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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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28th Jul 2020, 5:28 pm | #1 |
Triode
Join Date: Oct 2017
Location: Saltdean, Brighton, East Sussex, UK.
Posts: 34
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Ever Ready Sky Queen. PSU modifications
I have been working on this radio following the advice from previous threads such as these:
https://www.vintage-radio.net/forum/....php?p=1042753 https://www.vintage-radio.net/forum/...=118410&page=2 I have replaced all the out of tolerance resistors and the mustard type caps. Someone had added a high wattage resistor of 2.7 ohms between the LT+ brown wire from the battery connector and the switch where the LT+ connects into the circuit. There is another additional component not shown on the circuit diagram which is a 1000 uF electrolytic cap connected between LT+ and the chassis (LT-). Am I right in assuming the functions of these were probably a dropper resistor when someone was using a power supply higher than the 1.5 v required for LT and the electrolytic was to provide provide smoothing for this AC derived power supply? I have removed the 2.7 ohm resistor as I will be using a 1.5 V battery for the LT supply. I am wondering whether I need to remove the 1000 uF capacitor as well. |
28th Jul 2020, 10:18 pm | #2 |
Hexode
Join Date: Sep 2003
Location: Cheltenham, Gloucestershire, UK.
Posts: 298
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Re: Ever Ready Sky Queen. PSU modifications
Steve,
My guess is that either the original supply was a tad too high (by about 0.34V), possibly because it was loaded to 125mA instead of the 250mA more prevalent in earlier times; or, the owner was trying to give the valves a longer life by slightly under-running them (design voltage 1.4V, but they do still run at a somewhat reduced voltage). If your chosen supply is rock solid at 1.5V there's no need to keep the 2.7ohm resistor, unless you want to explore long-life techniques. As to the 1000uf capacitor, if fitted after the 2.7ohm resistor it would have slightly smoothed the turn-on inrush and decoupled the heater supply a bit more. Leaving it in place won't harm (if your power supply is happy); but it shouldn't be necessary. hth. dave
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Dave Teague |
29th Jul 2020, 7:23 pm | #3 |
Triode
Join Date: Oct 2017
Location: Saltdean, Brighton, East Sussex, UK.
Posts: 34
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Re: Ever Ready Sky Queen. PSU modifications
Dave,
Thanks very much for your advice. Steve |
30th Jul 2020, 1:26 pm | #4 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,078
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Re: Ever Ready Sky Queen. PSU modifications
The valve filament nonimal voltage is actually 1.4V. Higher disproportionately reduces life.
I've been using a Vidor CN420a (Dx96 series valves), the LT supply is battery-derived but regulated, at 1.38V. So just slightly below nominal. It's clocked up well over 1,000 hours with no valve problems over the last 10 years. |
30th Jul 2020, 1:49 pm | #5 |
Heptode
Join Date: Dec 2014
Location: Limerick, Ireland.
Posts: 901
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Re: Ever Ready Sky Queen. PSU modifications
1.3V is the Philips recommendation for mains PSU on them.
1.38V is at the upper limit. The end point of the Zinc cells is 0.9V, though usually the limit is the DK91, DK40, 1L6, DK92 or DK96 stops oscillating around 1.1V. A 0.9V to 1.4V variable LT supply is one way to test how good the DKnn is in circuit. 2 x 1N5408 in series to shunt an LT PSU with a series resistor is better than a regulator as it will also protect against inadvertent HT faults. |
1st Aug 2020, 1:54 pm | #6 |
Triode
Join Date: Oct 2017
Location: Saltdean, Brighton, East Sussex, UK.
Posts: 34
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Re: Ever Ready Sky Queen. PSU modifications
Thanks to Mike and Kalee for your posts.
As I will be using a an alkaline D cell for the LT which can be up to about 1.6 V, I reckon I should replace the 2.7 ohm resistor I removed with a 1.5 ohm resistor. This will drop 0.1875 V assuming 125 mA current is drawn. |