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kalee20 · 9th Dec 2009, 8:55 pm

If you add AGC (you already have the circuitry back to the IF valve) you'll want to change the EF91 for an EF92. The EF91 isn't variable-µ so changes in bias won't have much effect on gain.

Also, if you add a tuning indicator, you can have a lot of fun with it! Especially if you have delayed AGC, you can try the operation from the AGC line (when it probably won't deflect for weak stations) and from the demodulator (when it will).

Incidentally, I like the reasoning behind the volume control connection. The same's occurred to me, though as I've always had the control on an earlier stage with a resistive anode load so if the wiper does ever lose contact, there's no danger of the valve frying.

9th Dec 2009, 8:55 pm	#21
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: My new Superheterodyne! If you add AGC (you already have the circuitry back to the IF valve) you'll want to change the EF91 for an EF92. The EF91 isn't variable-µ so changes in bias won't have much effect on gain. Also, if you add a tuning indicator, you can have a lot of fun with it! Especially if you have delayed AGC, you can try the operation from the AGC line (when it probably won't deflect for weak stations) and from the demodulator (when it will). Incidentally, I like the reasoning behind the volume control connection. The same's occurred to me, though as I've always had the control on an earlier stage with a resistive anode load so if the wiper does ever lose contact, there's no danger of the valve frying.