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Leon Crampin · 7th Jun 2008, 7:03 pm

Bush also fitted a high value resistor in the HT feed to the two EF80s in the VHF41 - their first (and not very good) FM chassis. This disappeared on later sets such as the VHF61 which used an ECC85.

I think if cathode poisoning were a serious issue, many FM sets which had been operated for most of their lives on FM would show a poor E/UCH81 triode. This is not my experience, in fact the heptode is usually less good than the triode, possibly because it has spent most of its life biased nearly off by the AGC assuming most sets are tuned to a local station. This gives rise to all of the anode current being "sourced" from a small area of the cathode where the grid pitch (variable mu heptode) is at its coarsest.

I think the relative unreliability of the E/UCC85 is simply due to its running a bit hot for its envelope size. Fitting a blackened screening can makes a big difference and can reduce tuning drift as a result. It's a dead easy mod on Bush sets which use standard cans.

Leon.

7th Jun 2008, 7:03 pm	#16
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Another (possibly silly) valve question Bush also fitted a high value resistor in the HT feed to the two EF80s in the VHF41 - their first (and not very good) FM chassis. This disappeared on later sets such as the VHF61 which used an ECC85. I think if cathode poisoning were a serious issue, many FM sets which had been operated for most of their lives on FM would show a poor E/UCH81 triode. This is not my experience, in fact the heptode is usually less good than the triode, possibly because it has spent most of its life biased nearly off by the AGC assuming most sets are tuned to a local station. This gives rise to all of the anode current being "sourced" from a small area of the cathode where the grid pitch (variable mu heptode) is at its coarsest. I think the relative unreliability of the E/UCC85 is simply due to its running a bit hot for its envelope size. Fitting a blackened screening can makes a big difference and can reduce tuning drift as a result. It's a dead easy mod on Bush sets which use standard cans. Leon.