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Old 19th June 2019, 09:57 PM   #51
Troy Madden is offline Troy Madden  United States
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Originally Posted by GoatGuy View Post
Sweet. LEDs as the “no-cheaper-or-better-but-definitely-au-courant” bias method (as opposed to say a 8 volt Zener, LOL) is fine. They definitely are more of a craftsman's Art though… you have to actually measure the VF of an LED (and use the same brand and model) to achieve the VF you're happy with.

Having been “in the business” of using semiconductor devices for most-every application (as well as starting off with a keen respect for 300 V B+ supplies… for valves), I have a now-somewhat-misguided “suspicion” against expecting run-to-run uniformity in semiconductor widgets, even from the same manufacturer, with the same model number.

In the Dark Ages (certainly after 1970 — when crystal dinosaur scales were used, but before 1990 — when manufacturing 'chops' got quite a bit tighter) those “data sheet specs” demonstrated huge manufacturing variability ranges. ± 35 to 60% wasn't at all uncommon. And were you to order “a bag” of parts, they'd vary all over the place.

However, of more recent experience, I've found manufacturing tolerances to be much tighter. I got — as an example — 250 J301 junction n-channel FETs, for a few pennies apiece.

It took me by surprise on setting up a mass-testing-while-drinking-beer rig, to find that almost all of the devices were within 3% of each other, operating-spec wise. There were a few outliers, but most were quite tightly near the center of the data sheet spec.

So, time's are a'changing.

GoatGuy ✓
Both sides came in real close to 7.8 volts. The zeners didn't show today, and I had a bag of about 100 decent quality LEDS so I decided to give it a shot.
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Old 21st June 2019, 09:33 PM   #52
Troy Madden is offline Troy Madden  United States
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Originally Posted by Troy Madden View Post
Would you call this good enough? These are the distortion numbers on my amp project, loosely based on the GEC 400 watt tube amp plans. I'm using 4 KT120's in parallel push pull, and instead of cathode followers driving the output tubes, I'm using paralleled 6SN7's with CCS plate loads. Output trans is Hammond 1650T. Measurements taken with Keithley 2015, at 1 Khz.

3 watts-.07% , 5 watts-.11%,
10 watts-.17%, 20 watts-.29%, 30 watts-.31%,
40 watts-.28%, 50 watts-.22%, 60 watts- .27%,
75 watts-.52% 90 watts-.61% and starting to clip
New distortion numbers with LED bias on driver tubes, and balance adjust pot on phase splitter (AC volts adjusted using Keithley 2015 ):
4 watts- .03%, 12 watts- .13%, 20 watts- .20%, 30 watts- .26%, 40 watts- .27%, 50 watts, .18%, 60 watts- .14%, 80 watts, .36%, 100 watts- .25%, just beginning to clip
It sounds GREAT! It has an immediacy that is quite surprising and pleasing.

Last edited by Troy Madden; 21st June 2019 at 09:37 PM.
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Old 21st June 2019, 10:46 PM   #53
multi is offline multi  Australia
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Why not use the cathode follower?
I have always used The cathode follower
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Old 21st June 2019, 11:03 PM   #54
Troy Madden is offline Troy Madden  United States
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Originally Posted by multi View Post
Why not use the cathode follower?
I have always used The cathode follower
I am getting far less distortion this way, when actually driving the KT-120's. The orig GEC front end measures very well stand alone, but for some reason total distortion increases dramatically when its driving the output stage. I even reverted back to the GEC circuit earlier this week, as a sanity check, but 2nd and 3rd harmonic distortion was double or more. I'm wondering if the distortion of my modified circuit is such that it is being cancelled in the output stage. In any case, it measures and sounds good now, so I think I will leave it as is.

Last edited by Troy Madden; 21st June 2019 at 11:05 PM.
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Old 22nd June 2019, 07:43 AM   #55
Chris Hornbeck is offline Chris Hornbeck  United States
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Congratulations on your successful project. A very good case can be made that only the distortion in the first Watt, or the first .01 Watt, matters. Performance near clipping gives bragging rights, but really only needs to be much better than a loudspeaker - not that difficult.


But performance within the working range, say -60 dBVU to 0 dBVU, actually matters much more. Not as sexy, but more important.


All good fortune,
Chris
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