that's the problem.

Cool ill check it a bit, just to sya those pictures are not of the amp I have but I am sure its pretty old anyway and needs some work.

jpfamps

Looks like the standby switch is on the HT secondary ground connection (not a good place for it in my opinion).

A shorted valve would provide a path to ground.

Antique_Guitars

Right............

Firstly the switch was definitely on 240v

I removed the power tubes which to be honest are pretty much brand new Svetlana EL34's and the thing still blew the fuse.

I remembered what happened to this amp now as it was actually serviced about 4 years ago by a tech, I used it to record an album with my band and everything was cool then I lent it to my friends band and that it was it never worked after that.

Here is a pic of the inside, it doesn't look too bad.

I am just going to have to sell it as spares or repair I guess.

ICBM

Chances are it has a shorted rectifier diode, since the filter caps look replaced.

Worth finding out, because if it is the repair will be a few pounds. The other main possibility is a blown PT, in which case it may be close to uneconomical on a less valuable amp - although I would still think it worth doing.

And get that Bulgin cable fixed!! It's dangerous. They all are, actually… best to fit one of the TAD replacement IEC (kettle) sockets.

Antique_Guitars

Hmm the other one I have has a Bulgin thing as well but its detachable

I just dont know if I can be arsed to deal with these amps anymore, I have my matamp and the STAK that works which will do.

Ill probably whack it on ebay as spares when I get back from france.

Antique_Guitars

How long do I have to wait before I can put the valves back in?

ICBM

You don't.

But if you're selling it for spares or repair, keep them.

Antique_Guitars

Cool thanks, I might just put it aside for now but I shall sell the 230 watt version I have but that needs new vavles and a good clean/service.

ICBM

Find out what's wrong with this one first. You don't want to give away potentially the better of the two.

Antique_Guitars

yeah true enough, I have three in total 2 x 130w 1 x 230w.

I want to get rid of the 230 watt anyway as it is bloody heavy and I am never going to use it.

That one works but clearly all the power tubes need to be changed, they all light up but two of them are not very strong. Also all of the knobs are very noisy so it needs a very good clean.

ICBM

Is that a 4xKT88 amp? Those seem to be becoming quite desirable for DIY bass amp projects. As long as the transformers are OK it should be worth something.

Antique_Guitars

Yeah it is, it works and makes sounds but the tubes are crap, you can see burn marks on the glass but they do still work.

All the pots are crackling and it needs a good clean.

They are actually really amazing sounding amps similarish to Hiwatt's but not as good.

I am going to ask for about £300 for it collection from London

It is fucking heavy though.