Changing a Valve

Mark1960

ICBM said:

Mark1960 said:

Once power is off and it's unplugged, is there any residual power still retained in a valve amp. I've heard somewhere that you still have to be careful what you touch, or is that not the case?

Yes, it is the case. As ever, it varies depending on what amp it is - some automatically drain the charge from the filter caps, some don't. The Classic 30 is one that doesn't, although since there is no standby switch, simply turning off the power, unplugging the amp from the wall and waiting until the valves have cooled down will get you *most* of the way there. Simply being careful what you then touch will keep you safe after that.

Thanks for that I'll proceed with caution.

Mark1960

Any advice on where to get the valves (and what type / make) and fuse from or just any mail order on the internet?

ICBM

Valves - Karltone. I would get JJs, they're the most reliable EL84s in my experience. The fuse may be a little more tricky since it's a 30mm (1-1/4") type not something you can find in an electrical shop. The best bet is Ebay, probably - there are many sellers who do stuff like that in small quantities. If you have real trouble, PM me.

Mark1960

ICBM said:

Valves - Karltone. I would get JJs, they're the most reliable EL84s in my experience. The fuse may be a little more tricky since it's a 30mm (1-1/4") type not something you can find in an electrical shop. The best bet is Ebay, probably - there are many sellers who do stuff like that in small quantities. If you have real trouble, PM me.

Valves ordered, and fuse sourced, collecting from a local hardware store! Thanks once again.

Mark1960

I have now replaced the fuse, and fitted four new power valves, and am very happy to report that the amp works fine again. Many thanks to ICBM and others for technical support and moral support. I couldn't have done it without your help.

ICBM

Well done!

Mark1960

ICBM said:

Well done!

Thanks - With your help it was easier in the end than I thought. I think having the confidence to have a go is a major hurdle. The only thing I was worried about was touching something with a residual voltage and getting a shock>

ICBM

Mark1960 said:

Thanks - With your help it was easier in the end than I thought. I think having the confidence to have a go is a major hurdle. The only thing I was worried about was touching something with a residual voltage and getting a shock>

It is a risk, although a minor one. With most amps, simply turning off the power - without using the standby switch, if it has one - will drain the caps enough to be safe before the valves cool down, even if they do retain a bit of a charge. And of course making certain the amp is unplugged from the mains.

Where the confusion arises is that there are some which do retain fully-charged caps with dangerous voltages if you turn the standby off first, then the power - and others where turning the standby off first is the safest. It all depends on whether there are caps upstream of the standby switch or not, and whether those caps have resistors on them.

At the end of the day, as long as you *also* do the standard safe working practice of never touching the amp with both hands at once when you're working on the circuitry - which is why many old-school techs deliberately put the other hand in their back pocket when doing that - then you're extremely unlikely to come to any harm.

Mark1960

ICBM said:

Mark1960 said:

Thanks - With your help it was easier in the end than I thought. I think having the confidence to have a go is a major hurdle. The only thing I was worried about was touching something with a residual voltage and getting a shock>

It is a risk, although a minor one. With most amps, simply turning off the power - without using the standby switch, if it has one - will drain the caps enough to be safe before the valves cool down, even if they do retain a bit of a charge. And of course making certain the amp is unplugged from the mains.

Where the confusion arises is that there are some which do retain fully-charged caps with dangerous voltages if you turn the standby off first, then the power - and others where turning the standby off first is the safest. It all depends on whether there are caps upstream of the standby switch or not, and whether those caps have resistors on them.

At the end of the day, as long as you *also* do the standard safe working practice of never touching the amp with both hands at once when you're working on the circuitry - which is why many old-school techs deliberately put the other hand in their back pocket when doing that - then you're extremely unlikely to come to any harm.

Are the caps the white circular units that you push the valves into?

ICBM

Mark1960 said:

Are the caps the white circular units that you push the valves into?

No - "caps" = capacitors. The large cylindrical components marked with voltage and 'uF' values, grey and black in your amp. The ones marked with voltages over 100V are best not touched unless you're sure they're discharged .

Mark1960

ICBM said:

Mark1960 said:

Are the caps the white circular units that you push the valves into?

No - "caps" = capacitors. The large cylindrical components marked with voltage and 'uF' values, grey and black in your amp. The ones marked with voltages over 100V are best not touched unless you're sure they're discharged .

Are they the silver units just beyond the HT fuse on the pic you sent me?

ICBM

Mark1960 said:

Are they the silver units just beyond the HT fuse on the pic you sent me?

Yes, but if you touched them don't worry - the metal ends are the ground connections, so safe . In this amp... some others have what's called 'stacked' caps - to double the voltage rating - and then one of them is at half the full voltage, so be careful before assuming anything!

I know, there is a huge amount you need to learn and it can be very confusing at first. Most of us who work on amps for a living have occasionally learned some of it the hard way, which is why the back pocket rule matters.

Mark1960

ICBM said:

Mark1960 said:

Are they the silver units just beyond the HT fuse on the pic you sent me?

Yes, but if you touched them don't worry - the metal ends are the ground connections, so safe . In this amp... some others have what's called 'stacked' caps - to double the voltage rating - and then one of them is at half the full voltage, so be careful before assuming anything!

I know, there is a huge amount you need to learn and it can be very confusing at first. Most of us who work on amps for a living have occasionally learned some of it the hard way, which is why the back pocket rule matters.

Presumably the back pocket rule means the charge goes through the body to ground, as oppose to through the heart area if both hands are touching?

ICBM

Mark1960 said:

Presumably the back pocket rule means the charge goes through the body to ground, as oppose to through the heart area if both hands are touching?

No - it means the charge doesn't go through the body to anywhere. Or not unless you're standing in bare feet on a damp floor, so don't do that either! Any normal shoes will be effective enough at insulating you. You will still get a shock, because the capacitance of your body will allow a very small current to flow, but although it will hurt it's unlikely to kill you.

You're correct that the most dangerous is one hand touching something live and the other touching something grounded, like the amp chassis - even if it's not connected to the mains earth, that will complete the circuit - the current will flow straight across your chest and can indeed be fatal even at quite low currents, around 30mA is enough.

Mark1960

ICBM said:

Mark1960 said:

Presumably the back pocket rule means the charge goes through the body to ground, as oppose to through the heart area if both hands are touching?

No - it means the charge doesn't go through the body to anywhere. Or not unless you're standing in bare feet on a damp floor, so don't do that either! Any normal shoes will be effective enough at insulating you. You will still get a shock, because the capacitance of your body will allow a very small current to flow, but although it will hurt it's unlikely to kill you.

You're correct that the most dangerous is one hand touching something live and the other touching something grounded, like the amp chassis - even if it's not connected to the mains earth, that will complete the circuit - the current will flow straight across your chest and can indeed be fatal even at quite low currents, around 30mA is enough.

OK thanks for information. Most interesting