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While researching the world of vacuum tube audio amps, I noticed that many commercially available stereo amps have an odd number of tubes. Here's a random example.

How can a decoupled stereo sound be produced using a single tube?

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    \$\begingroup\$ The middle one is a rectifier tube, to provide the DC power supply to the other tubes. \$\endgroup\$ – τεκ Dec 23 '18 at 1:05
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We utilize PSVANE/Shuguang brand KT88 tubes for vigorous and forceful sound (PSVANE/Shuguang brand randomly distribute). The 6N8P tubes provide a soft and dynamic sound stage and the 5Z3P (This tube is singular)tube acts as the rectifier.

2 Tubes are "Pre-Amplifier tubes (1 for left speaker and one for right speaker in a stereophonic system)

2 Tubes take the Signal that has been amplified from the Signal Source by the Pr-Amplified Tube into a Signal that can be amplified by a power amplifier because a power amplifier requires a minimum amount of voltage peak to peak in hthe signal it's amplifiying and the pre amplifier provides the usually weak signal from the orignal sound source into a Voltage level large enough to feed the Power amplifier which provides enough power to power and 8-ohm Speaker. The KT99 are the two Power Amplifier tubes and according to this website:(https://www.thetubestore.com/power-tubes/el84-6bq5-tube-types) The Tube Depot Description of the KT88 Tube

According to this website the 6N8P is the Pre-Amplifier Tube: (https://www.radiomuseum.org/tubes/tube_6n8p.html)

6N8P Tube description

The last tube, the 5Z3P as the amazon.com description states, indicates it is for rectification which uses only one tube in amplifiers unl;ike the full bridge rectifier for alternating current to DC rectification because AC/DC rectification only happens at a frequency of 50-60-Hz whereas audio is, conceivably, rectifying up to around 20-kHz so full bridge rectifiers would create a 40-kHz Signal to be pre-amplified and then power-amplified along it's path from signal source through the amp and eventually through the voice coil and cone of the speaker being powered.

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  • \$\begingroup\$ Thanks for the great elaboration. I am surprised tubes are used as rectifiers. I am not sure I understand from your description why they are considered superior to diode bridge. Does it say that it acts as a low-pass filter, whereas a bridge would pass freq.s up to 40KHz? \$\endgroup\$ – ysap Dec 23 '18 at 9:19
  • \$\begingroup\$ They are not superior they were just "first" you can lookup the first semiconductor device and it's a rectifier: en.wikipedia.org/wiki/Crystal_detector Audiophiles use tube rectifiers simply because they like tubes. I like tubes too. but for rectification it generally isn't that important. The other tubes have more of an effect on the sound ( if any). The rectifier simply removes the negative portion of the waveform so that the DC Pre-Amp and Power tubes aren't messed up by the negative voltage. \$\endgroup\$ – Danny Sebahar Dec 23 '18 at 10:15
  • \$\begingroup\$ Thanks. Yes I am well aware what a rectifier does. This is essentially (ab)using a tube for cutting out half cycle of the AC signal (does it? I am not sure, looking at the schematic you added. Maybe it is symmetrical).Then the caps do the rest of the filtering. I was just surprised that someone would prefer this (seemingly inferior, as it has less of the efficiency) over a diode based bridge. Just for the sake of using tubes. Go figure... \$\endgroup\$ – ysap Dec 23 '18 at 13:14
  • \$\begingroup\$ The rectifier acts just like moder bridge rectifiers you can see in the schematic that the bottom tube has a transformer with two secondaries and on is center tapped and the ends both go into the rectifier. This simply changes AC to DC. It doesn't actually effect the audio signal path as it exists only in providing power. However It has a Power response that is more natural than a semiconductor diode because it can only change current slowly as the filament is heated and cooled. \$\endgroup\$ – Danny Sebahar Dec 24 '18 at 1:47
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The description of the amplifier in your example clearly states that one of the five tubes is the rectifier. It should also be pointed out that many vacuum tubes contain more than one function, e.g. dual triodes (12AX7, etc.), so that a stereo preamp, at least, could be built from one tube.

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  • \$\begingroup\$ Thanks. So, one tube with multiple elements? Is there no noticeable crosstalk? \$\endgroup\$ – ysap Dec 23 '18 at 9:16
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    \$\begingroup\$ The interelectrode capacitance specifications for the 12AX7 are on the order of only 1 picofarad. Thus I believe that at audio frequencies, the crosstalk would be well below the channel separation of any normal stereo sources. \$\endgroup\$ – Barry Dec 23 '18 at 14:01
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The example you show uses 5 tubes (valves).

Odd number of tubes? How about two tubes per channel and one rectifier tube?

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