Mic Kaczmarczik's TubeInformation - The Blue Guitar

From kurts@sr.hp.com Wed Dec 6 15:52:13 CST 1995
Article: 4005 of rec.audio.tubes
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From: kurts@sr.hp.com (Kurt Strain)
Newsgroups: rec.audio.tubes
Subject: Re: Need 5AR4 data
Date: 6 Dec 1995 16:58:51 GMT
Organization: Hewlett Packard Sonoma County
Lines: 59
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Joe Lowe (jlowe@hiwaay.net) wrote:
: TYPICAL OPERATION
: AC plate to plate supply voltage 450 V
: Effective plate supply impedance per plate 160 Ohms
: Average output current 225 Ma
: DC output voltage at input to filter 475 V
: Voltage drop for current of 225 Ma. 17 V
: No problem.
: : What transformer secondary voltage is needed for this? Like maybe 750VCT,
: : or 375Vrms per plate.
: AC input X 1.414 - 17 aprox.
For 420VDC, this formula says that AC input must be (DC output + 17) / 1.414,
or (420+17)/1.414 = 383 Vrms.
Above, where output = 475 VDC, expected input should be (475 + 17) / 1.414,
or 348 Vrms. But, instead, it says we need 450 Vrms. If the effective
plate resistance is 160 ohms and current is .225A, the drop looks like it
will only produce 160 ohms * .225 A = 36 V. Even that is not enough to
account for the large discrepancy between input 450 Vrms and 348 Vrms.
The difference is that series resistance before the input capacitor acts
to average down the voltage output, much like a choke input filter does.
So I modelled it on SPICE to check it out.
With no load, and an AC input of 450 Vrms, the voltage will rise to
1.414*450 = 636 VDC. It doesn't matter what Rrectifier and Cinput are.
With a 225 mA load, or a resistance load about 2200 ohms, the output voltage
with 160 ohm plate resistance yields not 636 - .225A*160 = 600 VDC, but
actually 450 VDC, as specified, with C = 40 uF. With C = 4000 uF, the
output remains 450 VDC again. The impedance of the tube refuses to allow
a full charge with that current drain.
To get 420 VDC output with 160 ohm rectifier resistance with a 170 mA load,
and 40 uF input capacitance or more, I went back to SPICE to find out it
needed an input of 400 Vrms. An easily obtainable transformer. But it
would be a mistake to buy a 375 Vrms transformer.
A 5U4, on the other hand, has less rectifier resistance, about 50 ohms.
It might be better to use the 375 Vrms transformer for it. Anybody using
5U4's to replace 5AR4 should expect higher DC voltages in capacitor input
supplies. Choke input supplies make rectifier differences produce much less
impact. That's because the input choke destroys the full charging capability
anyway, changing it from a peak voltage supply to an average rectified
voltage supply. Series resistance in the rectifier circuit has a similar
effect in ruining the peak voltage supply.
So, I am thankful for the data because I was able to calculate the needed
transformer for it. But I wanted to show how I differ in calculating
the expected output voltage.
Kurt
From kurts@sr.hp.com Thu Dec 7 10:43:55 CST 1995
Article: 4018 of rec.audio.tubes
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From: kurts@sr.hp.com (Kurt Strain)
Newsgroups: rec.audio.tubes
Subject: Re: Need 5AR4 data
Date: 7 Dec 1995 00:58:16 GMT
Organization: Hewlett Packard Sonoma County
Lines: 69
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Kurt Strain (kurts@sr.hp.com) wrote:
: Joe Lowe (jlowe@hiwaay.net) wrote:
: : TYPICAL OPERATION
: : AC plate to plate supply voltage 450 V
: : Effective plate supply impedance per plate 160 Ohms
: : Average output current 225 Ma
: : DC output voltage at input to filter 475 V
: : Voltage drop for current of 225 Ma. 17 V
: : No problem.