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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 5th Aug 2017, 11:27 am   #21
G6Tanuki
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Default Re: EF805S and EF85 Vs EF89 as IF tube for a HF receiver

Some colour-TV IF-stages using the 183 *did* use a form of bridge-neutralising - though I gather this wasn't to improve stability but instead to remove the narrowing-of-bandwidth that a hint of feedback causes (sort-of 'regeneration-in-reverse').

When your IF is in the 30-40MHz range using simple L-C tuned circuits to get a ripple-free bandwidth that's 10% of this is still problematic!
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Old 6th Aug 2017, 5:15 pm   #22
The-Triode
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Default Re: EF805S and EF85 Vs EF89 as IF tube for a HF receiver

Neutralizing an EF85 is something I've seen in commercial radios schematics. It does not make any particular problem to me, as I look for decent performances with what I have than with what it should be best to have.
Attached a schematic from LOEWE, with neutralized EF85.
Interestingly, Philips data for the ECH81 mention the EF85 about having a common screen grid resistor...

Let's suppose, then, to putting together an ECH81 + 2XEF805S (I.F.) receiver. When it comes to the detector/pre-audio stage, would it have some meaning using a double diode/triode tube to get a kind of amplified/compensated AGC so to get a reasonable AGC behavior?

For AM reception i'll think later.

Cris
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File Type: pdf schematic-EF85_2.pdf (169.0 KB, 120 views)
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Old 8th Aug 2017, 10:35 am   #23
G8HQP Dave
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Default Re: EF805S and EF85 Vs EF89 as IF tube for a HF receiver

Amplified AGC needs careful design. Actually, ordinary AGC needs careful design too but often gets overlooked. A variable-mu valve can provide maybe 40dB of gain control. With three of them (assuming for the moment that you will apply AGC to the ECH81) you can get 120dB of control. The snag is that if the final valve is backed down by 40dB in gain then that also means something like 40dB (i.e. 1%) in current too, so it can only deliver 1% of signal before clipping. This may not be sufficient signal into the detector to generate enough voltage to achieve -40dB gain control. One solution is to have weaker AGC on the final IF valve. There are also good reasons to avoid AGC on the mixer, or at least limit the AGC here. Hence you might add an RF gain stage (but with little gain) just to get more AGC action.

A classic mistake seen in some 1960's receivers (especially after amateur 'improvements' have taken place) is to have an IF strip using 6BA6 and an RF stage using 6BZ6 all running from the same AGC rail. 6BZ6 has higher gain and is only semi-remote-cutoff so can approach cutoff (and hence signal distortion) with an AGC voltage which will only have a small effect on 6BA6. People then come up with interesting (but wrong) explanations as to why their 'hot' receiver cannot handle strong signals.
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