6N1P Bass DI - Revisited

[dmp]

Yosh,
Are you following the schematic on the first post? With a shared cathode resistor, it is strange you need to go down to 80 ohms to get the right bias. Doublecheck your circuit to the schematic or post a picture.
Did you connect the AC Heaters to ground with 100 ohm resistors or a center tap? If your heaters were floating they will be noisier.

 

[dmp]

Getting close...
I had both the transformers standing before, but with one laying down it is very quiet. Andy, thanks for getting me going on this project again. Just need to tidy up the wiring and it's ready for the studio.

 

[andyfromdenver]

Getting close...
I had both the transformers standing before, but with one laying down it is very quiet. Andy, thanks for getting me going on this project again. Just need to tidy up the wiring and it's ready for the studio.

yes! *hi 5* you got it really tiny! nice  ;D


and @ yosh. 

it's time to look for my post on the grid curves and graph for biasing your particular amp.

This could get really deep and technical really quick and I defer to the ninja masters of the electronics world, but I'll hopefully help ya out. 
The important considerations are
1. staying below max dissipation.
2. centering (or hey! not) your bias for max swing before grid current limiting or cut off.
3. The DCR (resistance) that the OT primary presents to the circuit.

THEORETICALLY! with an ideal transformer and no regard for running your paralleled tube to it's new max dissipation (probably not good...) you would simply determine the slope of the load line given a particular supply voltage and primary impedance. Then, you simply draw a straight line up from the supply voltage to the point of max dissipation and use the graph and ohm's law to determine the cathode resistor (which sets the negative grid voltage). However, the line actually will not be straight up and down due to losses in the primary, it will slant over in actual testing as the supply voltage is reduced to the plate.

so use my graph and slope since your supply voltage is 200.

mark a point where your actual plate (anode) voltage is and the voltage drop across the cathode (1.84) as the negative grid voltage.

your signal will swing from that point along the slope to the limit of the grid voltage (0) to where it hits the horizontal axis.

this is super simplified and leaves out some details and I welcome any corrections.

I hope some of the more knowledgable will agree, it is more about how it sounds to you, because the more uneven the wave clipping is, the more distortion that is introduced, and it could be just what you are looking for? 

 

[dmp]

For the 6N1P, max dissipation is 2.2 watts PER triode. For class A you can bias pretty close to max dissipation.
As you experiment with biasing this hotter though, keep in mind  the transformer has to handle the bias current going through it. It is a transformer coupled tube design. Most tube circuits are resistor coupled with an output transformer connected after a DC blocking capacitor. 

Scenario 1 (CJ's schematic, first post): shared 220R cathode, V_cathode=1.84 V, V_plate=162 V, i=4.18 mA/triode
Power = 0.67 W/triode
transformer DC current = 8.36 mA

Scenario 2 (dirty1_1garry schematic): 220R per cathode, V_cathode=2.02 V, V_plate=162 V, i=9.18 mA/triode
Power = 1.48 W/triode
transformer DC current = 18.36 mA

 

[andyfromdenver]

Heaters are much noisier when I use AC so I might try using some schottky diodes and a low dropout regulator then perhaps I'll have enough room to get 6.3 V regulated DC.

sorry i missed this part and i apologize to ask, but did you leave out a center tap when you went AC? 

I would be so hesitant to use the the words "much noisier" when describing the difference in my experience.

I do realize that the world of pro audio is all about chasing the super percents, and what is measurable etc.

just wanted to clarify before you pursue the DC heaters.

 

[yosh]

Thank you again for sharing your knowledge! I will spend some time with those graphs and get my head wrapped around this.

I did leave out the center tap. Oops. I'm realizing how much I don't know about tube design. I think I'll be reading up a lot in the near future. I'll put the center tap in and see where that gets me. Thanks!

 

[CJ]

that transformer should handle 18 ma no problem,

18 ma time 5000T = 90 amp turns, or 113 Gilberts

figure a natural air gap from the butt stack = to about 4 or 5 mil = 1 Tesla or 10 K gauss.

core is good for 18 KG, and AC flux is only about 2 KG so you still have 6 KG of core left before saturation,

can't vouch for how long the tube will last, 

 

[andyfromdenver]

haha! great to know CJ, thank you very much for the detailed breakdown.

@ yosh, you're so welcome! I say give AC heaters another try. Does your PT already have a 6.3V winding center tap?  If not, and if you are a tube socket wiring ninja, you can fly two 1/2 W 100 ohm resistors from each of the heater pins (4 & 5) to both terminate at the shared cathode of the socket, this is safe and has convenient tie points.

