15-09-2021, 09:50 PM
Well I have managed to get somewhere, I did a bit of static testing on the two ECL86's I have both down with the best one providing 30mA Anode current rather that 36mA at 250 Volts and -7 on the grid, good enough for me.
I reduced the HT to 225 Volts (anode and screen) and worked out what I would need to get around Ia = 15 mA with the better valve, it also turned out to be -7 volts on the grid, or close anyway. Ig2 was = 3mA at this level So this set the cathode resistor to 390 Ohms, most other values as per the Data sheet on the valves.
I did have a major issue of self oscillating at something is excess of 20KHz greatly influenced by where my hand was in proximity to the transformer above the chassis, the only way I could stop this was with a snubber arrangement across the primary, started off with a 0.01uF going as a last test before I hit my night-time deadline of 0.01uF in series with 330 Ohm.
I do have feedback on board and the response is reasonable to say 5KHz and then starts to drop to half level around 9KHz, the feedback values seemed to have very little difference to the response.
Peak power before distortion gets to be harsh is at 1.6 Watts in to 3 ohms speaker this is with a 300mV peak to peak signal input. I have yet to add a 10,125Hz LC filter for the TV.
One thing that did surprise me and I would appreciate thoughts on this, the audio transformer has standard interleaved E & I laminations and I was expecting it to have all E laminations through the bobbin with I's at one end with a paper or card spacer, obviously not?
Adrian
I reduced the HT to 225 Volts (anode and screen) and worked out what I would need to get around Ia = 15 mA with the better valve, it also turned out to be -7 volts on the grid, or close anyway. Ig2 was = 3mA at this level So this set the cathode resistor to 390 Ohms, most other values as per the Data sheet on the valves.
I did have a major issue of self oscillating at something is excess of 20KHz greatly influenced by where my hand was in proximity to the transformer above the chassis, the only way I could stop this was with a snubber arrangement across the primary, started off with a 0.01uF going as a last test before I hit my night-time deadline of 0.01uF in series with 330 Ohm.
I do have feedback on board and the response is reasonable to say 5KHz and then starts to drop to half level around 9KHz, the feedback values seemed to have very little difference to the response.
Peak power before distortion gets to be harsh is at 1.6 Watts in to 3 ohms speaker this is with a 300mV peak to peak signal input. I have yet to add a 10,125Hz LC filter for the TV.
One thing that did surprise me and I would appreciate thoughts on this, the audio transformer has standard interleaved E & I laminations and I was expecting it to have all E laminations through the bobbin with I's at one end with a paper or card spacer, obviously not?
Adrian
Learning as I go!
Youtube EF91 Valve
Youtube EF91 Valve
