SE or PSE amplifier with 6N6P driving the 300B tube

[rotaspec]

Make sure pin 9 (shield) of 6N6P is grounded or you may have hum problems.

 

[sefa]

Hi Koonw,

First, thank you very much for the work and all support so far for the schematics for 6N6P driving the 300B

I have some question for this project as I'm planning to move forward with it and maybe tuning little if it's possible. As for the hum issue, I will check this weekend and so far I manage to decrease considerable amount of the hum when I attach two resistors for the cathode, 2x 1.5K/9W so, 750 ohm/18W and parallel I use electrolyte capacitor 220uF/100V; removing the cap, hum decreases big time.

What I want to ask you is;
- is mandatory to use resistor for the 300B cathode? I'm just thinking why shouldn't cathode connect straight to the ground?!
- Does the same design with the 6N6P, provide enough output if I want to use two 300B (PSE)? Maybe I connect another tube just to hear the difference in power..
- output capacitor 1uF - do you think the value is big? Reason I'm asking is, that mostly in this area is used 0.22uF or 0.47uF.

Please, don't get me wrong for any question as I'm still learning tube designs and trying to be informed.

sefa

 

[Koonw]

Hi Koonw,

First, thank you very much for the work and all support so far for the schematics for 6N6P driving the 300B

I have some question for this project as I'm planning to move forward with it and maybe tuning little if it's possible. As for the hum issue, I will check this weekend and so far I manage to decrease considerable amount of the hum when I attach two resistors for the cathode, 2x 1.5K/9W so, 750 ohm/18W and parallel I use electrolyte capacitor 220uF/100V; removing the cap, hum decreases big time.

What I want to ask you is;
- is mandatory to use resistor for the 300B cathode? I'm just thinking why shouldn't cathode connect straight to the ground?!
- Does the same design with the 6N6P, provide enough output if I want to use two 300B (PSE)? Maybe I connect another tube just to hear the difference in power..
- output capacitor 1uF - do you think the value is big? Reason I'm asking is, that mostly in this area is used 0.22uF or 0.47uF.

Please, don't get me wrong for any question as I'm still learning tube designs and trying to be informed.

sefa

- If you use AC supply for 300B heater you need a hum pot or at least 2 resistors to cancel the hum from supply. When you connect bypass cap there is more gain, hence more hum if presents. You need to deal with this separately by disconnecting the driver.

- The cathode resistor provides self bias for output tube. If you ground the cathode directly, you need to provide a fixed bias equal to self bias to the output tube. I send you a sch of such arrangement. You can remove the solid state and connect -ve bias supply directly to 300b grid.

- 300B has a grid current to 1.5mA at full drive swing. This sch provides 5+mA, so it's enough to drive one tube. I suggest you increase the driver HT slightly, to increase the current if there is obvious problem driving 2 tubes in parallel.

-1u and 100k forms a RC time constant down to 10Hz, so if it's the bottom end response you required, you can recalculate RC constant other than 10hz, if it's not 1u. Google for RC constant calculator for quick calculation.

 

[sefa]

- If you use AC supply for 300B heater you need a hum pot or at least 2 resistors to cancel the hum from supply. When you connect bypass cap there is more gain, hence more hum if presents. You need to deal with this separately by disconnecting the driver.

- The cathode resistor provides self bias for output tube. If you ground the cathode directly, you need to provide a fixed bias equal to self bias to the output tube. I send you a sch of such arrangement. You can remove the solid state and connect -ve bias supply directly to 300b grid.

- 300B has a grid current to 1.5mA at full drive swing. This sch provides 5+mA, so it's enough to drive one tube. I suggest you increase the driver HT slightly, to increase the current if there is obvious problem driving 2 tubes in parallel.

-1u and 100k forms a RC time constant down to 10Hz, so if it's the bottom end response you required, you can recalculate RC constant other than 10hz, if it's not 1u. Google for RC constant calculator for quick calculation.

Koonw,

Thank you for the information.

I'm using regulated DC for the heaters as I red on most websites that, 300B it's "difficult" to setup good with AC. So, I took short path on this with regulated DC.
Next days, I will have more time and will go through the things and schematics, and will see how all will go together.

Enjoy your weekend

sefa

 

[sefa]

Hi Koonw,

I manage to eliminate completely hum. Issues was, few gnd wires resistance were high and what's odd that, these wires were fresh, new never used. So, I did rewire completely and result is no hum! Anyway, still I didn't put amp altogether as I have to check for a chassis, but this can wait little bit as I didn't decide that should I go for SET or PSE.

