The problem with the 15K resistor is that it's always 15K, regardless of frequency. The 150H choke is about 19K at 20Hz, then that falls off. We can size the parallel feed capacitor to present more impedance as well as that happens to keep the tube lightly loaded. Above 20Hz, the impedance of the 150H choke goes up dramatically, and the impedance of the primary of the output transformer looks more like a constant reflection of whatever is loading it (provided the load isn't too far off the specifications of the secondary).
With the 15K resistor, the load on the tube is always less than 15K. If you wanted to use a plate load resistor, I would suggest a 47K plate load. This will bring your B+ up around 500V I believe. The Hammond 156C is about $15 USD, why not use a pair?
With the 15K resistor, the load on the tube is always less than 15K. If you wanted to use a plate load resistor, I would suggest a 47K plate load. This will bring your B+ up around 500V I believe. The Hammond 156C is about $15 USD, why not use a pair?
High value of plate load resistor 47K make B+ too high for me to do. I saw schematic of Chinese 3A5 Preamp in Aliexpress use 33K plate load resistor with B+ 175V to get 70VDC on 3A5 anode.
If resistance on anode can be varied depend on frequency of input, Choke 150H is easier.
Local shop has 140H 8mA 4360R which should be enough for 130VDC 8mA instead of 100VDC 10mA. B+ will be 165VDC.
Thank you very much Mr. Audiowise.
Dear Mr. Minhaj if provide Parallel 1.4V to 3A5 instead of 2.8V how the schematic of Filament part would be. Because I also use 2.1VAC pin 1+7 of my 4P1L PP instead of 4.2VAC.
Just curious of 1.4V filament to be design. Thank you in advance Mr. Minhaj
If resistance on anode can be varied depend on frequency of input, Choke 150H is easier.
Local shop has 140H 8mA 4360R which should be enough for 130VDC 8mA instead of 100VDC 10mA. B+ will be 165VDC.
Thank you very much Mr. Audiowise.
Dear Mr. Minhaj if provide Parallel 1.4V to 3A5 instead of 2.8V how the schematic of Filament part would be. Because I also use 2.1VAC pin 1+7 of my 4P1L PP instead of 4.2VAC.
Just curious of 1.4V filament to be design. Thank you in advance Mr. Minhaj
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supernut,
Post #42 -schematic is for line stage, it can be ref for HP amplifier. Even for line stage you can do much better otherwise, as audiowise mentions.
Post #43 -.5watt is not output power it is total dissipation at idle, I don't know exactly but it should be at least 3/4 times less than that.
Post #46 -my filament supply is parellel, two stages, 1st stage is well filtered regulated voltage source followed by 2nd stage high impedance constant current source.
post #47 -5814 alone can drive 50 ohms HP is loaded with a proper line out transformer or in parafeed transformer arrangement.
post #48 -wrong schematic please don't try it, you can not bias the DHT this way.
Post #42 -schematic is for line stage, it can be ref for HP amplifier. Even for line stage you can do much better otherwise, as audiowise mentions.
Post #43 -.5watt is not output power it is total dissipation at idle, I don't know exactly but it should be at least 3/4 times less than that.
Post #46 -my filament supply is parellel, two stages, 1st stage is well filtered regulated voltage source followed by 2nd stage high impedance constant current source.
post #47 -5814 alone can drive 50 ohms HP is loaded with a proper line out transformer or in parafeed transformer arrangement.
post #48 -wrong schematic please don't try it, you can not bias the DHT this way.
I guess 680R is important. I ever saw in web 4p1l PSE also use 17R resistor to ground resistor need more watt 50 watts. Due to large current flow through it. Not sure it is called filament bias? I guess the voltage before that resistor need to be deducted from anode voltage. From mr. Audiowize i think it should be same 135v anode - 5vdc above 625R we get 130vdc which same value in datasheet. I learn a little by little. So fun to learn from people.
but the value of resistor in another web is use by formula of R_{FIL}=\frac{V_{GK}}{I_{F}+I_{K}}
which is not equal to 625R but 10.95R
see link at Bartola UK
Filament Bias: a practical example with 3A5 DHT – Bartola(R) Valves
which is not equal to 625R but 10.95R
see link at Bartola UK
Filament Bias: a practical example with 3A5 DHT – Bartola(R) Valves
sorry again not correct. You need to connect pin1 and pin7 with a piece of wire and apply voltage across pin4 and shorted pin 1 and pin7. If we assume DC resistance of plate choke is 4000 ohms and b+ is 135 volt then passing 10ma current plate voltage will be 95 volt (4000x.01-135). 1.5uf cap need to be 4.7uf. 10K:50r will be better than 15K:50R. Why don't you go for a good output transformer such as 10K:50r gaped for 10ma dc current and be happy been simpler!
By the way yes this filament bias I was talking earlier, best sounding biasing method for my ears.
Regards
By the way yes this filament bias I was talking earlier, best sounding biasing method for my ears.
Regards
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