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Old 2nd Aug 2020, 3:56 pm   #21
paulsherwin
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Default Re: Dansette rectifier switch on surge

240V sounds plausible given the rating of the smoothers.
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Old 2nd Aug 2020, 5:04 pm   #22
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Default Re: Dansette rectifier switch on surge

Don't want to take this all hopelessly OT, but I've been struggling with the little bit of ac theory I know to work out how you could rectify 240V rms from the xformer secondary to get 240V dc across the smoothing capacitor. I have to admit, it's got me beaten!

Mike
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Old 2nd Aug 2020, 5:10 pm   #23
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Default Re: Dansette rectifier switch on surge

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Originally Posted by Boulevardier View Post
Don't want to take this all hopelessly OT, but I've been struggling with the little bit of ac theory I know to work out how you could rectify 240V rms from the xformer secondary to get 240V dc across the smoothing capacitor. I have to admit, it's got me beaten!

Mike
We don't know as yet what the secondary voltage is in the OP's amplifier, neither do we know what the voltage across the reservoir is supposed to be.

Lawrence.
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Old 2nd Aug 2020, 5:12 pm   #24
paulsherwin
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Default Re: Dansette rectifier switch on surge

The rule of thumb is to multply the AC voltage by 1.4 to get the rectified smoothed DC voltage if there are no inefficiencies and losses. There are always losses though, particularly in 1950s metal rectifiers but also within the transformer if present.
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Old 2nd Aug 2020, 9:37 pm   #25
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Default Re: Dansette rectifier switch on surge

I wouldn't worry about it. The capacitor can has been 'conditioned' from new to work at that voltage and will probably out live you. I had exactly the same issue with an amplifier in a record player with an EZ80 valve rectifier and a smoothing can rated at 350 volts continuous, which had an HT of 400 volts fully loaded, the can capacitor had a surge voltage rating of 425 volts, though. I queried it at the time, but it doesn't seem to have caused any problems since around 1960 when the unit was made. There's a forum thread regarding it a couple or three years back - picture of amplifier below where you can just see the ratings on the can:-
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Old 3rd Aug 2020, 1:06 pm   #26
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Default Re: Dansette rectifier switch on surge

I should probably just clarify things a little regarding the above post.

The problem (if you can call it that) is the same, but the other way round (if you see what I mean).

In your case with a solid state rectifier, the voltage starts high and then drops to a level below the working voltage of the can capacitor when the output valve warms up and conducts to load the supply. However, in my case described with a valve rectifier, the voltage starts at zero and rises to a higher level than the working voltage of the can capacitor as the thermionic valve rectifier warm up and conducts, regardless of the fact that the output valve has also warmed up and is fully conducting and loading the circuit. So in your amplifier the overload to the capacitor is very short term, whereas in my amplifier it is continuous once the amplifier has warmed up and is working. I checked all current drains and transformer tappings on my amplifier and everything was normal, so that's just how it was designed - to be right on the limit!
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Old 3rd Aug 2020, 9:32 pm   #27
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Default Re: Dansette rectifier switch on surge

I thought most of these old electrolytic cans had a surge rating....typically 275V working, 350V surge. It simply means they are able to withstand a short-term overload without harm.

Multiplying the AC voltage at the anode of the rectifier by 1.414 will give you the peak DC voltage (assuming no losses).
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Old 3rd Aug 2020, 10:51 pm   #28
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Default Re: Dansette rectifier switch on surge

Is the 'surge' mechanism really just a reflection of the relationship between voltage and lifetime ? Electrolytics are unusual in that manufacturers routinely specify a lifetime for them. I recall the rule-of-thumb for temperature (every 10C that the cap is run below its rated temperature doubles its life, at least down to room temp). I seem to remember there's one for voltage too, but I can't bring the numerical details to mind. I wouldn't be surprised to find that if a cap was rated for 2000 hrs at 85C and 275V that it would last for a decently long time at, say, 45C and 300V, and would take 350V for perhaps tens of hours, which would mean essentially for ever if each switch-on surge only lasted 20 seconds or so.

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GJ
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Old 3rd Aug 2020, 11:04 pm   #29
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Default Re: Dansette rectifier switch on surge

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Originally Posted by GrimJosef View Post
Is the 'surge' mechanism really just a reflection of the relationship between voltage and lifetime ? Electrolytics are unusual in that manufacturers routinely specify a lifetime for them. I recall the rule-of-thumb for temperature (every 10C that the cap is run below its rated temperature doubles its life, at least down to room temp). I seem to remember there's one for voltage too, but I can't bring the numerical details to mind. I wouldn't be surprised to find that if a cap was rated for 2000 hrs at 85C and 275V that it would last for a decently long time at, say, 45C and 300V, and would take 350V for perhaps tens of hours, which would mean essentially for ever if each switch-on surge only lasted 20 seconds or so.

Cheers,

GJ
Perhaps you've hit on the key to this whole riddle, GJ!

Mike
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Old 3rd Aug 2020, 11:40 pm   #30
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Default Re: Dansette rectifier switch on surge

Hi!

My experience with B8A & B9A 6.3V output valves in good condition was always that they started to draw h.t. current and produce sound from the speaker within 20 s of switching on from cold, providing the screen–grid volts and the –G.B. is correct!

Don't forget in the TV era that the booster diode, which had to have heater/cathode insulation suitable for a peak of +4.5kV on the cathode during line flyback, took at least 45s to well over 1 min to fully warm up and allow the line o/p stage to come to life, the larger proportion of a TV set's h.t. current is drawn by the line & frame TB circuits, and in any case, TVs were always series–run heaters with a surge–limiting thermistor by this time, which itself extended the warm–up period noticeably, so a T.V. set's main electrolytics may have the higher unloaded/part–loaded h.t. across them for a much longer period than a radio set or amplifier, and virtually all TV electrolytics were 275V working – even with the single–standard 625–line TVs at the end of the valve/hybrid era when h.t. supplies were commonly 230/240V, it was common for only the reservoir capacitor to be 300V or 325V rated in a multi–can – I often came across dual–voltage rated smoothing blocks (300V/325V reservoir, 275V for the other sections), so these electrolytics were well up to short–term unloaded h.t. voltage rises!

Today's electrolytics do have a "surge" voltage rating – it's never marked on the component these days, but it is quoted on the datasheet from the capacitor's manufacturers, and there is a (proportionally smaller) surge–rating for low–voltage rated capacitors as well!

Chris Williams
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