376
Item nr.
The tube you use after the spare tube. Or before.
| Production | The Netherlands, 2014. |
|---|---|
| Tubes | PC86. |
| Cabinet | Five pin socket. |
It is possible to produce fit-in replacements for old tubes from other tubes or parts. For a rectifier, this is particularly easy, as a rectifier can be replaced by two diodes plus series resistors. 
I found out that a C443 output tube can be replaced by another tube, the PC86. This is a 4V tube with internal resistance comparable to the C443. Because the PC86 needs just 1 to 1.5V of negative bias (and the C443 about 10-20V), the bias must be reduced. Also the transconductance of the PC86 is much higher: 13mA/V against 2mA/V. So the base contains a voltage divider to reduce both bias and signal. The special design reduces DC bias by a factor of 10, and AC signal by a factor of 6. The C443 is directly heated, but the PC86 has a separate cathode, and my design brings this out in a side contact that must be grounded.
Here the same in more detail. The voltage divider has the added resistors Ra and Rb in the tube base. PC86 has its plate on pins 1 and 9, and 9 is connected to the plate pin of the base with red wire. PC86 has its heater on pin 4 and 5, and these are connected to the pins of the base with grey wire. PC86 has its grid on three pins: 2, 8, and 6. 6 is connected to 7 (cathode) through a 23k resistor, over which falls a small fraction of B- and a fraction of the signal. Pin 2 goes to the grid pin of the base through Ra of 47k. PC86 has its cathode on pin 3 and 7. 7 is for the voltage divider and 3 is connected to the side screw. There a lead connects to the ground of the radio. For DC, R6 is part of the divider and for AC it isn't.
| Obtained | 4/2014 from scrap parts. |
|---|---|
| Condition | 8. |
| Disposed | Storage (inquire if interested to buy). |
| Sound sample | PLAY SOUND I was never in Lodz in Poland so far, and would like to visit it some time. But I think the correct pronounciation sounds more like Woodz. |
The radio sounds better with the replacement. Grid detection requires a transmission to be moderately modulated. Around 1930, modulation depth was about 30%, but in the 21st century, 80 to 90% is not unheard of. With these depths, there should be some AC load in the plate circuit of the detector, and the resistors Ra and Rb form a nice load (through T2).
After placing the despair tube, a small reversable modification in the radio makes the sound better still. The 2044 speaker in the Erres radio was designed for a triode tube and the use of a penthode output looses a lot of low audio frequencies. Philips compensated for the shrill sound by placing a capacitor over the speaker, to reduce high notes as well. The triode brings the basses back, and you can disconnect the capacitor on one side to bring the trebles back as well.
With a C443 tube, the radio draws 22.7 to 22.9 W (measured), depending on the HF gain control. With the despair tube, power consumption is reduced to 20.9 to 21.2W.
I have heard that the PC86 is notorious for wearing out quick, so I should buy some stock for the decades to come. In November 2020, I used my second PC86 to replace the broke E442 tube. I soldered the triode in the socket of the defective tube, and added a 500 Ohm kathode resistor (bridged with 100nF) to prevent the current to become too high.