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Old 4th May 2019, 11:22 pm   #21
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Bush VHF-41 Refurb.

Use of the ECH81 triode as an additional AF amplifier on FM (in FM-only and FM-AM receivers) and on the “gram” input on receivers generally was, as I understand it, included in the original “mission statement” for this valve. For example it was used this way (for FM and gram) in the Pye FenMan II. The same triode was also specified for use as a self-oscillating mixer on FM, in which case Philips envisaged that it would be preceded by an EF80 or EC92 RF amplifier. Whether any UK setmaker used it this way I don’t know. (The original Philips FM-AM valve set did not include the ECC85 - which came from Telefunken, I think – so a different approach was needed. However, it would appear that Philips did add the ECC85 to its range quite early on.)

In the case at interest, possibly an external RF preamplifier (or aerial booster amplifier) might have some chance of improving overall FM performance by getting a higher signal level into the receiver, enabling it to operate well along its quieting curve. Although the fully broadband front end is more likely to get into cross-modulation difficulties at lower signal strengths than typical sets, so one might need to be judicious in respect of the extra gain applied.


Cheers,
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