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Old 26th November 2010, 02:25 PM   #1
akis is offline akis  United Kingdom
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Default High voltages

never built anything with tubes, I was looking at a guitar amp schematic from Mesa and it struck me at its simplicitly. What also struck me was the fact there were very high voltages present in the circuit all the way up to the pots and switches, the only separation being a single capacitor. I was imagining a lentil sized ceramic cap being the only protection between 200-400 Volts and the potentiometer (and some unlucky guitarist)...

Now I know musicians using valve amps do not exactly get electrocuted in their dozens, but in other kinds of designs we go to lengths to keep high voltages (transformers, wires etc) at the back of the enclosure and physically isolated from anything else as much as possible, whereas in a valve system you have 400V running loose all over the place.

Just curious
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Old 26th November 2010, 03:35 PM   #2
jjman is offline jjman  United States
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Proper safety grounding should prevent shocks from a hot chassis. Even if some DC voltage gets to the pot, it's not on it's body. I worked on a friends amp that had DC getting to a pot from one of several bad caps. The pot was oxidized more than the others but no voltage was on it's body, which is also grounded in many/most amps.

Older guitar amps have a "death cap" and no ground connection on the mains plug. This is the scenario you are alluding to since the bad/leaking cap places mains voltage on the chassis which is not safety grounded since safety didn't matter back then.
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Old 26th November 2010, 05:11 PM   #3
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Old 26th November 2010, 06:44 PM   #4
M Gregg is offline M Gregg  United Kingdom
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akis,

You have to remember that the grid on a tube is a piece of metal which is distanced from the anode of the tube. If the grid is coupled to the HT for some reason via a resistor then there is normally a resistor to Gnd which should divide it. Then if the input has a coupling cap I would expect to see a polypropylene which has excellent insulation. The chassis should be grounded so if HT contacts the chassis then "Auto Disconnection of Supply" should take care of it via fuses or C/B.
The danger is "mechanical" damage to the inside of the tube, when the components i.e. grid shorts to anode, however if it has a coupling cap this would also have to be faulty.

Some old equipment was not built with safety in mind so if you are repairing I would expect most people would build in safety as a matter of course! Discharge resistors are an example!

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Last edited by M Gregg; 26th November 2010 at 06:48 PM.
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Old 27th November 2010, 01:54 AM   #5
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The 3 legs of the pot are all insulated from the case and shaft, the case and shaft are earthed, the potential fault current is minimal. So even if the capacitor failed the pot internals would see some more voltage but would still work fine. If the pot insulation failed the earth would shunt the minimal fault current away from the operator.

While I live in a country where unearthed metal work is proscribed even if the case is unearthed, provided there is no other path for the current to flow outside the case the fault current will be totally contained within the case thereby causing no danger to anything outside the case.
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Old 27th November 2010, 12:18 PM   #6
DF96 is offline DF96  England
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The really dangerous place in most electronics is the mains on/off switch. The insulation between live mains and the user is quite small, yet quite sufficient. Almost anythng else, transistor or valve, is safer as there is more isolation and the current capability behind the isolation is much smaller than the bare mains. If volume pots worry you - don't touch the switch!
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Old 27th November 2010, 05:58 PM   #7
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Quote:
Originally Posted by akis View Post
never built anything with tubes, I was looking at a guitar amp schematic from Mesa and it struck me at its simplicitly. What also struck me was the fact there were very high voltages present in the circuit all the way up to the pots and switches, the only separation being a single capacitor. I was imagining a lentil sized ceramic cap being the only protection between 200-400 Volts and the potentiometer (and some unlucky guitarist)...

Now I know musicians using valve amps do not exactly get electrocuted in their dozens, but in other kinds of designs we go to lengths to keep high voltages (transformers, wires etc) at the back of the enclosure and physically isolated from anything else as much as possible, whereas in a valve system you have 400V running loose all over the place.

Just curious

High voltage on an interstage potentiometer will not be of any immediate danger unless something unusual happens such as the hi side of the pot touches the chassis. Even then you have a plastic knob on the pot shaft. It is very common for older amps to have varying amounts of DC on the hi side of a volume pot due to leaky coupling caps, but the wiper and hi side are normally isolated from the chassis.
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Old 30th November 2010, 08:59 PM   #8
godfrey is offline godfrey  South Africa
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Quote:
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Now I know musicians using valve amps do not exactly get electrocuted in their dozens, ...
Sez who? How did you think their hair got like that?
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Old 30th November 2010, 10:49 PM   #9
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Quote:
Originally Posted by akis View Post
whereas in a valve system you have 400V running loose all over the place.
It's a consideration. A solid-state headphone amplifier isn't going to take you out. A tube one could. That said, I've wired myself up to more than one old communications transceiver including morse key without a second thought.

OTOH, if you have kids...

w

...might be one way of getting rid of them.
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