Cascode Vs Normal RF Amp

[pwdrive]

Hello all....Briefly what are the pro's and con's for using say a twin triode in cascode for an RF amp verses the standard pentode or triode arrangement to cover the SW band 1.6 to 30 mhz

Input would be from standard parallel resonant tuned circuit, output (untuned) feeding to the grid of a valve mixer (not pentagrid)

Any comments/ or experience of welcome

Lawrence

[Paula A]

Low impedance and wide bandwidth.. they give not much more than unity gain, but allow better aerial impedance matching than the "stock" damped tapping or winding on a coil.

Also worth checking out the grounded grid circuits for a similar setup using just one triode.

[pwdrive]

Thanks for the reply Paula "not much more than unity gain" the circuit I have in mind has the bottom triode of the cascode as a normal grounded cathode ie: not grounded grid as in VHF cascode, considering this would the gain still be unity ?
Comments welcome.

Lawrence.

[pwdrive]

Appologies Paula I have got my top and bottom cascodes mixed up! so what you say makes sense.

Lawrence.

[Skywave]

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.

[pwdrive]

Thanks for the information Al, so in all that do I take it that if I wanted an RF amp in my Regenerodyne project then there is nothing to loose by going cascode as it will take up no more physical space as a pentode, ie: both cascode triodes in one envelope a la ECC189 etc. like you say it offers better isolation beween the aerial and the rest of the circuit, which has to be a bonus in anything regen.

Lawrence.

[Paula A]

Yes indeed.. they were "stock" in valve VHF turret tv tuners after all. Theres nothing to lose by trying it, after all it's only a twin triode.. pcc88 was specially designed for the purpose, usually followed by a pcf80/86 etc and they should be plentiful.

I don't do very much rf stuff these days.. so it's all very rusty. The tuners in my friend the vc52/3 used them and iirc I never had to replace one in my few years.. they seem to have a very good life

[pwdrive]

Hello Paula, yes when I get this little project underway I will give the cascode a go, the gain of the RF stage is not that critical as regens have plenty of that, it's more to do with isolating the A/E input and RF preselection from the rest of the circuitry and visa versa.
The PCC88 was quite a reliable valve, I remember the turrett TV tuners well, clunk, clunk it was the Cyldon turret tuner in my early days followed by the Fireball tuner then the Phillips printed coil tuners.

Lawrence.