Safe method for measuring plate voltage

[trobbins]

Did you use a bias probe, with cathode current sensing, to measure cathode currents for the 34 and 29mA readings at idle ?

A clip probe is likely less accident prone than using a pointer probe. For either technique, the risk is accidentally shorting terminals 2 and 3.

Perhaps think about permanently soldering in a 100:1 resistor divider across B+, and taking a wire link from the mid-point (nominal 5V from divider) out to where it is easily connected to a meter for more regular maintenance checks. As an example, use two 220k 1W resistors in series for the 'upper arm' of the divider, and two 2k2 0.4W resistors in series for the 'bottom arm' of the divider.

Also worthwhile cleaning between terminals 2 and 3 on the socket of each power tube, say with a cotton bud and some methelated spirits or IPA. Over the years, dust and debris can build up and cause tracking/arcing between those terminals.

[Jquijano]

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.

Well if you have some book or video you could recommend on biasing properly without using dissipation percentage i would kindly appreciate it.

[DrGonz78]

Do You have a scope? Just look at the crossover distortion that occurs at various stages of tweaking the bias settings. It’s very much the exact spot that connects the positive and negative wave form through the zero crossing. In musical recording we refer to similar pattern called DC offset. The 50-60% is where set the bias on a lot of tube amps. I pretty much never set bias to 70% unless I’m trying to push it at first to get it warming up quicker then I dial it down. Just scope it if you have one.

[DrGonz78]

[Jquijano]

Do You have a scope? Just look at the crossover distortion that occurs at various stages of tweaking the bias settings. It’s very much the exact spot that connects the positive and negative wave form through the zero crossing. In musical recording we refer to similar pattern called DC offset. The 50-60% is where set the bias on a lot of tube amps. I pretty much never set bias to 70% unless I’m trying to push it at first to get it warming up quicker then I dial it down. Just scope it if you have one.

Sorry aint got a scope just a fluke digital multimeter and euro tube bias pro 1

[The Dude]

There's a wide range of acceptable bias levels- anywhere between "just out of crossover distortion" and "over-dissipation". IMO, it's not that critical, although lower bias = longer tube life. If you don't have a scope, this is a good start

https://robrobinette.com/Tube_Bias_Calculator.htm

[hylaphone]

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.

What conditions make some amps more prone to this? Can it be mitigated with some type of snubber?

[Helmholtz]

As this oscillation is only caused by a probe lead touching the plate, I don't consider it a problem as probing a power tube plate can (and should) be avoided and the amp might be perfectly stable otherwise..
I guess many amps are prone to this effect, but because of the risk involved don't recommend to test for it.

Principle behind is that an (unshielded) wire connected to the plate produces a capacitance between the plate and some other circuit point which acts as a non-inverting input. Result is positive feedback causing HF oscillation.
Typically only one side of the OT primary is critical while the other side is inverted phase and doesn't cause positive feedback.
I'd think that high gain or large NFB support the effect.

[g1]

With regard to video in post #19 (can't quote it for some reason):

I would still recommend checking power tube idle current after doing the crossover method. Before you have a experience with it, it's very easy to bias the amp way too hot.
And it's very important not to do it near full power, as opposed to what he suggests. At full power you will get the kink and not be able to remove it. I usually set at 1/2 to 2/3 power so just a hint of notch is showing (you won't hear it) and then check cathode current. Using this method I find I'm usually ending up in the 50 to 60% range. If factory setting calls for something colder, I stick with the factory setting.

[Jquijano]

Thanks