Is it necessary for safety and oscilloscope protection to use an isolation transformer when working on valve amplifiers that already have a mains transformer.
The mains transformer is grounded to mains earth and the chassis is also grounded. However, the only point in the amp where mains power is exposed is at the transformer terminals. The rest of the amp is running off high voltage DC and low voltage AC (6.3 volts).
John
The mains transformer is grounded to mains earth and the chassis is also grounded. However, the only point in the amp where mains power is exposed is at the transformer terminals. The rest of the amp is running off high voltage DC and low voltage AC (6.3 volts).
John
Not essential, but nice to have for general safety. Of course the high DC voltage is dangerous too.
That's what he's saying- the DC is dangerous too so it's only a partial solution for safety.
Yes, use only one hand (other hand in pocket, not on the equipment), no jewelry, drinks, other people, etc. around.
Have the power off, connect the probes, then turn the power on. Reverse the procedure to remove the probes.
Never touch a probe while the circuit is energized. Make sure your probe is rated high enough in voltage, too.
The units absolutely requiring an isolation transformer are the ones where the chassis is connected to one side of the AC line. This was common practice in the 50's and 60's. Touching metal chassis that is at 120 vac with 20 amps behind it, is deadly. Many houses/work benches had the hot and neutral pins reversed in the plug, as well. I rented one such house in 1983, found out while working on a hot chassis TV.
Many amps also had capacitors connecting the chassis to one pin of the power plug, which were called "death caps" for a reason.
An ohms check between power plug pins and the chassis will find the first condition. The second requires a capacitance tester.
After 3 pin power plugs became common, UL listed appliances had the chassis isolated from both power pins. The round 3rd pin was supposed to be connected to chassis.
Practice varies outside the USA, especially in 2nd & 3rd world countries.
Many amps also had capacitors connecting the chassis to one pin of the power plug, which were called "death caps" for a reason.
An ohms check between power plug pins and the chassis will find the first condition. The second requires a capacitance tester.
After 3 pin power plugs became common, UL listed appliances had the chassis isolated from both power pins. The round 3rd pin was supposed to be connected to chassis.
Practice varies outside the USA, especially in 2nd & 3rd world countries.
Thanks for the detailed info. I thought that was the case. Most of the vintage radios and amps produced in Australia use a mains transformer and there was no 240 volt ac in the radio, with the exception of the transformer/switch connection.
I actually had a "bird on a wire" episode a couple years back. I had to hold a 120VAC wire into a power box,
tighten the wire and NOT touch anything else so I was 'elevated' electrically for a minute or so. Gave me the
willies but no tingles.
G²
Take it from an old experienced technician...... using an isolation transformer is good practice when using any test equipment, regardless of the age/design of the equipment being tested.
It eliminates any possible source of potential shorts between the units.
It eliminates any possible source of potential shorts between the units.
Best bit of safety gear for working on valve amps is a cap discharger, that and a lamp limiter.
Andy.
Andy.