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In Reply to: RE: What to do with 490V??? posted by hifipaul on November 22, 2015 at 14:03:56
"I got 565-0-565v using a variac to decrease the primary volts to where the 6.3v taps measured 6.3v. Likely I'll get more V drop once loaded..."
Not if the 6.3V taps were also measured without a load. That would mean your variac wasn't turned up enough when you measured the HV. In any event, I would do as Paul suggested above, plus use a critical value choke. There's no reason not to continue with the project as planned. If you're interested, post the values for LC components you're planning to use (including DCR), and I can look at the output Z over frequency with SPICE. Better to do that now than after the design has been finalized.
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Buy Chinese. Bury freedom.
Follow Ups:
Thanx for commenting. The secondaries measure 141ohm to ct and 152ohms to ct or 293 ohms leg to leg. The choke is a Stancor swinging choke 76ohms dcr and rated for 400ma and 4Hys. The cap (after the choke) will likely be ~80uf or so (this can change easily). primarys for the Tangos are 100 ohms.
The original operating points for the KT88 were to be 400v at 75ma with fixed bias. This is no longer the case. New operating points and likely self bias are needed.
Likely I'll set up some kind of load for the amp within a week and measure my B+ at full load within a week.
" The choke is a Stancor swinging choke 76ohms dcr and rated for 400ma and 4Hys. ... The original operating points for the KT88 were to be 400v at 75ma..."
Sorry, I meant to get back to this sooner. Can I ask why you're using a swinging choke? The amplifier is Class A, so average current draw doesn't change much when the amplifier is driven. That means the choke won't "swing." In any event, it's necessary to know the approximate inductance at the actual operating current (150 mA) in order to analyze this. For the purpose of simply taking a quick look, I'll assume it's 5H (the 4H rating is at 400 mA). If this choke is followed by 80uF, the following curve represents relative levels looking back into the supply:
As you can see, the curve is nice and smooth, with no irregularities that might indicate resonances "visible" to the amplifier. Just for contrast, here is the same supply with a 10uF output cap:
In this case, insufficient capacitance creates a much higher overall output impedance. Attenuation at 1kHz is degraded 15dB relative to the previous example. In addition, resonance between the cap and inductor produces an impedance bump. The supply practically "lets go" at 25Hz, meaning it won't be able to properly "anchor" the amplifier. Amplifier output in this area will almost certainly be distorted and poorly defined.
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Buy Chinese. Bury freedom.
Mr. Kingdom
Thanx for your looking into this. the graphs are helpful; I think I'll up the cap value even more to lower the imp in the bass region.
I did finally get it up and running with just the KTs running in self bias. First try got me 492v. Dialing the variac back so that 6.3 volts were on the filaments got me 472v, changing the rectifier to 5v4 got me down to 460v. with 45v self bias I'm now down to 415v across the KT88s. I can live with that.
To answer your question about using a swinging choke. It's true that the average i doesn't change much in class A. To my way of thinking, music in a class A circuit is a deviation from the mean; the louder the music the greater the deviation. Thus a swinging choke is needed. Also, SE circuits sound better to me with choke loaded supplies.
The most important reason for using a swinging choke is; it was on the shelf, taking up room, and needed to be used. A major factor in my designs is how to use up all the stuff I acquired in the Hamfests of the 1990's.
"I'm now down to 415v across the KT88s. I can live with that."
Excellent!
"To my way of thinking, music in a class A circuit is a deviation from the mean; the louder the music the greater the deviation. Thus a swinging choke is needed."
The deviation is instantaneous and doesn't change the average. The choke will only "swing" in response to average changes in current. This conversation does make me wonder though - will a swinging choke respond to instantaneous changes in current draw when those changes occur at a low-frequency rate? This needs more thought...
"The most important reason for using a swinging choke is; it was on the shelf, taking up room, and needed to be used."
I'm sure lots of commercial manufacturers have used that same design philosophy. :)
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Buy Chinese. Bury freedom.
I see about 47v bias (triode mode), plus at least 40v lost in resistances. Not counting rectifier voltage drop. Should turn your 490v into 403 volts - sounds pretty close to me.
Edits: 11/23/15
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