Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Quickie => Topic started by: booangler on October 26, 2009, 05:58:01 AM
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Has anyone given any thought to using LED's in place of the Cap and Cathode Resistor? I had been reading through some forums on another site where they were discussing this as a general topic. So I had to ask.
Looking forward to you thoughts.
Alan
edit due to brain fart...
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That would be the cathode resistor, not the plate resistor - right? :^)
I think I posted on the old forum but can't find it right now. Anyhow, the problem is the battery voltage will drop as they age. Using the fixed bias of an LED in the cathode will cause an exaggerated change in tube current as the battery ages; the cathode resistor provides negative feedback at DC to maintain the appropriate operating point.
A quick look at the curves shows the plate current dropping from 2.5mA to 1.0mA as the battery drops from 36v to 24v.
A reasonable solution would be to use a current source to replace the plate load - that way the tube maintains a constant operating point, and operation will cease suddenly when the battery no longer provides sufficient compliance for the current source. I'd suggest 1.5mA as a good value to use. Note this will not work well for headphone operation where a substantial output voltage is going to be needed, unless you also add more battery voltage. Now this is starting to get complicated!
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We just C4S'd a Quickie on Friday. I find the bass of the Quickie lighter than my big preamp (thankfully not the other way around - that would be an embarrassment!) and so I thought maybe a C4S would give the 3S4 a bit more bass punch. For reference, Quickie was running into the 25K input of my active crossover. Anyway, from a brief listen we really couldn't hear much difference between the stock plate resistor and C4S plate load. I wonder if different types of batteries might sound different...?
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Plate chokes made a BIG difference in my Quickie - louder, punchier, and better bass. I used two Hammond 156C, I think, which were rated 150H for inductance and 3.7k ohms for resistance.
I had thought about C4S on plates, too, but the chokes seem to accomplish the same goals. And I didn't have to mess with the cathode side of things....
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I am going to redo that comparison of the resistor load vs the C4S in the Quickie. It turns out that were were using a cheapo pair of interconnects at the input of the quickie that seems to cut the bass way down. Found this out yesterday when using the cables elsewhere in the system. I will do a comparison once more, with a pair of known good cables.
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I am going to redo that comparison of the resistor load vs the C4S in the Quickie. It turns out that were were using a cheapo pair of interconnects at the input of the quickie that seems to cut the bass way down. Found this out yesterday when using the cables elsewhere in the system. I will do a comparison once more, with a pair of known good cables.
Doc,
Looking forward to your updated comparison. Sounds hopeful.
Cheers and thanks for all you do.
alan