Saturday, October 12, 2013

Slept In Again

     I'm starting to think I've picked up some kinda bug; I have been exhausted most of this week and I haven't done anything.

     Tam claims "A Bobbi at rest tends to stay at rest," maintaining that if nothing wakes me up, I'll stay somnolent forever.  Maybe that's it.  If so, it can be cured by getting myself into motion and actually doing something.


Robert Fowler said...

Cooking bacon is always a good thing.

GB Hoyt said...

Bacon, yes!
I'm designing a single tube MOPA using a 10DE7, and I think I'm going to use a xtal controled Colpitts oscillator on the high mu side of the tube. One thing my google fu seems to be weak on is calculating 1. the resistor that goes across the xtal and 2. the capacitor values in the voltage divider for the oscillator circuit. I know which is supposed to be more than the other, and that the ratio between them determines feedback.
Figured I would ask you before asking the rest of the glowbugs.

Roberta X said...

...And ask me *here,* too?

Rx "across the crystal:" that's the grid-leak resistor. Some versions used a potentiometer -- pick a value between 100K and 1M and see how the thing keys; if it is chirpy, you were wrong and need to change it. (Honest. It would be fun to run the math but it's faster to do it empirically and gives you a better feel for what does what.) The OTs often added a DC blocking cap -- the resistor goes on the grid side, not the crystal side.

Why a Colpitts? You can think of it as a capacitive-reactance version of a Hartley and the same general rules and ratios apply. Crystal current (and thus heating, drift and the chance of damage) goes up as the amount of feedback goes up. You can rule-of-thumb this. (At your own risk -- I used to use TV colorburst rocks for general fooling around, since they are plentiful and cheap.)

The capacitive voltage divider is across the crystal and will affect frequency stability (so will the series grid condenser, if used). Use good condensers. You grab some Z5U or X7R ceramic junk and you're on your own. Look for NP0 (C0G) ceramic, silver-mica or polyetheylene caps. Expect to pay more for them.

GB Hoyt said...

" that's the grid-leak resistor" Ah, ok!
got it!
Colpitts because Dark Crystal!
and I have plans for a Hartley circuit using a PTO later. Just want to make sure I can light this sucker and not blow the tube up. I'll be using a Colorburst xtal, and I may even ply AF4K for a 35060 or two so i can QSY, or if I feel confident enough with the overall workings of the tube, just build a hartley. I do have SOME condensors that are worthwhile. I'm always on the lookout though.
Might have to make a trip over to Skycraft in Orlando.