6N1P Bass DI - Revisited

[andyfromdenver]

I pulled all my old posts so i don't confuse matters for the folks and detract from CJs scheme drawing, and to not annoy the vets.
If peeps feel inclined they can delete their now context-less responses...

In other news!  The all-enclosed version (AC heaters with hum balance and tube inside) is aaalmost done with the *patented term by me alert  :* "measure thrice, drill nice"
10x5x3 flimsy Hammond chassis  , but it will sit up-right and be a lil' DI rock.

*edit in: this chassis is a cool size, but is too flimsy for anything other than being babied at home.  Seek out a stronger chassis . edit in 2: DO NOT ORIENT YOUR TRANSFORMERS THUSLY IF USING PT269ex and stand-up CJ OT, see pic below for sonic perfection*

 

[andyfromdenver]

I did want to add this pic with info relevant to triodes in parallel, cause online info is scant.
ref: Designing Tube Preamps For Guitar and Bass- M. Blencowe
        RDH4

This is just the starting point, to chart the slope prior to -grid voltage relative to cathode. For beginners and intermediates who want to tailor the bias point and calculate V gain, Miller capacitance, valve output impedance, etc.

any corrections or additions welcome

*edit in: add to my little column of parallel vs. single that gm (transconductance) doubles.  *

 

[dmp]

In other news!  The, all-enclosed version (AC heaters with hum balance and tube inside) is aaalmost done with the *patented term by me alert  :* "measure thrice, drill nice"
10x5x3 flimsy Hammond chassis  , but it will sit up-right and be a lil' DI rock.

Do you get any 60 Hz hum on the output from the transformers being too close together? I built one up similar to yours - transformers were a little closer together - and I was getting hum on the output.

 

[My3gger]

Hi everybody... the more I check this and previous thread about this DI more confused I'm becoming... in the previous schematics there were two 200R and 470uF (each on its half of the tube) and now in the new one just one. Also the current was twice as big as now. Is this really ok? I know CJ said this is corrected regarding to the measurements... did anyone tried this version? Compare to the previous one?
I have to know this because depending on the current (which is now also different - half the previous one) I'll know how much voltage drop will be on the resistors - so I can choose the correct secondary voltage (the difference between 170V and 200V secondary is quite big).
Am I complicating too much? Should I just build it and see what happens? 


p.s.: what happened wit andy's posts?  ???

It would help us if you add links to posts, external links, maybe even post # when there are two similar schematics on same page. Quote or attachment is another option.
Iirc, there was a bit of confusion with one of DI's schematic having two cathode resistors and cathode current written very close to one triode. Compared to revisited schematic it probably contained error regarding current through each triode. Datasheet shows it at 10,5mA typical, max ~20mA (could be for pulses/transients).
Revisited schematic shows 8,36mA through both triodes, this should help you find out power transformer requirements. 30V difference in HT is not really a big problem for tube at 170V, difference between 8 and 16mA is.

 

[bancho]

It would help us if you add links to posts, external links, maybe even post # when there are two similar schematics on same page. Quote or attachment is another option.

yes it was in the other thread about REDDI but now I also see people made it with one cathode resistor/cap... so I'll stick with schematics in this thread.

So the PTs are now ordered...  thanks everybody!  ;D

 

[andyfromdenver]

Do you get any 60 Hz hum on the output from the transformers being too close together? I built one up similar to yours - transformers were a little closer together - and I was getting hum on the output.

Hi DMP, no hum.
it's not as easy as with spkr matching OTs (4,8,16, etc.), but I always check with headphones on the secondary to ensure it is in a flux dead-zone with the PT.
Are you going in line level with the LVL knob at max? 
If I crank the following stage (ex: mixer) I can get to the noise floor, but that is way too hot for normal use.

