[HomeBrew] Chirpy 6V6 osc/filtering solution?

[N2EY@aol.com]

In a message dated 7/21/2003 11:07:55 AM Eastern Standard Time, catman351@yahoo.com writes:

> Just wanted to know how I should eliminate/mitigate a chirpy 6V6 (AC-1 type) oscillator. 

First thing to do is determine why it's chirping. Does it do so with every xtal or only with some? Do the plate and screen voltages drop a lot when the key is down? Does fooling with the feedback help? 

>I took a look at WA6VVL's book, "Vintage Anthology, Book 1" and he basically detuned the plate to lessen the chirp on the AC-1's signal. I have been doing that practice but would like to try and eliminate the problem electrically. I have been toying with the idea of canning my present filtering system and basically put in a 470 uF/450 VDC electrolytic I have. I'm currently using two 22 uF/350 VDC caps and a single 2H choke in pi configuration but somehow that doesn't seem to be doing anything with the chirp problem. Any comments? 

The "big capacitor" solution may help some if the problem is power supply regulation, but before going off on a quest, see how much the plate and screen voltages change when the key is pushed. Screen voltage regulation can be important because of how the oscillator circuit works. Some folks use a VR tube or two to stabilize the screen voltage for this reason.

Sometimes fooling with the values of the feedback capacitors and/or the gridleak helps. An RFC in series with the gridleak can help, too. 

The 6V6 is really not the best tube for a keyed xtal osc for a number of reasons. It is a beam tube, not a true pentode, so it can have strange behaviors in cathode-hot-for-RF circuits. If you like octals, a 6AG7 is a better choice. 

73 de Jim, N2EY