29-07-2020, 07:31 PM
I opened the attaché case Invicta to find that it had a DK96, but the 1j24b replacing a DF96 (with a pull down resistor on g2), a 1j24b with an 1N60 replacing the DAF96 and a 1j29b with only one filament wired replacing the DL96.
I'd looked for those last week and couldn't remember where they'd gone, but I did remember that the dual 1j24b mixer-oscillator "blew up" on the AVO VCM163. So obviously that's why there was a DK96 in it. BBC R4LW blasting in when the DK96 was replaced with the new Franken mixer-osc, without realignment. No AGC action at all. So overloads on RTE 1 LW unless you rotate to null the signal!
The only simple way to do AGC with the Rod tubes is controlling g2 from 20V (almost no gain) to 50V or 55V (maximum gain), or add the 1j37b or the 1j42a and use a differential signal to reduce gain. Some Russian sets certainly used several 1j37b, some as mixers and some as AGC.
So if retrofitting you do either the heptode or the IF amp, but not both. Certainly not an RF preamp also unless there is a manual gain option.
Next I'll see which of the TO and TO clones are worklng (I've two different Transoceanics, and RCA clone and a Hallicrafters clone). I know they all take 1L6 and I've only one. The DK92 capacitances are sufficiently different that only the lowest bands work without re-alignment. So I'll measure real current of the 1L6 including the worn out ones with working filaments at 1.3V and the Franken mixer-osc and add a parallel ballast for the same current (nominally 50 mA).
The set with the most battery valves I have is maybe the Hallicrafters S72
8: 1T4 1U4 1R5 1U4 1U4 1U5 1U5 3V4
The 1U5 is a different pin out to DAF91. One is the IF BFO for CW originally, now works for SSB too.
Two of the 1U4 (DF904) are RC coupled as a higher gain IF amp. The manual RF gain varies g2 on these.
It's 100mA @7.5V LT! I think replacing more than one valve in a series chain with a Rod Pentode is fraught with horror. Or replacing anything other than audio stages due to AGC issue even if the capacitances are matched.
I'd looked for those last week and couldn't remember where they'd gone, but I did remember that the dual 1j24b mixer-oscillator "blew up" on the AVO VCM163. So obviously that's why there was a DK96 in it. BBC R4LW blasting in when the DK96 was replaced with the new Franken mixer-osc, without realignment. No AGC action at all. So overloads on RTE 1 LW unless you rotate to null the signal!
The only simple way to do AGC with the Rod tubes is controlling g2 from 20V (almost no gain) to 50V or 55V (maximum gain), or add the 1j37b or the 1j42a and use a differential signal to reduce gain. Some Russian sets certainly used several 1j37b, some as mixers and some as AGC.
So if retrofitting you do either the heptode or the IF amp, but not both. Certainly not an RF preamp also unless there is a manual gain option.
Next I'll see which of the TO and TO clones are worklng (I've two different Transoceanics, and RCA clone and a Hallicrafters clone). I know they all take 1L6 and I've only one. The DK92 capacitances are sufficiently different that only the lowest bands work without re-alignment. So I'll measure real current of the 1L6 including the worn out ones with working filaments at 1.3V and the Franken mixer-osc and add a parallel ballast for the same current (nominally 50 mA).
The set with the most battery valves I have is maybe the Hallicrafters S72
8: 1T4 1U4 1R5 1U4 1U4 1U5 1U5 3V4
The 1U5 is a different pin out to DAF91. One is the IF BFO for CW originally, now works for SSB too.
Two of the 1U4 (DF904) are RC coupled as a higher gain IF amp. The manual RF gain varies g2 on these.
It's 100mA @7.5V LT! I think replacing more than one valve in a series chain with a Rod Pentode is fraught with horror. Or replacing anything other than audio stages due to AGC issue even if the capacitances are matched.
