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Old 26th August 2021, 08:47 PM   #31
OldHector is offline OldHector  Sweden
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Join Date: Jun 2018
Location: Täby, Sweden
The power supply has been adapted, and the voltages are as predicted, so that is a step forward. The plate voltage on the PL84 is still 216V, and now the ECC81 B+ is 275V, the plate has increased by 30%, 175V and 95v on the two anodes. I think this has filled out the sound a bit.

The next step is connecting the feedback. I tried a 5.1K resistor in series with a 390pF for starters, similar to the standard SPP, but I ended up with a mild hum, scratchy volume control, and some sort of crackling, or oscillation. The scratchy volume control is a sign of some grid voltage, so that probably means I need to calculate the resistor value a bit more smartly.

I am fairly confident it is not the wiring of the OPT - I have experienced the howling that can result from the OPT being wired out of phase, and it is not that type of noise.

Getting closer!
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Old 27th August 2021, 08:14 AM   #32
OldHector is offline OldHector  Sweden
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Join Date: Jun 2018
Location: Täby, Sweden
Should have been 'I tried a 5.1K resistor in parallel with a 390pF for starters' ...
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Old 26th September 2021, 01:35 PM   #33
OldHector is offline OldHector  Sweden
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Join Date: Jun 2018
Location: Täby, Sweden
I could not get the feedback to work at all with the normal FB resistor value (5.1k). I'm a long way from understanding this. The FB resistor injects a proportion of the AC voltage from output signal to the cathode of the driver, and I would have thought that the ratio was related to the operating point of the driver tube and the output to the loudspeaker, which in my build should be more or less the same as a standard SPP.

In the end I used a 100k potentiometer and selected a point where the sound seemed nicest. This turned out to be around 43k, so I ended up fitting a 47k resistor just so I can move on to another project, and enjoy using the amp in the mean time.

The sound now does feel more precise, crisper, and the range is bit fuller, but it feels a little bit like designing using chicken entrails; it would have been nice to have been able to derive a value by simulation or calculation. Not having my oscilloscope does hamper things a bit.

I have a 490pF mica capacitor to fit at some point, but I wanted to listen to a variety of stuff first before trying that.
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