01-08-2020, 11:10 AM
(This post was last modified: 01-08-2020, 11:11 AM by Mike Watterson.)
Maybe they had excess 0.1uF 350V caps to use up? It doesn't even have DC on it. Because it's in series with a 390 pF even if there was AC (there is RF), any RF or other AC is going to be all across the 390 pF. So it does nothing.
Maybe the 390 pF was originally to go there and the two connected ways of SW1b for LW were supposed to go to 0V. That's how I'd wire it.
It's generally a terrible layout of chassis, despite the space. The Pygmy and Murphy I did recently are both better layouts, though the Pygmy is a bit mad at the volume control and the output valve wiring. Hence the tendency it has to have feedback whine unless the wire from anode to transformer is perfectly dressed.
I remembered I had a box of Marconi/HMV (EMI) battery valve PSU parts that came with some old IFTs I bought. It had four power resistors. Two were obvious scrap. One looked OK but was open circuit, so I cracked off the cement at the ends, crimped the solder at the base of the wires and it measured 1795, as written on it. I put fresh solder at the base of the wires and it was still OK, so it went in to replace the 1K with dubious 750 Ohms in series.
I disconnected the two main HT caps in one can and rechecked them; they are perfect. The four section main dropper is well within tolerances.
So now getting 6.46 V LT, vs the trader sheet 6.75. Except the 1R5/DK91 LT is at 1.45V!
1T4/DF91: 1.27
1S5/DAF91: 1.21
3V4/DL94: 2.48, with 1.31V across the 470 Ω, earlier I mis-labelled that as a DL92. It's a 3V4 in listing and fitted.
I took out the valves after unplugging isolation transformer. The 470 Ω is 522, which is 11.1% high. The 220 resistors are within 4.5% and the 180 Ω across the 1R5 is only 5.6% high.
Perhaps some sockets or pins are more tarnished than others. It is normal to run the mixer/osc filament a little higher than the IF amp and detector/preamp so as to get the very last out of the battery, but it's too high on the 1R5, especially if the LT was at a higher voltage.
I'll need to think about this. Suggestions welcome.
Maybe the 390 pF was originally to go there and the two connected ways of SW1b for LW were supposed to go to 0V. That's how I'd wire it.
It's generally a terrible layout of chassis, despite the space. The Pygmy and Murphy I did recently are both better layouts, though the Pygmy is a bit mad at the volume control and the output valve wiring. Hence the tendency it has to have feedback whine unless the wire from anode to transformer is perfectly dressed.
I remembered I had a box of Marconi/HMV (EMI) battery valve PSU parts that came with some old IFTs I bought. It had four power resistors. Two were obvious scrap. One looked OK but was open circuit, so I cracked off the cement at the ends, crimped the solder at the base of the wires and it measured 1795, as written on it. I put fresh solder at the base of the wires and it was still OK, so it went in to replace the 1K with dubious 750 Ohms in series.
I disconnected the two main HT caps in one can and rechecked them; they are perfect. The four section main dropper is well within tolerances.
So now getting 6.46 V LT, vs the trader sheet 6.75. Except the 1R5/DK91 LT is at 1.45V!
1T4/DF91: 1.27
1S5/DAF91: 1.21
3V4/DL94: 2.48, with 1.31V across the 470 Ω, earlier I mis-labelled that as a DL92. It's a 3V4 in listing and fitted.
I took out the valves after unplugging isolation transformer. The 470 Ω is 522, which is 11.1% high. The 220 resistors are within 4.5% and the 180 Ω across the 1R5 is only 5.6% high.
Perhaps some sockets or pins are more tarnished than others. It is normal to run the mixer/osc filament a little higher than the IF amp and detector/preamp so as to get the very last out of the battery, but it's too high on the 1R5, especially if the LT was at a higher voltage.
I'll need to think about this. Suggestions welcome.
