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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 19th December 2017, 04:57 PM   #111
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by Shef View Post
I second on TIP50 filter. Cheap, simple, and reliable as a brick. While the mosfet provides kinda better filtering it does not tolerate any offending. The TIP50 would put up almost with any accident.
PS … ... ... we really agree on this, friend. While one can imagine any number of ingenious ways to use a tough-as-nails NPN as a pass (series) regulator, the emitter-follower is intrinsically trivial to do.

And in this regard, I like (because they're so darn cheap) to use a cascade of them. Make a resistor chain:

VUNREG = 385 V
RTOP = 2.2 kΩ (340 V)
RMID = 1.0 kΩ (320 V)
RLOW = 1.0 kΩ (300 V)
RBOT = 15 kΩ

The Thevenin equivalent impedance (Kirchoff's laws, etc) combined with an hFE of 25 or so for an "O" bin TIP–50 yields a 2 volt drop on the 340 V tap, a 1.8 volt drop on the 320 tap, a 0.9 V drop on the 300 tap. Input currents to the BASE of each TIP–50 emitter follower. Nice small caps (10 μF) gets the leveling in place.

One could build all that on a small circuit board what, 2 square inches? With operating (output) loads of 27 ma (340 V), 17 ma (320 V) and 7 ma (300 V), the TIP–50s should have tiny heat sinks. less than 2 watt dissipation, per.

Anyway yes. I am a firm believer in E = IR and P = IE, and the utility "in the modern sense" of using spreadsheets to figure out the math. (That's where these values, above, came from. It took about 3 minutes to cobble together. No circuit, sim, drawing, mistake-editing needed.)

GoatGuy
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Old 19th December 2017, 10:18 PM   #112
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by GoatGuy View Post
Thanks. I keep forgetting that most everyone (except a lot of us DIY goats) have completely forgotten their algebra skills. With

E = I R you easily rearrange to:
I = E/R and
R = E/I

etc...

GoatGuy
Good evening,

I'm not sure if you're directing this specifically at me, or at people in general...

If the latter (and having re-read my previous response to shef), it occurs that I may have given the impression I'm ignorant of these formulae, which is not the case, I just hadn't seen them coralled in so handy a fashion .

I've spent time on a fair few audio forums over the years, I won't need to tell you that most of what is bandied about is utter guff (especially in any guitar related forum, which as a guitarist I'm guilty of visiting). It doesn't take long to work out who knows what they're talking about and who doesn't.

As a relative newcomer to diyAudio, I've been impressed not only by the number of people who obviously (and demonstrably) know their onions, but also how well they know them and how willing they are to share that information. It's a breath of fresh air.

Getting to my point, I'm obviously new to designing/building hi-fi audio. In spite of what you may think, I'm not the sort of person who expects to have it all spelt out for him. I've been doing my best to put in the hours and learn the theory behind the circuit (indeed I write V=IR in the top right hand corner every page), but my brain is not naturally mathematically inclined, and it's been slow going.
Nevertheless, it's safe to say I've done more maths over the past few weeks/months than I ever did in school, and have learned more to boot.

With regards to my difficulties designing my power supply, it was as I struggled through the maths in Morgan Jones that he informed me (and I paraphrase) 'nowadays we'd just plug the numbers into PSUD2'. Seeems I'd have been better off using pen and paper, or as others have suggested testing the supply on the bench.
Long (dull) story short, I've spent most of the weekend doing just that, and while I'm no closer to a finished design, I think I've learned something. Exactly what, remains to be seen...

Matt

Last edited by LeftHandFool; 19th December 2017 at 10:21 PM.
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Old 19th December 2017, 10:59 PM   #113
LeftHandFool is offline LeftHandFool  United Kingdom
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Originally Posted by LeftHandFool View Post

If the latter

Matt
I meant former...
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Old 20th December 2017, 01:26 AM   #114
PRR is offline PRR  United States
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Originally Posted by cnau View Post
....Hammond 372HX 600v CT, 230 ma. ....the PSUD model matches this result using the unloaded voltage (325v) and twice output resistance from spec sheet.....
The real series resistance would include the DCR of the 120/240V *primary*, as reflected by the voltage ratio.

That's hard math, and 3X harder because 372HX has three significant outputs.

A "perfectly optimum" design would use the same pounds of Copper on primary and secondary(s), just different gauges to get the right turns count for the desired ratios.

And then, ignoring ratios, the effective secondary resistance is indeed double the secondary resistance alone, because the primary resistance will reflect-across as the same value.

Let's check. For simplicity, take primary wired 240V. Secondary is 651r. Primary is 7.7r resistance. This reflects as the *square* of the voltage ratio. 651/240 is 2.7. 2.7 squared is 7.36. 7.7r at primary reflects as 57r added to secondary. Sec alone is 77r. So just like that, it is not quite "double". However there are two more load-rated windings, which the primary has to supply. Adjusting for the 651V winding being about 138VA of a 190VA total load, we get primary resistance "just to power the 651V winding" of 79r, which indeed makes "double" the 77r of the sec alone.

