I have finished my attempt to use the tubes, 6C5, 6SF5, and 6V6, in place of the oroginal tube lineup of 12AT7 and 50C5 in my near clone of the Heathkit GR-81. I have 150 volts on the plates instead of 115. The radio seems to work ok, but I would like some input on the voltage requirements on the cathode and the grids of the above mentioned tubes as compared to the oroginal tubes. This is my first working radio, and I really like this "simple circuit" radio. Will someone look at the online schematic of the GR-81, and, if you are familiar with the 6V6, 6SF5, and 6C5, would you leave the resistor and cap values alone, or would you change the values on all three tubes? If you are knowledgable about radio circuits, I would like your thoughts on my project. It may be best for me to leave well enough alone, since the radio works pretty well! Thanks, Lenny

Just one thing: the original GR81 design uses half of a 12AT7 for the first audio. Your 6SF5 is more of a 12AX7 triode, higher mu, higher plate resistance. For that reason, I would expect the cathode bias resistor there to be in the 1k to 2k range, rather than the 270R Heathkit chose for the 12AT7. The other values (470k grid resistor, 100k plate resistor) are typical for a high mu audio triode. The component values for the audio output stage are typical, I think they can stand as they are.

How does the regenerative detector stage work? I don't like that 680k grid leak resistor Heath chose for that stage. I'd use something in the 2M to 5M range. I have a GR81 and I got quite a jump in sensitivity when I changed it to 4.7M.

Rob

Thank you for replying.The regen seems to work ok. It kicks in about half turn position of the regen pot. I believe it is doing fine! I first built the radio with two 6C5 triodes and the 6V6. Someone on this site thought that I needed a hi MU triode, so I switched to the 6SF5. I guess I could go back to the duo 6C5 setup. Lenny