You keep taunting me, one day. Come to think of it, I have 30 of those 12BK5's. No reservations on blowing them up. I was curious and compared the 6AQ5 to the 12BK5 and the 15CW5 to boot. Left to right 6AQ5, 12BK5, 15CW5.
At double the heater current of the 6AQ5 and bigger plate than even the CW5 you would think it might have more going for it than its rated 3.5W in SE Class A. What the heck is holding it back? Why the plate resistance double the 6AQ5?
7.5 watt heater in a big cathode sleeve, large enough plate to catch all the electrons it throws out, and dissipate plenty of heat, it looks like it should ROCK. I have 5 or 6 of the 6 volt flavor but the oddball pinout has kept me from sticking them in a socket so far, so I really don't know how much I can squeeze out of them.
Someone thinks that they do something good. After selling a few dozen a year for years, all 2800 6BK5's that were in stock at a popular tube store vanished last year. There are still over 1000 of the 12 volt flavor, and a few hundred of the 25 volt version left in stock.
Other tubes that are optimized for low drive requirements so far have been less than generous in the power output department.
Another tube with similar characteristics and even higher transconductance is the 60FX5. It looks like a 32ET5 or a 50C5 but no matter how hard I pound on a pair, all they want to give me is maybe 10 watts. Maybe the optimization for low drive hurts the maximum power output. The clue is that curves for both tubes don't go past 80 mA. That limits the power output right there.
The 6AQ5 is limited by the wimpy 2.8 watt heater. There are only so many electrons coming off that cathode and even if you collect them all in the plate you are still going to have higher tube losses than a similar tube with a hotter heater in a fat cathode.
I haven't dived into tube geometry much and do not understand why this tube limit's around 80 mA with its higher impedance. As you said, lots of electrons and a place to catch them. With a 5V signal needed for full output could you not be able to drive this guy directly with a opamp into AB2?
> do not understand why this tube limit's around 80 mA
G1 has "too much control". They did this so they could build a phono with ceramic needle direct into power tube, no preamp tube.
Taking G1 positive "would" get more out. The high Gm predicts that G1 will suck heaps of current. The low (original) price suggests that G1 is cheap and liable to melt easily.
General rule: you can get Power, or Gain, but not both in the same device. The different optimizations conflict.
1 Watt of 60FX5 is a VERY sweet sound. Dunno why you'd goose them to 10 Watts when many-many more suitable tubes are available.
G1 has "too much control". They did this so they could build a phono with ceramic needle direct into power tube, no preamp tube.
Taking G1 positive "would" get more out. The high Gm predicts that G1 will suck heaps of current. The low (original) price suggests that G1 is cheap and liable to melt easily.
General rule: you can get Power, or Gain, but not both in the same device. The different optimizations conflict.
1 Watt of 60FX5 is a VERY sweet sound. Dunno why you'd goose them to 10 Watts when many-many more suitable tubes are available.
I had some, and a board that they would drop right into, so I wound them and a lot of other series heaters up to see how far they would go. The lowly 50C5 was the clear winner here making 20 to 25 watts per pair with no hint of distress. The 60FX5 was the worse. The application is a budget guitar amp running on an isolation transformer. The tube of choice is the 32ET5. I can get 25 watts per pair at 1.5% THD and id doesn't melt.
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