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In the schem shown above, I am using a 6dj8 for a driver. In the tube manual the pin out is numbered such that you would think that the second half of the tube is on the left and the right half is the the first section. How do I know for sure? I assume it makes a difference when hooking things up?
Edits: 07/31/17 07/31/17 07/31/17Follow Ups:
if you are making a stereo amp, the phase splitter could be a different tube. The 6DJ8 is great for that first gain stage. The splitter isn't going to impart tonal qualities to the circuit. a 12AU7 would fit in there in which case the resistors would be 22K and not 30K.
6DJ8 are desirable and spendy tubes. I wouldn't waste it on a splitter.
The circuit looks kinda like the WIlliamson.
See link for final resolution of the problem. Also see references to using 6.3 vac instead of 12 in that same thread. It works well and that is what I am now using.
Edits: 08/11/17
what did you do with pin 9?
are you mindfull of heater cathode potential difference in this circuit when using 6dj8?
I see now that the problem was in the layout, but why is it they show different values for the two triodes in the tube manual if they are both the same? They show a unit one and a unit 2. That is what made me think that one side must function as a driver and the other as an amplifier.
I guess the first clue is they show the Class A info as per each unit, but why is the other info different?
Edits: 08/01/17 08/01/17 08/02/17
To add to what CB said below, the 6dj8 was developed for RF work with the two triodes in cascode (one triode on top of the other).That's why the data sheet refers to each of the two triodes the way it does.
That has nothing to do with us using the 6dj8 as a dual triode for audio work.
Just like with a 12ax7, each triode in a 6dj8 is the same.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/01/17
Now I understand. That clears up the confusion for me and it all makes sense now. Each day I learn something new. My thanks to all.
Edits: 08/02/17
Like Tre mentioned, the datasheet gives various capacitances for each half separately, which isn't applicable to what you're doing. For audio frequency applications, each triode inside the bottle is identical.
Thanks per above.
Got things sorted out a bit. For the schem shown, the pins shown in the tube manual as 6,7 & 8 are for the left hand side of the tube and pins 1,2 & 3 are for the right hand side of the tube as shown. I know this because if you hook them up the other way 'round the gain is very mucdh reduced. So I guess if you hook it up the other way you are trying to run it backwards?
Still need to get the voltages adjusted as the plate to cathode voltage is still 150 vdc. May be time to try blackplates suggestions and see what that does.Curious to see how it sounds with the proper voltages.
Edits: 08/01/17 08/01/17
The 6dj8 has two triodes that totally independent except for a shared heater.
They are identical except for small differences in the inter-electrode capacitance's that will make no difference in your application.
The image above shows that one of the two triodes uses pins 6,7 and 8 with pin 6 being the plate and pin 7 being the control grid and pin 8 being the cathode.
The other triode uses pin 1 for the plate, pin 2 for the control grid and pin 3 for the cathode.
Other than layout, it will not matter which triode you use for which stage of the amplifier.
I hope that made sense to you. If not, please ask questions.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
You are getting good advice from a lot of guys who are much more knowledgable than I, but your comment about a difference in gain makes me wonder whether you are attending to not just which pins subserve which "side" of the tube but also which pins are attached to the plate, grid, and cathode of each section, respectively. Any good tube manual should show you a diagram of the pin arrangement in which the function assigned to each pin is labeled. You could possibly get the very different results you hint at if, for example, the plate and cathode pins for one side have been wired up backwards (plate circuit to cathode pin, cathode circuit to plate pin). So, you might want to double check that. Any error in wiring will of course make a huge difference in the result.
Yes, there is a physical left and right side of any dual section tube depending on your viewing angle. But that has nothing to do with electrical performance in a twin section tube.If the two sides are the same, such as a 12AX7, the the only differences would be minor manufacturing tolerances. And even at that the left or right side could be the favorable section depending on the application.
Now there are quite a few dual and triple element tubes with drastically different sections like the driver tube in a Dynaco ST35, a 7247, which IIRC is half a 12AX7 and half a 12AU7. In this case you do have to be careful how it's wired.
But it should not matter much at all which side of a twin tube is used provided you don't violate proper circuit layout as mentioned above. If the two sides are much different in a twin tube, I'd say you have a bad tube!
Edits: 08/01/17
I think you're making a mistake subbing the 6dj8 for the 6n1p. BTW, if you can find them the 6n1p-e flat plates from the 60's are the best.
twystd
I have 6dj8's, I don't have the others. If I get some I may give them a try.
Both sections are the same so you really don't need to know which one is which.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"you really don't need to know which one is which."
I would add that it's important not to allow wiring connected to a low-level section to intersect or cross over wiring carrying a higher-level signal (or to pass in close proximity to a higher-level stage). If you wire everything so signals travel in one direction only, starting with the input, this won't be a problem.
--------------------------
Buy Chinese. Bury freedom.
So which is the low level section?
"So which is the low level section?"Which ever side of the tube is wired as the first gain stage in your circuit. In a twin tube there is no difference outside of published tolerances.
Edits: 08/01/17
Thanks Tre' that's what I needed to know.
PJ's commentary is certainly sage wisdom.
I would add that if you adjusted the 30K plate load and 340R cathode resistor on the first stage to get 75V on the plate of the first 6DJ8 triode, then you could dump a whole lot of parts from the amp (the two 1M resistors and 0.1uF cap are no longer necessary).
I tried doing this but the volume started breaking up out at about half throttle. I was able to get the plate to cathode voltage down on the first section by changing the plate resistor and leaving everything else the same per the schem. Tomorrow I will work on the other section.
Edits: 08/01/17
You'll want to adjust both the plate load and cathode resistor together. If you increase the plate load, your bias voltage will drop (potentially a lot), and that will limit the incoming voltage into the amplifier if you go too far (what's your bias voltage currently?). Also don't forget to leave that stopper resistor between the first stage's plate and the second stage's grid.
Yeah, I still have a bunch of work to do to get it working within specs, but now that I have the layout correct it should be easier. It already sounds pretty good too.
Not quite sure I follow you on that. I like to use as few parts as possible, so I would like to understand what your reasoning if you care to explain. Thanks.
The second stage needs to see +75V at its grid, and Broskie has added in the two 1M resistors and coupling cap to provide this.
If you adjust the operating point of the first stage you can change the voltage at the plate from 150 to 75, then this plate can be connected to the grid of the next stage (leaving the 300 Ohm resistor to prevent oscillations).
To make this change, you'd want to draw some load lines. I'd recommend looking at 47K and 75K.
Thanks, I will give it a try and a listen.
Try a 75K plate load and a 530 Ohm cathode resistor. That should give you about 2V of bias and 75V on the plate.
The triodes are basically identical.
I see the circuit as shown is for a 6N1P, not a 6DJ8. Both sections are operating at around 150v plate to cathode. The 6DJ8 is rated for a maximum of 130v, and there have been many, many reports over the years of short lifetime at any voltage over 90 volts.
The 6922 is rated for much higher voltage, 220 or 250 in the data sheet I have on hand. It is otherwise similar to the 6DJ8, but the 6N1P is not similar. To use the 6922, you would still need to adjust the cathode bias resistor of the first stage to get the right voltage.
I forgot mention that I am lowering the plate to cathode voltage and making several other changes to accomodate the 6dj8, including the cathode resistor which I have at 135 ohms now. Will be working to get everything in specs for the 6dj8 as I like it's sound. Thanks for the input.
Edits: 07/31/17
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