But basically you'll need 100ohms from each heater end to ground, it shall be super quiet after.  If by chance you are using a 6.3V lamp, you can add the heater artificial CT off that, if there is a nearby ground. 

Time to drop some bass... j/k

Andy

To add cause I am a pics junkie: Here is a pic of my first all-enclosed, the Cobra Box *makes the devil horns gesture and headbangs for a second until i get a crick in my neck*
Sigh, I jacked up the paint job, but I etched in that cobra I have been drawing since I was a kid, probably going to sand it all and repaint.

If only Hammond made this chassis...oh...4 x thicker  , it would be stellar.
this one's mine so no grd lift and no polarity flip.

 

[dmp]

Looks good Andy, how does it sound?
Did you go for the 18mA bias or 8mA?

 

[andyfromdenver]

Looks good Andy, how does it sound?
Did you go for the 18mA bias or 8mA?

Thank you!  Mine is stock CJ scheme with higher supply/B+
~10 mA

It sounds exactly like the red object. exactly! ;-)
Also, thank you for sharing information on the circuit and biasing; very helpful!

Here is a test measurement summary (pic) and a scheme I drew up for an elevated Heater CT ref DC V.
This is for the next DIY shennanigan... A separate 6N1P EQ box so i don't exceed heater-cathode V with a cathode follower circuit.

 

[andyfromdenver]

and here is a summary I did for varying Cathode resistance. (apologies: big pic d/t small writing...) with an earlier box on, lvl max, and no connection. I interrupted the plate to primary connection with my meter, just to feel the wind in my hair  :

I did like the 100ohm version too.

I was thinking, what if you had like a 270ohm and another 180ohm on a spst switch for either 270ohm or 108ohm action.

I thiiiiink it would be audibly noticable?
can call it the "Hair" switch 

 

[yosh]

I have a finished DI and I used it all day on a tracking session yesterday. Sounded fantastic! Installing the center tap for the heater winding made it very quiet again. At the moment I'm using a 100 ohm resistor on the cathode. But I might play with that value a little more to find exactly what I like. Maybe even install a 200 ohm trimpot.

For enclosures I used Hammond P-H1441-20BK3 with cover P-H1431-20BK3. Ordered from CE Distribution. I think it was around $90 for two enclosures including shipping. They work great!

and here is a summary I did for varying Cathode resistance. (apologies: big pic d/t small writing...) with an earlier box on, lvl max, and no connection. I interrupted the plate to primary connection with my meter, just to feel the wind in my hair  :

I did like the 100ohm version too.

I was thinking, what if you had like a 270ohm and another 180ohm on a spst switch for either 270ohm or 108ohm action.

I thiiiiink it would be audibly noticable?
can call it the "Hair" switch 

I did this with a pair of tube mics that I built years ago. A switch on the power supply flips a relay inside the mic and changes the cathode resistor. The switch is labeled "Clean and Pretty / Hot and Dirty". I use them all the time 

 

[bancho]

Ok, so I'm at the final touch of this DI (I know it takes too long but anyway...) and there is some question/doubt about grounding the whole thing. For more complicated / advanced circuits I know it is very important how you make the power supply and signal grounding because of all the ground loops and things.
Is it here also that important? Can I make a single node for the power supply (RC network) and the signal circuit?

 

[My3gger]

Is it here also that important? Can I make a single node for the power supply (RC network) and the signal circuit?

It is very important, not only because you want to avoid noise on supplies, it has to be Safe in the first place. Judging by pics you showed there are a few things to inslulate, take decision about heater voltage and check where supplies are taken. + line from the end of power supply, never close to diode bridge because of interferences. You haven't mention ground on IEC and HT bleeding resistor, those are first two things to do imo and is good practice in general.

 

[damianschwartz]

Will give a go on this quite soon. Just gathering all the info i can, but i'm sure i will have to ask for some help, even though i did many DIY projects, 90% of them were Synthesizers, and no tube experience apart from fixing my old Bassman a couple of times.

I did realize the DW Fearn "All tube DI" (that they have in a studio i work sometimes) is a double channel DI sporting 1 6n1p on each channels, by Mr Fearns webpage. Bet it's not a much different design, wondering which output trafo that might be using.

 

[andyfromdenver]

Will give a go on this quite soon. Just gathering all the info i can, but i'm sure i will have to ask for some help, even though i did many DIY projects, 90% of them were Synthesizers, and no tube experience apart from fixing my old Bassman a couple of times.

