I found some Garnet EL34's locally, and plunked them in. Biasing them up to B+ 459V, 38.1 mA, 36.2 mA
Two questions :
1. seems like a reasonable bias level? Warm without shortening tube life too much?
Sure, the hottest tube is not quite 70%, so I doubt that will shorten either of their lives. Bottom-line, as long as you don't see redplating under strong signals, you're fine with any bias you choose ("under strong signals" implies you also get no redplating at idle).
2. matching - I don't have many options other than a completely new pair of tubes. Is 6% considered a good match? I'm thinking 10% is a match.
Depending on how you do the math, the present tubes are matched roughly to 4-5%.
I would be fine with that level of matching. Absolute perfect matching only gets you a hair more clean output power. Perfect matching & output stage balance also cancels all even harmonic distortion
generated in the output section. Then again, your output transformer and phase inverter may not be perfectly balanced, negating the effect of perfectly matched tubes.
Gross mismatching (like 70mA, 10mA) may rob some power through-put capability of the output transformer, especially at low frequencies. It may also hinder the hum-cancelling of push-pull (or not; I'm not sure at what point a severe imbalance will result in loss of the cancellation of hum that normally happens in the OT).
In Radiotron Designer's Handbook, 4th Edition, it was shown that using 2 dissimilar triode output tubes with characteristics 50% apart only resulted in about 5% THD instead of a much lower THD due to perfect even harmonic cancellation in the output transformer.
Also note that for power output tubes, idle current was at best the secondary method of matching tubes; the primary way was testing for output power given a specified input signal and load impedance (according to RDH4). That told the tester that the tubes operated the same under dynamic conditions. This is not impossible to do now, but I can't think of any vendor that specifically claims to match by this method.
Read into the above comments however you like. I tend to try to buy matched tubes, maybe because of a false sense that perhaps they'll wear evenly or behave the same over time. I also have bias adjust and bias balance controls in my latest amp build, so that 2 non-matched tubes can be biased to the
exact same idle current. However, even if I do that I still would not get the maximum possible clean output power, because of differences between the tubes, unless I have dynamically matched tubes. And with all of this said, I still only plug & bias the tubes once, and hardly ever look at biasing again (even though I know tubes can and do drift).
So I'm saying I'm contradictory: I like having the ability to bias tubes to a gnat's-ass, but I really don't use that ability much.
Topic: EL34 Biasing (Read 2927 times)