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Old 10th Nov 2021, 8:37 pm   #101
ms660
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Default Re: Pilot Little Maestro restoration project.

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Ok i'm hoping to get the dropper wired up soon,. Thinking of using some spare tie points on the 25Z4 Rectifier to mount the dropper parts, Ideally pin 6 to heater, Is pin 6 internally connected to the valve ?
Pin 6 = NP which normally means no pin 6 is fitted to the valve base.

Lawrence.
Well it is fitted on mine, Get what your saying now about the limiter power,



Jay
Can you post a photo of the under side of valve base showing all the pins?

Lawrence.
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Old 10th Nov 2021, 8:43 pm   #102
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Pin 6 = NP which normally means no pin 6 is fitted to the valve base.

Lawrence.
Well it is fitted on mine, Get what your saying now about the limiter power,



Jay
Can you post a photo of the under side of valve base showing all the pins?

Lawrence.
Was looking at the wrong valve, Eyes not what they were, Yea pin 6 is missing...
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Old 10th Nov 2021, 8:55 pm   #103
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Default 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.
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Old 10th Nov 2021, 8:58 pm   #104
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Default Re: Pilot Little Maestro restoration project.

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Thanks sideband. Was just going by what i read online, I ended up buying the caps mentioned in my last post,


Jay
Yes well on-line is not the same as practical experience. Decca used a ceramic audio coupling capacitor in the DM2C TV in 1958 (and probably all the derived types) and I've come across them in radio's as well. None of this was Hi Fi stuff of course and there may be problems using ceramics in quality amplifiers (though I can't quite see what). However for general audio they should be fine.

There is a lot of 'pseudo-science' buzzing around the Web so beware.
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Old 10th Nov 2021, 8:59 pm   #105
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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.
Got a big finned heat sink that'll bolt to the space where the canned smoothing caps used to be.

As for the surge limiter, One schematic says 33 ohm,3 watts.. I have 50 ohms 5 watt ceramic.?
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Old 10th Nov 2021, 9:10 pm   #106
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Thanks sideband. Was just going by what i read online, I ended up buying the caps mentioned in my last post,


Jay
Yes well on-line is not the same as practical experience. Decca used a ceramic audio coupling capacitor in the DM2C TV in 1958 (and probably all the derived types) and I've come across them in radio's as well. None of this was Hi Fi stuff of course and there may be problems using ceramics in quality amplifiers (though I can't quite see what). However for general audio they should be fine.

There is a lot of 'pseudo-science' buzzing around the Web so beware.
Yea iv'e noticed mis-information a few times online, Google ain't Gosspil, Cheers for the info.


Jay
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Old 10th Nov 2021, 9:41 pm   #107
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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.
No problem, for a more accurate figure for the power dissipated by the rectifiers surge limiter (without using a true RMS meter) you can use Schade's curves in conjunction with the procedure given in RDH4 Part 5 for calculating the rectifiers RMS current (and thence the power dissipated in any series resistance):

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.
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Old 10th Nov 2021, 9:49 pm   #108
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Default 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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Old 10th Nov 2021, 10:38 pm   #109
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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 ?
I have been referring to the rectifiers surge limiter....the one that's connected between the mains input and the anodes of the rectifier.

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.

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Old 10th Nov 2021, 10:44 pm   #110
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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 ?
I have been referring to the rectifiers surge limiter....the one that's connected between the mains input and the anodes of the rectifier.

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.
Ok theres only one resistor between the mains and the rectifier anode ? (470ohm) ? I was thinking there may have to be another one preceding the 470 ohm, But if i'm right the 470 ohm IS the surge resistor



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Old 11th Nov 2021, 12:05 am   #111
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Default 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 ?


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Old 11th Nov 2021, 11:14 am   #112
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Default 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.
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Old 11th Nov 2021, 11:41 am   #113
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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.
Cheers simon,

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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Old 11th Nov 2021, 1:53 pm   #114
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Default Re: Pilot Little Maestro restoration project.

HT resistor heat sink, Bit close to the speaker
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Old 11th Nov 2021, 3:51 pm   #115
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Default 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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Old 11th Nov 2021, 5:39 pm   #116
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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.
the circuit i'm working from is in post 1 Except the HT Resistor will be 470 ohms and 10 watts, Not the 120 ohm / 7 watt pictured, Lamp bypass will be 15 ohms 5 watts, The heatsink won't be the size pictured above, It'll be cut down to fit, Depending on the heat produced i might just bolt it straight to the chassis. Just chucking some ideas about..


Jay
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Old 11th Nov 2021, 6:42 pm   #117
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Default 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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Old 11th Nov 2021, 8:27 pm   #118
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Like below, then?


Is the 33R just there to trim the heater chain current from what the bare 4.2uF lets through?
Possibly, But on the page i took the image from its referred to as a surge limiter.

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Old 11th Nov 2021, 9:05 pm   #119
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Default 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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Old 11th Nov 2021, 9:08 pm   #120
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Default 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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