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PCL86 Tube substitution

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PgrAm

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I recently acquired a vintage spring reverb tank (gibbs 4FB2A1C), so I began to look around for a circuit to drive it and found this: module_reverb_schematic_full.png. It uses a pretty specialized tube, a PCL86 which is a pentode and triode in one tube. Since I already had a couple 6AU6s and 12AX7s lying around I figured it might be worth adapting the circuit to use those tubes. Unfortunately I'm pretty new to working with tubes and don't really know how to go about this, I'm fairly sure I just need to adjust the bias, but not sure exactly how. Could someone give me some guidance on this?
 

betwixt

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It will probably work as it is without adjustment. There was nothing special about the PCL86 except it's unusual 13.6V heater filament voltage, a 6.3V ELC86 was also made for conventional heater wired systems. The 'P' at the start of the device number indicated the heater worked at 0.3A and was intended to be used with other 'P' devices which had their heaters wired in series, the voltages were chosen according to the required cathode temperature while the current remained constant. It made it easier to wire up in television receivers where a 6.3V transformer would have added to costs and weight.

Brian.
 

PgrAm

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Thanks for the help. Just to be sure, aside from obviously changing the heater voltage to 6.3v, a 6au6 and 12ax7 should be drop in replacements for a PCL86?
 

FvM

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a 6au6 and 12ax7 should be drop in replacements for a PCL86?
You mean using one 12AX7 triode system and the 6AU6 pentode together to replace the PCL86/ECL86? The pentode data are different, you would at least change the cathode resistor to get the same bias point. PCL86 has higher output power respectively maximum plate dissipation, but 6AU6 should still work.

Is it a tube nostalgy project or would you consider transistor amplifiers as well?
 

PgrAm

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You mean using one 12AX7 triode system and the 6AU6 pentode together to replace the PCL86/ECL86? The pentode data are different, you would at least change the cathode resistor to get the same bias point. PCL86 has higher output power respectively maximum plate dissipation, but 6AU6 should still work.

Is it a tube nostalgy project or would you consider transistor amplifiers as well?
Yeah the idea is to replace the PCL86 with two other tubes. I'm currently trying to calculate the new value of the cathode resistor for the 6au6 but this is proving to be somewhat mind boggling. I can't seem to really wrap my head around how to bias a pentode properly. I would really appreciate it if someone could explain the math I would use to calculate the new value of this resistor. As for your last question its pretty much just tube nostalgia.
 

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The cathode resistor can be read off the control charcteristic in datasheet. You have intended Ia and Ua, determine Ugk and calculate Rk.

Check if the the plate dissipation is below the maximum rating.
 

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