I have been looking for an improved bias method on my D3a phono preamp to avoid the cathode capacitor.
The four D3a tubes were previously bridged with 1000uF electrolytic capacitors. The grid bias voltage is 1.5Volt for all 4 tubes.
The tubes are all parallel DC heated.
I have connected the positive terminal of the Rod Coleman regulator to one side of the heater and the other side of the heater is connected to the cathode. From there a common resistor goes to signal ground. The negative terminal of the Rod Coleman regulator is connected to the signal ground.
The calculation of the resistor is:
1.5V (Bias) / ( 4x 0.315A (Heater Current)
1.5V / 1.26A = 1.2 Ohm
I use a Powertron metal foil resistor that is screwed to the cabinet.
The sound has improved enormously with this measure. Next I will improve the C3o driver tubes of my power amps in this way as well.
The four D3a tubes were previously bridged with 1000uF electrolytic capacitors. The grid bias voltage is 1.5Volt for all 4 tubes.
The tubes are all parallel DC heated.
I have connected the positive terminal of the Rod Coleman regulator to one side of the heater and the other side of the heater is connected to the cathode. From there a common resistor goes to signal ground. The negative terminal of the Rod Coleman regulator is connected to the signal ground.
The calculation of the resistor is:
1.5V (Bias) / ( 4x 0.315A (Heater Current)
1.5V / 1.26A = 1.2 Ohm
I use a Powertron metal foil resistor that is screwed to the cabinet.
The sound has improved enormously with this measure. Next I will improve the C3o driver tubes of my power amps in this way as well.
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In this case I used a regulator for 801a
6.3V + 1.5 V = 7.8V and 1.26A the 801a have 7.5V and 1.25A so its a perfect match
6.3V + 1.5 V = 7.8V and 1.26A the 801a have 7.5V and 1.25A so its a perfect match
It's a good resistor to work with.
I have used this Bias method successfully when I was experimenting with (indirectly heated) pentodes (example: 6BW7) for driving power tubes.
I have used this Bias method successfully when I was experimenting with (indirectly heated) pentodes (example: 6BW7) for driving power tubes.
I am going to try Filament Bias (read, no cap) with my C3g. The resistor value is:
1.35V (bias) / 0.375 (heater current) = 3.6 ohm
I have a 10 watt 3.6ohm resistor coming my way this week. I will report back.
1.35V (bias) / 0.375 (heater current) = 3.6 ohm
I have a 10 watt 3.6ohm resistor coming my way this week. I will report back.
Hi Rod
I use a Lundahl filament choke and 47000uf capacitor in front of your regulator. Do you think its benefitial to use the choke in common mode?
When you are feeding a low value resistor (<30Ω for example) a large common-mode choke can be helpful to reduce any leakage current from the mains transformer (PRIMARY → SECONDARY leakage capacitance).
But if the load connected to the regulator is higher value Ω, and the Common-mode choke has high impedance (at 50-100Hz) it is possible that the load (wiring, resistor, IDHT heater) circuit can acquire some electromagnetic fields (from other trafos, inductors).
If you have any problem with trafo leakage capacitance (>50pF, for example) using an R-core trafo, or shielded audio-grade Power Trafo (including shielded toroidals) - is a better solution that common-mode impedance, for lowest noise.
for differential noise, the regulators will remove almost all !
Cagomat - can the IDHT C3g (triode mode) be used with a filament bias? One of the posters on the other thread that IDHT can not be filament biased. Yes, your response on this thread suggests other wise. If the IDHT filament bias can work, please share how the low ohm value resistor is connect (without a bypass cap).
Whatever this is, it’s not filament bias. If it works so much the better. Another way to avoid the pesky cathode capacitor.
I tried this today and it did not work with AC filament supply. HUGE hum. My current chassis does not have enough room for the power transformer needed for the colement filament regulators, so I won't be able to try DC at this point.
Hello Pat,
If you want to try this with my regulator:
1. Change the Regulator's R1 to 2.7Ω 3W wirewound. This will set the current to ca. 370mA at the centre of the trimmer range.
2. Install the filament bias resistor. Buy 3.9Ω as well as 3.6Ω if you can.
Your connexion drawing is correct.
3. Mount the regulator with power transistors cleanly attached to the chassis;
4. Apply 12 to 14V DC to the input of the regulator.
5. Adjust the regulator trimmer to get 6.15 to 6.30V on the C3g heater pins.
If the bias voltage is not quite correct (for heater voltage somewhere in this heater-voltage range), add parallel resistance to the filament bias resistor to get more plate current, or add series resistance to get less plate current.
