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Old 19th May 2004, 03:31 PM   #1
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Default Biasing a cathode follower?

Hi all,

I've decided that I need to build a Buffer linestage instead of a plate follower linestage since I need no gain. Merely impedance matching. While I have designed plate followers and understand how tp bias them this will be the first cathode follower circuit I have tried. I would like to bias the 5687 at 20mA (a cathode follower sounds better at high current from what I understand) but I'm not sure about how to calculate the cathode resistor in order to achieve this. Would anyone care to give me a lesson in cathode followers? Pretty please.

Gavin
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Old 19th May 2004, 04:33 PM   #2
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This is a type of cathode follower that naive builders have done many times and has given CFs an undeserved bad reputation. You need to be dropping some serious voltage across the cathode resistor, and that means either returning it to a negative rail or biasing up the grid with a voltage divider and capacitively coupling the input.

Let's say you want to go the negative rail route. And you've got -200V available. If the grid is roughly at ground, the cathode will be roughly at ground (within a few volts). So for a desired current, you size the cathode resistor according to Ohm's Law. Want 20 ma? Then for a 200V drop across the resistor, R = 200V/20ma = 10kohm.
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Old 19th May 2004, 06:21 PM   #3
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Yeah, the way I do something floating like that is instead of a grid leak to ground, I use two leaks, one to +V and the other to ground.

Tim
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Old 19th May 2004, 06:23 PM   #4
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Quote:
Originally posted by Sch3mat1c
Yeah, the way I do something floating like that is instead of a grid leak to ground, I use two leaks, one to +V and the other to ground.

Tim
That works fine, but you do have to capacitively couple the input. Depending on the application and one's religion, that may or may not be a disadvantage.
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Old 19th May 2004, 07:45 PM   #5
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Quote:
Originally posted by Sch3mat1c
Yeah, the way I do something floating like that is instead of a grid leak to ground, I use two leaks, one to +V and the other to ground.
Yes, sometimes, you're forced to do it that way, but it does inject HT noise directly into the grid. A quieter approach is to add a small cathode bias resistor in series with the main cathode load, then take a 1M grid-leak from there up to the grid. Not only does this give better rejection of HT noise, but the 1M resistor is bootstrapped, making it appear much larger, giving a higher input impedance, which can sometimes be useful.

Bootstrapping is positive feedback, so if you use it, a grid-stopper resistor is a good idea. A grid-stopper is a resistor soldered between the grid and all other grid connections. It should be soldered so that its body is only 2mm away from the grid pin (the inductance of even a short bit of wire is significant at VHF). 1k is usually sufficient, but if you don't have an oscilloscope (to check that it worked), 4k7 might be a good idea. Use a carbon film resistor (lowest inductance at RF).
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Old 19th May 2004, 08:23 PM   #6
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Hi,

The best way to hook up a CF for a buffer in a pre amp is the old fashioned way of using a good quality choke IMHO. You donít need 20 mA, 5 mA to 10 mA will suffice. The idling current depends largely on the load resistor which is usually high ( >100K ).

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Old 19th May 2004, 09:30 PM   #7
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The circuit that Pjotr provided is an EXCELLENT starting point for a buffered volume control.

Replace the I/P coupling cap. and grid leak resistor with a 100 K Ohm pot. Obviously, the wiper of the pot. gets connected to the grid stopper resistor. You can replace R3 (the cathode bias resistor) with a stack of 3X red LEDs. Such a stack places the cathode at a "perfect" 6 V. above the grid. The full 2 V. RMS O/P of a CDP is 5.66 V. peak to peak. With 6 V. of grid bias, a CDP will not clip the grid circuit.

The inductive reactance of the loading choke at 20 Hz. should be at least 3X Rp. A good alternative to a loading choke is a constant current sink (CCS). You can construct a CCS from either a MOSFET or a pentode. In pentodes, the 6AU6 is fine for modest current, while the 7054 is good for currents too large for the 6AU6 to handle. A single "bottle" solution to a CCS loaded CF is the 6BM8. The triode in the 6BM8 is 1/2 a 12AT7. The power pentode in the 6BM8 will LOAF at 12AT7 currents.

Increase the O/P coupling cap. to 4.7 muF., as that value accomodates low (10 K) I/P impedance loads.
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Old 19th May 2004, 10:14 PM   #8
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I guess I should have been more specific about my needs. I am using a Parasound preamp with my SE EL84 amp at the moment. I drive the amp and a M&K subwoofer amp with the preamp. I wanted to go all tube but I was concerned that a anode follower circuit would not drive the two parallel loads (plus I don't need all of the gain). THat is why I wanted to use a buffered volume control. With the low output impedance of a buffer I wouldn't be concerned about the lower than optimal load that it would be operating into. I may have this all wrong and my original preamp circuit would work fine. Opinions?

Gavin
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Old 19th May 2004, 10:21 PM   #9
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Your SE EL84 amplifier probably has quite a high input impedance, but what's the input impedance of the M&K amplifier? There's no point in having a wonderful CCS or choke load if it's shunted by an amplifier having 5k input impedance...
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Old 19th May 2004, 10:27 PM   #10
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I have no idea. The amp is integrated into the subwoofer cabinet and I lack the equipment to test for input impedance. I may have to stick with a SS preamp. THere is a McCormack I'm looking at.

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