• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Can plate chokes be THAT different?

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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."
 

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tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
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
 

euro21

Member
Paid Member
2004-09-17 12:16 pm
Budapest
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.
 
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