I've done three varieties now, ac and dc heaters, one with PT outside enclosure too, as long as the transformers are oriented right, they are all super quiet.

hope you get it where you want it

Andy

 

[andyfromdenver]

On the subject of inputs, if one is forgoing the xlr/ 1/4" combo-jack.
Here is a (large  )  pic of my cliff jack wiring, one switched and one not (for the "thru").

aaaand on this subject, my heart wants to use 1M..., but I have settled for 560-680k... the Russian datasheet sez 1M and another sheet too, aaaand the *red object* lists 1M inpt imp on their specs. However the Svetlana brief datasheet that I used above lists 500k as the max....

 

[ruffrecords]

On the subject of inputs, if one is forgoing the xlr/ 1/4" combo-jack.
Here is a (large  )  pic of my cliff jack wiring, one switched and one not (for the "thru").

aaaand on this subject, my heart wants to use 1M..., but I have settled for 560-680k... the Russian datasheet sez 1M and another sheet too, aaaand the *red object* lists 1M inpt imp on their specs. However the Svetlana brief datasheet that I used above lists 500k as the max....

Usually the grid resistance spec is for use with grid leak biasing. This circuit does not use grid leak biasing, it uses cathode biasing so you can safely use 1M.

Cheers

Ian

 

[dmp]

I totally take it at face value if you say your PT is inducing a hum in the OT, and if so, I imagine you might be thinking about rebuilding?  Or, if you have a stand-up PT, and can possibly try a 90 degree turn.

I did some tests that made me think it was the transformer orientation but I haven't looked at it since. 
Thanks for your observations  -  I will have to revist this project to try to figure out a solution.
Since I already did the metal work in the compact box, I REALLY hope I can get it to work with the close transformers. 
I have an edcor PT, but since it is the same style I don't think that would make a difference. 

 

[PRR]

> Usually the grid resistance spec is for use with grid leak biasing.

That is not how I understand it.

6N1P data does not say. Some US sheets show 1meg max grid resistor, then 10Meg in the amplifier table for Rk=0 (gridleak bias).

 

[CJ]

note that there is an internal shield that should probably be grounded,

there are a lot of 6N1P tubes out there, some better than others as far as noise is concerned,

Svetlana might be he way to go,

 

[andyfromdenver]

Huge update guys!  CJ, dmp, et al eager builders or refiners.

dmp, you got me really obsessed with the noise floor. I was able to borrow one of my other builds back when CJ was finishing the OT with horizontal feet.

I realized that those builds were, in fact, quieter with the Hammond PT 269ex.

Look at this orientation everyone. This is the absolute, super-duper-est, hifi studio fanatically quietest way to orient these two transformers. Ka-PERIOD! lol (I like to add Ka like KAPOW!!)

look back at my work in progress pic a few posts up (reply #41) when I had the OT standing. That is not the ideal orientation. The new way is much much quieter as far as flux induced hum.

So dmp! can you swing your OT this direction and re-attach?  I think you will be very pleased

 

[dmp]

dmp, you got me really obsessed with the noise floor. I was able to borrow one of my other builds back when CJ was winding the OT with horizontal feet.
I realized that those builds were, in fact, quieter with the Hammond PT 269ex.
Look at this orientation everyone. This is the absolute, super-duper-est, hifi studio fanatically quietest way to orient these two transformers. Ka-PERIOD! lol (I like to add Ka like KAPOW!!)
look back at my work in progress pic a few posts up (reply #41) when I had the OT standing. That is not the ideal orientation. The new way is much much quieter as far as flux induced hum.
So dmp! can you swing your OT this direction and re-attach?  I think you will be very pleased

I remember at one point reading about it and seeing something about that. if you look at Fender guitar amps, that's how they orient the transformers.

 

[andyfromdenver]

I remember at one point reading about it and seeing something about that. if you look at Fender guitar amps, that's how they orient the transformers.

Well, the stand up way is how Marshall did the 2204s, so really it just depends in my experience! and listening tests rule all. 
As I was saying before (may have deleted it) the headphone trick is great for guitar amps, but it's much harder to hear the hum levels with headphones and this OT.

Solution: For experimenters with other PTs, with the OT secondary output circuit built up and leaving the primary disconnected, it is much easier to plug into a mixer and listen to the hum change as you move the OT around.

