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constant current filament regs

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Hi Folks,

I feel like I'm missing something obvious with this constant current filament regulator issue. When I connect a constant current supply across a valve filament- eg ECC88- set to desired current -300mA- the current level is acheived at a voltage of typically around 2 volts. This seems to make sense as the resistance of the filament will be low when it is cold. Therefore how can a constant current supply ever give the necessary 6.3 volts resulting in the filament getting hot enough for correct emission?

Cheers

Matt
 
Ex-Moderator
Joined 2003
Welcome to the forum. Agreed, there's a very low starting voltage, but even so, you are heating the filament, so its temperature (and resistance) rises. After a minute, or so, it will be very nearly at full voltage (assuming that your current source has the necessary compliance to sustain 6.3V across the load). ECC88 draws 300mA, it's E88CC that draws 365mA, but only if made by Mullard.
 
Konnichiwa,

EC8010 said:
ECC88 draws 300mA, it's E88CC that draws 365mA, but only if made by Mullard.

I believe you are mistaken. Any single Datasheet I have for ECC88, E88CC, 6DJ8 and 6922 all agree with each other as to the filament requirements. They all (Philips, Mullard, Sylvania, Siemens, Valvo) list the ECC88 as having a 6.3V/365mA Heater NOT 300mA.

So please check your sources, they seem to be inaccurate.

Sayonara
 
Ex-Moderator
Joined 2003
You're quite right, I typed them in the wrong way round. My Mullard data sheets (August 1959) say ECC88 draws 365mA and the E88CC sheets (October 1959) say 300mA. I've measured both, and they tally with the data sheets. However, although Brimar 9th Ed. states 365mA for the ECC88, when I measured them, I found they drew 300mA, so yes, the data sheet is wrong.

Perhaps you'd like to measure some Sylvania or Valvo if you have any?
 
Konnichiwa,

EC8010 said:
You're quite right, I typed them in the wrong way round. My Mullard data sheets (August 1959) say ECC88 draws 365mA and the E88CC sheets (October 1959) say 300mA. I've measured both, and they tally with the data sheets. However, although Brimar 9th Ed. states 365mA for the ECC88, when I measured them, I found they drew 300mA, so yes, the data sheet is wrong.

Perhaps you'd like to measure some Sylvania or Valvo if you have any?

I don't like to measure any. I merely pointed out that the datasheets where in disagreement with your statements made.

If you had wanted to make a useful contributiona in the context of the question it might have been to note that the E-Series valves have heaters that are intended to operate from a specified voltage (also in parallel) and will draw whatever current is needed, while the various "series string" type heater valves (P & U Series are most common) operate their heaters on a current basis.

That means when using a CCS to supply the heaters of a Valve with 6.3V (or indeed 12.6V) parallel operation operation heaters the CCS should be adjusted so that after a reasonable time (I'd wait for at least 5minutes) the measured vopltage is within +/-10% of nominal, I personally would recommend the -10% point as this significantly increases lifespan.

Past that, you will likely find that the datasheets specify a heater current tolerance of +/-20% at rated current, which means a valve drawing 300mA is in effect within the +/- 20% tolerance.

Sayonara
 
Ex-Moderator
Joined 2003
Kuei Yang Wang said:
I don't like to measure any.

That just about sums it up. It may please you to make snide attacks, but by your own admission, your statements are not based on measurement.


Matt: Don't expect the curves for a valve with heaters run 10% below rated voltage to look the same as when run at the rated voltage. To quote directly from Mullard's November 1958 "General Notes" on Special Quality Valves:

Heater voltage: Life and reliability of performance are a function of the value and degree of regulation of the heater voltage. In order to achieve the maximum useful life the heater should be maintained as close as possible to its rated value, and unless specific recommendations are made on individual data sheets, designers should aim to maintain the voltage at the valve pins within +/-5% of the published nominal value. The tolerance quoted includes variations in the supply voltage.

That seems clear enough. Bearing in mind that receiving valves were designed for less exacting duty, the voltage tolerance for individual valves was relaxed to +/-7% in their "General Operational Recommendations" for receiving valves. Nevertheless, they went on to say:

The heater current of series connected valves should be within +/-3.5% of the rated value when the supply voltage is at its rated value and valves with bogey heater characteristics are employed.

Because you are in the happy position of using a CCS, you need not worry about the differing warm-up characteristics between valves, just so long as they all require the same current. I have carried out numerous measurements to verify this assertion.

Because of the Richardson-Dushmann equation, valve characteristics are very dependent on cathode temperature and therefore heater power, valve heaters were very precisely made. As an example, Mullard's E180F data sheet quotes a maximum variation of heater current at 6.3V of +/-15mA, which equates to +/-5% of 300mA.

In the end you are doing a very simple thing. You are heating a cathode to the correct temperature. So long as it is at the correct temperature, it does not matter how the electrical heating was implemented, it could have been constant voltage, constant current, or constant power. Electrical imperfections within the valve and its associated heater wiring such as stray capacitances and leakage resistances mean that differing heater strategies vary second order effects such as susceptibility to mains-borne noise.
 
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