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Old 18th March 2006, 01:39 AM   #11
rdf is offline rdf  Canada
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Hi arnoldc! Afraid I can't help you with a book recommendation, I've been doing audio measurements for so long the source of that knowledge is completely forgotten. My recommendation is to download the Rightmark Audio Analyzer (http://audio.rightmark.org/products/rmaa.shtml) and use your sound card to play around. For a free tool it's very powerful. Any decent audio card will work for signals within its limits. Unfortunately any measurements involving DC or high levels will require some interfacing to keep from taking out the card and/or computer.

You may already realize this, but when playing with bypass caps remember the AC impedance of the LED will be in the 10 ohm range. To completely bypass the LED at audio frequencies requires a massive cap, which is a bit contrary to the point of using a LED. In my current circuits the bypass is only intended to provide a solid shunt between grid and cathode at RF frequencies because, to be honest, that's the kick I'm on at the moment. To my ear the quality of that cap is surprisingly audible, though I reserve the right to claim is wasn't after further auditioning.
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Old 18th March 2006, 01:52 AM   #12
arnoldc is offline arnoldc  Philippines
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hi rdf, I've downloaded that software quite sometime ago but have not played around with it. I'll check.

In your experienced opinon, will the LED require cap bypass on -

a) resistor loaded
b) choke loaded

single triode section, in driver duties?
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Old 18th March 2006, 02:27 AM   #13
SY is offline SY  United States
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The 5-8 ohm impedance provides quite marginal degeneration, especially with low mu tubes. Just out of curiosity, I measured the harmonic spectrum of a couple of red LEDs in cathode circuits- well below my measurement limits. All I could see was a few millivolts of fundamental, exactly what would be predicted from the dynamic impedance, i.e., the thing looked like a 5 ohm resistor. Any bypass cap that has a significant effect at those low impedances will likely be more nonlinear than the LED.

I'm including some of these measurements in an article I've been trying to finish on a power amp design.
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Old 18th March 2006, 02:49 AM   #14
rdf is offline rdf  Canada
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Can't say with certainty arnoldc since I haven't tried either yet. One circuit (with additional oddities of its own!) is a 6C45 into a 5 kohm SE OPT, the other is a 6C45 loaded by an IXYS solid state CCS in series with 10 kohm. Don't see why a small bypass wouldn't work in choke and resistor loaded circuits though. The values I use are usually well under 0.1 uf.

I don't want to claim too much science behind it as nothing I have at home measures very high frequencies. Spice simulations using available models for LEDs suggested their impedance rises with frequency above the audio band. When used as a cathode load this implies increasing cathode feedback at ultrasonic frequencies and beyond. That to me implies the potential for decreasing stability so I shunt the LED and let the tube roam free up there. Neither circuit uses grid stoppers with a very high transconductance tube and neither is at all prone to RF pickup or makes any noise, that's about all of a 'scientific' confirmation for the principle I can offer at the moment and it's not much. It's such an easy thing to try, I recommend giving both a go and find out if it matters for your circuit.

I see that SY posted while I wrote this. What he said! I'm not bypassing at audio frequencies, only ultrasonic and beyond.
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Old 18th March 2006, 03:07 AM   #15
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Whoops, I see I wrote the LED bypass is intended to provide a "shunt between grid and cathode" a couple of posts back. That should of course be "cathode and ground". Sorry about that.
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Old 18th March 2006, 03:21 AM   #16
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I haven't done a dynamic impedance measurement over 20kHz, but it's certainly flat to there. And switching response time (using square wave excitation) is better than I can measure. In "Valve Amplifiers," Morgan Jones showed some models of LED nonlinearity but didn't measure it at useful currents (10mA). I note, though, that he didn't use a bypass cap in the LED-biased phono stage. In fairness, the signal levels there are low, much lower than for a driver stage. But in his 6SN7 distortion measurements and my similar 5692 measurements, the LEDs seemed to contribute negligable nonlinearity. Jones's advice was to keep currents high and maximize plate loads.

All that said, I certainly don't see how a high frequency bypass could hurt anything. It's the idea of a whomping electrolytic that spooks me.
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Old 18th March 2006, 03:23 AM   #17
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I'm getting a bit frustrated, all my LEDs are blue

I picked one that looks like Agilent and hooked it up to my 45 amp, as expected, I'm not getting 2V bias but rather 3V spot on.

Grrrrrr
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Old 18th March 2006, 06:43 AM   #18
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Attached is what LTSpice claims as a comparative plot of the cathode voltage for a 6C45 biased at 10 ma with a QTLP690C LED, the latter pulled at random from LT's default library pack. The solid green trace is for an unbypassed LED, the blue represents a perfect 0.1 uf cap, no inductance, leakage, etc.. Manufacturer's specs for most film caps that value show a self-resonance between 10 and 20 MHZ anyway. Possibly 'math gone wild' with no true correlation to reality but it's what got me trying bypasses.
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File Type: png led biased 6c45 - bypass cap vs none.png (4.6 KB, 1762 views)
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Old 18th March 2006, 08:09 AM   #19
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Guys, I just have to post this.

I bought some LEDs from a local shop and picked the orange ones for my experiment.

I have a decent 45 amplifier, which I can say can go head to head with commercial ones like Sun Audio in its current form. My driver is an Raytheon 5842, with choke load on plate, cap coupled (Jensen) to globe 45s with grid choke, small F-475 Tamura OPTs as output.

I replaced the 100 ohm resistor and its Panasonic 470uF bypass cap and in goes the orange LED which gave me 1.9V bias. Just perfect (would have loved 2V).

WOW!

I played my usual torture CDs, The Blue Man Group The Complex and Audio CDs. WOW! Played some more, Switchfoot, and Cowboy Junkies... WOW! Played some vinyls, Sergio Mendez, The Great White, Crowded House, Clearaudio Test Disk (Classiscal), REM... WOW!

I have more bass depth and weight. The highs are cleaner. The resolution and dynamic contrast improved!

Geez! For *just* an LED??!!

I am so happy, I still have a wide grin while posting this. Thanks to Sy and to all who supported me on this.

I'll be leaving still wearing this grin... Off to a Sergio Mendez and Brasil 2006 concert. They're in town
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Old 18th March 2006, 10:09 AM   #20
SY is offline SY  United States
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Glad that worked out, but really, all praise should go to MJ, whose wonderful books convinced me to try this in the first place.

Enjoy the concert; I spent last weekend in Vienna and managed a nice opera and a symphonic concert. It's so much better than even the best hifi ever made....
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