Bill Turner escribi:
On the "other" reflector a statement was made about TPTG oscillators
and I posted a follow up question about the time of the Great
Migration to this reflector so I never did get an answer. I'd like to
ask it again here.
The 'Gentleman From Georgia' (you know who) made the comment that a
TPTG oscillator will not oscillate if the grid tank is tuned higher in
frequency than the plate tank. Under that condition, apparently the
phase of the fed-back voltage is wrong and oscillation can not occur.
According to Bill Orr's 1959 handbook descritpion for TPTG oscillators:
"The feedback of energy from the plate to the grid circuits is accomplished by the plate-to-grid inter-electrode capacitance within the tube. The necessary phase reversal in feedback voltage is provided by tuning the grid tank capacitor to the low side of the desired frequency and the plate capacitor to the high side."
This means lower than resonance at the grid tank, higher than resonance at the plate tank; so based in those conditions, the above statement by the "you know who" guy is right.
From Terman's Radio Engineering Handbook 1943 Ed.:
"In the tuned-grid tuned-plate circuit, the grid tuned circuit LgCg and the plate tuned circuit LpCp, are both adjusted to offer an inductive reactance at the frequency to be generated. The circuit can accordingly be redrawn as shown in Fig. 1h [E.N.:not posted in this e-mail], and is seen to be the equivalent of a Hartley circuit in which the ratio of exciting voltage to alternating plate-cathode voltage is determined by the relative amounts of detuning of the plate and grid circuits.
The capacity Cgp [E.N.: shown in parallel with the coil that ressembles the lower portion of the Hartley oscillator coil] is usually supplied by the interelectrode capacity of the tube, although at lower frequencies added shunting capacity is helpful".
But, this gets me confused; if both circuits are adjusted to be inductive,
this means XL is lower, so both are adjusted at a frequency lower than
resonance. Not at either side as stated in Orr's text.
I'll look the equivalent Hartley circuit perhaps I'll see this better. Each tuned circuit adds its phase reversal to make the circuit oscillate.
Best regards
Guillermo - LU8EYW.
TPTG oscillators - the intentional kind - have not been used in ham
circles for decades so I wouldn't be surprised if knowledge of them
has been pretty much lost.
All comments welcome.
73, Bill W6WRT
