I recently bought a pcb to build a hybrid tube/chip power amp. 6dj8 type cathode follower buffer feeding a pair of lm3886. Clearly the tube buffer is meant to add warmth and even-order harmonic distortion to the chip amp. My supposition is that the tube is meant to be nearly unity gain.
The relevant portion of the schematic is attached. (What? I can't attach photos?)
The designers, since they are chinese, designed around the chinese 6n11 tube. I can't find curves for this tube, but the vague internet rumors are that it's a chinese 6n1p, which would mean that it's pin compatible with a 6dj8 type but not really a 6dj8.
I've no intention of buying a 6n11 for $15. I have some 6n1p, 6dj8, and 6n23p here. I'm hoping that the 6n23p is a good choice for this application.
I've built a few valve amps based on other peoples designs but i am still essentially a tube dunce. I'm capable of not killing myself with high voltage but i don't know how to calculate the resistor values around a tube to set the operating point.
So, here is where i ask for help: Assuming that a 6n23p is a good match for this application, what are the ideal resistor values for R7, R8, and R9 at 90vdc b+? What should i consider adjusting if B+ turns out to be closer to 80v or closer to 100v?
Would i be better off with a more run-of-the-mill 6dj8 type? I've heard that 90v isn't really a great voltage for the 6n1p.
The relevant portion of the schematic is attached. (What? I can't attach photos?)
The designers, since they are chinese, designed around the chinese 6n11 tube. I can't find curves for this tube, but the vague internet rumors are that it's a chinese 6n1p, which would mean that it's pin compatible with a 6dj8 type but not really a 6dj8.
I've no intention of buying a 6n11 for $15. I have some 6n1p, 6dj8, and 6n23p here. I'm hoping that the 6n23p is a good choice for this application.
I've built a few valve amps based on other peoples designs but i am still essentially a tube dunce. I'm capable of not killing myself with high voltage but i don't know how to calculate the resistor values around a tube to set the operating point.
So, here is where i ask for help: Assuming that a 6n23p is a good match for this application, what are the ideal resistor values for R7, R8, and R9 at 90vdc b+? What should i consider adjusting if B+ turns out to be closer to 80v or closer to 100v?
Would i be better off with a more run-of-the-mill 6dj8 type? I've heard that 90v isn't really a great voltage for the 6n1p.
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Some simple math should get us in the right ball park. With a 90V supply you want the cathode to be around half that for maximum output swing - say about 45V. With 33K in the cathode lead the tube current is not likely to be much more than one milliamp (45/33) which is way too small for a 6DJ8 in a cathode follower. You should aim for something like 10mA so you could change the 33K to 4.7K. Trouble is, looking at the 6DJ8 anode curves, that would mean a rather low grid bias voltage mainly because the voltage across the tube is so small. So let's try 5 mA instead and use a 10K resistor. from the anode curves, 45V across the tube and 5mA current requires a bias of -1V - that's more like it. We now need to change the 470R resistor to drop 1V at 5mA - R = V/I =1/5m (K) = 200 ohms.
So try changing the 33K to 10K and the 470 to 200 ohms for a 6DJ8. The 6N23P is pretty close to this so the same values should work.
Cheers
Ian
So try changing the 33K to 10K and the 470 to 200 ohms for a 6DJ8. The 6N23P is pretty close to this so the same values should work.
Cheers
Ian
Thanks for the suggestion.
I should have also asked for a pointer to a good website or book to learn more about this stuff, in terms that someone bad at math can understand. I've seen recommendations for Morgan Jones' book more than once - would you agree that it's a good place to start?
About the b+ -- the schematic doesn't specify it (I added that) but the board is configurable for either 65vac tube supply (= about 90 volts after rectification and smoothing) or to use the full wave of the dual supply for the chip amp.
I have selected a 25-0-25-0 toroid for the chip amp supply which would give me about 70v, which is almost certainly too low, so I am considering an off-board voltage doubler on the top half of the AC giving me closer to 100-105v.
It sounds as though i would not likely have to adjust your suggested resistor values for 15v greater b+. All the HV caps on the board are rated 160v so i have no worries there.
I should have also asked for a pointer to a good website or book to learn more about this stuff, in terms that someone bad at math can understand. I've seen recommendations for Morgan Jones' book more than once - would you agree that it's a good place to start?
About the b+ -- the schematic doesn't specify it (I added that) but the board is configurable for either 65vac tube supply (= about 90 volts after rectification and smoothing) or to use the full wave of the dual supply for the chip amp.
I have selected a 25-0-25-0 toroid for the chip amp supply which would give me about 70v, which is almost certainly too low, so I am considering an off-board voltage doubler on the top half of the AC giving me closer to 100-105v.
It sounds as though i would not likely have to adjust your suggested resistor values for 15v greater b+. All the HV caps on the board are rated 160v so i have no worries there.
+1000.
Also see the cathode follower article ("The Heretical Preamp") on my website.
SYclotron Audio The Heretical Preamp
Wait. R9 in the above schematic is 470K - corroborated by the BOM and the printing on the pcb.
Did you mean 200R or 200K?
Easy to misplace numbers on a schematic. In any case, use his suggested resistor values in the cathode circuit (R7 and R8). The grid resistor is less critical, it can be anywhere from 10k-1M. It's bootstrapped in this circuit, so you can make the coupling cap a lot smaller if you keep the 470k grid resistor.
I did not mention R9. I was talking about the 330 ohm resistor R7 which I mistakenly wrote down as 470. My apologies. There is no need to change R9.
And by the way, Morgan Jones book is ideal for beginners.
Cheers
Ian
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I don't where you are right now with this project but I came across a tube reference guide which indicated a 6n11 equivelant to a E88CC.
6N23P is equivalent of ECC88, E88CC etc.
B+ 90V is too low. Go for B+ 200V in cathode follower (buffer). About 10mA, 11mA for E88CC (6N23P).
B+ 90V is too low. Go for B+ 200V in cathode follower (buffer). About 10mA, 11mA for E88CC (6N23P).
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Well,that's to say,practically any tube will do 🙂
To keep some tubes from oscillating there is a 1k R on the grid.
The disadvantage of cathode-bias is you have to figure out witch cathode resistor is needed.Here the cathode goes up to get the right bias automatically.
Mona
To keep some tubes from oscillating there is a 1k R on the grid.
The disadvantage of cathode-bias is you have to figure out witch cathode resistor is needed.Here the cathode goes up to get the right bias automatically.
Mona
I could probably work this out myself, but someone might have already done it: which of the two biasing methods (post1, post 13) results in less power supply noise (audible or subsonic) appearing at the output?
Yes,perhaps some noise from V+,but is that much ?
With the 33k input resistor there is a divider of 1/15 V+ to grid.On the output there is a divider Ri tube/ Rcathode from V+,much worse !
If the little extra bothers,an RC filter on the resistor grid to V+,or connect the divider somewhere on a V+ in the (better filtered) preamp.
Mona
For audio yes - in fact it will be better than that assuming a low impedance source. Not subsonics, where the 100nF cap is no longer effective - you get half the supply rail noise.Ketje said:
I suspect the post 1 circuit may be better in this respect, which would be an answer to your post 13 question. However, it would depend on resistor values and valve mu.
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