The PCC88 tube is often claimed to be a direct replacement for the ECC88 / 6DJ8 tube. However, the heater of the PCC88 is specified to operate from a set current (300 mA), while the ECC88 / 6DJ8 heater is specified for a set voltage (6.3 V). So, replacing an ECC88 / 6DJ8 with a PCC88 usually results in the PCC88 operating with 6.3 V heater supply designed for the ECC88 / 6DJ8.
Is this a problem? I read a lot about this subject on the interwebs, but I found no clear answer. So I pulled out a few PCC88 from my boxes and did some measurements. First of all, I measured the heater voltages that develop from different heater currents. The readings were taken from five old, unused Philips PCC88 tubes. The readings were quite consistent across the five samples, and I'll just show the means across all five samples. I attached the full data for those who want to take a look.
It's quite obvious that at the nominal rating of 300 mA heater current one does not get 6.3 V. Vice versa, by applying a heater voltage of 6.3 V, the heater current comes out at 263 mA, quite a bit below the nominal spec.
While 6.3 V on the heater won't kill the tube, it might affect its operation. I measured the characteristic curves and compared the same tube operating at the nominal spec (300 mA) and at a typical ECC88 / 6DJ8 situation (6.3 V):
The curves are not very different, but they are also not the same. You might not notice straightaway, but the grid voltage between the two curve sets is offset by about 0.5 V (the grid voltage labels pertain to the blue curves only, the red curves have no labels). This means the bias / operating points will be offset accordingly. For example, if the tube is biased at 100 V anode voltage and -2.5 V grid voltage, the plate current will be 10 mA with 300 mA heater current, or about 6..7 mA with 6.3 V on the heater. Also, at lower plate voltages, the curves with the nominal heater spec (300 mA) are a bit steeper than those with the ECC88/ 6DJ8 spec (6.3 V).
So, there you have it:
No, the PCC88 is not a 100% equivalent of the ECC88 / 6DJ8 due to it's different heater.
Yes, the PCC88 may work as a replacement for an ECC88 / 6DJ8 -- but I can't tell if it works well enough for you in any given application. Hopefully my measurements will help you to figure things out.
Is this a problem? I read a lot about this subject on the interwebs, but I found no clear answer. So I pulled out a few PCC88 from my boxes and did some measurements. First of all, I measured the heater voltages that develop from different heater currents. The readings were taken from five old, unused Philips PCC88 tubes. The readings were quite consistent across the five samples, and I'll just show the means across all five samples. I attached the full data for those who want to take a look.
It's quite obvious that at the nominal rating of 300 mA heater current one does not get 6.3 V. Vice versa, by applying a heater voltage of 6.3 V, the heater current comes out at 263 mA, quite a bit below the nominal spec.
While 6.3 V on the heater won't kill the tube, it might affect its operation. I measured the characteristic curves and compared the same tube operating at the nominal spec (300 mA) and at a typical ECC88 / 6DJ8 situation (6.3 V):
The curves are not very different, but they are also not the same. You might not notice straightaway, but the grid voltage between the two curve sets is offset by about 0.5 V (the grid voltage labels pertain to the blue curves only, the red curves have no labels). This means the bias / operating points will be offset accordingly. For example, if the tube is biased at 100 V anode voltage and -2.5 V grid voltage, the plate current will be 10 mA with 300 mA heater current, or about 6..7 mA with 6.3 V on the heater. Also, at lower plate voltages, the curves with the nominal heater spec (300 mA) are a bit steeper than those with the ECC88/ 6DJ8 spec (6.3 V).
So, there you have it:
No, the PCC88 is not a 100% equivalent of the ECC88 / 6DJ8 due to it's different heater.
Yes, the PCC88 may work as a replacement for an ECC88 / 6DJ8 -- but I can't tell if it works well enough for you in any given application. Hopefully my measurements will help you to figure things out.
