[maxhifi]

Hey - try change the 0.5uf coupling caps, the blip on mine was caused by a shifting operating point of the phase inverter tube following the oscillator. After warm up the bias would drift due to a leaky capacitor and the tube would go into asymmetrical clipping, causing this blip. The feedback of the circuit helped clean up the waveform at the output into just a blip but at the cathode of the phase inverter it was really ugly! If your caps were bad a while it could have caused excess plate current and damaged the plate or cathode resistors, so check all voltages in this section.

Of help to me find in the problem was to allow it to fully warm up for an hour with the heaters on, until it began misbehaving, and then look at all the waveforms with my oscilloscope. Also, set the regulated voltage, and check the plate voltage on the 6V6. Mine had toasted the three parallel 10k plate load resistors, causing two to open and one to drop in value to 600 ohms, understandably ruining the THD spec of the oscillator. I also changed the 3W lamp as mine was stuck in the socket and I wrecked it. Now the oscillator works flawlessly and THD is less than 0.2% according to my heathkit IM-58 and that may well be the heathkit's residual distortion.


Now what I really want is an R-100/URR !