[Milsurplus] Crystal Eater?

[Kenneth G. Gordon]

On 31 Mar 2016 at 10:01, David Stinson wrote:

> This 6L6 crystal oscillator, 250 V B+.
> Will it crack FT-243s?
> 
> https://goo.gl/photos/Uhke9dbq7oAWpruV6

That is a Pierce crytal oscillator circuit from the BC-669. The Pierce circuit is possibly the 
easiest on crystals. However, the MODIFIED Pierce is far easier on them.

I have never understood why designers used a beam-power tube as a crystal oscillator, and 
especially as an ECO: the beam-forming plates (often shown as a screen-grid, which they 
most certainly are NOT) couples the RF energy around the screen and directly to the plate, 
completely bypassing any possible shielding effect which arises by using the screen as the 
oscillator's "plate".

Hallicrafters did this constantly. The BC-610 is another example of this. So is the "Paraset", 
for that matter, but the power is so low in that rig it really doesn't matter much.

At +250 VDC, I would expect the shown circuit to not damage crystals, even the FT-243 
variety, but when first using that circuit, you should probably watch cathode current. Keep it 
below 30 mA if possible. IMHO, 60 mA is the absolute maximum current which should be 
allowed to FT-243 crystals, and even that is, to my mind, dangerous.

Adjusting the value of C-62 would give some modicum of control over cathode current.

FYI, the MODIFIED Pierce is a crystal oscillator in which the "plate" of the oscillator is the 
screen-grid of the tube in question. An ECO. Any non-beam power pentode would generally 
make a pretty good Modified-Pierce crystal oscillator. Although the 6AG7 is the most 
common (and FAR easier on crystals) tube used in this circuit, there are many other tubes 
which work extremely well in this circuit. 6CL6, among others.

The 802 was supposedly purposely designed for such service. The 6F6 is (as I recall it) not 
a beam tube, but is a power pentode. It has the same base diagram as the 6L6. Minor 
rewiring of the socket would provide a Modified-Pierce circuit using that tube.

Ken W7EKB