It rarely works out this close. Wire only comes in set gauges, we always round-up/down from ideal #31.2 to available #30 or #32. We like to find turn-per-layer which fills the whole width so the winding is not lop-sided or wastes available space. High volt windings may need more insulation (a 10KV winding may be >50% varnish and paper).
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Old 20th December 2017, 02:54 AM   #115
cnau is offline cnau  Canada
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PRR, thanks for the explanation. I was wondering about that.
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Old 21st December 2017, 12:24 PM   #116
gabdx is offline gabdx  Canada
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I don't think there is harm to try the choke input.

It will be very nice to experiment and see what the result B+ is vs. the simulations.
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Old 28th December 2017, 03:36 PM   #117
LeftHandFool is offline LeftHandFool  United Kingdom
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Default Progress, of sorts...

Afternoon all,

Progress has been made over the festive period, although not all of it in the right direction.

Having finally arrived at a design I thought might be a worthwhile starting point, I sat down and totted up the cost. At this pint, quickly realised that to make the project worth embarking on would cost far more than I anticipated and - more importantly - more than I have to spend.
As a result, I'm shelving the project for the time being, until I have amassed enough:

A: Money

and

B: Knowledge

to make it a success, which I don't think in all honesty it would have been had I continued with it at present.

I'd like to thank everyone for their help and advice along the way, and indeed apologise for having wasted your time. I suspect a lot of that advice will find it's way into the final project, however far down the road that may be...

On a more positive note, I haven't been idle. I've traded the SET for a simpler, cheaper 'parts drawer' design, running EL84's as pentodes for around 4 Watts per channel.
I'll start a new thread for that soon, any help will be greatly appreciated .

Thanks again,

Matt.
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Old 29th December 2017, 11:17 AM   #118
waxx is offline waxx  Belgium
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This tread was not for nothing. I was trying to design a similar amp as you and it helped me to get to this design . I don't know if i'm going to build it. I would like to simulate it before i try, but i don't know how to do that...

Did i make obvious mistakes? And how do you simulate this?
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Old 29th December 2017, 03:13 PM   #119
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by waxx View Post
This tread was not for nothing. I was trying to design a similar amp as you and it helped me to get to this design . I don't know if i'm going to build it. I would like to simulate it before i try, but i don't know how to do that...

Did i make obvious mistakes? And how do you simulate this?
WAXX:

(1) your input jack is "upside down". The tip is connected to input of tube, not the "ring" or shell.

(2) C1 strictly doesn't need to be 400 volts. There is no HV path to U1's grid. Even if tube fails.

(3) The gain on ECC–81 is high. gm = 5.5 ma/V nominally; with plate resistance, maybe (g = 44) or so.

(4) The C2 of 330 μF is overly large. 47 μF delivers –1 dB at 15 Hz. In general, smaller capacitors are better capacitors, linearly

(5) The R4 = 24 kΩ … only dissipates about 0.5 W nominally. 5 W is a bit overkill. I might edge toward a 2 W carbon-comp or metal film. Much improved sonics compared to a wire-wound. And easier to get.

(6) C3 is a poor choice, given R5 of 47 kΩ. The –3 dB knee is at 72 Hz. You'd want that closer to 10 Hz, ideally. Simple scaling gives the answer: 0.047 μF • ( 72 ÷ 10 ) = 0.33 μF.

(7) R5 of 2 W dissipation is ridiculous. It is a near-zero current almost-at-ground resistor. Just use a ¼ or ½ watt standard resistor. Also 45 kΩ? You want the closest 'standard value' resistor. E–24 or E–48 scale, so 47 kΩ. Not 45 kΩ.

(8) It really isn't clear why you'd specify a 220 μF cap for U2's cathode bypass, and 330 μF for U1's. “More for the final”. Now … I'm not saying 220 μF is a bad value. It just is stark contrast to the U1's cathode bypass.

(9) on your power supply, it is ALWAYS better to have 2 "filter" stages at half-inductance, than 1. Drive the final off the first stage, drive the input off the 2nd stage. 5 Hy is enough for each.

(10) Personally, having had my fair share of diode failures, I'd DOUBLE UP the 1N4007 rectifiers. 2 in series for each position. Just IN CASE "one goes".

GoatGuy
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Last edited by GoatGuy; 29th December 2017 at 03:16 PM. Reason: typos
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Old 29th December 2017, 03:36 PM   #120
waxx is offline waxx  Belgium
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Thank you for the comment. I need to learn a lot it seems... Your answer is a good start for that.
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