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A Tube amp without coupling capacitors? Possible?
A Tube amp without coupling capacitors? Possible?
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Old 11th August 2019, 01:40 PM   #11
starkeyg is offline starkeyg
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Agree with DF96:
"No. You have been reading too many audio websites and reading too few electronics textbooks.

If you want to improve a tube amp then the first thing to get rid of is the tubes. Then ditch the transformers. After that it is difficult to decide whether to get rid of the resistors or capacitors - maybe the caps should go. Finally, some would want to get rid of the wires.

The reason you don't see tube circuits wired like modern opamps is that nobody has invented opposite polarity tubes using positrons instead of electrons."

Transformers have far worse characteristics than any decent cap used well within its design limits. Why the love of those and distaste for caps???
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Old 11th August 2019, 02:01 PM   #12
DF96 is offline DF96  England
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I suspect it is nostalgia for the past, combined with lack of understanding.
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Old 11th August 2019, 02:31 PM   #13
50AE is offline 50AE  Bulgaria
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A Tube amp without coupling capacitors? Possible?
Yes, it's possible. You can use interstage 1:1 transformers, which if built correctly, have the best bandwidth.
Or use DC coupling, which is harder.
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Old 11th August 2019, 02:46 PM   #14
kevinkr is offline kevinkr  United States
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A Tube amp without coupling capacitors? Possible?
My findings are pretty much in line with tubelab's comments, when I was still designing and building PP amps I used dc coupled driver stages which I capacitively coupled to the output stage. Optimizing performance for the available supply voltages and building in tolerance for drift and line voltage variations was quite the task.

As DF96 states, the reason you see comparatively few fully dc coupled designed is the lack of a complementary device and substantially more complex design and power supplies. Have a look at a vintage Tektronix scope vertical amplifier design for examples of a "fully developed" DC coupled amplifier design.

Acrosound also had a PP 6BQ5 which was DC coupled which is noted both for its unreliability and propensity for eating output tubes.
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Old 11th August 2019, 03:29 PM   #15
jhstewart9 is offline jhstewart9  Canada
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Audio Magazine published a DC coupled amp in the 50s. My recollection it was PP all the way, 2X6B4s as outputs. Look for it later.
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Old 11th August 2019, 03:59 PM   #16
alayn91 is offline alayn91  France
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Hello,
There is, at least, one capacitor in the audio circuit: the reservoir capacitor.
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Old 11th August 2019, 04:50 PM   #17
CBS240 is offline CBS240  United States
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With IPS and VAS perhaps if you pour some sand into the circuit it can be DC, but direct coupling output tubes I would think might be problematic.
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Old 11th August 2019, 04:56 PM   #18
plasnu is offline plasnu  United States
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Mr. Kaneda has been building DC coupled amp since 80's, both tube and SS, SE and PP. Looks like he has a lot of followers in Japan. His 300B single ended published in 97' looks like this.
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Old 11th August 2019, 05:00 PM   #19
Sheldon is offline Sheldon  United States
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Built this a few years ago. It's my headphone amp now.
https://www.diyaudio.com/forums/atta...sn7-schema-gif
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Old 11th August 2019, 05:21 PM   #20
rongon is offline rongon  United States
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A Tube amp without coupling capacitors? Possible?
If the definition of "tube amp" includes headphone amps then it's actually easy (except for any large value capacitors in the power supply, of course). The recipe for a single-ended, single-tube headphone amp is simplicity itself:

1) Use an output transformer with about 5k to 10k primary impedance and a secondary impedance that is close to the impedance of your headphones (something like a 50 ohm secondary would work for most).

2) Use a high transconductance, frame grid RF pentode that will have a gain of at least 30 and plate resistance of about 1000 ohms when wired triode. Wire it triode. 12GN7A or 6e5P would work.

3) Use paralleled LEDs to cathode bias the tube. (I ended up preferring the sound of a cathode resistor with bypass capacitor, but maybe I actually like bad 'capacitor sound'.)

4) Regulate the plate supply. Something like 150V to 200V DC that can deliver 100mA or so should work for a stereo amp. (I wound up getting lazy; after trying mine with a choke input passive power supply with RC decoupling I ended up keeping it that way because it's dead quiet and sounds good to me. It even has AC heaters, but I can't hear any hum at all.)

That's a single-stage, single-ended triode, output transformer coupled headphone amp with no coupling capacitors.
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