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In some cases where 6.3V DC heating is applied, the voltage can be raised from 6.3V to 7.6V as needed for the PCC88 to get the required 300mA. If there is not enough voltage on the secondary of the transformer, schottky diodes and LDO regulators can be placed. With AC heating this is not possible.
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They are about one and the same within normal tolerances except the filaments. Current versus voltage. It is not a clever idea to mix them up as each type is for a specific purpose which is why they have different type numbers. In fact it is pretty clueless to do so. The voltage over the series/current PCC88 is simply not relevant as it is about the heater current. This saved extra windings on the power transformer for the TV set manufacturer. Often filaments were connected in series directly to mains voltage. They needed to be specified for the same current and warming up time.
Both PCC88 and ECC88 were TV tuner RF tubes. E88CC is the SQ version with guaranteed 10,000 hours lifetime (the normal ones are often also pretty sturdy). Old tube oscilloscopes used to be filled with E88CC.
The PCC88 can indeed be used as ECC88 provided the filaments will be current controlled to 300 mA. They are NOT plug in replacements for ECC88. If they were they would have been called ECC88 and vice versa. Mixing parts up without changing the filament situation will not lead to success. A better approach was done with some military tubes that simply got a different pinout from the consumer version. This made exchange of technically equivalent but mechanically way more sturdy military versions by plugging out/in consumer versions impossible to the average tube DIYer.
Both PCC88 and ECC88 were TV tuner RF tubes. E88CC is the SQ version with guaranteed 10,000 hours lifetime (the normal ones are often also pretty sturdy). Old tube oscilloscopes used to be filled with E88CC.
The PCC88 can indeed be used as ECC88 provided the filaments will be current controlled to 300 mA. They are NOT plug in replacements for ECC88. If they were they would have been called ECC88 and vice versa. Mixing parts up without changing the filament situation will not lead to success. A better approach was done with some military tubes that simply got a different pinout from the consumer version. This made exchange of technically equivalent but mechanically way more sturdy military versions by plugging out/in consumer versions impossible to the average tube DIYer.
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I don't see any particular reason to design new devices with PCC88, they are not as cheap as they used to be. We used to have here as many EI PCC88s as you want for a pittance, but now that's gone. For that money, it's probably better to get some better quality 6N23Ps instead of ECC88/E88CC.
Most of the xCC88 was used in TV-sets where series filament was the natural choice. That's why there was a lot of PCC88 left over.
Today the only new production is the JJ E88CC which by the way is less expensive then the remaining stock of NOS xCC88
and can be bought with warrany from several vendors, eurotubes.com is one in us. 6n23p is not an exact equivalent.
Today the only new production is the JJ E88CC which by the way is less expensive then the remaining stock of NOS xCC88
and can be bought with warrany from several vendors, eurotubes.com is one in us. 6n23p is not an exact equivalent.
Once I had a large quantity of Telefunken PCC88 all still in sealed boxes. These were then absolutely useless/worthless but so was ECC88 as it was/is an RF tube. In fact many scrapped NOS/NIB "TV tubes" and "RF tubes" yes by having them crushed. It were then just outdated technology "lamps" without value taking too much space. I discarded quantities of P tubes which is hard to grasp now 🙂 As the normal known ECC81/82/83 were becoming scarce/expensive some tried out ECC88 and its sudden popularity (only because it was available, no more no less!) soon presented higher demand, higher prices and scarcity. People are a flock walking behind each other copying behavior so ECC88/E88CC also became endangered species. I sold my lot to a pretty well known audio designer that made his phono preamp design with current controlled filaments by means of a current source. No one made a point of it as it had its qualities mainly due to the excellent tubes used.
Invent a good story with it, mention Telefunken, diamond logo, getter shape and such tube pilot lingo and no one will even think of differences 😀
Invent a good story with it, mention Telefunken, diamond logo, getter shape and such tube pilot lingo and no one will even think of differences 😀
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A bit off topic, but may I ask? I heard that the difference between ECC88 and E88CC is the symmetry of S and/or gm, and there was a "burn-in" procedure at the factory to make a symmetrical E88CC from slightly asymmertical ECC88 by making the better side worse. Is it true?
