yeah, it will run at a lower (or in fact any) voltage. However, you need to look to where the most linear part of the curves are. I haven't looked in any detail, but I understand that the 12A*7 runs more linearly at a higher voltage.
Take a look at the curves and see what you think.
As far as the biasing is concerned, I'd look to put around 500k on the top resistor, 390k on bottom resistor and the pot in the 100k range. This makes sure you are always negative bias but gives a good range of adjustment to play with.
Make sense?
Take a look at the curves and see what you think.
As far as the biasing is concerned, I'd look to put around 500k on the top resistor, 390k on bottom resistor and the pot in the 100k range. This makes sure you are always negative bias but gives a good range of adjustment to play with.
Make sense?
That schematic is from the drawing I made while designing the circuit 10 years ago. I believe my actual B+ is more like 285V and the circuit values may be tweaked accordingly. You don't need as much B+ if you drop the active load section. Since it is a cathode follower, it has no gain and so it really doesn't have much of a sound per se.
Ashton,
That is very true, plus if I start with a higher voltage I can always go down with out having to change power transformers, right?
Am I correct in my understanding that R2, R3 and R4 are setting the bias?
R3 is a pot that has its variable output going to the grid. Is this servering the same function as the pot in your circuit. I am going to build this on a turret board so I can always rebuild it, but which do you feel would be the better circuit to play with first?
rknize,
What part of the circuit is the active load? In a perfect world it would not have a sound of its own, but we all know that there is no such thing as perfect. 😀
I am doing this to better understand audio circuit, especially valves.
That is very true, plus if I start with a higher voltage I can always go down with out having to change power transformers, right?
Am I correct in my understanding that R2, R3 and R4 are setting the bias?
R3 is a pot that has its variable output going to the grid. Is this servering the same function as the pot in your circuit. I am going to build this on a turret board so I can always rebuild it, but which do you feel would be the better circuit to play with first?
rknize,
What part of the circuit is the active load? In a perfect world it would not have a sound of its own, but we all know that there is no such thing as perfect. 😀
I am doing this to better understand audio circuit, especially valves.
yes t othe higher voltage. Yes to the bias resistors. Yes, in both our circuits R3 is varying the bias voltage to G1 and hence the operating point of the tube. Both circuits are doing essentially the same thing - just using different methods to get the bias centralised relative to the cathode.
On yours, for any given setting of the pot R3 the bias will always be at the same % of supply voltage regardless of the applied voltage. This is because its a voltage divider network across the entire supply voltage. This has obvious advantages if you are "messin' wiv its mind" a bit.
On my setup, the bias is derived as a function of the current flowing in the cathode circuit. If you change supply voltage, or other circuit parameters that change the cathode current, bias will change. Cool as long as you know...
Build 'em both and see what the difference is- its only a few resistors that are different so its no real cost!
In Russ' circuit the tube S2 BELOW the cathode follower S1 is acting as the load (instead of using a resistor to do this). This tube is effectively a CCS (constant current sink) and it alters its conductance to maintain a constant current flow through the circuit. The 630R resistor is setting bias on S1.
On yours, for any given setting of the pot R3 the bias will always be at the same % of supply voltage regardless of the applied voltage. This is because its a voltage divider network across the entire supply voltage. This has obvious advantages if you are "messin' wiv its mind" a bit.
On my setup, the bias is derived as a function of the current flowing in the cathode circuit. If you change supply voltage, or other circuit parameters that change the cathode current, bias will change. Cool as long as you know...
Build 'em both and see what the difference is- its only a few resistors that are different so its no real cost!
In Russ' circuit the tube S2 BELOW the cathode follower S1 is acting as the load (instead of using a resistor to do this). This tube is effectively a CCS (constant current sink) and it alters its conductance to maintain a constant current flow through the circuit. The 630R resistor is setting bias on S1.
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Yep...what he said. Triode doesn't make the best CCS, but it gives the spare section something useful to do, which gives you a tad more gain (0.97 in my case) and supposedly lower distortion (though I never measured it).
That circuit and the other one posted on my site for that preamp (a mu follower) are designed using the procedure laid-out by Morgan Jones. He used a 6DJ8 and I used a 12AU7 because I had a whole bag full of them. This was about 10 years ago and they were pretty much the first tube circuits I ever "designed". More like following a cook book, but it was a great learning experience. I strongly suggest you get his book. I have the 2nd ed, but I hear the 3rd is even better.
That circuit and the other one posted on my site for that preamp (a mu follower) are designed using the procedure laid-out by Morgan Jones. He used a 6DJ8 and I used a 12AU7 because I had a whole bag full of them. This was about 10 years ago and they were pretty much the first tube circuits I ever "designed". More like following a cook book, but it was a great learning experience. I strongly suggest you get his book. I have the 2nd ed, but I hear the 3rd is even better.
Ashton,
I will definitely build both, was just wondering if one had a pro/con over the other. As you said they are doing the same thing just in a different way.
Ok, I saw a lot of circuits that had two valves and wondered what the 2nd tube was for. When I get a little bit more time with the circuits I will look at redoing it with two tubes.
Russ,
I will definitely look for Morgan's book, several other people have told me about him.
I will definitely build both, was just wondering if one had a pro/con over the other. As you said they are doing the same thing just in a different way.
Ok, I saw a lot of circuits that had two valves and wondered what the 2nd tube was for. When I get a little bit more time with the circuits I will look at redoing it with two tubes.
Russ,
I will definitely look for Morgan's book, several other people have told me about him.
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