I've had another tinker with the Portadyne Princess that I'm restoring, and as I suspected, the mains dropper is burnt out. One section has been by-passed with a wirewound resistor perched on top of it - the other section is O/C. The two sections are 260R & 550R, a total of 810R. The closest modern values would be 270R & 560R, a total of 830R, which would be fine as it's just 2.5% varience, and on the right side. Without doing the sums on Wattage, I thought 11 Watt ceramic ones would suffice and with Joe's kind assistance I obtained some. However, having now done the sums, I realise that they need to be at least 25 Watt, preferably 30 Watt.
The valve heaters chain consists of 35Z4, (35V), 35L6 (35V), and two 12BA6 at 12.6V each, a total of 95.2V at 0.15A. Thus, on 240V mains, this leaves 240V - 95.2V = 144.8V to drop across the two sections of the dropper. 144.8V/830R = 0.175A. So far so good. As to the Wattage, 144.8V x 0.175A = 25.3W.(There's also the small amount drawn by the HT to consider when working out the Wattage of course, so to be on the right side, 30 Watts would be better).
I've had a good trawl of internet, and can't find any 560R & 270R 25-30Watt wirewound resistors. The nearest I've come to it are some remarkably cheap (£2.95, free P&P) new 1,000R 'rheostats' which at 4" long and almost 1" diameter are a tad on the large size. I could fit an extra slider and adjust that and the slider which comes with the resistor to give the desired 560R & 270R. So cheap that I thought I'd order a couple anyway, but for some reason my paypal transaction was rejected, so something a bit odd is going on - I use the same credit card on paypal all the time and did so yesterday with no problems.
http://www.ebay.co.uk/itm/30W-Watt-1K-Oh...1001272157
The bottom line is that I wondered I anyone has any ideas as to a source of suitable 25/30 Watt resistors of these values please?
I've attached the circuit and a couple of pics of the position of the existing dropper, surmounted with the one bodged onto it.
(As an aside, note that the single pole mains switch is in the neutral line. I've seen an explanation somewhere as to why they often did that, but I can't recall it. Any ideas anyone?)
The valve heaters chain consists of 35Z4, (35V), 35L6 (35V), and two 12BA6 at 12.6V each, a total of 95.2V at 0.15A. Thus, on 240V mains, this leaves 240V - 95.2V = 144.8V to drop across the two sections of the dropper. 144.8V/830R = 0.175A. So far so good. As to the Wattage, 144.8V x 0.175A = 25.3W.(There's also the small amount drawn by the HT to consider when working out the Wattage of course, so to be on the right side, 30 Watts would be better).
I've had a good trawl of internet, and can't find any 560R & 270R 25-30Watt wirewound resistors. The nearest I've come to it are some remarkably cheap (£2.95, free P&P) new 1,000R 'rheostats' which at 4" long and almost 1" diameter are a tad on the large size. I could fit an extra slider and adjust that and the slider which comes with the resistor to give the desired 560R & 270R. So cheap that I thought I'd order a couple anyway, but for some reason my paypal transaction was rejected, so something a bit odd is going on - I use the same credit card on paypal all the time and did so yesterday with no problems.
http://www.ebay.co.uk/itm/30W-Watt-1K-Oh...1001272157
The bottom line is that I wondered I anyone has any ideas as to a source of suitable 25/30 Watt resistors of these values please?
I've attached the circuit and a couple of pics of the position of the existing dropper, surmounted with the one bodged onto it.
(As an aside, note that the single pole mains switch is in the neutral line. I've seen an explanation somewhere as to why they often did that, but I can't recall it. Any ideas anyone?)
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
