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07-05-2023, 02:13 AM
Hi Guys
Bias is set without any signal present.
We stated this several places on the forum and in the TUT-series. No signal needed or wanted, so turn the MV or Volume controls to zero.
Yes, there are methods of bias setting for lowest THD where a signal is used, but initial setting is always done without a signal present.
In a cathode-biased amp, the idle current is ideally the same as the peak current ever drawn from the supply at maximum audio output. However, almost universally we see an increase in power draw at full output because the output stage is under-sized for the PSU and OT combination, requiring a class transition from class-A to class-B. The AC-30, for example, is only 22W as class-A, but produces 30W before clipping.
In a pure cathode-biased amp - one without a bypass cap - the voltage across Rk will vary with the signal. As mentioned previously, adding a high-value Ck will reduce or eliminate this voltage variation and was the first attempt to "fix" or stablise the idle condition. 22uF does not cut it as far as stabilisation goes and you see schematics where the voltage is given for idle and full output, as say, 10V and 13V respectively.
Champ81: Note that the 450V rating you placed on Ck is completely wasteful, as I commented previously. Maybe it is what you have in stock? Definitely will do what you expect, which is as much as it does in an AC. A 100V cap is physically a lot smaller and can be a much higher value if you want good results.
Personally, I do not like cathode-bias for output stages except for tone AND set for far less than maximum power in this bias mode. For the complexity of the bias adjustment circuit, it could be a lot easier and more stable using true fixed-bias with a proper negative bias supply. In general, there will be less interaction between the individual adjustments using negative grid voltage than with positive cathode voltage.
In my previous post where I mentioned the dissipation of a 1k cathode-bias resistor being about 4mW, that is for a preamp stage. A 1k used for EL-84s et al would dissipate watts and need to be a power resistor.
Bias is set without any signal present.
We stated this several places on the forum and in the TUT-series. No signal needed or wanted, so turn the MV or Volume controls to zero.
Yes, there are methods of bias setting for lowest THD where a signal is used, but initial setting is always done without a signal present.
In a cathode-biased amp, the idle current is ideally the same as the peak current ever drawn from the supply at maximum audio output. However, almost universally we see an increase in power draw at full output because the output stage is under-sized for the PSU and OT combination, requiring a class transition from class-A to class-B. The AC-30, for example, is only 22W as class-A, but produces 30W before clipping.
In a pure cathode-biased amp - one without a bypass cap - the voltage across Rk will vary with the signal. As mentioned previously, adding a high-value Ck will reduce or eliminate this voltage variation and was the first attempt to "fix" or stablise the idle condition. 22uF does not cut it as far as stabilisation goes and you see schematics where the voltage is given for idle and full output, as say, 10V and 13V respectively.
Champ81: Note that the 450V rating you placed on Ck is completely wasteful, as I commented previously. Maybe it is what you have in stock? Definitely will do what you expect, which is as much as it does in an AC. A 100V cap is physically a lot smaller and can be a much higher value if you want good results.
Personally, I do not like cathode-bias for output stages except for tone AND set for far less than maximum power in this bias mode. For the complexity of the bias adjustment circuit, it could be a lot easier and more stable using true fixed-bias with a proper negative bias supply. In general, there will be less interaction between the individual adjustments using negative grid voltage than with positive cathode voltage.
In my previous post where I mentioned the dissipation of a 1k cathode-bias resistor being about 4mW, that is for a preamp stage. A 1k used for EL-84s et al would dissipate watts and need to be a power resistor.
