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Aiken "Long-Tailed-Pair" Article Q's


 
3/6/2000 4:10 AM
Morris
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Aiken "Long-Tailed-Pair" Article Q's
I'm in the process of building an amp with a Fender blackface-style preamp (using a 6SL7 instead with a 250p/.02/.02 tonestack) and a 2x6V6 output section. I was going to use a 2xEL84 output section but the guys here thought my PT might be a little stiff for that (it's a Hammond 270FX, 275-0-275). I plan to run the 6V6's in UL with no negative feedback (except for the UL of course). My last problem is the resistor/cap values in the PI, which will also be a 6SL7. I read the great article on the "long-tailed-pair" at Randall Aiken's site but I still had one question. He states:  
 
"If no global negative feedback is used, the tail resistor should be made as large as practical, with respect to the amount of current being drawn, and the desired headroom of the amplifier. This will give the best balance to the PI outputs. This resistor has little effect on gain, but a major effect on balance and headroom."  
 
OK, I don't have a scope, so could anyone help me with some ballpark values for the tail resistor in a ZNFB configuration like the one I have? Actually, help in deciding the best resistors/caps to use throughout the PI for the amp I have would be greatly appreciated.  
 
Thanks in advance for your help!  
 
3/6/2000 7:04 AM
Randall Aiken
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Aiken "Long-Tailed-Pair" Article A's
Well, since I apparently got you into this, I'll try to get you out! :)  
 
First of all, the 6SL7 has an internal plate resistance of around 44K. Using the general 2x Rp rule of thumb for the "optimum" plate load, it would be happy with an 88K load, so we'll choose 82K as the nearest standard value. Use this for the two plate resistors. This also looks good on the load line.  
 
In a normal common-cathode stage, assuming a 300V plate supply, the 6SL7 looks like it would be happiest with a bias point of around -2V, which would result in a plate current of 1.35mA and a quiescent plate voltage of around 190V. This would require a cathode resistance of 2V/1.35mA = 1.48K, so 1.5K would be the nearest standard value. This would give a voltage swing of around +/-62V, or 124V p-p.  
 
However, since you will be using this as a self-biased phase inverter, the plate-to-cathode voltage will be lower by around 80V or so, due to the required voltage drop across the "tail" resistor. Why 80V? Well, this is an educated guess, based on the fact that we want to drop around 25-30% of the total available plate voltage across the resistor, in order to make it a fairly large value, which is good for balance, as it better approximates a constant current source. We don't want to drop more than that, or the amount of headroom will be compromised.  
 
This is the equivalent of running with a plate voltage of 220V instead of the 300V supply we started with. With a 220V supply and an 82K load, the 6SL7 would be best biased around 1.25V and 0.85mA, with a quiescent plate voltage of around 150V. This would give a voltage swing of around +/-51V max, or 102Vp-p, with an 82K load. In order to achieve this bias point, a cathode resistance of 1.25V/0.85mA = 1.47K would be required. Half this value (because the resistor is shared between the two tubes) would be around 735 ohms, so a good standard value would be 680 ohms. This bias point will then result in a quiescent plate voltage of around 80V + 150V = 230V. (The 80V is the cathode voltage, and the 150V is the quiescent plate voltage relative to the cathode as determined from the plate curves).  
 
Now, the last thing to determine is the value of the "tail" resistor. Since we decided to raise the cathode by around 80V, and the total cathode current is 2*0.85mA = 1.7mA, the required tail resistor is 80V/1.7mA = 47K. This is a relatively large value in comparison to the bias resistor value of 680 ohms, so it should approximate a constant current source well enough for our needs.  
 
Use a 0.1uF cap to ground on the unused grid side, and the "standard" 1Meg grid resistors. Since the effective input impedance will be around 2Meg, the input coupling cap should be no more than around 0.005uF, for a -3dB point of 16Hz. Too high a cap can lead to unwanted "blocking" distortion, but you can probably to up to 0.022uF without too much trouble, depending on the low frequency bandwidth of the circuitry in front of the phase inverter.  
 
To recap, good values to use are:  
 
Cin1 = 0.005uF  
Cin2 = 0.1uF  
Rg1 = 1Meg  
Rg2 = 1Meg  
Rk = 680  
Rtail = 47K  
Rp1 = 82K  
Rp2 = 82K  
 
You may want to "trim" these values to taste after constructing the circuit, but without a scope,it will be a bit difficult! Let me know how it turns out.  
 
Randall Aiken
 
 
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3/6/2000 1:07 PM
Morris
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Thanks you so much! A breakdown like this would be a great addition to your article at your site.  
 
Thanks again!  
 
Morris
 
3/7/2000 6:13 AM
Randall Aiken
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You're welcome! I'm working on the article now...  
 
Randall Aiken
 
3/8/2000 3:20 AM
Morris
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After thinking about it for a bit I had a few more questions.  
 
1. Was there a reason you suggested matching plate load resistors rather than the typical Fender/Marshall values of 82K/100K?  
 
2. What are the main design differences for the PI when using or not using negative feedback to the PI? Is my desire to run UL with no feedback why you suggested such a large tail resistor (47K) compared to Fender/Marshall PI's that have feedback to the PI?  
 
Thanks in advance!
 
3/8/2000 3:31 AM
Randall Aiken
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(1) Yes, I suggested matching the two resistors in case you want to use the second input, such as for a reverb summing point, or a second channel input. If you aren't going to use it, and are just grounding the 0.1uF cap, making the first side (the inverting output) plate resistor smaller than the other side will match the gains a bit more tightly.  
 
(2) The feedback balances the outputs a bit better, and doesn't require as large a tail resistor. Once again, I assumed you might want to use both inputs (I like to!). The larger tail resistor contributes to better balance and better common-mode rejection, because it better simulates a current source, which has a very high impedance.  
 
Note also, that the required drive level to the non-feedback PI input will be approximately half that of the feedback PI for the same output clipping level. This of course varies depending upon how much feedback you are using.  
 
Randall Aiken
 
3/11/2000 9:13 AM
Randall Aiken
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Okay, the article is now up on my site! It contains a detailed explanation of the graphical load line design technique for the long-tail pair phase inverter, including pictures of the load lines superimposed on the plate curves for the 6SL7GT tube.  
 
Randall Aiken  
http://www.aikenamps.com
 
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