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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 14th Mar 2019, 3:31 pm   #1
G6Tanuki
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Default Ge or Si transistors - any stability-difference if used in a VFO?

I'm looking at building a 'classic' Vackar-style transistor VFO that will be operating around 3MHz and be powered from a 12V supply (suitably stabilised).

The circuits I've seen in 1960s RSGB and ARRL books - some use Ge transistors, some Si.

I've got both (no, whiskery AF11x-series are not included!) - are there likely to be any stability-benefits/disbenefits in going for Si over Ge? Or will any difference be more down to the physical characteristics of the particular transistor's encapsulation? I'm thinking of clamping the transistor-case to the case of the VFO-box to provide a smidgin of thermal-inertia.
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Old 14th Mar 2019, 3:53 pm   #2
kalee20
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Default Re: Ge or Si transistors - any stability-difference if used in a VFO?

I tend to think Si will be more stable as the gain stages, and temperature effects will have less effect on them.

However, it is possible that a temperature drift caused by the tuned circuit may just happen to be compensated by an opposite drift in a Ge device. So AS A WHOLE the circuit may just happen to be more stable with Ge. Unlikely, but possible.
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Old 14th Mar 2019, 5:37 pm   #3
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Default Re: Ge or Si transistors - any stability-difference if used in a VFO?

G3PDM did a nice write up on a version of the Vackar for his receiver (Radcom/Bulletin) and RSGB handbook. The fun thing to find is an Oxley tempatrimmer. I put an alternative tempco trimmer in the ARRL handbook osc chapter... designed to be easy to set and free from unobtainium.

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Old 14th Mar 2019, 6:51 pm   #4
G6Tanuki
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Default Re: Ge or Si transistors - any stability-difference if used in a VFO?

Yes I'm looking at the G3PDM version, though that uses a 2N3819 FET which would not be in the age-related spirit of my 'design'.

I have some old "MAT" germaniums!
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Old 14th Mar 2019, 8:58 pm   #5
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Default Re: Ge or Si transistors - any stability-difference if used in a VFO?

How about a 2N2926? - that overlaps far in the the years of the geranium.

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