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Designing a Single-Ended Triode EL34 Power Amp
Designing a Single-Ended Triode EL34 Power Amp
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Old 29th December 2017, 06:06 PM   #121
GoatGuy is offline GoatGuy  United States
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WAXX:

You are welcome. I did none of the above with 'modeling' or 'simulation'. Back “BC” (before computers), one designed circuit after circuit on paper, with a pencil, and perhaps either a “scientific” calculator … or even older, just a slide-rule. Because ± 1% was just fine. For everything except the most technical things.

So … we “remembered formulæ”, so to speak. Simple stuff:

(1) Z = 1/(2πFC) … as the impedance calculation for capacitors vs. frequency.
(2) Z = ( 2πFL ) … for inductors
(3) P = IE … power
(4) E = IR … Ohm's law, resistance.

And a few things like when "Z" is equal to "R" (for pass capacitors), then one can expect a –3 dB response. So, for the instance of your 47 kΩ 2nd stage grid pull-down resistor and the 0.047 μF DC blocking signal-pass capacitor,

(10) Z = 1/(2πFC) which rearranges to
(11) F = 1/(2πZC) which substitutes in
(12) F = 1/(6.28 × 47,000 Ω × 0.047×10⁻⁶ farad)
(13) F = 72 Hz

That's where I got that recommendation number (6).

_______

The point is, to learn to “handle” equations in (1) thru (4) above. And to be confortable and confident in using basic algebra to rearrange 'em. As I did in (10 thru 13). Seriously: do try it on paper. With a pencil. And a handy erasor. Because "it is easy" and somehow writing down the numbers on the back of a junk-mail envelope is … satisfying.

GoatGuy
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John Curl's Golden Rule…: 100 kHz bandwidth, 3 μs risetime, 100 W mean output, 100 V/μs slew rate, 2 Ω dynamic load, 20 amp min current source/sink

Last edited by GoatGuy; 29th December 2017 at 06:08 PM. Reason: lots of typos
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Old 29th December 2017, 07:57 PM   #122
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by GoatGuy View Post
… or even older, just a slide-rule.
Still got mine, but it doesn't get used much these days
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Old 29th December 2017, 10:18 PM   #123
waxx is offline waxx  Belgium
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Quote:
Originally Posted by GoatGuy View Post
WAXX:

You are welcome. I did none of the above with 'modeling' or 'simulation'. Back “BC” (before computers), one designed circuit after circuit on paper, with a pencil, and perhaps either a “scientific” calculator … or even older, just a slide-rule. Because ± 1% was just fine. For everything except the most technical things.

So … we “remembered formulæ”, so to speak. Simple stuff:

(1) Z = 1/(2πFC) … as the impedance calculation for capacitors vs. frequency.
(2) Z = ( 2πFL ) … for inductors
(3) P = IE … power
(4) E = IR … Ohm's law, resistance.

And a few things like when "Z" is equal to "R" (for pass capacitors), then one can expect a –3 dB response. So, for the instance of your 47 kΩ 2nd stage grid pull-down resistor and the 0.047 μF DC blocking signal-pass capacitor,

(10) Z = 1/(2πFC) which rearranges to
(11) F = 1/(2πZC) which substitutes in
(12) F = 1/(6.28 × 47,000 Ω × 0.047×10⁻⁶ farad)
(13) F = 72 Hz

That's where I got that recommendation number (6).

_______

The point is, to learn to “handle” equations in (1) thru (4) above. And to be confortable and confident in using basic algebra to rearrange 'em. As I did in (10 thru 13). Seriously: do try it on paper. With a pencil. And a handy erasor. Because "it is easy" and somehow writing down the numbers on the back of a junk-mail envelope is … satisfying.

GoatGuy
I prefer Excel above an old enveloppe. But it's true that knowing the math is very important when designing something like a tube amp. That's why i'm studying it and make exercise designs like that above.

The amp is something like i want to build, but it is now mainly an exercise. The build won't be for the next months i think if i build it, and the design may (and has already due to your remarks) change(d) a bit. I'm full recalculating everything with your remarks in my mind...
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Old 29th December 2017, 11:32 PM   #124
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by waxx View Post
I prefer Excel above an old enveloppe. But it's true that knowing the math is very important when designing something like a tube amp.
Excellent. Excel is. I've loved the power, the "immediate answer power" that spreadsheets deliver. I was a total fan of Lotus 123, of Quattro Pro, then eventually Excel. And still, Excel.

And most unfortunate is the move I made to Mac Excel. Unlike Windows Excel, it has no ability (or support for) writing code-as-callable-functions.

Which is really REALLY sad: I had a whole library of common formulæ that have no direct built-in Excel functions; I had (don't even know where they are) a whole library of hand-coded tube and transistor transfer functions. Gone. It is maddening. But The Big White Box company has decided that its level-of-customers simply are more gifted at getting themselves into support-hêll with callable code than its worth supporting.

