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Old 30th April 2008, 11:08 PM   #1
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Default single-tube (6GN8) amplifier?

hey all. i've hacked together an interesting idea for a basic class a amplifier using a 6gn8 triode/pentode...

how's it look? any comments or suggestions?
http://www.geekysuavo.org/throwbacks...lifier-1.0.pdf (it's also attached)

some figures, which i'm assuming (??) to be relatively correct:
triode section
HT = 350V
Rl = 120k
Ia = 1.125 mA
Va = 215V
P(Rl) = 152mW
Vin = -1Vpp
Vout = 60Vpp
Av = 60
Ra = 44.29
mu = 90
gm = 2.032mA/V
Rk = 1.8k
Vg-k = -2.025V
P(Rk) = 2.3mW
rk = rk || Rk = 901
Ck = 220uF
f(-3dB/LF) = 0.8Hz

pentode section
Zl = 6.5k
Va = 145V
Vin = -2Vpp
Vout = 125Vpp
Av = -62.5
Ia = 31.54mA
Pout = 6.5W ... is this correct?
Ig2 = 15.5mA
Ik = 47mA
Rk = 53.6
Vg1-k = -2.5192V
P(Rk) = 118mW
Ck = 3000uF
f(-3dB/LF) = 1Hz

miller effect
for pentode, Cag = 0.1pF
Cmiller = (60 + 1) * 0.1pF + Cstray = 17pF
for pentode, Rgl = 1M
f(-3dB/HF) = 288 kHz

i know that's just a lot of numbers, but i figured they may be of use.

peace,
~ brad.
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Old 30th April 2008, 11:46 PM   #2
Jeb-D. is offline Jeb-D.  United States
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Your schematic doesn't match the data listed. Schematic has a B+ of 350V, but you list the pentodes anode voltage of 145V. Your transformers 6.5k impedance is AC, the DC resistance is probably less than 100 ohms. 350V B+ would exceed the plate and g2 voltage ratings, which is 330V.

The 6gn8's Pentodes plate is rated for 5W. That means the absolute most power you will be able to squeeze from it is 2.5Wrms (5Wpeak) in Pentode mode. But you will realistically probably end up with about 2W Pentode and maybe a little less in UL. That's with a properly designed circuit.

There are some similar tubes rated for more power. I can provide numbers if they are of use to you.
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Old 1st May 2008, 12:32 AM   #3
Jeb-D. is offline Jeb-D.  United States
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In case you are new to transformer coupled stages, check out:
http://members.aol.com/sbench102/po-dis.html

Keep in mind that your plate voltage will be the B+ (HT, whatever you want to call it) and subtract, bias voltage (for cathode bias) and subtract - (idle current*transformers DC resistance).

You will find the voltage from the plate to ground to be pretty close to B+ at the operating point. Now the tricky thing is (when compared to resistor plate load), the plate voltage swings above(way above) your supply voltage (B+) when an AC signal is applied.
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Old 1st May 2008, 01:10 AM   #4
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Quote:
Originally posted by Jeb-D.
Now the tricky thing is (when compared to resistor plate load), the plate voltage swings above(way above) your supply voltage (B+) when an AC signal is applied.
Yep. It took me the LONGEST time to figure this out before I could design my own amps Great site by the way.
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Old 1st May 2008, 08:50 AM   #5
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wow, thanks for the feedback! i wasn't getting any emails from the forums, so i'd assumed incorrectly that nobody had replied... oops.

ok, i do see that my operating voltage is much too high now. thanks for pointing that out. but since reactance (and thus impedance) of the output primary changes with signal frequency, how is there any chance that the frequency response of the power amplifier will be linear in the 20-20k range? i must be missing something here...

just to make sure, the triode looks o.k., right?

~ brad.

p.s. thanks about the site!
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Old 1st May 2008, 08:50 AM   #6
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as a side note, i'm probably not looking for too much power output, since this is kind of my first design anyways... i suppose i could always go with a 12ax7/el34?

BUT, would 2W per channel even be enough to push two small 8 ohm speakers into reasonable volume?? thanks again for the help...

~ brad.
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Old 1st May 2008, 08:50 AM   #7
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sorry for the barrage of posts... lol. i just figured out how loud you can make 2W depends on the sensitivity of the loudspeakers... good to know.

~ brad.
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Old 1st May 2008, 08:50 AM   #8
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well, now that i'm thinking about it, what are these mystical similar tubes to the 6gn8/6eb8?

~ brad.
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Old 1st May 2008, 03:10 PM   #9
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You might want to check out the latest issue of audioXpress (out today?). I've got an article on a similar amp in it. There is some data (schematics, etc.) on the design here:

http://www.pmillett.com/midget.htm

The magazine article has the full construction details in it...

Pete
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Old 1st May 2008, 05:11 PM   #10
Jeb-D. is offline Jeb-D.  United States
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Quote:
but since reactance (and thus impedance) of the output primary changes with signal frequency, how is there any chance that the frequency response of the power amplifier will be linear in the 20-20k range?

With a resisitive 8ohm load on the secondary- The reflected impedance on the primary is 6.5k. The inductance of the transformer is high enough to where its reactance isn't influential until the low frequencies.

A good way to look at it is like this- the plate is loaded with a 6.5k resistor in parallel with a large inductor. At DC, the inductor is pretty much shorts out the resistor. But as frequency increases the inductors reactance becomes large, to the point of where it's like an open circuit. All that the plate will see is the resistor (which is the reflected impedance). The transformers inductance will determine the low frequency cutoff. For a given impedance, a transformer with a larger inductance will be able to extend lower in frequency.

For a real world speaker load, where the impedance fluctuates, yes, the reflected primary impedance does vary. This will cause a non linear frequency response from a voltage standpoint. The amount of how much the output voltage fluctuates with impedance variation is how amplifiers output impedance is determined. An amplifier with a very low output impedance acts as a voltage source, where-as a amplifier with a high output impedance acts like a current source.
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