I was looking at the attached schematic then read the write up and noticed the schematic shows a plate choke of 100H with 850r. Below is a portion of the write up that says the plate choke should be 15mh with .5r. So 100h down to 15ma and 850r down to .5r. That seems like too big of a difference.
"A choke acts like a passive current source. Of course one could use an active CCS like 317 and co but remember at 3A, there a lot of heat dissipation to handle there and the chips would not less long this way. So I went ahead and use a choke instead. I recommend that the inductance be increase but this is what I have you instead. Values around 50mH would be better. However remember to reduce power loss the resistance should not be too big and should be rated for at least for 4Amps. Resistance of less than 0.5 R must be used. But the value of 15mH works fine too. Preferably the choke should be interleave and section to increase its capability to filter the HF hash emitting from the charging of the large filter cap that is needed to be used."
"A choke acts like a passive current source. Of course one could use an active CCS like 317 and co but remember at 3A, there a lot of heat dissipation to handle there and the chips would not less long this way. So I went ahead and use a choke instead. I recommend that the inductance be increase but this is what I have you instead. Values around 50mH would be better. However remember to reduce power loss the resistance should not be too big and should be rated for at least for 4Amps. Resistance of less than 0.5 R must be used. But the value of 15mH works fine too. Preferably the choke should be interleave and section to increase its capability to filter the HF hash emitting from the charging of the large filter cap that is needed to be used."
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Since the passage you quoted mentions a current of 3A, which is definitely more than that driver stage is running, I'd guess the passage refers to a choke in the filament supply and not the plate choke shown in the schematic.
From the data sheet of the 12GN7A, it looks like the tube is intended to run at anode currents well below 100 mA, so I'm not sure where the writer gets 3-4 A from.
Plate chokes tend to be upwards of 100 H, if not more. I suspect the actual value is related to the output impedance of the tube. One would probably want rp to be significantly lower than the impedance of the inductor at the lowest frequency of interest (so 20 Hz for audio circuits). Also note that the ESR of the inductor forms an LR filter with the inductance. Hence, for the inductor to work as an inductive load, the ESR must be significantly lower than the impedance of the inductance at the lowest frequency of interest.
Tom
Plate chokes tend to be upwards of 100 H, if not more. I suspect the actual value is related to the output impedance of the tube. One would probably want rp to be significantly lower than the impedance of the inductor at the lowest frequency of interest (so 20 Hz for audio circuits). Also note that the ESR of the inductor forms an LR filter with the inductance. Hence, for the inductor to work as an inductive load, the ESR must be significantly lower than the impedance of the inductance at the lowest frequency of interest.
Tom
Thanks Guys. I think you are right that the passage is for the GM70 heaters not the plate chokes. I have a pair of Hammond 156c chokes I'll use for the plate of the 12GN7A.
The Hammond 156C is rated for 8mA Maximum DC Current. Mind the cathode potential will be at 0,8 volt with R4= 100 ohm. Easily overdriven with actual input voltages..
Magnequest and Lundahl both make chokes that would be relatively suitable for use in this circuit. (Not identical to specified, but quite workable..)
I read this a number of times but don't understand the meaning. Can you explain it in simple terms that I can understand.
Tube grid potential -almost- negative compared to cathode.
Anode (and most grids) current flowing through cathode resistor make cathode potential to Uk= R4*Ia. If (first) grid is in zero potential (AC grounded via grid resistor), it is negative compared to cathode.
I cathode potential is 0.8V, when signal exceed it, the grid-cathode "diode" will be conduct, grid current would be flowing. In most cases distortion would be rising.
Anode (and most grids) current flowing through cathode resistor make cathode potential to Uk= R4*Ia. If (first) grid is in zero potential (AC grounded via grid resistor), it is negative compared to cathode.
I cathode potential is 0.8V, when signal exceed it, the grid-cathode "diode" will be conduct, grid current would be flowing. In most cases distortion would be rising.
As you can see 12NG7a datasheet, this (pentode) tube optimal operating range is between 100-400V and 5-100mA.
I think it's not Hammond 156C (8mA) friendly tube. 🙂
I think it's not Hammond 156C (8mA) friendly tube. 🙂
I've found the Hammond 126 series to be really good value just used as plate chokes, not full interstages. They're bifilar wound and quite substantial. For full interstages I like Lundahl, esp. LL1660.
I've used the 126C for 26 and tubes with higher Rp, with its max current of 15mA. And the 126B is great for 4P1L with its max. current of 30mA.
I've used the 126C for 26 and tubes with higher Rp, with its max current of 15mA. And the 126B is great for 4P1L with its max. current of 30mA.
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