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In Reply to: RE: Quickie info?? posted by taylor on September 27, 2009 at 10:15:51
The plate impedance of the 3S4 is about 4K and so is the load resistor so the output impedance is closer to 2K ohms. The low plate load resistor also reduces the gain; mu is about 4.5 so the gain ishalf that, 2.25 or 7dB. That low plate resistor modifies the numbers quite a bit.
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Great! This was what I was interested in. Im considering it for buffer/small gain duty for a portable system.
Thanks.
Ah, that explains a lot! I was thinking about gain vs. mu today in the Quickie, and I wasn't super convinced we were even getting 10db. What would you expect using a choke instead of the 4k resistor? (12v isn't enought compliance for C4S boards, is it?)
Here's my idea for a cheap current source that adjusts its current as the battery voltage falls. Should give about 2mA with fresh batteries. The diode is any silicon diode. Impedance is not huge like a cascode current source, but it's plenty for the Quickie and there's no need to isolate power supply noise so a really high impedance isn't necessary.
Looks like you should expect 13dB gain for a high impedance plate load such as a choke or current source.
That's another great upgrade.
I don't have my Quickies yet. As I understand it, the HT is always on, but no current flows unless the filament is on. With this upgrade, it seems that this will always be conducting .3 ma, even when the filament switch is off. So this would necessitate a switch for the HT in order not to drain the battery, I think.
Triamp... Take a load off!
My bad - posting too quickly, too late at night. I might have had a martini by then, too!
As I look at it, the capacitor really should go around the resistor and diode, not just the resistor.
Maybe an LM334 would do the job. 40v max; a single 33 ohms resistor will set 2mA, makes a 2-terminal current source - like a depletion-mode FET but with precision current setting.
I'd think a single LND150 would be your best bet. Probably wouldn't need a resistor at all -- just tie the gate to the source. Single 2SK170 would work, too, though you'd probably have to select for low Idss, or use a resistor.
And if 2 ma is set, then we don't have to worry about it rising, and we can use series feed in the Speco and Paramour upgrade trannies!
Triamp... Take a load off!
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