I did realize the DW Fearn "All tube DI" (that they have in a studio i work sometimes) is a double channel DI sporting 1 6n1p on each channels, by Mr Fearns webpage. Bet it's not a much different design, wondering which output trafo that might be using.

You can do this project, if you can work safely w/ high voltage . The circuit is as simple as a first tube project gets.

I checked the DW site. Their DI is a different flavor. two gain stages (well, one gain stage and one cathode follower) cap coupled between and cap coupled cf to OT (read no DC on primary and no need for gap and large size).
The CJ iron 6N1P tube DI is a single ended design with no caps in the signal path. It's a single gain stage with the valve paralleled and the signal taken from the anode with the OT serving as the anode/plate load. It has to have a huge OT to handle DC on the primary, a gap, and low freq friendly specs.

The picture on the DW site is a little funny, but perhaps conveys something meaningful to the inexperienced musician.

In conclusion, CJs iron is a must. I think there is a suitable Sowter that I mentioned before and someone else mentioned a cinemag, but CJs price is 1/3 less than Sowter, and is so custom it makes me cry/smile.

Andy

 

[Hank Dussen]

I'm having some trouble with the heaters.
Without the tube I get 6,3v DC but with the tube inserted I can not get it higher then 5,9v.
The PT has 4A on this winding so that should be enough. Is it correct to have pin 4 connected to the 6,3v and pin 5 to ground?

 

[andyfromdenver]

I'm having some trouble with the heaters.
Without the tube I get 6,3v DC but with the tube inserted I can not get it higher then 5,9v.
The PT has 4A on this winding so that should be enough. Is it correct to have pin 4 connected to the 6,3v and pin 5 to ground?

Hi Hank. your connection, as stated, is correct for DC heater operation.  Question: what is the AC voltage of the heater tap directly off the power transformer (PT)? perhaps it is not high enough or perhaps the rectified DCV is not high enough?  If you are using a PT with a 6.3VAC winding, you could perhaps just use AC heaters, however at lightly loaded and with wall voltage fluctuation/ increase you may exceed a comfortable max for the tube.


keep at it! this is a cool tool!  and to update, I am getting close to having the free time to explore the 6N1P treb mid bass eq add-on for CJs awesome iron project. tbc!
Andy

edit in: I forgot to add that 5.9VDC on the heaters is perfectly acceptable imo

 

[Hank Dussen]

Hey Andy, thanks for your suggestions.
I tried 2 power transformers for this. The first had 8v on the secondary but is only rated 600mA so that's not enough after the rectifier. The second transformer I tried is rated as 6,3v (4A) on the secondary but unloaded I get 7,4v. After the rectifier that's 8,9v so that should be enough for the voltage drop of the LM317. But when I insert the tube the voltage after the rectifier drops to 7,8v and after the LM317 it drops to 5,8v.
And this is resulting in a too loud hum. Or maybe that's caused by something else. I'm trying to find out.

I'll try to run the heaters with AC. But I think I better try to drop the 7,4v a bit.

BTW, your plan for adding an EQ to this design sound great. Keep us posted!

 

[andyfromdenver]

Hey Andy, thanks for your suggestions.
I tried 2 power transformers for this. The first had 8v on the secondary but is only rated 600mA so that's not enough after the rectifier. The second transformer I tried is rated as 6,3v (4A) on the secondary but unloaded I get 7,4v. After the rectifier that's 8,9v so that should be enough for the voltage drop of the LM317. But when I insert the tube the voltage after the rectifier drops to 7,8v and after the LM317 it drops to 5,8v.
And this is resulting in a too loud hum. Or maybe that's caused by something else. I'm trying to find out.

I'll try to run the heaters with AC. But I think I better try to drop the 7,4v a bit.

BTW, your plan for adding an EQ to this design sound great. Keep us posted!

Interesting.  I did DC heaters without a Vreg, I imagine you will need to adjust it per data sheet and formula. Just to clarify, if your PT has a heater CT you cannot connect it and also chassis ground post rectifier.
I implemented DC heaters with big resistors and 10k mfd caps, as an alternative.
The hum sounds like you need to chassis ground the DC heater circuit (again, not a with PT heater center tap (CT) simultaneously).
Here is a relevant link: http://www.valvewizard.co.uk/heater.html  apologies if you have already read this.  DC heaters are discussed at the bottom.
edit: here is a thread from a while back on my dc heater circuit, I made several changes when finally finished that I didn't add and the device is gone, I think it was just another 10k mfd cap, but it shows another means and has helpful input from the super-users. Just scroll towards the end when I switched to unregulated as an alternative.
https://groupdiy.com/index.php?topic=64560.0