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Old 2nd Aug 2020, 1:44 pm   #1
EF80TVVALVE
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Default Dansette rectifier switch on surge

Hi all,

Just sorting through one of these single EL84 Dansette amplifiers and I'm replacing the rectifier with a 1N4007 and series resistor. I'm getting a high voltage at switch on, the HT is exceeding 300v DC, the HT I believe should be around 240v DC under normal operating conditions. I don't really want to leave it on and see what it drops to once the EL84 warms up as the smoothing capacitors are only rated at 275v and I don't want to damage them with a too high voltage being left on them during warm up time.

I've been stringing in resistors in series to work out a suitable value, I'm currently at 8k for the resistor. Any thoughts?
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Old 2nd Aug 2020, 1:52 pm   #2
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Default Re: Dansette rectifier switch on surge



Lawrence.
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Old 2nd Aug 2020, 1:56 pm   #3
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Default Re: Dansette rectifier switch on surge

At initial switch on the valve won't be drawing any current, so you can add all the resistors you like and the voltage will still be high.

Try something like 150R and let the player warm up.
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Old 2nd Aug 2020, 1:57 pm   #4
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Default Re: Dansette rectifier switch on surge

You cannot add 8k to the string...
Assume the EL84 passes about 40mA so you will end up with about 100 volts HT. (And you would need a 12 watt resistor.)

Do you see a surge rating on the cap? Or up rate the cap to 350 volts?
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Old 2nd Aug 2020, 2:01 pm   #5
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Default Re: Dansette rectifier switch on surge

Resistors won’t help during the valve warm-up period - by Ohm’s Law they won’t drop any voltage until there is current going through them - which won’t happen until the other valves have warmed up and start drawing current. I don’t have a schematic, but I’m assuming that the original rectifier was a valve. If so, this would at least have delayed the HT until the rectifier had itself warmed up, by which time (with luck) the other valves would have also warmed up and be drawing current. I’ve always tried to replace a failed valve rectifier with another valve rectifier to preserve the original design conditions.

Mike

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Old 2nd Aug 2020, 2:17 pm   #6
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Default Re: Dansette rectifier switch on surge

In my opinion it will be fine, maybe even better as the main cap get a bit of "reforming" at every switch on.
 
Old 2nd Aug 2020, 2:20 pm   #7
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Default Re: Dansette rectifier switch on surge

Quote:
Originally Posted by merlinmaxwell View Post
In my opinion it will be fine, maybe even better as the main cap get a bit of "reforming" at every switch on.
I'd still be nervous at the thought of 300V across ageing 275V capacitors.

Mike
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Old 2nd Aug 2020, 2:31 pm   #8
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Default Re: Dansette rectifier switch on surge

You could add a resistor to chassis so about 10mA of current is being drawn before the valve warms up. This should be enough to pull down the HT voltage to more reasonable levels.

Alternatively just uprate the smoothers. 275V is pushing it a bit even with a metal rectifier in place.
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Old 2nd Aug 2020, 2:36 pm   #9
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Default Re: Dansette rectifier switch on surge

Thanks for all of the replies thus far. It is replacing a metal rectifier, it is the single EL84 type amplifier as used in the Major de luxe, Bermuda etc. I do know that the voltage will drop as the EL84 warms up but I do worry about the excess voltage across the smoothing caps on start up and I didn't want to leave it on long enough to find out.

At the moment it is the original hunts can fitted, rated at 275v but with no indication of peak working voltages. When I get around to it I imagine 400v types will go in its place, so not as much of a problem for them but for testing purposes at the moment I don't want to cause any damage!
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Old 2nd Aug 2020, 2:36 pm   #10
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Default Re: Dansette rectifier switch on surge

The original rectifier was a half wave contact-cooled type. There is only one valve. The 3 in one can is only rated at 275V - it's amazing more did not regularly fail. There was no surge resistor ever fitted to this amplifier when manufactured.
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Old 2nd Aug 2020, 2:48 pm   #11
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Default Re: Dansette rectifier switch on surge

I imagine the original rectifier won't have dropped a huge number of volts off-load, and EL84s have always been a bit slow to warm up. So in all likelihood the capacitor has been surviving this switch-on surge ever since the amp was new.

If you think the cap is now failing then you might as well replace it sooner rather than later. In the meantime would a thermistor work better than a plain resistor ?

