Just how hot does a cathode have to be to properly emit??
EDIT: Removed a comment meant humorously, but which could have been misinterpreted.
How much "hot" is hot enough?
The data sheet was linked earlier in the thread; take a look at it. Right up at the top, it says a 6AU6 wants 6.3v @ 0.3A, and the 12AU6 wants 12.6v @ 0.15A. If you multiply either of those, you get about 1.9 watts. That's how hot the cathode wants to be to properly emit, 1.9w worth. The cathode and the rest of the tube don't care how you make that happen.
Unfortunately, the heater dictates what you can do. The 12.6v heater looks like 12.6v/0.15A = 84 ohms when it's hot. The 6.3v heater looks like 6.3v/0.3A = 21 ohms when hot. That's a big difference. If you apply 6.3v to the 12.6v heater, you get 6.3v/84 ohms = 0.075A, and 6.3v * 0.075A = 0.4725w. That's 1/2 the current, and 1/4 the heat!
Yep, the tube may light (dimly) but won't do it's job right.
But we're *ass*uming here... Did you measure with a meter lead on pin 3 and the other lead on pin 4? Or did you measure from one of these pins to ground? The "right way" to see the full heater voltage is from pin-to-pin. When you measure from pin to ground, you see half of the total heater voltage present.
And we're also assuming heater wiring like the usual guitar amps. It is possible, if the winding puts out 12.6v, to use a 12AU6, a 12AT7 with pin 4 to one side and pin 5 to the other, and the 6BQ5 heaters in series, with the whole shebang in parallel. That might be goofy, but I've seen weirder.
... could it be that the amp has the wrong tube in it? Please give me more than that.
Or it could just be the wrong tube. 6AU6 is still near dirt-cheap, but not as dirt-cheap as a 12AU6 might be (the 6v version was used in more stuff).
The values on the pentode are kind of odd. You may try a 220k inplace of the 1M load resisitor and use the 1M in place of the 3.9M on the screen. Then sub a 2.2k for the 6.8k cathode R and put a capacitor across that.
Well, you could do that...
The road forks here; you'll have to decide which trip sounds more appealing to you.
This whole thing looks like a power amp to me. Yeah, I see a tone control, but it looks a lot like input gain stage -> phase inverter -> output tubes. And that's the basic plan for 97.6% of all stand-alone power amps, whether they're PA, instrument or Hi-Fi. So, the question is what are ya gonna use this for? If for bass, you might leave it be. If for guitar, the right pentode may or may not be raucous enough as-is.
Without calculating, the 1M plate load does look high. But the way to get the most gain out of a pentode is to raise the plate load. The screen resistor should be a value related to the plate load. Normally, the total cathode current goes mostly to the plate, with some portion splitting off and going to the screen. The ratio of plate current to screen current is pretty consistent within a given tube type. It would appear that a 6AU6 would be a 4-to-1 ratio of plate current to screen current, judging by the resistor values and assuming the designer wanted similar voltage on each element.
The data sheet typical conditions shows 5mA plate 2.1mA screen, 7.6mA plate 3mA screen, and 10.6mA plate 4.6mA screen. Thpse all land pretty near 2.5:1 for the ratio of plate to screen current. So we can see without measuring that the designer wnated the screen voltage lower than the plate voltage, and useda screen resistor 4x the plate load instead of 2.5x the plate load.
So that would mean a lot of gain, maybe. Big screen resistor means less screen voltage which means less plate current. When plate current in a tube drops, Gm drops too, so that could drop our gain some. The unusually big 6.8k cathode resistor seems wrong, maybe too big. Is it Blue-Gray-Red (6.8k) or Blue-Gray-Brown (680)? It might be the bigger value to still arrive at a healthy bias voltage with low plate current.
We'll save ways to butcher the pentode for later...
So the 2 roads I was thinking about: You could try to make this an all-in-one chassis like every other guitar amp. Or... you could accept is as just a power amplifier, and make seperate preamps to mate up with it. You could possibly play with a number of different designs and not have to re-work the main amplifier chassis, just the couple-tubes for your preamp chassis.
What do you think?
Topic: 12AU6 as V1 (Read 12350 times)