The cascode twin-triode amplifier first came to light in a Proc. I.R.E article, June 1948, by Wallman. He described his circuit in terms of an h.f. amplifier, quoting noise factors of 0.25 dB @ 6 MHz and 1.35 dB @ 30 MHz. His arrangement basically consisted of a grounded-cathode triode followed by a grounded-grid triode to provide the low noise of a triode combined with the amplification and stability of a pentode. This arrangement has been more or less adhered to over the years, but there are two distinct variations: series-connection and parallel (shunt) connection. In practice, at freqs. below about 70 MHz, site and other noise has tended to make those noise performance figures not provide any real advantage. Nevertheless, the arrangement does have some distinct benefits: it has such a reserve of sensitivity that at h.f. neutralization of the first stage is unnecessary; cross-modulation effects are very low; wide bandwidth and the layout is not critical. With the plethora of strong signals that can be found within - and just outside - the Amateur bands, a front-end that has good cross-modulation performance can be of real benefit.
For an h.f. receiver implementation, where this cct. is used as the r.f. amplifier, it is usual for tuned transformer-coupling to be used between the aerial input and the grid of the first triode and a tuned transformer-coupling from the anode of the second triode to the next stage - typically the mixer. AGC can easily be applied to the cascode cct. following conventional practice.
My Eddystone 888A has such a modified front-end which uses an ECC85 (original design featured a 6BA6). This modification was made many, many years ago. On test with two coupled signal generators, I do recall measuring the cross-mod. performance of this cct.: I don't have the figures to hand (it was over 30 years ago) but I do recall that the results were quite impressive. In terms of reliability, I have not had any concerns: I only replace the ECC85 during annual performance checks, if appears that the noise figure of this set has fallen off drastically - and even then that can occasionally be due to a defective 1st. mixer: an ECH81 - as opposed to a 'tired' ECC85.
Al.
For an h.f. receiver implementation, where this cct. is used as the r.f. amplifier, it is usual for tuned transformer-coupling to be used between the aerial input and the grid of the first triode and a tuned transformer-coupling from the anode of the second triode to the next stage - typically the mixer. AGC can easily be applied to the cascode cct. following conventional practice.
My Eddystone 888A has such a modified front-end which uses an ECC85 (original design featured a 6BA6). This modification was made many, many years ago. On test with two coupled signal generators, I do recall measuring the cross-mod. performance of this cct.: I don't have the figures to hand (it was over 30 years ago) but I do recall that the results were quite impressive. In terms of reliability, I have not had any concerns: I only replace the ECC85 during annual performance checks, if appears that the noise figure of this set has fallen off drastically - and even then that can occasionally be due to a defective 1st. mixer: an ECH81 - as opposed to a 'tired' ECC85.
Al.
