Ge or Si transistors - any stability-difference if used in a VFO? - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 14th Mar 2019, 3:31 pm

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.

kalee20 · 14th Mar 2019, 3:53 pm

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.

Radio Wrangler · 14th Mar 2019, 5:37 pm

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.

David

G6Tanuki · 14th Mar 2019, 6:51 pm

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!

Radio Wrangler · 14th Mar 2019, 8:58 pm

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

David

14th Mar 2019, 3:31 pm	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	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.

14th Mar 2019, 3:53 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	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.

14th Mar 2019, 5:37 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,877	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. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

14th Mar 2019, 6:51 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	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!

14th Mar 2019, 8:58 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,877	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. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done