GoatGuy
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John Curl's Golden Rule…: 100 kHz bandwidth, 3 μs risetime, 100 W mean output, 100 V/μs slew rate, 2 Ω dynamic load, 20 amp min current source/sink
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Old 29th December 2017, 11:49 PM   #125
GoatGuy is offline GoatGuy  United States
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Originally Posted by 12×10¹ View Post
Still got mine, but it doesn't get used much these days
Same here. NEED combined with ubiquity obsoleted the slide-rule for me. Maybe too, "being able to do a lot of very simple math in one's head". And Excel. And a teeny-tiny bit of PERL for finding values that require iterations to converge.

GoatGuy
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John Curl's Golden Rule…: 100 kHz bandwidth, 3 μs risetime, 100 W mean output, 100 V/μs slew rate, 2 Ω dynamic load, 20 amp min current source/sink
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Old 30th December 2017, 01:44 AM   #126
zigzagflux is offline zigzagflux  United States
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Been enjoying my two-week session of learning Python. Will never use Excel again for computations. Excel certainly has its place, and can do complex numbers with a little effort. But nothing beats being able to say a/b+c when they are all complex, and it 'just works'. To say nothing of arrays.
Learned matlab back in college, but not interested in paying the price.
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Old 30th December 2017, 10:28 AM   #127
waxx is offline waxx  Belgium
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Quote:
Originally Posted by GoatGuy View Post
Excellent. Excel is. I've loved the power, the "immediate answer power" that spreadsheets deliver. I was a total fan of Lotus 123, of Quattro Pro, then eventually Excel. And still, Excel.

And most unfortunate is the move I made to Mac Excel. Unlike Windows Excel, it has no ability (or support for) writing code-as-callable-functions.

Which is really REALLY sad: I had a whole library of common formulæ that have no direct built-in Excel functions; I had (don't even know where they are) a whole library of hand-coded tube and transistor transfer functions. Gone. It is maddening. But The Big White Box company has decided that its level-of-customers simply are more gifted at getting themselves into support-hêll with callable code than its worth supporting.

GoatGuy
That's one of the many reasons i avoid those computers. The Big White Box company makes good phones and tablets (but too expensive for what it is), but their pc's and laptops are way too limited for what i (as IT-er and nerd) want to do with it. Lenovo makes also very solid machines and windows is not that bad anymore today like it once was. And Linux is even better as OS but lacks the software i need. But i run both and keep OSX for my phone...

But like with most things, each to his own.
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Old 30th December 2017, 08:18 PM   #128
PRR is offline PRR  United States
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Quote:
Originally Posted by GoatGuy View Post
...Back “BC” (before computers)
… we “remembered formulæ”...........
Or just bench marks.

"Without a PC", I instantly spotted the C3 R5 dubious values. Self-bias EL34 can use 500K grid resistor; why be 10X smaller? Just makes the driver work harder. Which also means a big cap. No math: remember these happy combinations:

0.01uFd + 1Meg == 16Hz
10uFd + 1K == 16Hz

So 0.05ufd against 50K (always round-off) compared to 0.01u+1Meg. 20X lower R. 5X higher C. Both in the direction of less bass. 20/5 is 4. So the pole is near 16Hz*4 or 64hz.

Maybe 70 if you use the full numbers and proper pi. But already we do not care-- 64/70hz is way too high for "Hi-Fi". You can't even run a lot of 16Hz poles without them adding up to shaved bass.

Assuming that a 0.047u cap is in hand, try the max grid resistor. 0.05u+0.5Meg is 5X better and 2X worse than 0.01u+1meg. Counting on thumbs, 3.3Hz doubled is 7Hz, a fine value. We could even use a less than max Rg, 330K, and hit 10Hz. The very light loading of 330K on the 24K driver reduces driver distortion.
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Old 1st January 2018, 01:34 AM   #129
Chris Hornbeck is offline Chris Hornbeck  United States
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Here's an alternative view of cathode bypass and grid coupling cap values: with no (intentional) loop feedback, cap values don't need to be staggered, making cathode bypass cap values independent of each other. Both can be made very large on the grounds that the larger the value, the smaller the signal voltage that will appear across it. As long as the cap acts like a cap, brute force wins.

Choice of grid coupling cap value might also take into consideration overload recovery time. A properly designed amplifier's output stage clips first, and at point of grid drawing current. Grid current charges grid coupling cap, dynamically changing bias of output stage until bled back down. A bleed-down time contant of about a quarter second gives a good compromise.

Also, does your HV DC supply include another bypass cap after the 70R dropping resistor?

All good fortune, a Happy New Year to all,
Chris
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Last edited by Chris Hornbeck; 1st January 2018 at 01:37 AM.
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