Here's a little glory shot if it inspires anybody (cept it really really needs a new paint job now that i drilled the front...). if one is going to put a new exit hole in CJs OT, please use a grommet

Take care!

Andy

 

[yosh]

I just finished a build of one of a pair that I'm making and it sounds great! Though voltages aren't quite where they should be. I'm getting 6.7 VAC on my PT heater winding and 7 DC out of the rectifier. This is going into an LM317 which is obviously not enough voltage to get me my 1.25V drop needed for regulation. Then while I'm getting 250 volts AC out of my high voltage PT winding, after rectification I'm getting 350V! More increase than I expected. After the 3 2k resistors, I'm getting about 250V. For the B+ I can just order different resistor values to get me closer to 170 there, but I'm not exactly sure what to do about the heater voltage. I'm thinking about just trying AC? Any suggestions? Thanks!

 

[dmp]

I just finished a build of one of a pair that I'm making and it sounds great! Though voltages aren't quite where they should be. I'm getting 6.7 VAC on my PT heater winding and 7 DC out of the rectifier. This is going into an LM317 which is obviously not enough voltage to get me my 1.25V drop needed for regulation. Then while I'm getting 250 volts AC out of my high voltage PT winding, after rectification I'm getting 350V! More increase than I expected. After the 3 2k resistors, I'm getting about 250V. For the B+ I can just order different resistor values to get me closer to 170 there, but I'm not exactly sure what to do about the heater voltage. I'm thinking about just trying AC? Any suggestions? Thanks!

I am using a 9v heater winding before rectification. AC heaters should work well if they are twisted and oriented properly.

It doesn't sound like your tube is pulling the right amount of current. Unloaded, the B+ will be much higher. Maybe you don't have the tube hooked up to the power supply yet?  What is the cathode (pin 3 / 8) voltage of the tube? Compare to the voltages posted by CJ on the first post.

 

[yosh]

I am using a 9v heater winding before rectification. AC heaters should work well if they are twisted and oriented properly.

It doesn't sound like your tube is pulling the right amount of current. Unloaded, the B+ will be much higher. Maybe you don't have the tube hooked up to the power supply yet?  What is the cathode (pin 3 / 8) voltage of the tube? Compare to the voltages posted by CJ on the first post.

Yes, I have the tube installed    The cathode voltage is 3.45 VDC or so, which looks like about double what it should be. Is this because my B+ is too high? I'll try those AC heaters, thanks!

 

[dmp]

Yes, I have the tube installed    The cathode voltage is 3.45 VDC or so, which looks like about double what it should be. Is this because my B+ is too high? I'll try those AC heaters, thanks!

The higher the cathode voltage the more turned off the tube is. With the bias low, the tube draws less mAs, and you have less voltage drop across the power supply resistors (and higher B+). So it's all related. Instead of changing the power supply to get a lower B+, you need to figure out why the tube is biasing so low. What is your cathode resistor (s)?
If you have more cathode resistance, the cathode voltage is higher, the grid is more negative, and the tube is biased colder.  The B+ doesn't set the bias - the grid to cathode voltage sets the bias.
If your tube isn't properly heated, it won't operate correctly either - so maybe you need to get the heaters hooked up properly before the B+

 

[andyfromdenver]

yosh, +1 with dmp on going AC heaters. just keep the wiring tight and twised and wedged in a corner.

what is the plate voltage (pins 1 & 6)?

and did you use 220ohm on the cathode.

sounds like your build is in the zone to me.
though you can alter the dropping string like you stated. just the last 2k ohm will do.

If you want to experiment and leave nothing on the table, you can make a rheostat with a 500-1k ohm linear pot in series with a 100ohm resistor (so you don't ground the cathode) and vary your bias (cathode resistor) in real time, to find where you are happiest. can observe/measure change in V and I too for educational purposes

 

[yosh]

Thanks for the advice! If I lower the cathode resistor to 80 ohms then I get 1.84 VDC on the cathode. B+ is then down to 198 VDC or so. That all seems much more along the lines of what to expect, but I'm not sure why the big difference between 80 ohms and 220 as designed. Any ideas?

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.

Thanks again!