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10th Nov 2021, 8:37 pm | #101 | |||
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Pilot Little Maestro restoration project.
Quote:
Lawrence. |
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10th Nov 2021, 8:43 pm | #102 | ||
Heptode
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Location: Larkhall, South Lanarkshire, Scotland.
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Re: Pilot Little Maestro restoration project.
Quote:
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10th Nov 2021, 8:55 pm | #103 |
Heptode
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Re: Pilot Little Maestro restoration project.
Thanks Lawrence, I didn't know (or had forgotten!) that you have to allow for the ripple current.
It looks as though there will be about 6W dissipated in the 470 ohm resistor and the datasheet says it's only ok for 5W without a heatsink so better with a heatsink really. |
10th Nov 2021, 8:58 pm | #104 | |
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Re: Pilot Little Maestro restoration project.
Quote:
There is a lot of 'pseudo-science' buzzing around the Web so beware.
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10th Nov 2021, 8:59 pm | #105 | |
Heptode
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Re: Pilot Little Maestro restoration project.
Quote:
As for the surge limiter, One schematic says 33 ohm,3 watts.. I have 50 ohms 5 watt ceramic.?
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10th Nov 2021, 9:10 pm | #106 | ||
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Re: Pilot Little Maestro restoration project.
Quote:
Jay
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10th Nov 2021, 9:41 pm | #107 | |
Dekatron
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Re: Pilot Little Maestro restoration project.
Quote:
http://www.tubebooks.org/books/rdh4.pdf Or there's a simpler way (without using a true RMS meter) so far as calculations go if the DC load current and the DC load voltage are known along with the peak AC voltage input to the surge limiter by using Kauzmann's curves. The curves/formula's etc are all contained in the link below and should be self explanatory. Lawrence. Last edited by ms660; 10th Nov 2021 at 9:49 pm. Reason: extra info |
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10th Nov 2021, 9:49 pm | #108 |
Heptode
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Re: Pilot Little Maestro restoration project.
Hang on, Seems to be a bit of confusion creeping into the picture, I thought the surge limiter was on the supply that fed the heaters.. Not the rectifier ?
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10th Nov 2021, 10:38 pm | #109 | |
Dekatron
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Re: Pilot Little Maestro restoration project.
Quote:
It's there to limit the maximum transient peak anode current to a value that doesn't exceed the valve manufactures maximum specification, sometimes that value is given in the valve data and sometimes it isn't. Lawrence. |
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10th Nov 2021, 10:44 pm | #110 | ||
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Re: Pilot Little Maestro restoration project.
Quote:
Jay
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11th Nov 2021, 12:05 am | #111 |
Heptode
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Re: Pilot Little Maestro restoration project.
Right, I pretty much know all i need to know to be cracking on with this thanks to everybody who has contributed to the thread,
But one final thing remains, The heater chain surge limiter is listed in my original schematic in post one as 33 ohm & 3 watts, I have available a 50 ohm at 5 watts, Cement resistor. Obviously it'll knock a bit more voltage off the circuit, But can it still be used ? Jay
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11th Nov 2021, 11:14 am | #112 |
Heptode
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Re: Pilot Little Maestro restoration project.
Yes, it will be fine for an initial test of the valve heaters which I suggest you do on its own before moving on to connecting the supply to the recitifier anode. The additional 17 ohms will drop an extra 5 volts.
Back in Post 46, we established the pilot lamp had been moved to between chassis and V3 and has no shunt. With the new capacitor dropper and 50 ohm resistor in place you will be able to try the heater circuit. A 1 amp or 3 amp fuse in the plug is a good idea. The valve heaters and the pilot lamp should light with a normal glow. The next check should be using your meter on a low ac volts range to check the voltage firstly across the pilot lamp and then across V3, they should both be about 6.3 volts. Please let us know what you find at this stage. You may need a shunt across the lamp if it's too bright. |
11th Nov 2021, 11:41 am | #113 | |
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Re: Pilot Little Maestro restoration project.
Quote:
I'm gonna dig out the old fluke 8020-B For the ceremonial lighting of the valves. I can make up a 33 ohm using the 50 ohm mentioned above and a 100 ohm 100 watt power resistor in parallel But thats a bit OTT, I'll just buy one. If i'm right the total heater chain should be 75 volts, Thats as good as my maths get.. Weird thing is the 50 0hm/5watt resistor i'm talking about was being used to power the rectifier, Not sure how that worked, As for the lamp shunt i can make one up from 2 X 8 ohm resistors if needed. If i don't report back its gone pete tong Cheers
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11th Nov 2021, 1:53 pm | #114 |
Heptode
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Re: Pilot Little Maestro restoration project.
HT resistor heat sink, Bit close to the speaker
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11th Nov 2021, 3:51 pm | #115 |
Dekatron
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Re: Pilot Little Maestro restoration project.
Surely the HT surge limit resistor can be done with a simple axial wirewound resistor under the chassis? Adding a great heatsink seems a bit OTT to me!
Any chance of a circuit of what you've actually got now / trying to do? A 33R resistor in the heater chain would be pointless as a surge limiter but might make sense as a pilot lamp bypass.
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11th Nov 2021, 5:39 pm | #116 | |
Heptode
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Re: Pilot Little Maestro restoration project.
Quote:
Jay
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11th Nov 2021, 6:42 pm | #117 |
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Re: Pilot Little Maestro restoration project.
Like below, then?
Is the 33R just there to trim the heater chain current from what the bare 4.2uF lets through?
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11th Nov 2021, 8:27 pm | #118 | |
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Re: Pilot Little Maestro restoration project.
Quote:
Not getting new post notifications for some reason
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11th Nov 2021, 9:05 pm | #119 |
Heptode
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Re: Pilot Little Maestro restoration project.
Hoping to have her playing over the weekend provided the valves are ok and theres no open or shorted coils.
Still have the speaker to patch up, Tear from the voice coil right up to the edge of the speaker , Can see me tuning in a station and the speaker turning to dust
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11th Nov 2021, 9:08 pm | #120 |
Dekatron
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Re: Pilot Little Maestro restoration project.
I guess it just limits the potential capacitor charging current peak maybe?
Seems a bit pointless, it's lower resistance than the series heater chain itself.
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