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Defiant M770

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Nuvistor
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Posted by: abctelevision

Does a HT capacitor dropper reduce the power consumed by the radio?

Looking at the problem in my simple way, if the radio doesn’t get as hot, it’s using less power.

The other thought as far as the electric meter is concerned, with I and V being at 90 degrees, will it affect the reading on the meter?

 

Frank

 
Posted : 19/02/2018 6:32 pm
hamid_1
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Yes, a capacitor dropper will reduce the overall power consumption of the radio.

The valve heaters will have the same amount of current flowing through them and will consume the same amount of power, but no power will be dissipated in the capacitor (in theory, anyway). So there will be some saving of power, compared to a dropper resistor which would convert the excess voltage into heat. The radio will run cooler as a result.

Of course, the radio will no longer work on DC mains, but that's not a big problem these days since DC mains supplies disappeared years ago.

 

As for whether the additional capacitance will affect your electricity meter readings, well it shouldn't, as long as it takes power factor into account. I'm pretty sure this is the case with all domestic electricity meters. If you use a clamp-on meter, it only measures current and not power factor, so you may get a high reading.

 
Posted : 19/02/2018 6:51 pm
turretslug
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Thanks Hamid- as I was drying the dishes and booting the computer up for a  response! Yes, the current through the dropper/heater combination are the same for resistor and capacitor dropper cases, as is the overall voltage across them- but as they are no longer  exactly in phase with a cap dropper, the effective power ("hot watts") is less, only the resistive heaters part of the chain absorbs power. An olde-worlde spinning eddy current disc 'leccy meter is a genuine watt-meter and will show W, rather than VA, consumed. Modern electronic meters should do the same, but there has been muttering that they don't always calculate true power with reactive loads.... The power companies might argue that there is a degree of justified penalization for reactive loads- as their supply network still feels the full current and its (I squared)R etc. system losses. Someone who knows more about supplier electrics would be better placed to comment.

 
Posted : 19/02/2018 7:13 pm
PYE625
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Posted by: sideband
 
js1280 SAM 1554
The dropper caps fitted at present...2 x 1.5uF 450V AC. I'll probably change this later

 

 

 

 

That style of capacitor does not look out of place.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 19/02/2018 10:10 pm
sideband
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Yes...they are growing on me...I think I'll just wire them up neatly and fit a 100 ohm wirewound across a pair of spare terminals for the supply to the anode of the rectifier. I've also found a 100K    1 watt resistor so that will do for the discharge resistor. All I need to do now is order some caps as I seem to have run very low on common values notably 0.1uf and 0.022uF.

Out of interest I measured the heater voltages today and they are just a fraction off specified but well within tolerance...the two 13V heaters are measuring 12.8 and 13.1, the rectifier (40V) is measuring a smidge over 40V. The output valve is difficult to get to but will assume that as the others are virtually spot on, then it's likely that the heater (38V) will be the same.

Also out of interest, I checked the voltage across one of the heaters at switch-on......it starts low and builds up over about 10 seconds so there is no 'surge'. Makes sense as the resistance is low to start with and then gets higher as the heaters warm up. In a way it's self-limiting.

 

 
Posted : 19/02/2018 10:48 pm
sideband
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Very quick update, too cold to work for long in the loft even with heating. Critical caps changed, (screen grid decouplers, HT decoupling, audio coupling, cathode decoupler (output valve)), 100 ohm fitted between switched mains and anode of rectifier. Mains applied carefully via variac...start at 150V, HT rose to about 75V....mains set to 200V, HT now 107V, low hum from speaker and......heterodyne whistle....back off reaction....try tuning, station heard in background, Talk Sport received which can be boosted in volume by increasing reaction, mains at 230V, HT now near normal at 190V, volume increased but tuning around it seems very deaf, too cold to investigate further and besides a hot meal awaits......at least it's a start.

It's quite a while since I've tuned a TRF with reaction, probably about 50 years when I built my first SW set.

 

 
Posted : 23/02/2018 9:06 pm
sideband
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Further update. Set appeared very deaf even for a TRF. R4 and Talk Sport was about all I could get. Managed to find a weak station near 250 metres and tweaked the two pre-set caps on the tuning gang. Immediate improvement and now around half-a-dozen stations can be heard. It just about tunes down to Capital Gold but struggles to bring it in with any real strength.

LW is not very good and there is a lot of background noise on both wavebands. I don't have any replacement valves to try but I suspect it wouldn't make a lot of difference. It's nowhere near as good as the much older Philips Superinductance 274A that I have.

Anyway it's working and I'll play around with the aerial length. 

 
Posted : 26/02/2018 10:34 pm
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