If you use a 10V heater supply with a 10R series resistor you can plug in ECC88 or PCC88 in the same socket and both will receive the correct heater voltage (current close enough)...
See also https://www.tubecad.com/july2000/page7.html
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P tubes have current/series filaments. It is NOT about filament voltage, it is about the right current flowing through the series connected filaments. In the past LM317 was used as current source for current/series filament tubes when not used in their typical environment.
This keeps turning up literally for decades and it is in fact quite simple. Start by comparing voltage and current sources.
https://www.allaboutcircuits.com/technical-articles/an-introduction-to-current-source/
This keeps turning up literally for decades and it is in fact quite simple. Start by comparing voltage and current sources.
https://www.allaboutcircuits.com/technical-articles/an-introduction-to-current-source/
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If 7.6V is supplied to the PCC88 heather, how much is the heating current deviation expected? It's just one resistance wire after all. I don't think CCS is mandatory.
For 10V supply, I calculated 10ohm for ECC88 (6.3V; 365mA; resistor voltage drop 3.7V) and 8ohm for PCC88 (7.6V; 300mA: resistor voltage drop 2.4V). how did you calculate?
The heating power is the same for both tubes, 2.3W approximately.
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Not mandatory but easier when using them in low tube count audio devices where usually not all tube filaments are connected in series for various reasons and (slow start) DC is preferred. In the average audio device the number of tubes is usually too low for P tube filament series connection (to mains voltage!) anyway. Please check maximum cathode voltage to filament specifications too per tube type and per application.
Filament voltage is not a parameter of relevance here, it may fluctuate as long as the current is as specified. If this is all too hard don't bother.
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I used E88CC, 6N6P and many other tubes with series connected heaters and heating 12.6V. I did not find big differences in the voltage on the heaters, 6.3 +-0.2V max. I usually select the tubes to have about the same voltage drop on both heathers (6.2-6.4V). Lifting heathers to some potential I always do if necessary.
I don't see many problems there, except in some circuits where it is not advisable to replace the ECC88 with a PCC88 without changing the heating voltage, because the operating point moves a lot. Changing PCC88 with ECC88 is easier, just need to add a series resistor.
If one already has a stock of PCC88s, he should foresee the ability to regulate the heating supply voltage, so one day he can use E88CCs and the like.
CCS is also a good idea, but rarely used. Once upon a time I used a LM723 as a voltage regulator with a current limit for tube heating.
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Take a look at the diagram in the first post. That's why I posted it.
Your attachment says "PCC88 needs 7V at 300 mA". That's not correct. The PCC88 wants 300 mA. There is not set voltage. I am pretty sure the voltage you get from 300 mA through a PCC88 heater will be different for different makes/brands/batches/samples. Just use 300 mA and don't worry about voltage if you want to be on the safe side.
A cold filament takes pretty high current if it is heated by fixed voltage. I suppose the P series lasts longer because there is a fixed heating current, whether it is cold or hot.
Filaments in series meant one less winding, lower current so thinner (and less) wire, cheaper switches, lower inrush current etc. Chances of a tube having a broken filament were also reduced with so many “resistors” in series. All for lower cost, higher reliability and longevity. These TV sets were mostly abandoned later on because of their smell not because they were defective 🙂
It worked very good in real life but even today current/series filaments are often misunderstood.
It worked very good in real life but even today current/series filaments are often misunderstood.
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Yes, I looked at the xls file now. It is similar for the ECC series, +- a few % deviation of the current on the heaters.
The filament is a single heating wire, same on ECC and PCC just slightly different resistance. It will last longer on any tube only if you have provided a limitation of the starting current. Before it burns out, the coating on the cathode will probably lose its emission.
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