Quote:
Originally Posted by Robert Gribnau
I suggest that you try out the input stage and phase splitter using the original values first (so 180K instead of 100K, only 470 Ohm in the cathode lead of V1a and 15K for the feedback resistor; what is at the grid of V1a can stay like it is now). If that works OK, you can always try out modifications.
You could try 270 Ohm instead of 330 Ohm for the cathode resistors of the output stage. See page 58 of the book I linked to in post #8 (B+ on the centre-tap of the output transformer there is 317 V so as long as you don't exceed that the PCL86's will be fine; see page 62).
Filaments should have a proper path to ground. When the filaments are being fed from the same transformer that provides the B+, than there is some capacitive coupling between the filaments and the rest of the amplifier (that's probably why you get away with it now) but good practice is to ground them properly.
|
Over the past few days, I had worked intermittently on this single channel PCL86 PP amp. As suggested by Mr. Gribnau, I readjusted resistor values and connections as close as possible to the original Dansette schematic: R6 and R7 are 180K, R4 is 470 Ohm with R6 (positive feedback) and negative feed back from R11 connected to the upper end of R4 at the cathode of V1 triode, and the center tap of the 13 volt heater secondary winding of the power transformer was connected to the ground bus.
This resulted in less buzz when I connect my old iPhone as a music source, and output volume is slightly louder. Total current draw from B+, as measured by the voltage drop across the first 1K power resistor after the bridge, was about 50mA. So I started reducing the cathode resistors (a single R12 in Dansette schematic) of the output pentodes, from 2 x 330 to 300 Ohms. This brought a small increase in current, so I changed to 270 Ohms. Now it is drawing 60-65 mA. Screen grid current has increased from 3.5 to 4-4.5 mA per tube. I have measured voltage across the cathode resistors but I can't remember now. There was a slight disparity from calculations done on global current draw. I will measure it once more.
After connecting as above, the amp started to oscillate at a low frequency: at first I disconnected the filament ground but it didn't work, then I disconnected feedback, and oscillation stopped. I removed the 100pF capacitor that was paralleled with the 10K resistor and reconnected feedback, the oscillations were absent.
Now negative feedback had a significant effect when I connect and disconnect it. When connected the volume is reduced with more effect at high frequency sound. The bass is full and powerful. Treble is low. I tried to reduce the magnitude of negative feedback by increasing the value of R11: from 15K in the original, in steps up to 100k. Treble is louder and the reduction in volume with the NFB is less, and still no audible distortion. I hope I can make some "measurements" in the near future.
I also got more daring and reduced R1' to 1K, to increase B+ to around 285-290 V, which also contributed to increased current draw.
Should I reduce cathode resistors more, to try to increase current? May be try also to increase B+ to 300-305V?
Regards.