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In the Forum: Analog Playback
In the Thread: The last phonocorrector: “End of Life" Phonostage
Post Subject: Nature's BiasPosted by Brian Clark on: 6/5/2008
 deemon wrote:
Do you mean a battery bias , Brian ? We didn't try it because of 1 V bias voltage - I don't know if the battery with this voltage does exist . I know only 1,5 V and 1,2 V cells ...... maybe using some chemical reactions we can obtain 1 V or little less , but I don't know how to do it ..... Anyhow , if we use a battery in the grid chain - we still need a resistor ( 2 M ) after the coupling capacitor , and we must add 2 contacts of the battery socket and the battery itself . But the fixed bias , as I think , is a more suitable solution in this case - we have the most simply sound chain from the plate of V1 to the grid of V2 , and the short grounded cathode of V2 . I think that it's the best idea ...
Dima


Sorry Dima, missed your posting first time round this morning.
By "contact potential biasing" I meant the technique that Thorsten likes for 2nd stages - no biasing resistor in the cathode circuit but a very large resistor (10MegOhms upward) for the grid return. This exploits the Volta potential between two differing metals (those of grid wire and cathode surface) in a vacuum to produce a constant small biasing voltage sufficient for the relatively low signal level encountered in this position.

OK as long as grid current doesn't flow of course so not suitable for just any old valve under any old conditions.
However I do note that Thorsten didn't employ this technique when modifying the 834P circuit so I guess there must be good reason in this particular application.
I must ask him.

Brian.

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