[ARC5] [Milsurplus] 6AQ5 Tank

[mac]

Dave,

OK, you've got a Pierce oscillator operating as a tripler with an  
output frequency of 24 MC which you want to operate on 3.5 MC  
instead.  Presumably you would be operating "straight through" on 3.5  
MC and you are concerned about what changes to the current the plate  
circuit parameters would have to be made to allow operation at the  
new, lower frequency.  Presuming, again, the current circuit is of the  
"modified Pierce" type with the 8 mc crystal connected between the  
control and screen grids and with an LC circuit in the plate lead that  
resonates at 24 MC (3rd harmonic).  Additionally, you are concerned  
about the reactance value (calculated) the current 40 pFd plate  
tuning(?) capacitor exhibits at 25 MC and the ramifications of this  
value vis-a-vis operation on 3.5 MC.

First off, the circuit may operate FB straight through with only an RF  
choke in the plate lead and, of course, a 3.5 MC xtal inserted in the  
place of the original 8 MC unit.  With the Pierce circuit, in fact, a  
resonate circuit in the plate lead may cause the circuit to stop  
oscillating when tuned to exactly the crystal frequency (see 60's  
handbooks for discussion of this and other crustal oscillator types).   
Also, beam tetrodes (6AQ5, 6V6, 6L6, 807. etc.) are not good choices  
for Pierce oscillators.  A well screened pentode such as the 6AG7  
operates with lower crystal currents and more stable oscillation.

Moving on to the broader matter of resonate circuits, your calculation  
of 166 ohms for the 40 pf tuning cap is correct.  Practically, though,  
the cap as the variable element of the circuit would not likely to be  
operating fully closed at 24 MC so the operating capacity would be  
something less and the reactance value something greater than 166  
ohms, but in any case, this value (or anything close to it) is not  
what the plate circuit "sees" at resonance.  The resonate impedance  
value will be much higher  and will be purely resistive.  Without  
getting bogged down in a discussion of Q, "loaded" Q and the like,  
John came up with a figure of 16 K which is certainly in the  
ballpark.  John also calculated that with your present tank coil a  
capacitor of something like 1400 pf would be required to resonate at  
3.5 MC which would be pretty clumsy from a practical standpoint.   
Going back to the handbooks again, a coil of 22 turns,  one inch in  
diameter and a 300 pf variable should work just fine which is, again,  
right in line with John.s 240 pf calculation.

Hope this is useful.

Dennis D.  W7QHO
Glendale, CA

****************
On Jul 20, 2011, at 3:50 PM, David Stinson wrote:

> Ok all you smart people-
> I'm looking at a 6AQ5 Pierce crystal oscillator/tripler
> with a plate tank circuit which outputs at 24 MC.
> The plate tank capacitor is 40 pFd.
> The capacitive reactance at 24 MC is only 166 ohms.
> This seems low for a tube tank.  Output capac.
> of 6AQ5  is 6 pFd,  so that doesn't change the picture much.
> I was looking at changing the tank to output at 3.9 MC,
> but if Xc = 166 ohms is correct, the tank cap
> at 3.9 MC would be 6.7 uFd.  That can't be right.
> What's up with this?
>
>
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