Plate Resistor ratings.

[dragonbat13]

Looking to stock up on some 100k plate resistors. I was looking into the wattage and voltage ratings.

For instance the Vishay PRO1 metal film resistors are, while rated at 1 watt, have a max voltage ratings of 350 volts.

I would think this resistor would be plenty acceptable for a third stage plate Resistor in the ab165 bassman I'm working on. But the amp, at the third stage node, shows 390 volts. According to Vishay, that would exceed the resistors rating.

I'm confused. What's going on here?

pr010203.pdf

bassman_ab165 (1) (1).pdf

[martin manning]

The rating refers to the voltage across the resistor, so you need to calculate or measure it for the specific application. Preamp tubes typically draw 1mA, so a 100k plate resistor only sees about 100V.

[dragonbat13]

Thanks Martin. You have been very helpful to me lately.

I see what your saying, and I had a clue on another type resistor. I guess the rating is "working voltage", correct? Such as the voltage drop across the resistor?

[martin manning]

Yes, exactly.

[trobbins]

At power up the resistor starts with 0V across it and then increases to idle condition, which would be far less than 350V (circa 390-250=140V from the schematic).

However, the resistor does see signal excursion peaks, and the B+ could be somewhat above 'nominal 390V', and the idle bias could be below nominal 250V, and the drive signal could be excessive, and the tube could conduct down to below 50V across it. When you couple all that with the PRO1 rating of 350V max (includes any AC peak) then a PRO1 can be a marginal choice for some driver stages, depending on the tube choice. I typically use PRO2 parts - they are physically quite small compared to vintage 2W parts - and would recommend them perhaps as a more practical generic part to have in stock.