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69.71.1.1
I have a preamp that originally used a 12ax7 as a cathode follower. The tube is cascaded: ie both sides are tied in parallel. After reading up on cathode followers in a Naval Preferred circuits Manual, I have decided to use a 6BK7B or a 5687 instead of the 12ax7. Have any members used these tube as a cascaded cathode follower? I'm also interested in applying a negative voltage to the cathode. I would like to hear from any members who have had any experience with speeding up the negative traveling waveform. All I need is .775 RMS AC volts out into an output transformer that is (1200 to 600 ohm impedance). Is a white cathode follower faster in it's square wave fall time than a standard (non totem poled) cascaded cathode follower? Tektronix used a 6DJ8 cascaded cathode follower in a scope preamp with a negative voltage (-125V) applied to the cathode through a 5watt 22K resistor. There was CONSIDERABLE CURRENT DRAW. It would seem to me that transconductance affects rise-fall time or am I wrong? Thanks, Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Follow Ups:
I'd be interested in knowing what the application is for your cathode follower (especially why you are using the output transformer).
The 5687 is a nice choice of a tube, and using a negative rail can allow the grid of the cathode follower to be at ground potential (which may or may not be necessary).
Thanks! My preamp is a long way (30+ feet-large room) from the power amps and I want balanced lines. I have several transformers I can try: UTC O8 which is 15000 to 600 ohms,+8dbv or a Freed/Scully that is 1:1 split 600-600.I have also seen a white cathode follower in a Wavetek oscillator that used the 5687 with 240 ohm cathode resistors. This little tube generates a lot of heat! The power amps are at the speakers. The preamp is in front of me at my listening post/chair. I have tube cad and running the numbers, there is not much difference between the four examples of cathode follower that Broskie shows using the 6BK7B + and - 150 volts across the tube: output impedance 105 ohms. I just want to know what improvement there is to be had with adding active CCSink to the bottom of the negative leg. It involves big heat sinks and I just don't know if it's worth it. Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
You don't need output transformers to run a balanced line (see Jensen Transformers white paper on balanced lines).
What is the input impedance of your amplifier?
The input jack goes to a 100K pot then to a 12ax7a diff amp. So 100K-1M, Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Miller capacitance on a 12AX7 is pretty high, and will need to be driven from a relatively beefy preamp for good bandwidth.
Still, check out the Jensen paper on getting a balanced out without a transformer. Do you need any gain in this preamp? Or are you just buffering?
Thanks! I don't like to use opamps with the + input grounded and unity gain. This is the worse case operating condition of any amplifier. A good transformer is better IMHO. Do you have a link of the Jensen article? Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
The op-amp case has nothing to do with a cathode follower. You are using a cathode follower anyway, I don't see how this has anything to do with what you're up to.
"My preamp is a long way (30+ feet-large room) from the power amps"
That's quite a distance. I'd be more concerned about the capacitance per foot than whether the run is balanced or coaxial. If you're certain the driver output Z is 105 ohms, the result can be simulated in SPICE. Have you already purchased the cable?
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Buy Chinese. Bury freedom.
Cannare star quad mic cable is what I have on hand. Thanks Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Canare L4E6S 185pf/ft. Nominal impedance 44 ohms. I tie the 4 conductors into two. But I'm open to changing. I've just had this around. Thanks, Jim
http://www.canare.com/ProductItemDisplay.aspx?productItemID=53
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
"Canare L4E6S 185pf/ft. Nominal impedance 44 ohms. I tie the 4 conductors into two."44 ohm cable only looks like 44 ohms if it's terminated in its characteristic impedance at the far end. I don't think a 44 ohm load on your preamp is the best solution, even with a cathode follower. Alternately, if the cable is terminated with a high-Z load, 10K or more, the cable will appear reactive, and you'll have to drive all that capacitance, roughly 0.0055uF for 30'. You could drive that value pretty well with a 105 ohm source, but you're likely to have problems with the transformer(s) used to balance the drive. Those transformers are designed to work into a known impedance, and their performance will become increasingly skewed as the load impedance increases. Worst case, they'll not only produce a very tilted response, there may be significant peaks and dips due to series and parallel resonances that would ordinarily be dampened with the correct load.
So, my suggestion for this application would be to simply use a single-ended cable (coax) of the lowest capacitance practical. One of the best (and most economical) for this in my opinion is Belden RG-58/U, rated nominally at 24pF/ft. That would produce 720pF total over a 30' span. Driven with a 105 ohm source, the loss at 20kHz relative to 1kHz is less than 0.001 dB. Even at 100kHz, it only increases to about 0.01 dB. In practice, this means you could in fact drive it with a much higher Z source and still achieve very good response. That would likely allow you to dispense with the expense of a negative supply and/or a very low Rp tube that needs lots of current.
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Buy Chinese. Bury freedom.
Edits: 04/02/15 04/02/15
Thank you very much! I'll look for some Belden RG58U and use the Canare for microphone use. The preamp will have both unbalanced and balanced outputs and will only use the transofmers if I have to feed my Ampex and with short cable. Did you look at the Wavetek output stage using 5687? Thanks, Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
When you buy the RG58, just be sure it has a copper braid that you can solder to, not aluminum. Nearly all of this is copper, but I do remember an off-brand some years ago that wasn't. Yes, the Wavetek output stage looks good. I installed their values into a SPICE simulator with 6SN7s, and it drives the capacitive load easily. Frequency response and distortion should both be excellent. I think this will prove to be a perfect solution for your application.
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Buy Chinese. Bury freedom.
Hey thanks! I really appreciate it. I'm not an engineer and this helps a lot! Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Holy crap, that's a lot of capacitance!
I'm using a solid state preamp now with Class A output, very low impedance. I want to try this electron tube preamp with the cathode follower output. Thanks, Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Here is the Wavetek white cathode follower using the 5687.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
With the 6AU6 in place, you'll have way too much gain for the circuit to be that useful.
In the case of the schematic above, I find White Cathode Followers to be fun to study and mess with, but quite fussy and somehow not quite as "together" as a normal cathode follower. I wouldn't expect good performance with the 5 Ohm load on the top triode.
Thank you! Yes, I'm not using the 6au6 stage, just considering the cathode follower output stage using the 5687. My first stage is a 12aY7/6072A as a diff amp. Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
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