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| Ground_Loop:
I'm bringing an old Philco radio/record player back to life and have a question about the phono signal. Boxed in red below is the phono signal going into the RF section. I assume this is done to avoid a dedicated amplifier for the low level signal. What I don't see is how the audio portion is mixed to a proper IF to be selected out. Please explain. It looks like a neat trick. |
| SeanB:
It is not fed into the diode demodulator, but instead amplified as AF audio by the IF strip, with the transformers Z1 and Z2 being used as interstage coupling transformers. The local oscillator is disabled when playing the phono side, instead being used to bias the 7A8 as audio amplifier. After a single stage of audio amplification, probably also with the transformers providing the record de emphasis, the audio is fed direct to the volume control, with the 7C6 likely being used to provide a small amount of negative feedback to control gain and improve linearity, also likely doing more de emphasis to get a better approximation of the RIAA curve, and driving the 35L6GT power amplifier tube. |
| floobydust:
The trick is using the 7A8 as a simple triode. In Phono mode, the LO is disabled and audio bypasses the IF, gets coupled into switch WS1 (near test point A) at the volume control. So they're using the 7A8 for some gain, grid2 at pin 3 as a plate and switching in a cathode resistor for bias. The turntable's cartridge, with octals Loctals it seems to be low output in the era. Later decades you would see high output ceramic cartridge used. |
| Ground_Loop:
Thanks for the explanation. I'm going to inject some test signals when I get it back up. I'm curious how much amplification that first stage provides. I'm by no means a tube expert, but I never would have thought one of the grids could be used as a plate. |
| bob91343:
It does seem that T1 acts as an audio transformer. Very strange circuit. |
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