The data sheets are not very explicit about the constant current. The heater current / voltage values sometimes have the current value marked in some way (underlined, arrows). The point is that different data sheets may indicate different heater voltages (even from the same manufacturer), whereas the currents are always specified at 300 mA.
The heater voltages given in the PCC88 data sheets often range somewhere from 7 to 8 Volt. The PCC88 tubes I have need approximately 9 Volt to get to the 300 mA.
Allen Wright wrote this about the PCC88 heaters (and some other tubes with current-rated filaments).
The heater voltages given in the PCC88 data sheets often range somewhere from 7 to 8 Volt. The PCC88 tubes I have need approximately 9 Volt to get to the 300 mA.
Allen Wright wrote this about the PCC88 heaters (and some other tubes with current-rated filaments).
Perhaps I wasn't clear clear enough, but my previous comment concerns the question whether the eTracer should have provisions for a current controlled heater power supply. To me it makes little sense There are several reasons why:
1. Due to the positive tempco of the filament resistance, a constant current will augment the deviations on cathode temperature as a result of variations in the filament resistance. On the other hand, a constant voltage will just diminish this effect on cathode temperature.
2. Data sheets (of P type tubes) never suggest, or even oblige, to use a constant current source (CCS). They only specify that such tubes are primarily intended for series connection of the filaments.
3. In commercial radios and TV sets these tubes are never fed by a CCS. However, in most cases you will see a small series resistor, whose only purpose is to bridge the gap between the sum of filament voltages and the mains voltage. As a welcome side effect it also limits the inrush current. Anyhow, it has nothing to do with a CCS.
4. According Alan Wright: "the 7DJ8/PCC88 tube is designed for current operation, 300mA to be precise, not voltage."
But in the next sentence he contradicts himself:
"It was originally intended to be used in TV sets with all the tube heaters in series, the idea being that all the tubes heater voltages would add up near to the local AC supply voltage, and hence not need a heater winding on the power trans - ..." (underlined by me)
IOW, an AC supply voltage would be used to accomplish constant current operation? Utter rubbish! BTW, if the heater voltage wasn't specified, you never could use these tubes in such applications anyhow!
Notwithstanding that a CCS is rather disputable, some means to measure the filament current with the eTracer does make sense. For example to identify outliers or rebranded bootleg tubes or to detect other anomalies.
Bottom line: Just use a voltage source, preferable with an inrush current limiter.
Cheers, E.
1. Due to the positive tempco of the filament resistance, a constant current will augment the deviations on cathode temperature as a result of variations in the filament resistance. On the other hand, a constant voltage will just diminish this effect on cathode temperature.
2. Data sheets (of P type tubes) never suggest, or even oblige, to use a constant current source (CCS). They only specify that such tubes are primarily intended for series connection of the filaments.
3. In commercial radios and TV sets these tubes are never fed by a CCS. However, in most cases you will see a small series resistor, whose only purpose is to bridge the gap between the sum of filament voltages and the mains voltage. As a welcome side effect it also limits the inrush current. Anyhow, it has nothing to do with a CCS.
4. According Alan Wright: "the 7DJ8/PCC88 tube is designed for current operation, 300mA to be precise, not voltage."
But in the next sentence he contradicts himself:
"It was originally intended to be used in TV sets with all the tube heaters in series, the idea being that all the tubes heater voltages would add up near to the local AC supply voltage, and hence not need a heater winding on the power trans - ..." (underlined by me)
IOW, an AC supply voltage would be used to accomplish constant current operation? Utter rubbish! BTW, if the heater voltage wasn't specified, you never could use these tubes in such applications anyhow!
Notwithstanding that a CCS is rather disputable, some means to measure the filament current with the eTracer does make sense. For example to identify outliers or rebranded bootleg tubes or to detect other anomalies.
Bottom line: Just use a voltage source, preferable with an inrush current limiter.
Cheers, E.
I tend to agree with Edmond's opinion on the heater requirement for a PCC88. It is hard for me to believe that a vacuum tube introduced in year 1957 requires a constant current source to function properly. Before the semiconductor and OPAMP era implementing a CCS is not an easy task.
