GT Hardness rating and biasing

[roushc]

Does one need to bias if replacing GT tubes of the same hardness?

Example: KT66-HP #4 Duet in Dr. Route 66

[Blue Strat]

Originally posted by roushc
Does one need to bias if replacing GT tubes of the same hardness?

Example: KT66-HP #4 Duet in Dr. Route 66

Not if the amp has previously been biased for tubes of the same hardness rating.

Note that each GT hardness rating has a fairly wide span so rechecking bias is always a good idea.

[JDJ]

What's a hardness rating? Where do you find it on the tube? Thanks.

[Kiwi]

Originally posted by JDJ
What's a hardness rating? Where do you find it on the tube? Thanks.

Layman's answer:

GT tests its tubes and assigns a "hardness rating," so buyers can estimate the response or breakup point.

Their scale is 1 to 10, with rating 1 being very "soft" and quick to break up, and 10 being very "hard", with max clean headroom and late to distort.

It's written on the tube itself, either a sticker on the base or printed on the tube glass. The end cap of a GT tube package should also have it labeled.

Guitar store employees usually do not know this rating system and will hand you the first package they can grab. If possible, ask to see what other GT hardness ratings they may have for that type of tube.

Kiwi

[Old Tele man]

...the "Hardness Rating" loosely equates to the tubes' operating transconductance (gm) value, ie: the HIGHER the gm-value, the MORE sensative it is to input signals; and, conversely, the LOWER the gm-value, the LESS sensative it is to input signals.

...you can readily visualize "how" the gm-value affects the tubes' (and thus amps') output power (Po) by inspecting the following equation:

Po = Zo' * ( gm * Vg.ac )^2

where:

Po = Output power, Watts.
Zo' = Effective plate load impedance, Ohms.
gm = Tube transconductance, Amps-per-Volt.
Vg.ac = Control-grid AC-signal, Volts(peak).

Notice that both tube transconductance (gm) and drive signal (Vg.ac from the Phase Inverter) are SQUARED functions, so "little" changes in either of their values produces "BIG" changes in output power....hence, the increased "sensitivity" to input signals (from PI).

P.S.--typically, the HIGHER the gm-value ("Hardness Rating" number) the MORE negative bias-voltage needed to "idle" down the tube to the same idle current and plate voltage drop; however, it is NOT a linear relationship.