GEC BC506 Non electrolytic capacitor question

[Mark Hennessy]

I spotted that too, right at the start. It does rather suggest that they really weren't taking any chances with C11.

Yes, you're probably right about early/very early. Rightly or wrongly, I've always based that on the technology rather than the release date of the sets, and regard the split between "early" and "very early" as the arrival of the alloy diffused AF117 et al in 1961. So to me at least, these sets came out towards the end of the "very early" era

If you could take a look at your 505, that would be interesting. I could dig out an early Hacker RP10, which uses a very similar configuration and the same Ediswan transistors, but it would take a long time to go through all the boxes to find one. As well as monitoring the base voltage, it'll also be interesting to see the DC voltage at the detector so it can be compared with my earlier findings.

[Kalee20]

Wow - some measurements forthcoming, thanks folks!

(Don’t forget to include a snippet of the circuit diagrams of what's being measured!)

I'm looking forward...

[Mike Watterson]

My sons would likely split at metal can transistors and canned ICs vs plastic and DIL ICs :D Or Silicon vs Germanium.

One suggested the start of the Modern Era was 1970s. I pointed out that rectangular plugs, TV and computers all existed before 1950. Fax, electric telegraph, phone, mechanical TV, records, radio telegraphh, typewriter, submarine, turbines, electric light, neon lamps etc all are 19th C.

Historians:
Early Modern is about 14th to 16th C, depending on region. Gunpowder reaches England.
Modern Era started about 17th C. Steam power squeaks in.
Electrical Age starts about 1800 (Volta and Battery experiments 1799).

[Mike Watterson]

Hmm, I can't see the BC505 in the attic. I'm sure that's where it is. So there will be a day or three delay...

I've a Decca TP60, which seems similar to the BC505 for AGC and transistors, but I don't see it either. It and BC505 were side by side on a shelf. I found a Decca TT33, which actually is similar to the BC505 anyway.

   

I can't imagine what X1 diode is for, unless to boost the IF voltage?

[Mark Hennessy]

X1 is a dampening diode. It increases the range of gain reduction the AGC can achieve before reaching the point where the transistor is no longer working in class A. I mentioned them earlier (post #12).

As the AGC drives TR2 towards cutting off, the voltage across R9 falls. At some point, this allows X1 to conduct, reducing the voltage from TR1. There's often a low-ish value resistor (hundreds of Ohms) in series with the diode.

I see it "stage 2" in the evolution of AGC in transistor sets (after what was seen in sets like the BC506). Stage 3 was - to me, at least - when the AGC voltage was applied to 2 IF stages - at this point the dampening diode was no longer used. After that, ICs took over.

Of course, that's a simplification, and there is much variation in practice, but a lot of manufacturers did follow this pattern, which of course is straight from the Mullard books of the time. I did have a flick through a couple of mine yesterday, and it's striking just how similar many of these sets were to the examples given in the books - right down to resistor values in many cases. Fair enough, of course...

This is one of my favourites: https://www.worldradiohistory.com/UK/Mis...s-1960.pdf

Page 199 (72 of the PDF) talks about AGC with and without the dampening diode (see figure 5).

[Mike Watterson]

The BC506 has only Vbe across the electrolytic with no signal, which is about -0.15V (wrt emitter)
The BC504, BC505, Decca TP33, Decca TT33 and Vidor Gem CN440 all have two caps. A small 40nF to 100nF effectively on base-emitter via IFT and electrolytic on the AGC line (effectively base to 0V via IFT). The electrolytic thus has about -1 V with no signal.


   

Actual Vidor Gem / CN440 on capacitor has about -1.4V with very little signal (background noise). About -1.1V on strong interference. I'd need to connect sig Gen. No AM here.

Emitter about -1V with no signal and -0.85V with the interference.

Supply from 6V battery pack about 4.7V at R8, and 4.9V at main power in.

PCB
   

Maybe tomorrow I'll connect sig gen

[Mike Watterson]

No damping diode on some (not on Gem). The emitter resistor 510 to 680 probably depending on supply. The table model Decca uses two spring top 6V 996 packs to give -6v for speaker. The Gem uses a 6V VT50 pack (mine has 4x AA inside).

The Gem (1958?) is same case as the earlier valve model (1957?*), so both have ferrite rod in pop-up handle. The earlier one tunes the LW, but the Gem simply has a Light Program presets for LW via a switch (TC1 & TC4).

I might be misrembering. They might be 1959 and 1958. Huge IFTs, but single tuned. The earlier valve model is all double tuned twin core IFTs, so only needs one IF amp (DF96). The Vagabond CN439. I have both and the amazing Vanguard VHF/MW/LW portable, maybe 1957? It has no DK96. All DF97s.

[Mike Watterson]

It's possible the concern was the very low bias on the electrolytic at no signal rather than reversal as the bipolar ones work with millivolts and some of the electrolytics would go leaky without sufficient polarising voltage to maintain the oxide layer.

[Mike Watterson]

X1 is a dampening diode. It increases the range of gain reduction the AGC can achieve before reaching the point where the transistor is no longer working in class A. I mentioned them earlier (post #12).


This is one of my favourites: https://www.worldradiohistory.com/UK/Mis...s-1960.pdf

Page 199 (72 of the PDF) talks about AGC with and without the dampening diode (see figure 5).

Yes, that's a lovely little booklet. Added to my library. Thanks.

Though in the Philips/Mullard cct, the diode is from emitter of IF amp to the IFT on the mixer. I suppose X1 does the same thing, but it is reversed as it's from the IF Amp collector to the Mixer's IFT. Most of the radios I have of that era don't have it.

[Mike Watterson]

It's possible the concern was the very low bias on the electrolytic at no signal rather than reversal as the bipolar ones work with millivolts and some of the electrolytics would go leaky without sufficient polarising voltage to maintain the oxide layer.

If we assume the transistor is never turned off then the capacitor voltage is about 110mV to 150mV, the Vbe of the transistor.