Valve Circuits 2
This page contains information on prototype valve circuits.
1. This circuit shows a crystal controlled, self oscillating single balanced modulator used to generate double sideband signals. The details shown are specifically for a 6AR8 beam deflection valve. This circuit will also oscillate with the anode and deflector voltages removed as the accelerator on pin 3 acts as an anode -
The prototype circuit as shown gave a minimum of 40dB carrier suppression and the final layout gave 56dB. Another 15-
T1 for 3.395Mhz was 15t + 15t bifilar, 9/47 strand Litz close wound on a one quarter inch diameter formar with a dust iron tuning core and C1 = 2-
R1 and C1 are the balance controls and a symmetrical layout will help with obtaining a good balance.
Audio may be single ended and fed to point A with point B decoupled to ground or push-
The anode current contains significant harmonics of the oscillator frequency which may appear to make the balance worse than it actually is on broadband measuring instruments like an oscilloscope, hence the second tuned circuit in T1 for measurement purposes.
During final construction a small PCB containing a relay switching facility has been added to allow for selecting crystals for each mode. As a result of the additional circuit capacitance, the feedback capacitors in the above circuit have been changed -
2. The low level amplitude modulator using the heptode section of an ECH81 has been moved to the Amplitude Modulators page elsewhere on this website.
3. The circuit on the right uses the triode section of an ECH81 as a unity gain phase inverter to provide the out of phase component to drive the balanced modulator at the top of this page. The circuit is not critical but some RF decoupling may be required depending on each user’s application and layout. The incoming audio signal drives deflector electrode A of the balanced modulator and the AF I/P to the right. The AF O/P drives deflection electrode B of the balanced modulator. Non-