29-04-2020, 06:11 PM
(This post was last modified: 29-04-2020, 06:12 PM by Mike Watterson.)
0 on AGC is no signal, eye largest dark. Reduce the anode resistor if the eye isn't fully open (not much more than 1/3 dark on a Y63).
AGC goes negative as signal is stronger. Once there is no dark, fully lit, a more negative AGC voltage won't do much on the eyes, though some will overlap.
Check leakage of capacitors across HT. One simple way with a parallel heater supply is remove all valves except the rectifier:
1) Measure and write down all series resistors in HT rail with power off.
2) Measure HT at rectifier cathode, first HT cap. Calculate what the other voltages should be.
3) Measure voltage across each capacitor. If it is lower than expected the capacitor is leaky.
Note that ANY leaky screen grid capacitors can dramatically lower the gain. The G2 can be used to vary gain over a 5:1 range for some valves. It's rarely done, though the so called RF gain on my Hallicrafters S72 actually varies IF and RF gain by adjusting the g2 volts on the 1T4 valves. Unlike g1 AGC, it does work on most Pentodes, not just variable mu / remote cut off types with the progressive wound g1 grid wire.
It could be that the Barker 88 is now working as well as can be expected.
I also doubt I've often seen AGC go as far as -6V. You can easily have more than -20V on an FM detector!
A very few valve sets have a slight negative bias for no signal. A car radio with the 13.6V HT may have +1 volts for no signal and certainly operates the ECH83 (if there is no inverter or vibrator, but about +12V anode) at about +1v, it's really just a rebranded ECH81. Such sets always had a pair of Germanium power transistors for audio, you need at least 80V from a vibrator or inverter for enough audio from valves on a car loudspeaker. A small portable can be only 45V HT, though won't be loud!
AGC goes negative as signal is stronger. Once there is no dark, fully lit, a more negative AGC voltage won't do much on the eyes, though some will overlap.
Check leakage of capacitors across HT. One simple way with a parallel heater supply is remove all valves except the rectifier:
1) Measure and write down all series resistors in HT rail with power off.
2) Measure HT at rectifier cathode, first HT cap. Calculate what the other voltages should be.
3) Measure voltage across each capacitor. If it is lower than expected the capacitor is leaky.
Note that ANY leaky screen grid capacitors can dramatically lower the gain. The G2 can be used to vary gain over a 5:1 range for some valves. It's rarely done, though the so called RF gain on my Hallicrafters S72 actually varies IF and RF gain by adjusting the g2 volts on the 1T4 valves. Unlike g1 AGC, it does work on most Pentodes, not just variable mu / remote cut off types with the progressive wound g1 grid wire.
It could be that the Barker 88 is now working as well as can be expected.
I also doubt I've often seen AGC go as far as -6V. You can easily have more than -20V on an FM detector!
A very few valve sets have a slight negative bias for no signal. A car radio with the 13.6V HT may have +1 volts for no signal and certainly operates the ECH83 (if there is no inverter or vibrator, but about +12V anode) at about +1v, it's really just a rebranded ECH81. Such sets always had a pair of Germanium power transistors for audio, you need at least 80V from a vibrator or inverter for enough audio from valves on a car loudspeaker. A small portable can be only 45V HT, though won't be loud!
