06-09-2025, 06:16 PM
Also they DID use an 8uF polarised cap on the volume control. Someone got the more expensive bipolar past the cost-cutting and inventory team.
In theory I could model it in LT Spice, but likely quicker to lash it up or measure a similar set I have.
The BC 504 has has 8uF polarised to 0V (-Ve to the positive diode) and 40nF from AGC line to emitter. You'd imagine a very strong signal might be an issue. R&TVS pg 122, 1961-1962
The BC505 (which I have) is very similar, except now 100nF on AGC to emitter.
Both have 2 stage IF, as has the 1 year later BC506. I don't see any AGC voltage on 2nd IF.
The three models are very similar and change on the BC506 "saves" one capacitor. The BC505 has an extra audio pre-amp compared to the BC504 and BC506. The BC504 & BC505 use GET873 IF and GET114 for all audio. The BC506 uses 2x PXA101 for IF and PXB103 driver and PXC121 PA. All 3 use a "totem-pole" push pull output stage so there is only a driver transformer.
I'd call these 3x GEC early (1961-1962?) rather than very early (1956-1957?)?
So I can certainly hook up the BC505 and see what a strong signal does. It claims -0.8V on 1st IF base with no signal. I can monitor voltage on R8 via an RFC to see what input signal drives VT2 to cut off.
Note some Ever Ready 1950s valve sets had AGC on mixer/Osc (DK9x) and on the IF amp (DF9x), but at least one had no AGC at all!
The transistor sets with 2 IF amps are poorer than many valve portables and all mains valve sets with 1 IF due to the the 2x IFTs on the valve sets have very much higher Q, and maybe lower loss. It's not just an issue of device gain. The valve IFTs have looser coupling (two cores) and are tuned on primary & secondary. Many transistor sets (and eventually all of them) had only single tuning and a common core.
In theory I could model it in LT Spice, but likely quicker to lash it up or measure a similar set I have.
The BC 504 has has 8uF polarised to 0V (-Ve to the positive diode) and 40nF from AGC line to emitter. You'd imagine a very strong signal might be an issue. R&TVS pg 122, 1961-1962
The BC505 (which I have) is very similar, except now 100nF on AGC to emitter.
Both have 2 stage IF, as has the 1 year later BC506. I don't see any AGC voltage on 2nd IF.
The three models are very similar and change on the BC506 "saves" one capacitor. The BC505 has an extra audio pre-amp compared to the BC504 and BC506. The BC504 & BC505 use GET873 IF and GET114 for all audio. The BC506 uses 2x PXA101 for IF and PXB103 driver and PXC121 PA. All 3 use a "totem-pole" push pull output stage so there is only a driver transformer.
I'd call these 3x GEC early (1961-1962?) rather than very early (1956-1957?)?
So I can certainly hook up the BC505 and see what a strong signal does. It claims -0.8V on 1st IF base with no signal. I can monitor voltage on R8 via an RFC to see what input signal drives VT2 to cut off.
Note some Ever Ready 1950s valve sets had AGC on mixer/Osc (DK9x) and on the IF amp (DF9x), but at least one had no AGC at all!
The transistor sets with 2 IF amps are poorer than many valve portables and all mains valve sets with 1 IF due to the the 2x IFTs on the valve sets have very much higher Q, and maybe lower loss. It's not just an issue of device gain. The valve IFTs have looser coupling (two cores) and are tuned on primary & secondary. Many transistor sets (and eventually all of them) had only single tuning and a common core.
