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Old 13th June 2018, 07:46 PM   #11
rongon is offline rongon  United States
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5842 Headphone Amp
Yes, it's the current drawn by all power supply filters at startup that would overload the EZ80 plate dissipation capability. Very likely with a string of large-ish value like 100uF.

What does PSUD2 say if you use 4X 1k and 22uF instead?

One solution would be to just buy a 115V:115V isolation transformer like the Triad N51X. That would give you about 150V for your B+. Then get a separate transformer for the 5842 heaters, maybe like this 12.6VCT 1A trannie.

If you don't mind more complexity, you could burn off that extra voltage using a voltage regulator. That's what I've been wrestling with lately... +150V DC Vacuum Tube Regulated Power Supply?
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Old 13th June 2018, 07:56 PM   #12
jdrouin is offline jdrouin  United States
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Well, maybe I'm not using PSUD2 correctly, but consulting the 5842 datasheet shows me that the plate resistance is about 1.7K at 150V (https://frank.pocnet.net/sheets/049/5/5842.pdf). So made the value of the end resistor 1.7K (see attached screenshot).



That results in 77V on the final resistor -- about half what I need -- but the ripple is incredibly low at 1.9uV.
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File Type: png Screen Shot 2018-06-13 at 2.53.26 PM.png (161.0 KB, 134 views)

Last edited by jdrouin; 13th June 2018 at 08:01 PM.
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Old 13th June 2018, 08:08 PM   #13
rongon is offline rongon  United States
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That 1.7k rp is with 150V plate voltage, 25mA plate current, with a 60R cathode resistor, so -1.5V grid bias.

According to the plate curves, plate voltage of 150V with plate current of 18mA should mean grid bias of about -1.9V, so cathode resistor of about 105R. Transconductance would be a bit lower at that operating point, so I'd figure the rp would be closer to 2.5k.

http://www.westernelectric.com/spec_sheets/417A.pdf

If you're sure your 5842 will be drawing 18mA plate current, try using an 18mA current sink as the load in PSUD2.

--
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Old 13th June 2018, 08:22 PM   #14
jdrouin is offline jdrouin  United States
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OK, posting results. After going through the first calculations, it turned out that my peak plate current was actually more like 5x the average current. So I recalculated the other key parameters.

Here's what I originally got:

B+ (Vl) -- 146.3V
(Il) -- 36mA
Vm -- 120V
Fm -- 60Hz
Average plate current -- 18mA
Peak plate current (assume 4x average) -- 72mA
Max peak inverse voltage -- 353.2V
Peak diode resistance -- 480R
Average diode resistance -- 547.2R
Effective PT secondary resistance (Rs) -- 453.66R
Voltage step-up ratio (N) -- 2.29167
Effective peak transformer resistance -- 933.66R
Effective average transformer resistance -- 1000.86R
Equivalent load resistance (Rl) -- 7.639K
Frequency time constant (wRC) -- 28.78
Peak source resistance to load ratio -- 0.1222
Average source resistance to load ratio -- 0.1310
Check of peak diode current in Reich/Schade's plots reveals it to be 5x

Recalculating key parameters with the new peak current reveals the following:

Peak plate current -- 90mA
Peak diode resistance -- 444.4R
Average diode resistance -- 506.6R
Effective peak transformer resistance -- 898.1R
Effective average transformer resistance -- 960.33R
Peak source resistance to load ratio -- 0.117
Average source resistance to load ratio -- 0.1257
Check of Reich/Schade plot reveals no significant departure from original plot

With the new information, some more calculations:

Rectifier efficiency (Er) -- 0.725 (looks good to the right of the knee in the chart)
PT peak secondary voltage -- 379.3V
PT RMS secondary voltage required -- 268.25V (very close to my PT's 275V per section)

The `80 datasheet says max. peak steady state plate current is 400mA, so the 90mA is still well within spec (unless I'm mistaken or have botched any of these calculations).

Quote:
Originally Posted by rongon View Post
If you're sure your 5842 will be drawing 18mA plate current, try using an 18mA current sink as the load in PSUD2.
Making the load an 18mA CCS results in 243V B+ and 877nV ripple. (attached)
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Old 13th June 2018, 08:36 PM   #15
rongon is offline rongon  United States
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Quote:
Making the load an 18mA CCS results in 243V B+ and 877nV ripple. (attached)
Try making C1 0.1uF (in other words, make it a choke input psu).

Quote:
The `80 datasheet says max. peak steady state plate current is 400mA
Confusing. "peak steady state plate current"? 400mA?? That's a ton of current for any tube rectifier. I don't think that's right.

According to this data sheet (Philips EZ80) the max continuous output current (Io max) with 250V per plate is 90mA.

--
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Old 13th June 2018, 08:46 PM   #16
jdrouin is offline jdrouin  United States
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Quote:
Originally Posted by rongon View Post
According to this data sheet (Philips EZ80) the max continuous output current (Io max) with 250V per plate is 90mA.
--
I'm using the `80, not the EZ80. http://www.r-type.org/pdfs/80.pdf

And I think what we're looking at is plate current, not output current.

According to the linked datasheet chart, though, if it's 90mA output then that would seem to exceed the limits of the tube.
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Old 13th June 2018, 08:56 PM   #17
jdrouin is offline jdrouin  United States
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Some more calculations following the procedure document.

Ripple factor (Rf) = 0.022
Output voltage (Edc) = 289.1V
Ripple voltage (Vr) = 6.2V (2.2%)

Now I need to figure out the smoothing factor and -dBs of all this, and the document seems to assume I know how to do that already (I don't).

For any noobs like me who are reading this, I'm looking at this document for audio db calculations now: http://eaw.com/docs/6_Technical_Info...lculations.pdf

Last edited by jdrouin; 13th June 2018 at 09:07 PM.
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Old 13th June 2018, 09:18 PM   #18
rongon is offline rongon  United States
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Quote:
I'm using the `80, not the EZ80.
Ah, 5Y3GT equivalent. OK. That's what I'm using in a preamp build where I had to burn off some volts.

Did you try changing the psu to choke input? You can either remove C1 entirely, or change it to 0.1uF (100nF). That should give you about 160V DC to the load.
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Old 13th June 2018, 09:26 PM   #19
kodabmx is offline kodabmx  Canada
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Originally Posted by jdrouin View Post
Choke -- Hammond 157H -- 10H, 414R (higher than mfr. claimed 408R)
LOL +- 20%. I'd say that's pretty damned good...
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Old 13th June 2018, 09:30 PM   #20
kodabmx is offline kodabmx  Canada
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Quote:
Originally Posted by rongon View Post
Ah, 5Y3GT equivalent. OK. That's what I'm using in a preamp build where I had to burn off some volts.
A resistor would have been a lot cheaper... Why waste the extra 10W of filament power?
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