minerva: Why two magic eyes?

Hello, 

recentyl I bought this radio and I wonder - why there are two magic eyes? Seems a little pointelss to waste EFM11 only as low frequency amplifier if usually it functions both as amplifier and magic eye.

Regards

Piotr

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Probably to use tubes with not working display function, but with good driving built-in pentode. Such eyes could be cheper than EF11 tube

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Perhaps the pentode section controls the gain of the triode section (via the deflector), so it's a little like a 7660 tube. Maybe part of AGC / AVC system. I'd have to look at schematic/circuit.

So a separate pentode and triode would not have same function, though using a pentode on detector to drive IF amp screen grids to control gain has been done.

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EFM11 has no triode and pentode section. It has pentode and display section, display driving electrode is internally connected to pentode's screen grid. EFM11 is combined tube to work in dual function - AF amplifying and eye driving. (like older AM2). In this radio display screen is not used (display electrode is connected to ground), and only pentode section is used, to work as normal pentode. This is method to use partially working tubes (with defunct display screen). Similar situaction was with ZX Spectrum memory chips. There was used 32Kb chibs, they was in fact 64Kb chips with upper half-array non-working.

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Still I can't find any rational reason to put two tubes instead of one. Was there really so much defunct EFM11 that they decided to use them instead of simple EF11? As I look at the prices of tubes in old magazine, EF11 were slightly cheaper than magic eyes which in turn were cheaper than end pentodes. 

I can't check right now in what condition that EFM11 is, but screen looks like new. 

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The EFM11 is being used as a pentode. The only reason I can think of is that the tubes were purchased for production of a "short superhet" with EFM11 as magic eye, then they decided they wanted more gain, so bought EM1 as magic eye? It doesn't seem to make sense.

The Philips Colette is later set using a magic eye as power indicator and gain -1 audio amp to drive push pull, even though the tone contol amp colud have been used if a passive tone control had been used.  This seems even stranger.

I downloaded and printed the schematic to see for myself!

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