Amp blowing? Amp problem

thebreeze

I've often heard the expression "I blew my amp" but never really understood what it referred to or meant.

What should I do about mine?  Having not used it for about 3 months (I've been using a different amp) I carried it to a different room in the house this afternoon.  I plugged it in to the wall/mains and without checking anything turned the switch on at the wall.  There was an immediate almost explosive phut which was quite alarming and then a burning smell, so I immediately turned it off at the wall.

On checking the amp I could see that the On/Off switch was in the opposite direction to the Stand-by switch.  For the amp to be On they both need to be in the same position.  I can't tell or remember though which direction is On and which one is Off.  It tripped one of the fuses in the house fuse box too.  I also noticed (when I bothered to look) that the Vol., Bass and Treble were all maxed out (10) (could have been a child over Xmas?...) but I'm not sure that should make any difference should it? 

There's a fuse on the amp and it says "3/4A SLO-BLO" - although there wasn't anything slow about how it blew  .  The glass bit is kind of dangly and the spring bit inside is all at one end and unattached to anything at the other (I'll post a pic below).

The amp is American.  Is this a vaguely normal thing that happens once in a while and it's just a matter of replacing the fuse?  Or is this likely to be something serious and I should get it to a tech pronto?

Many thanks for your advice/help etc.

thebreeze

bertie

 the glass is normally blue/black if they blow, plus as it went "phut"  its probably not the slow blow but another (have you checked the lead/plug ?)

what amp is it ?  is it a combo ? head ?  was the head plugged into a speaker ?

Id wait for ECC83 or ICBM  to pass professional judgement tho  

BillDL

thebreeze said:

Is this a vaguely normal thing that happens once in a while and it's just a matter of replacing the fuse?  Or is this likely to be something serious and I should get it to a tech pronto?

I am not an amp technician, but I would say that the fuse is there for a good reason.  It is there to protect the rest of the amplifier components and also anybody plugged into it if the amp of power supply into it develops a fault.  The recommended action is to have an electrical appliance checked for faults before replacing the fuse and powering it back on again.

You could, of course, just replace the fuse and try powering it on again to see if the fuse blows, and watch, listen and smell while you do so, but that would be entirely your risk.

I would do the following before being tempted to replace the fuse and try again.

• Check the wiring inside the socket that you plugged it into after isolating the supply to your sockets at the fuse box/consumer unit.
• If it looks OK, check the supply using a multi-meter, BUT ONLY IF you know what you are doing and how to use a mains rated meter.  Otherwise you could try plugging in a cheap mains powered appliance that has its own fuse to see what happens.
• If the lead for the amp is a removable one, try another of the same kind.  Make sure that the fuse in the plug of the lead isn't too high a rating because that should really be the first thing that blows if there is a fault with the power supply.
• Carefully vacuum any cobwebs and dust out of the amp and the circuitboard if you can gain easy enough access, but DON'T touch anything inside with your hands.  Look for any loose nuts, bolts, paperclips, etc lying around near any of the electrical circuitry.
• Remove and reseat your valves several times and look at their condition for signs of damage..

thebreeze

bertie said:

 the glass is normally blue/black if they blow, plus as it went "phut"  its probably not the slow blow but another (have you checked the lead/plug ?)

what amp is it ?  is it a combo ? head ?  was the head plugged into a speaker ?

Id wait for ECC83 or ICBM  to pass professional judgement tho  

Cheers bertie.  I haven't checked the lead plug but will do so now.  The burning smell was definitely coming from the amp though.  It's a very early Tone King Imperial combo - the 10th made and something of a prototype so has slight differences to the production models.  It's about 30 years old and was given a thorough going over by Chris at @RiftAmps in 2020.  Never had a problem with it before. 

ICBM

That's something fairly serious. Probably not a good idea to replace the fuse, as it's a near certainty it will just blow again and could stress other (as yet) undamaged components before it does.

If it blew the main fuse immediately on power-up, regardless of which state the standby switch was in, it's probably not a valve fault - unless it has a valve rectifier.

So (please...) for about the hundredth time on threads like these, WHAT AMP IS IT? 'American' covers rather a lot of amps, and it would be extremely helpful to know what make and model it is, since that would save asking questions like whether it has a valve rectifier! Why does everyone seem to want to keep it secret?

The knob settings have nothing to do with it whatever though.

thebreeze

Sorry @ICBM quite right, we just crossed - there's more detail above but say if you need more.

bertie

ICBM said:

 WHAT AMP IS IT? '

thebreeze said:

It's a very early Tone King Imperial combo - the 10th made and something of a prototype so has slight differences to the production models.  It's about 30 years old and was given a thorough going over by Chris at

@RiftAmps in 2020.  Never had a problem with it before. 

thebreeze

^ haha - belt and braces

@BillDL thank you

ICBM

thebreeze said:

Sorry @ICBM quite right, we just crossed - there's more detail above but say if you need more.

OK, so it is valve rectified - the 5AR4/GZ34, which will usually be at the end nearest the power cable/socket. There is a reasonable chance that this valve has failed, and the burning smell was coming from the base of the valve, rather than the socket. A short in the rectifier can blow the fuse even before the valves have warmed up.

