• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

* 35 watt * 300B SE = super 300B + Class A2

I learnt of Class A2 only the other day
What are the catches, how much more $?? Is bulb life shortened?
How do you know if a tube is suited?

In particular would TJ’s premium 300B+ Premium Graphite Plate – normally capable of 16-18 watts with 450 volts B+ www.diyhifisupply.com/diyhs_tj_tubes.htm be suitable for running class A2 and doubling output to around 35 watts?

. . datasheet (apologies for crude truncation to get within forum size limits) attached

googling reveals:

"Class A2/AB2/or B2 requires a very low impedance, transformer-coupled or DC-coupled driver stage . . .
The disadvantage is the extra complexity of the output stage required to source current to drive the output tube grids into the positive region."

Many more/ more expensive parts in the output stage?



  • tj 300b+ c.jpg
    tj 300b+ c.jpg
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Previously: Kuei Yang Wang

rick57 said:
I learnt of Class A2 only the other day
What are the catches, how much more $?? Is bulb life shortened?
How do you know if a tube is suited?

Class A2 relies on driving the Grid significantly positive, this means significant grid current flows. That clearly has implications in the driverstage. Usually Class A2 amplifiers (that is amplifiers designed to work in Class A2 by design for significant levels of the power range) clipp harder than Class A1 Amplifiers optimised for good overload behaviour.

It also means using an operation condition that has a comparably low anode voltage for the used valve combined with above average current. High cathode current means more wear on the cathode and thus reduced life.

As for "how do you know a Valve is suited", you know if the valve has a rating for grid current that includes a significant level of grid current. The original 300B is rated for no more than 1mA Gridcurrent, so it should not be driven into class A2/AB2.

Modern replicas and "super 300B's" generally do not specify gridcurrent limits, but i would suggest that by design the grids of these valves are not meant to dissipate significant power, so in effect these are not suited for Class A2/AB2 operation, except occasionally on peaks.

Finally, if you try to wring 35W from a 300B+ using A2 you will have made the amplifier considerably more complex, most likely with a negative impact on sound quality and all that for at best less than 3db more SPL (Doubling power = 3db more SPL).

I believe you would get the same increase in average ludness by making the clipping on peaks less audible, which means you need to design the amplifier for best overload recovery. I take it as read and understood that in most cases the audible problems of clipping (regardless of SE Amp or not) are not due to the actual clipped transient with music, but the "fallout" from the clipping, which depending on the amplifier can take many forms, including "latchup" for a short (but audible) time period, which is equivalent to briefly interrupting the music.

There are Valves that work only well in Class A2 and deliberate Class A2 operation should be confined to them. For a conventional Amplifier, using Valves designed for Class A1 operation the best route IMHO is to optimise the overload recovery, which usally also adds a little guilt and costfree Class A2 operation on the highest peaks anyway.