12-08-2020, 08:41 AM
(This post was last modified: 12-08-2020, 08:53 AM by Mike Watterson.)
Actually with a strong signal out of band and the AGC going to maximum, the 3rd harmonic can be generated in the Hexode.
An RF preamp isn't just about gain. It can be a buffer for an extra tuned stage to reduce out of band intermodulation (and a signal can "modulate" itself). One of the reasons for the Triode-Hexode is actually to reduce LO leakage on the RF in pin, which is severe on heptodes, octodes and self oscillating mixer using triodes. Hence VHF valve amps using either a balanced input or an RF stage with almost no gain to stop LO radiation on the roof top aerials. US Broadcast only receivers, battery or mains, often used an RF amp before the heptode/octode purely to reduce re-radiation.
Any triode-Pentode, hexode-triode or heptode etc with a separate oscillator is superior to a combined mixer/oscillator using a single active device. The ECH81 (=ECH83) is very good, but not as good as a multiplier (7360, ring diodes, Gilbert Cell) with a separate oscillator. It's good because the triode part is a separate oscillator. This also removes FM of the LO by a strong signal, a failing of single device mixer/oscillators such as the heptode and triode/transistor mixer-oscillator.
I've an open mind as to if it's really a third harmonic. I'd put a low pass filter (1.8MHz approx) on the input and also compare a signal generator at that frequency and other on MW at higher level.
Or add a tuned SW circuit with a unity gain J-FET buffer (follower) to see if it's an artefact of the hexode. You CAN use any amplifier, hexode or not, to get a 3rd harmonic by the non-linear action of multiplying, which may be more likely at certain bias levels, so also disconnecting the DC AGC bias (if used on the ECH81 hexode) and applying 0 to -9V via 1 M Ohm might be interesting.
Oh, and I had a set where the Triode part (oscillator) of the UCH81 wasn't running due to a resistor gone high. It would work at the low end of MW and high end of LW on the generator, with tuning. I think simply leakage past the 470 KHz IF and it acting as a TRF! It was a very high level.
An RF preamp isn't just about gain. It can be a buffer for an extra tuned stage to reduce out of band intermodulation (and a signal can "modulate" itself). One of the reasons for the Triode-Hexode is actually to reduce LO leakage on the RF in pin, which is severe on heptodes, octodes and self oscillating mixer using triodes. Hence VHF valve amps using either a balanced input or an RF stage with almost no gain to stop LO radiation on the roof top aerials. US Broadcast only receivers, battery or mains, often used an RF amp before the heptode/octode purely to reduce re-radiation.
Any triode-Pentode, hexode-triode or heptode etc with a separate oscillator is superior to a combined mixer/oscillator using a single active device. The ECH81 (=ECH83) is very good, but not as good as a multiplier (7360, ring diodes, Gilbert Cell) with a separate oscillator. It's good because the triode part is a separate oscillator. This also removes FM of the LO by a strong signal, a failing of single device mixer/oscillators such as the heptode and triode/transistor mixer-oscillator.
I've an open mind as to if it's really a third harmonic. I'd put a low pass filter (1.8MHz approx) on the input and also compare a signal generator at that frequency and other on MW at higher level.
Or add a tuned SW circuit with a unity gain J-FET buffer (follower) to see if it's an artefact of the hexode. You CAN use any amplifier, hexode or not, to get a 3rd harmonic by the non-linear action of multiplying, which may be more likely at certain bias levels, so also disconnecting the DC AGC bias (if used on the ECH81 hexode) and applying 0 to -9V via 1 M Ohm might be interesting.
Oh, and I had a set where the Triode part (oscillator) of the UCH81 wasn't running due to a resistor gone high. It would work at the low end of MW and high end of LW on the generator, with tuning. I think simply leakage past the 470 KHz IF and it acting as a TRF! It was a very high level.
