Quote:
Originally Posted by Synchrodyne
Is it necessary to use an RF double triode? The ECC83 has been used in AF cascode circuits. And Rogers used the ECC808 for the cascode input stage of its Master Mk II control unit. I think that the ECC807 was also specified for cascode applications ...
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I agree it would definitely be worth considering a valve which was designed specifically to do this job. I couldn't put it better than Morgan Jones does in his book
Valve Amplifiers:
"An important point to note with cascodes is that there is only one valve that is really suitable for use as a cascode; this is the ECC88/6DJ8 or the E88CC/6922 (special quality version) which was
designed to be used as a cascode. Try other valves by all means but do not expect the performance to be as good."
As well as considering the relationship between the resonant frequency of your mounted valve assembly and the noise spectrum in the room you should also take into account the resonances in the metal structure of the valve itself. As far as I know the only example of Mullard mentioning this specifically was in the case of their EF86 where they did everything they could to minimise the microphonics. They said
http://www.r-type.org/addtext/add036.htm:
"The electrode structure has been made particularly rigid to keep the microphony of the valve to a very low level. There are no appreciable internal resonances below 1000 Hz, the vibration at higher frequencies being effectively damped out by the chassis and the valveholder."
I note that the ECC88 is a frame grid valve which, all other things being equal, is a stiffer structure than the usual 'grid rod' approach.
If you want a softer suspension than rubber grommets then you could consider cutting a relatively large hole in the chassis and fastening a disc of thin rubber sheet over it. The valveholder could then be mounted in the centre of that sheet. One issue with soft suspensions though is the connecting wires to the valveholder pins. There's no point in mounting the holder softly only to short (vibrationally) the whole soft isolator out with stiff wires coupling it to the surrounding circuitry.
When I was a child I remember visiting the house of a dedicated audiophile who was concerned about microphony driven by the loudest source of sound in his listening room - stating the obvious, this was the music he was playing. He simply put his record deck and electronics in the next, much quieter, room and fed the speaker leads through a small hole in the wall. I think he even organised a delayed lowering device for the tonearm so he didn't miss the first few seconds of the music as he was walking back to his listening chair !
Cheers,
GJ