Cheers,

GJ
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Old 2nd Aug 2020, 3:04 pm   #12
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Default Re: Dansette rectifier switch on surge

For years metal rectifiers and their ilk have been replaced with a silicon diode and series resistor without making other changes to the circuit. Is there any evidence that this practice has lead to failure of the reservoir/smoothing capacitors? I can't say that I've ever heard of that happening. Caps should be able to withstand an over voltage for a short period until the valve/valves warm up.

If reservoir/smoothing caps are being replaced because they're faulty it would make sense to use components with a higher voltage rating. Otherwise "If it ain't broke don't fix it".

The title of this thread is a bit misleading. Switch on surge or inrush current should be reduced by an (anti) surge resistor. What we're discussing here is high initial HT voltage.
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Old 2nd Aug 2020, 3:05 pm   #13
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Default Re: Dansette rectifier switch on surge

It's never crossed my mind to use a thermistor but it sounds like it could be a good idea for these diodes. I have a spare metal rectifier of the same type that is working so I quickly strung that in and it behaved similar with a peak of 360v DC, so it does look like these caps have been coping with this from new.

I don't suspect the caps but as they're 50 years old and I don't want to be doing more work to it in the future (along with avoiding the clean up if one dies in a spectacular fashion) so I'm just going to replace them. A 120R resistor and 1N4007 gives me 220v HT which is fine so I'll stick with this.
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Old 2nd Aug 2020, 3:13 pm   #14
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Default Re: Dansette rectifier switch on surge

A thermistor won't help. No resistance will drop any volts if there's no current passing through it.
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Old 2nd Aug 2020, 3:15 pm   #15
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Default Re: Dansette rectifier switch on surge

It puzzles me a bit that the original capacitors are only rated at 275V, when the dc output I'd expect from that rectifier arrangement would be around 350-360V - which is what the OP is getting according to #13.

Mike
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Old 2nd Aug 2020, 3:25 pm   #16
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Default Re: Dansette rectifier switch on surge

That's puzzled me from time to time too. Maybe it was done to save money. At any rate the caps seemed to survive during the normal working life of the player and were often still good 50 years later.
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Old 2nd Aug 2020, 3:36 pm   #17
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Default Re: Dansette rectifier switch on surge

Whenever I restore one of these Dansette Amplifiers I always replace the multi can capacitor. I have never liked the idea that in with the two sections which are at HT there is the low voltage cathode cap for the EL84. I know all Dansettes hum a bit but having the low voltage cap separate seems to help reduce it a bit in some examples.
The original rectifiers on these are better than some, (I always replace the ones in the Bush valve record players on sight) But for long term reliability they are best replaced. I usually fit around 100 to 150 Ohm surge limiter which keeps the HT sensible.
The carbon resistors are usually best replaced as well especially in the two valve amplifier which is sometimes fitted to the Conquest . (It is the same amp as the Radio-Gram version uses) they are normally a bit high.
These amps are so simple with so few components that its very easy and quick to rebuild so it's not worth leaving anything to chance.

One point I would check is the temperature of the mains transformer especially if originally the record player didn't have an earth.
These transformers really were poor and run hot, sometimes after fitting an earth they get hotter. I assume due to leakage...
I think the same transformer was used from the early single valve amp through to the later models using an ECL82, then they stuck a lamp on the front.. So the load on the tranny increased, most of the transformers on the later models have a big wax "bogey" hanging from the winding.
Rich
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Old 2nd Aug 2020, 3:39 pm   #18
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Default Re: Dansette rectifier switch on surge

Quote:
Originally Posted by paulsherwin View Post
A thermistor won't help. No resistance will drop any volts if there's no current passing through it.
There will be the charging current into the reservoir and smoothing caps. The question then becomes whether RC could ever get to be comparable with the valve's warm-up time. I've never seen it happen with an output valve, and that's all this amp has, but it's not unusual with the smoothers at the input stage end of the HT rail, where the small-signal valves are also quick to warm (passing current in a small number of seconds).

Cheers,

GJ
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Old 2nd Aug 2020, 3:53 pm   #19
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Default Re: Dansette rectifier switch on surge

But, does anyone know what the normal working HT voltage is on these amplifiers? The OP says he believes it's about 240V - is that right? And what is the rms voltage of mains transformer secondary?
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Old 2nd Aug 2020, 3:56 pm   #20
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Default Re: Dansette rectifier switch on surge

Secondary voltage on the 14-3 is given as 240 volts on the schematic.

Secondary voltage on the Monarch is given as 225 volts and the DC across the reservoir is given as 240 volts as best as I can make out but that one uses an ECL82.

Lawrence.

Last edited by ms660; 2nd Aug 2020 at 4:06 pm. Reason: extra info
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