SCC88/PCC88 heaters in series?

Ampdog

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I posted this problem years ago and cannot now locate the thread.

I had the inconvenient experience to discover that there is an unacceptable current variation in heaters of certain ECC88 valves. Trying to operate heaters in series I found  from 5,7 - 6,9V drop across certain heaters. This would indicate an inconsistency heater wire gauge/length!

I have not experimented with PCC88s (which would be more convenient for me) but am loathe to spend the money only to find the same inconsistency there.

Has anybody experienced this?  (It also avoids that one could use a handy string of ECC88 heaters  in series for convenience of d.c.feed.)

(I could not find specific reference to this on internet.)
 

handsome

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Most "P" equivalent tubes were designed for series usage - the Phillips datasheet specs PCC88 as suitable for series or parallel heating. At least "P" series tubes we know were manufactured back then and to spec - today's manufactured ECC88s simply could be badly made - were you using modern or ye olde dusty originals in your experiments?
 

Moonraker

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E specification : denotes 6.3V heater. 

P specification: 300mA heater.  Typically used in series where the total voltage of a long string of U tubes (100mA) would exceed the mains voltage - for instance TV sets.
 

Ampdog

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Moonraker,

Yes - but that does not explain why the ECC88 heaters should often draw currents more than 15% off the spec.value!  One would expect the heater wire to be of the same gauge!??

My point with series connecting heater(s) is that it is easier to design a high-voltage lower current d.c. heater supply than a 6,3V .d.c. high current one to e.g. avoid introduction of hum in pre-amps.
 

Moonraker

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Ampdog said:
Moonraker,

Yes - but that does not explain why the ECC88 heaters should often draw currents more than 15% off the spec.value!  One would expect the heater wire to be of the same gauge!??

My point with series connecting heater(s) is that it is easier to design a high-voltage lower current d.c. heater supply than a 6,3V .d.c. high current one to e.g. avoid introduction of hum in pre-amps.

What voltage is applied when the current draw is observed to be maybe 15% off spec?  The voltage indicated on the data sheet?  The point is, that voltage is an indicative value only and the 300mA is the overriding spec.

So ideally, one would use a 300mA current source to drive P- tubes for the correct heating power. They are tubes where heater current and not voltage is the primary spec.  The current will be correct and the voltage should then be near the datasheet-indicated value. 

If the voltage deviates, so be it.  After all, consider the analogous situation : when E-tubes are driven off 6.3V the current drawn should be at or near the indicated value but deviations are seldom reason for anxiety, and indeed seldom observed in the first place.

For even lower hum one could use the U series tubes (100mA).  Or even lower, as Neumann did with the VF14 microphone preamplifier. 




 

Ampdog

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Moonraker said:
What voltage is applied when the current draw is observed to be maybe 15% off spec?  The voltage indicated on the data sheet?  The point is, that voltage is an indicative value only and the 300mA is the overriding spec.

So ideally, one would use a 300mA current source to drive P- tubes for the correct heating power. They are tubes where heater current and not voltage is the primary spec.  The current will be correct and the voltage should then be near the datasheet-indicated value. 

If the voltage deviates, so be it.  After all, consider the analogous situation : when E-tubes are driven off 6.3V the current drawn should be at or near the indicated value but deviations are seldom reason for anxiety, and indeed seldom observed in the first place.

? ? ? ?  A thermionic valve operates basically off the intensity of electrons 'released' by the cathode. This is a function of the temperature or then 'wattage' of the heater, which consists equally of voltage and current values!

and yes, kind sir! The spec-ed voltage was applied (I did pass matric physics!  :D) The general application of 6,3V to heaters (from a transformer winding - a constant voltage source) appears to contradict your statement of current priority. A further statement that I have read, that 6V heaters can seldom be operated in series - further strengthens the impression that it is either difficult to control the diameter and/or length of heater filament wire, or something else is the matter.  One is able to connect wire-wound resistors in series with predictable results - so what be the problem here?

My initial question was thus: Why a noticeable variation at all!  Is the heater wire of such uncontrollable diameter, - or was the length not cut to a specified figure?  If that cannot be achieved there is certainly reason for anxiety!

 

marantz123

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Some deeper information about vacuum tube heaters... may shed some light... :nfi:

https://www.thevalvepage.com/valvetek/heater/heater.htm
 

handsome

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NI believe that some heaters are specced as current being the overriding factor and in others voltage are the defining factor meaning that yes, for a P valve the correct current will not guarantee the expected voltage but should give the correct wattage. Agreed that the product of these two factors should always result in the the same wattage but as we have seen significant variations can occur. I do know that some ECC88 variants actually have 365mA ratings and not the expected 300mA that the nomenclature would suggest. So it would be interesting to see what the "other side" of this conundrum (volts and/or current) is with your particular *88s. Possibly the (relatively) extreme temperatures of a heater make the comparison to a resistor less reliable.
 

gavinbirss

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Late to the party but have a related question.

The Earmax/Pro and "clones" also used the 18V AC winding to power 12AT7/ECC81 and 2X6922 heaters in series it amongst other things seems also make life on the tubes short lived.

Is it simply because of being run at starving B+?

201601070919402.jpg


202106130709310.jpg


 

Moonraker

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Ampdog said:
The general application of 6,3V to heaters (from a transformer winding - a constant voltage source) appears to contradict your statement of current priority. A further statement that I have read, that 6V heaters can seldom be operated in series - further strengthens the impression that it is either difficult to control the diameter and/or length of heater filament wire, or something else is the matter.  One is able to connect wire-wound resistors in series with predictable results - so what be the problem here?

My initial question was thus: Why a noticeable variation at all!  Is the heater wire of such uncontrollable diameter, - or was the length not cut to a specified figure?  If that cannot be achieved there is certainly reason for anxiety!

Voltage spec priority for E tubes, current for P tubes.

Why worry so much? 

Maybe running production changes necessitated an increase in current (increase in wire thickness) to ensure a given cathode temperature at 6.3V. 

No damage done if your power transformer can accommodate - and if you do not have this heater in series with a different one. 
Hence the whole point : better to apply E tubes as they were intended.  Parallel heaters...
 

Moonraker

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handsome said:
I do know that some ECC88 variants actually have 365mA ratings and not the expected 300mA that the nomenclature would suggest

The nomenclature simply suggests 6.3V for ECC88.  The 300mA is 'also specced', expected, hoped for , .. :)
 

Moonraker

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ghostinthemachine said:
In the winter of 2021, I revised an EAR 518 power amplifier that used PCC88 and PCL82 tubes in the front-end. The heaters were series connected. No issues there.

Correct.  Being P tubes, they are designed and specced and what not to operate in series.  As was done in hundreds of thousands of tube TV sets in the 1950-60s.
 
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