What to do is remove the rectifier valve, replace the fuse - you should be able to get the correct value on Ebay if you don't have one, you only need a couple - and power up again. If the fuse blows again with no rectifier in, it's a definite tech job.

If it doesn't, it's probably worth buying a new rectifier valve (spares are useful anyway, and these aren't too expensive) and try again - make sure the amp is on standby when you power up. If it now blows immediately it's still a tech job. If it works until you switch from standby to on, it needs further investigation - there's probably enough to be going on with here!

thebreeze

ICBM said:

thebreeze said:

Sorry @ICBM quite right, we just crossed - there's more detail above but say if you need more.

OK, so it is valve rectified - the 5AR4/GZ34, which will usually be at the end nearest the power cable/socket. There is a reasonable chance that this valve has failed, and the burning smell was coming from the base of the valve, rather than the socket. A short in the rectifier can blow the fuse even before the valves have warmed up.

What to do is remove the rectifier valve, replace the fuse - you should be able to get the correct value on Ebay if you don't have one, you only need a couple - and power up again. If the fuse blows again with no rectifier in, it's a definite tech job.

If it doesn't, it's probably worth buying a new rectifier valve (spares are useful anyway, and these aren't too expensive) and try again - make sure the amp is on standby when you power up. If it now blows immediately it's still a tech job. If it works until you switch from standby to on, it needs further investigation - there's probably enough to be going on with here!

Thank you.  I'll do all that.  I guess I can fine the fuse value from a manual, there doesn't seem to be anything written on the current one?

ICBM

thebreeze said:

 I guess I can fine the fuse value from a manual, there doesn't seem to be anything written on the current one?

It will be engraved on the side of one of the end caps - looks like the end that's currently in the holder cap - but it may be hard to read. If not, it should be printed on the amp's back panel next to the holder, or in the manual.

If there's absolutely no information you can find at all, use a T1A. The US 120V value in the manual online is 2A, and the 230/240V value is usually half that.

I'm assuming there isn't a voltage selector on the outside and your helpful small assistant hasn't set it to 120V, by the way...

bertie

sounds a bit "ambulance chaser" ish  - but be interesting to see what it is 

thebreeze

bertie said:

sounds a bit "ambulance chaser" ish  - but be interesting to see what it is 

I’ll report back.  Can’t say when though

thebreeze

ICBM said:

thebreeze said:

 I guess I can fine the fuse value from a manual, there doesn't seem to be anything written on the current one?

It will be engraved on the side of one of the end caps - looks like the end that's currently in the holder cap - but it may be hard to read. If not, it should be printed on the amp's back panel next to the holder, or in the manual.

If there's absolutely no information you can find at all, use a T1A. The US 120V value in the manual online is 2A, and the 230/240V value is usually half that.

I'm assuming there isn't a voltage selector on the outside and your helpful small assistant hasn't set it to 120V, by the way...

Haha, no there's no tempting voltage selector.  By the power socket input it says, "220VAC 50Hz".  By the Fuse input it says, "3/4A (as in three quarters) SLO-BLO".  There is a sticker from a service in 2014 that says, "Fuse Rating....3"  Disconcertingly the "3" has been written over Tippex covering what had been written before.

On the current fuse I've managed to read (just!) "A (funny symbol?)TDC 11750mA.

The more modern production Imperial manual says "A standard 2A slo-blo fuse (style 3AG)" should be used.

My Imperial is No.12 v.11 (Mark Bartel reflecting in an email to the previous owner that V.11 reflected he was making a lot of changes to the design at the time.)  So it has 6 preamp valves rather than 5 for example.  I don't know if that matters?

Would it be possible to post a link to the correct fuse I should buy?  Only if it doesn't take up too much of your time, I'm slightly confused at this stage though.

ICBM

The existing value is T750mA - have a look on Ebay. I’m at work just now but if you can’t find one I’ll have a look later.

If you can’t find one, T1A will be safe enough.

thebreeze

ICBM said:

The existing value is T750mA - have a look on Ebay. I’m at work just now but if you can’t find one I’ll have a look later.

If you can’t find one, T1A will be safe enough.

I've bought these: 750mA Amp T750mA T750mAL Fuse AntiSurge 20mm x 5mm - Pack of 10 UK Stock 

ICBM

Is that a 20mm fuse? It looks more like a 30mm (1-1/4"), US amps often use those. Sorry, I forgot to say that... 

thebreeze

Don’t worry, they weren’t expensive.  I’ll have a check.

thebreeze

ICBM said:

Is that a 20mm fuse? It looks more like a 30mm (1-1/4"), US amps often use those. Sorry, I forgot to say that... 

Are these the right ones?  They have a voltage rating of 250v.  If you buy a pack of 10 though the voltage rating seems to be 500v.  Not sure why or whether they're actually different?

https://uk.rs-online.com/web/p/cartridge-fuses/8527146

ICBM

Yes. No practical difference, just different manufacturers test spec I think - no issue anyway if it’s a mains fuse, since it’s 240V. I wouldn’t even worry about using a 250V fuse for an amp HT where it should really be 500V.