6V6 cathode-bias level & tube idle-bias...

[In The Light]

Hello! I'd really appreciate some expert insight into this question:

How does a tube's idle-bias level rating effect a cathode-bias amp's self-biasing. For instance, if an amp is set at say 35 mA on the cathode, but the tubes are both rated 25mA idle bias - at what mA will the tube bias at?

Thanks much,

-Mike

[MWJB]

I'm assuming that when you say the tube is "rated at 25mA idle bias" you are talking about pre graded tubes, bought from a vendor who has marked them up as such.

If so, the tubes will only actually draw 25mA in the vendor's test circuit, or in an amp with identical voltages (not very likely and can only be confirmed by taking measurements). 30mA tubes from the same grading system will run hotter and 20mA tube wil run colder, but none of these figures necessarily correllate to what the tubes will actually run in your amp.

An amp can only be rated for a specific current rating if the tubes have been pre graded/selected by that manufacturer, for that amp. Even so, it is likey that there will still be a margin for error (AC wall voltages differ from place to place and even at different times of the day at the same place).

In short the tubes will bias at a level that is dependent on voltages in the amp they are actually going to be used in, combined with the value of the cathode resistor. There is no real way of pre-determining what this figure will be to the milliamp, but cathode biased amps are typically quite forgiving. If the amp sounds good then you probably have enough current, if you are not suffering with unusual amounts of hum/power fade/plates of the tubes glowing red or orange then you probably do not have too much current.

If you are particularly bothered about your plate current in your cathode biased amp, buy some bias probes or get a tech to check it for you.

Some older cathode biased amps are now running their tubes in excess of the maximum plate dissipation, because wall voltages have risen significantly since they were designed.

[In The Light]

MWJB, thank you sincerely for your great insight. I've been trying to get more understanding into this by doing my own research, but you really summed it all up in a superior fashion.

Much appreciated,

-Mike