08-18-2014, 08:06 PM
I think I need to let it sit for a bit while I think about things. New voltages are puzzling to me for several reasons:
80: Filaments=5v, plates=716v.
Heater voltages throughout are now running around 6.3v for all other tubes.
42: plate=265, screen=282, grid= -17, cathode=.7mV
6A7:plate=286, screen=76, anode grid=135, control grid=2.9 (positive voltage), grid cap was negative, but the voltage fluctuated up to -400mV and back down to like -5mV (kept cycling).
78: plate=283, screen=76, suppressor and cathode (pins 4 & 5 are connected)=2.9, grid=0, grid cap was much like the 6A7.
75: triode plate=193, diode plate=-4, cathode=.2mV, grid=.4mV, grid cap=-1.5
I'm not getting negative voltage on the control grid of the 6A7, the fluctuating voltage could be the Klein DMM being cheap, but it is worrisome. Plate voltages are too high now for the 78, 42, 75, and 6A7...not to mention the 80. The 80 and the 42 get pretty hot to touch just while I'm doing the voltages. The shielded tubes do not. I pulled the 80 and tested it on the Triplett 2314...it showed in the 70s for both tests. I might try a different 80 and a different 6A7 a bit later.
What would be the effect of a shorted trimmer (the 66 has plate/mica trimmers mounted along the back of the chassis)? Or perhaps a shorted tuning gang trimmer? They would all measure as open if they were shorted, right? Should I be hearing buzz/hum when touching grid pins on several tubes with the DMM probes? Also, getting a soft buzz touching the plates on the 80. Still no white noise, but the 42 and 75 seem to be operating OK, I get a loud buzz touching the ungrounded lug of the volume pot and a pop removing the 42. I looked at resistances for R44 and for R28 & 29, all are fine.
How much of the voltage is explained by the difference in equipment used to measure? This is why I like sets from the 20s.
80: Filaments=5v, plates=716v.
Heater voltages throughout are now running around 6.3v for all other tubes.
42: plate=265, screen=282, grid= -17, cathode=.7mV
6A7:plate=286, screen=76, anode grid=135, control grid=2.9 (positive voltage), grid cap was negative, but the voltage fluctuated up to -400mV and back down to like -5mV (kept cycling).
78: plate=283, screen=76, suppressor and cathode (pins 4 & 5 are connected)=2.9, grid=0, grid cap was much like the 6A7.
75: triode plate=193, diode plate=-4, cathode=.2mV, grid=.4mV, grid cap=-1.5
I'm not getting negative voltage on the control grid of the 6A7, the fluctuating voltage could be the Klein DMM being cheap, but it is worrisome. Plate voltages are too high now for the 78, 42, 75, and 6A7...not to mention the 80. The 80 and the 42 get pretty hot to touch just while I'm doing the voltages. The shielded tubes do not. I pulled the 80 and tested it on the Triplett 2314...it showed in the 70s for both tests. I might try a different 80 and a different 6A7 a bit later.
What would be the effect of a shorted trimmer (the 66 has plate/mica trimmers mounted along the back of the chassis)? Or perhaps a shorted tuning gang trimmer? They would all measure as open if they were shorted, right? Should I be hearing buzz/hum when touching grid pins on several tubes with the DMM probes? Also, getting a soft buzz touching the plates on the 80. Still no white noise, but the 42 and 75 seem to be operating OK, I get a loud buzz touching the ungrounded lug of the volume pot and a pop removing the 42. I looked at resistances for R44 and for R28 & 29, all are fine.
How much of the voltage is explained by the difference in equipment used to measure? This is why I like sets from the 20s.
Charlie in San Antonio
