• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Isolation Transformers

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).

Joined 2011
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.