Safe method for measuring plate voltage

[Jquijano]

Whats the safest method for measuring plate voltage with a digital multimeter on my marshall 1959HW 100 watts? I really dont want to get fried or shocked. Its good to have knowledge prior to attempting this?

[xtian]

Safest? Start with amp off, unplugged, and caps drained (and confirmed zero volts DC with a multimeter). Next, clip black meter lead securely to chassis or other obvious ground point, and red lead to power tube plate (pin 3 for 6L6/EL34/6V6, pin 7 for EL84). Turn on meter and set for DC volts. Turn on amp and let it warm up. Note voltage. Power down, drain and confirm caps at zero volts, remove meter leads.

But really, I probe live amps all the time. Each amp presents different access points to measure voltages--sometimes it's difficult to reach power tube pins, sometimes easy. Often it's easier to touch the OT primary's center tap—this voltage will be just a couple volts less than the plates.

[DrGonz78]

Be knowledgeable about the gear you are working on. Ask questions ALWAYS if it involves safety issues. Sounds like you are off to a good start. The thing is that after measuring plate voltage over 100 times it starts to feel like a walk in the park. But don’t start getting clumsy or lazy with safety habits. I’m every bit afraid to get zapped as I was the first time working with voltage. However the word afraid has two different meanings over time.

[Helmholtz]

As already mentioned there's no real need to measure power tube plate voltage as B+ at the OT primary CT is close enough.
Only exception is if you suspect an open OT primary connection, but that can be tested with an Ohmmeter.
Now, if you think you need to test plate voltage, I recommend to pull the PI tube first.
Reason is that some amps like to oscillate when a probe/meter lead is connected to a power tube plate, resulting in wrong meter readings.
Also such oscillations can generate extremely high (kVs) peak voltages which could damage the meter as well as the OT primary insulation.
Pulling the PI will prevent oscillation.

[pdf64]

... Often it's easier to touch the OT primary's center tap—this voltage will be just a couple volts less than the plates.

OT CT voltage will be a fraction higher than the anodes.
But yes, as per Helmholtz 's explanation, that's a much better 'node to probe'

[Jquijano]

Should i bias this amp at 70%?

[Jquijano]

Safest? Start with amp off, unplugged, and caps drained (and confirmed zero volts DC with a multimeter). Next, clip black meter lead securely to chassis or other obvious ground point, and red lead to power tube plate (pin 3 for 6L6/EL34/6V6, pin 7 for EL84). Turn on meter and set for DC volts. Turn on amp and let it warm up. Note voltage. Power down, drain and confirm caps at zero volts, remove meter leads.

But really, I probe live amps all the time. Each amp presents different access points to measure voltages--sometimes it's difficult to reach power tube pins, sometimes easy. Often it's easier to touch the OT primary's center tap—this voltage will be just a couple volts less than the plates.

Which is pin 3 in these sockets? Sorry for not being as experienced as you guys!

[Helmholtz]

Look at the sockets. I can see numbers there.
I case of doubt look up the tube datasheet (I have to do this all the time).

[Jquijano]

Look at the sockets. I can see numbers there.
I case of doubt look up the tube datasheet (I have to do this all the time).

Well i measured it correctly and professionaly. 500 vdc

[Jquijano]

These power tubes are 7 to 8 years old. I rarely use this plexi. Weird is the tube near the transformer and below the power jack mearures 34 Ma while the other ones measure 29 Ma. I striving for 70%. Should i get new tubes?

[Helmholtz]

Swap the tubes in their sockets to see if it's the tubes or something else.

[Jquijano]

Well i broke one of the key guides!

[xtian]

These power tubes are 7 to 8 years old. I rarely use this plexi. Weird is the tube near the transformer and below the power jack mearures 34 Ma while the other ones measure 29 Ma. I striving for 70%. Should i get new tubes?

70% is an arbitrary target from ages ago. I've found guitar amps sound great much cooler, 40-50%, and the tubes last much longer.

[J M Fahey]

Yes, biasing by %point shows ignorance or least lack of understanding.
That is a tip straight from You Tube University or "Guru" pages.

Biasing must reach just above the knee where plate current curve becomes linear to avoid/minimize crossover distortion, anything beyond is just waste and overheating.

Notice *many* complain about Factory bias being "too cold" ... by this silly rating that is.

Truth is Factory knows proper Bias (duh!) and use it.
Hint: their Design Engineers (at Fender Marshall Ampeg etc.) went to a proper University, did not learn watching Uncle Doug or Weber videos or browsing Rob Robinette site.

VOX being the exception, they love to cook tubes.
Not famous by their reliability or longevity though.

[loudthud]

Back in the old days, before DVM's, you used a VTVM, a Vacuum Tube Volt Meter. These typically had a 1Meg resistor build into the probe so that when you probed a Plate lead, the long wire going back to the meter didn't act like an antenna to cause oscillation. You couldn't use the same probe to measure resistance though.

If you try attaching a 100K to 1Meg resistor to your DVM probe, you'll have to apply a correction factor to whatever readings you take. Better to just measure B+ at the OT center tap and use that number.