Anyways, etracer provides heater current reading. It is easy to adjust the heater voltage to get the desired current.
It's also possible to add a slow-start feature to the heater in the software to slowly bring up the heater voltage. I remember utracer has such a function but I am not sure how useful it is since etracer's heater supply is fully regulated DC instead of chopped 19V waves as in utracer.
Chris
Anyways, etracer provides heater current reading. It is easy to adjust the heater voltage to get the desired current.
It's also possible to add a slow-start feature to the heater in the software to slowly bring up the heater voltage. I remember utracer has such a function but I am not sure how useful it is since etracer's heater supply is fully regulated DC instead of chopped 19V waves as in utracer.
Chris
CCS's surely weren't state ot the art in the 1950ies. The heater chain in TV's was calculated as follows: The approximate heater voltages of all tubes, as given in the datasheets, were added, the difference to the mains voltage was divided by 0.3, giving the series resistor's value.
Later, when tubes' count gradually decreased, the series resistor was replaced by a diode plus a smaller resistor or a capacitor.
Best regards!
Really? Maybe in contrast to audiophools I certainly do not speak of sonic differences, but I'd consider the prolonged heat up time without any current surge an advantage w.r.t. the tubes' lifetime.
And, last but not least, P series (and U series etc.) tubes were dedicated to operate at a certain heater current instead of a fixed voltage.
Best regards!
And, last but not least, P series (and U series etc.) tubes were dedicated to operate at a certain heater current instead of a fixed voltage.
Best regards!
Sure, the absent of a current surge is an advantage and that is the only advantage.
But a constant current, as already explained, has a destabilizing effect on the cathode temperature, which might be even more harmful than a high inrush current.
No! They are intended to put the heaters in series and to connect that bunch of heaters (plus a small series resistor) to a fixed voltage, being the mains voltage.
Series connection should not be confused with CCS operation.
It's better to feed heaters with a fixed voltage and use a NTC resistor or equivalent means to limit the inrush current.
Cheers, E.
Of course not. But the statement of "use a CCS for the filament" is quite different from saying that with some tubes the filament was designed for a given current rather than for a given voltage.
We are getting off topic here. Let's just say that there are different opinions about the best method to drive a tube filament, and that it's easy to adjust the filament voltage (and the current) with the eTracer.
Well, let's have a look at how b/w TV's were designed (at least in Germany) right before the final curtain fell for tubes: The remaining ones were a PL504 (sweep output, Uf ~ 27 V), PY88 (booster diode, 30 V), PCF802 (sweep osc., reactance, 9 V), PCL805 (frame osc. and output, 17.5 V), PL95 (AF output, 4.5 V), giving a total heater voltage of 88 V. To save power and heat, a 4µ7 series capacitor was integrated, dropping 201 V, due to it's reactance. So, with respect to the mains voltage of 220 V in those days, this capacitor can be regarded as an AC CCS with sufficient accuracy.
Despite of this 'CCS' heater failure, probably due to thermal runaway, was a rather scarce event. An explanation can be seen in the self-limitng NTC characteristics of the tungsten heater filaments.
Best regards!
I hooked up my DVM to the HTR supplies of my eTracer, and I found that the voltage is about 0.3 V lower that the setting in the software. How can I improve this? After reading the manual, it seems I could just change the "compensation" value in the software, but I am not sure the offset is just due to the diode drop. Is there anything else to consider?
New article on tube failures that can only be detected by high test voltages. Comments welcome.
Why you need a tube tester that goes up to 750V?
Why you need a tube tester that goes up to 750V?
There are two values in the config menu you can play with. The voltage drop (default 0.7V) and the series resistance (default 0.3 ohms including the fuse, the 0.1 ohms current sensing resistor, the resistance of the protection diode and all the series resistance in the trace). The equivalent circuit of the heater supply can be found in the PCB manual and the voltage is calibrated before the diode as indicated in the manual. The default setting is good enough for me.
Chris
Chris
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