Amp experts help please!

leerocker

Hey guys , Ive just purchased a used ezg50 and unfortunately there is a problem with it  which i wonder if anyone could point me in the right direction to what it is, when i purchased it there was a pack of spare fuses in the back didnt think anything of it at the time, i turned on today and no power replaced the fuse as it was blown and the next one blew and so forth.
 ive replaced the power tubes and fuse and it turned back on played and sounded fine, but every time i turn it off and turn back on it blows another fuse,
what do you think could be causing this because im out of ideas and hoping its not an expensive issue!

John_A

A rectifier valve on the way out is my guess

ICBM

Does the fuse blow on initial power-up or only when you switch the standby to play?

Is there any visible arcing in any of the valves - especially the rectifier - when the fuse blows?

leerocker

ICBM said:

Does the fuse blow on initial power-up or only when you switch the standby to play?

Is there any visible arcing in any of the valves - especially the rectifier - when the fuse blows?

Hey icbm, The power tubes looked like they had blown, i took them out changed them and the fuse and all fine, turned off then on again and fuse blows, every time i turn it off and turn back on it blows a fuse but when on all sounds fine!

cheers!

ICBM

leerocker said:

Hey icbm, The power tubes looked like they had blown, i took them out changed them and the fuse and all fine, turned off then on again and fuse blows, every time i turn it off and turn back on it blows a fuse but when on all sounds fine!

cheers!

Yes, but does the fuse blow when you turn the *power* on, or when you turn the *standby* to 'play'?

Assuming you don't just turn the amp on without using the standby, of course...

What do you mean by the power valves looking like they had blown?

leerocker

one side of each of the tubes was silver inside if that makes any sense, i leave standby on then turn on and play all fine, turn off then turn on again and wont power up and fuse is blown

ICBM

leerocker said:

i leave standby on then turn on and play all fine, turn off then turn on again and wont power up and fuse is blown

Sounds very like the rectifier valve is arcing when the standby is turned on when the amp is already warmed up. That's common on many modern amps where the standby switch is in the wrong place in the circuit - why they do that I don't know! I've had a look at some online photos of the inside of this amp and it looks likely that this is the case, although it's hard to be totally certain because the wires run under the circuit board.

The power valves may be fine, silver on one side is normal for some types of 6L6 - although it can sometimes be a sign of trouble too, depending on exactly that it looks like.

leerocker

forgot to say , when old power tubes were in it wouldnt turn on at all and blowing fuses instantly

ICBM

That does sound like trouble…

Did you buy this form a shop, or privately? If from a shop I would suggest raising a complaint and probably sending it back. One failed valve is one thing and 'more or less normal' for valve amp ownership, three is not so normal.

leerocker

Bought privately icbm

DJH83004

I think as you said right at the outset, the amp came with a spare pack of fuses in the back, which being cynical is a bit suspicious to say the least. I have never worked on one but from the images, there is just a single 3amp mains fuse, so makes isolating the fault slightly more difficult, and its quite big for a 50watt amp. All that said, as previously stated, the rectifier valve would be the prime suspect. 

jpfamps

The power valves shorting out can and does damage rectifiers.

As per advice above I'd try a new rectifier first.

In future you may be better off eschewing the standby switch.

leerocker

Cheers for all the info and wisdom @jpfamps @ICBM @DJH83004  @John_A ; its what makes the fretboard a great place to be on!

 Dr z have also been in touch almost instantly to my email and hes suggesting rectifier too , also the fuse on the back is only 1amp so might try a bigger one.

The doctor has recommended a 1.5amp fuse also,

The person i bought off i have bought from before and her husband who owned the amp sadly passed away and she had the amp serviced in january after he passed away and it hasnt been used since, so not really thinking that ive been conned because she has no need to pull a fast one.
 The amp is immaculate and come from a shop i use a lot and the owner spoke highly of the guy and family who owned it.

Hopefully i can get barry at vyse amps to look at it over the weekend and get it sorted because when its not blowing fuses it sounds great! 



So @jpfamps you are suggesting i dont use the standby at all and just turn on and off?

ICBM

If you're going to have it looked at I would suggest checking where the standby switch is in the circuit, and fixing the problem if it's upstream of the first filter cap. If it's too difficult to move it to after the first cap without a lot of work, it might be worth using it to turn off the feed to the power valve screens instead, which will also work.

That way the standby switch can be used normally.

I'd also be inclined to fit an HT fuse, which *should* be upstream of the cap.

DJH83004

It Does sound genuine from its history, and a 1.5amp slo-blo mains fuse is more of what you would expect to find in an amp of this size, good luck.

ICBM

DJH83004 said:

It Does sound genuine from its history, and a 1.5amp slo-blo mains fuse is more of what you would expect to find in an amp of this size, good luck.

Agreed. Most UK-spec 50W amps use a T1.5 or 2A fuse. (The 3A value shown in some pics should be for a US 120V model.) 1A is almost certainly too low, even if there's no fault with the amp.

leerocker

You guys are right at vyse amps now with new power tubes and higher fuse all is fine so far. So power tubes the culprit....   Thanks for your help guys