I'm using always schematics on post #11 http://www.diyaudio.com/forums/tube...ier-6n6p-driving-300b-tube-2.html#post4681100 and I can say that, this amp performs good and maybe needs any tweaking, that I don't know, but I have few concerns when I connect two 300B tubes;

- I connect mA from 300B plate to OT and I read around 77mA. I have matched tubes 79mA and 72mA. When I connect second tube, it increases max 83mA. I though this should go double. Voltage stays the same.
What's odd that, when two 300B are in circuit, both tubes are cooler than when only one is used.

- I believe has to do with first question, what is recommended Cathode resistor - Rk value, or is any method to calculate it.

- I use 1 kOhm grid resistor, again can this be calculated.


I really appreciate all the work Koonw.


sefa

 

[Koonw]

Hi Koonw,

I manage to eliminate completely hum. Issues was, few gnd wires resistance were high and what's odd that, these wires were fresh, new never used. So, I did rewire completely and result is no hum! Anyway, still I didn't put amp altogether as I have to check for a chassis, but this can wait little bit as I didn't decide that should I go for SET or PSE.

I'm using always schematics on post #11 http://www.diyaudio.com/forums/tube...ier-6n6p-driving-300b-tube-2.html#post4681100 and I can say that, this amp performs good and maybe needs any tweaking, that I don't know, but I have few concerns when I connect two 300B tubes;

- I connect mA from 300B plate to OT and I read around 77mA. I have matched tubes 79mA and 72mA. When I connect second tube, it increases max 83mA. I though this should go double. Voltage stays the same.
What's odd that, when two 300B are in circuit, both tubes are cooler than when only one is used.

- I believe has to do with first question, what is recommended Cathode resistor - Rk value, or is any method to calculate it.

- I use 1 kOhm grid resistor, again can this be calculated.


I really appreciate all the work Koonw.


sefa

Hi, glad to see you back. You just need to add another 1K resistor, (making it a total of 1000/2=500 ohms), same wattage each as you're using for one tube. This will double the current but the cathode voltage will be the same, resulting same bias as one tube model. The grid and plate are tied together as you have done. Since cathode resistor value is halved, you can try to double the value of bypass capacitors, if bass response is weaken. For best result you'll need to add 1k series resistor to each 300b grid, for better sharing of drive current.

 

[sefa]

Hi, glad to see you back. You just need to add another 1K resistor, (making it a total of 1000/2=500 ohms), same wattage each as you're using for one tube. This will double the current but the cathode voltage will be the same, resulting same bias as one tube model. The grid and plate are tied together as you have done. Since cathode resistor value is halved, you can try to double the value of bypass capacitors, if bass response is weaken. For best result you'll need to add 1k series resistor to each 300b grid, for better sharing of drive current.

Ok.

I just remember something now and forgot to mention on previous email.

Resistors for cathode (Rk), I use parallel 2x 1.5 kOhm (9W) which equals 750 ohm.
Now, when I apply same resistors for two 300B tubes, resistance drops down to 375 ohm; 750/2=375.
This happens as result of the heaters power supply which is single rail ~15 VAC about 4 Amps and each tube is connected with regulated LM1084 circuit.

What happened before (when I had hum issue) that, if all Rk are connected as I describe above, I can see the 300B tubes are driven hard and I can see plate/anode getting red as well voltage dropped down - cant' remember.

thanks
sefa

 

[bekim]

Hi all,

Recent days I have try the design as is shown on post #11 and, I can say it's really good comparing to other tubes I used as driver for 300B tubes.
My setup was almost complete and I am using regulated P/S from Neurochrome Audio: Precision high performance audio circuits for the DIY market. audio, 21st Maida regulator ver. 2.0, so tuning voltage to 380 VDC is done without any problem, and regulator does it job perfectly well, no hiss or hum, just clean sound.

@sefa

As Koonw stated in email, you can use 2.2K resistors for grid on your PSE configuration.

@Koonw

I'm curios, if more current is applied on 6N6P, will be any benefit on this design?


thanks

 

[sefa]

Hi all,


@sefa

As Koonw stated in email, you can use 2.2K resistors for grid on your PSE configuration.

@Koonw

I'm curios, if more current is applied on 6N6P, will be any benefit on this design?


thanks

Thanks bekim.

2.2 Kohm resistors I have applied for the grid. Looking other schematics, at first I thought I should use 2W resistor, but 0.5W or even 1/4W resistor is just fine, or at least I didn't notice any heating of the resistor.
I'm using same schematic which Koonw provided on post #11.

sefa