Thanks Rod! helpful.
My challenge is the space in my chassis. Not enough to take the AC transformer and the Coleman raw DC supply board. So, what do you think about using one of these devices for the DC supply? This fits within my chassis. These would feed your filament regulator. Would the Coleman filament regulator filter out the noise from these units? According to one of the specs on these units, the DC noise is 150mV p-p. Thoughts?
https://www.amazon.com/MEAN-WELL-IRM-30-12-miniature-module/dp/B00YMA6WTC/ref=sr_1_34?dchild=1&keywords=miniature+ac+power+supply+12v&qid=1626002596&sr=8-34
https://www.amazon.com/NOYITO-Precision-100V-264V-Step-Down-Ultra-Small/dp/B07FNFJTHH/ref=sr_1_33?dchild=1&keywords=miniature+ac+step+down+transformer&qid=1626002752&sr=8-33
https://www.amazon.com/Halogen-Converter-Electronic-Transformer-Adapter/dp/B071W2XC82/ref=sr_1_83?dchild=1&keywords=miniature+ac+step+down+transformer&qid=1626002851&sr=8-83
My challenge is the space in my chassis. Not enough to take the AC transformer and the Coleman raw DC supply board. So, what do you think about using one of these devices for the DC supply? This fits within my chassis. These would feed your filament regulator. Would the Coleman filament regulator filter out the noise from these units? According to one of the specs on these units, the DC noise is 150mV p-p. Thoughts?
https://www.amazon.com/MEAN-WELL-IRM-30-12-miniature-module/dp/B00YMA6WTC/ref=sr_1_34?dchild=1&keywords=miniature+ac+power+supply+12v&qid=1626002596&sr=8-34
https://www.amazon.com/NOYITO-Precision-100V-264V-Step-Down-Ultra-Small/dp/B07FNFJTHH/ref=sr_1_33?dchild=1&keywords=miniature+ac+step+down+transformer&qid=1626002752&sr=8-33
https://www.amazon.com/Halogen-Converter-Electronic-Transformer-Adapter/dp/B071W2XC82/ref=sr_1_83?dchild=1&keywords=miniature+ac+step+down+transformer&qid=1626002851&sr=8-83
The 150mV pp is a differential-mode noise, and is no problem. Connecting the Coleman regulator to this will reduce the audio-band noise to the microvolt region, and provide the high-impedance drive to the filament required to keep the audio-signal across the filament from being crushed or mutilated by the switching converter's output.
The only thing to watch is the leakage current specification. In the Meanwell IRM-30 data sheeet:
https://www.meanwell-web.com/content/files/pdfs/productPdfs/MW/IRM-30/IRM-30-spec.pdf
we see a value of <0.25mA at a line voltage 277VAC. This is relatively good for a mains→DC unit, and will reduce for 120V line voltage.
The leakage is common-mode noise, and can carry wideband noise. This current will head for safety ground through the shortest path, so it's important to make sure that there is a low - inductance safety-ground connexion right to the driver tube ground-point.
This precaution will reduce the risk of the noise getting into the ground of your DAC, or running through DAC→ Amplifier cables, which may be degraded by wideband noise.
The IRM-30 look good enough and low enough cost to be worth a try. The Halogen lamp drivers are likely to be lower quality.
Trying to follow this calculation of the heater bias R...
Hi Rod, banpuku,
1. O.K.
2. banpuku's drawing in post#9 indicates an operating point of 170 Va-k and 19 mA Ia. Siemens plate characteristics put the bias voltage at -2.25 V, so Vk is 2.25 V, and is the voltage across the bias resistor. Total current through the resistor is 19 mA Ia plus 370 mA for the heater, so 389 mA. Then, Rbias = 2.25/0.389 = 5.8 ohms. No?
3. - 5. No comment
Hi Rod, banpuku,
1. O.K.
2. banpuku's drawing in post#9 indicates an operating point of 170 Va-k and 19 mA Ia. Siemens plate characteristics put the bias voltage at -2.25 V, so Vk is 2.25 V, and is the voltage across the bias resistor. Total current through the resistor is 19 mA Ia plus 370 mA for the heater, so 389 mA. Then, Rbias = 2.25/0.389 = 5.8 ohms. No?
3. - 5. No comment