Fever FE-6J1-2 tube preamplifier kit review + listening test files

[mhardy6647]

Numbers and price really. Those HF types were made by the millions and cost virtually nothing. As opposed to 12AX7/ECC83: Those cost as much as the kit, each.

Immense numbers of 12AX7 tubes have been made, too. The difference is that there was/is a continuing demand for them.
Many/most of the TV tubes (especially of the 1960s, and especially those with odd heater voltages for use in series-string, power transformerless, "killer" chassis designs) are essentially worthless.

There has been a cottage industry of repurposing plentiful, truly cheap NOS (new old stock) for "hifi"/audio. Some of those efforts make more sense than others. The problem is that, successful designs can stress the supply of the tube type(s) required, driving up prices.

 

[Mike123]

Move to two/multi-tones or music, and most could not tell at 1%, and even 10% was harder.

I have the same experience. Now, let's decide on a position. My position is that if I can even occasionally hear a difference under ideal conditions and with specific signals, then such equipment is useless. Moreover, it's not difficult to assemble equipment these days where I personally can't hear a difference under any conditions. The second position: if the distortion isn't so noticeable that it's annoying all the time, then it's okay. This also present. It's just for what? Moreover, this test clearly shows that if there is a tube sound, it's not in the 2nd and 3rd harmonics, but most likely in the high output impedance of the power amplifier. Adding a tube to a preamplifier makes no sense at all.

 

[solderdude]

Yeah. Seems to me all these AliExpress and similar pizza lunch cost products must be funded. Fund and destroy competition, but cooperate at local level.

Some more images, including distortion in the tube path used for these test files. The hum that I have complained about is audible in fact only without music signal, or in extremely silent passages at high (unrealistic) volume setting. Otherwise we have ear masking. Ear masking works to mask non-linear distortion as well. Please note I am saying non-linear, not a mere "harmonic".
The woman voice used in the music sample is very sensitive to distortion - as we can hear from the delta file. Still - who can hear it when comparing the music files??

View attachment 552083 View attachment 552084View attachment 552086

Some questions that popped up.

What peak voltages were present, when making the recording, at the output of the buffer and what input range was the ADC set to

RMS values of the supplied files are around -17.6dB and peak levels at -0.9 and -1.1 for the different files.
This means the actual distortion most of the time was much, much lower than what's shown above as that is around 0.14x 0dBFS.
Now I am getting curious how the distortion was at around -17dB when 0dBFS was set at the same value as the recording was made.
This is generally the reason why tube amplifiers don't sound nearly as bad as distortion plots near maximum power suggests.

Pavel, did you correct for the polarity inversion of the tube preamp?

Seems so when analyzing the files

 

[pma]

@solderdude 2.7V input Cosmos range, peak sample -1dB as shown in the DW pdf report posted earlier. Post #25.
THD vs. Voltage curves were already posted in the thread, please go through the thread. I am out now with very limited time for the forum. Delta diff file was also posted so please check it to see and hear the distortion. So it goes that tube distortion decreases with lower amplitude. But we do not listen to 0dBFS music all the time. If yes, it is so distorted by compression that it is pointless. The conditions in the test are realistic. 2.5Vrms is about maximum you can reasonably get from this preamp.

 

[solderdude]

Thanks, will look into the rest. Not doubting anything other than why I could not detect it,
Yes, I had listened to the null before and now that I think I found the biggest 'tell' will try again.

When the null had not been amplified (which I don't think is is) and peaks were 2.4V and max. output voltage of the circuit would almost have been reached.
Looks indeed like the biggest culprit is compression near the top of the signal.
I knew 10% distortion in the bass in music is not easily audible.

RMS level of the distortion was -45dB so 0.5% which I always assumed would be a detectable level.

Tube goodness ... my ...

Thanks for the test. The tube amps I have here are far better performing and could never tell them apart, unless driving a headphone through high Z or using noisy tubes or tubes that were on the verge of crapping out.

Tried a few times but sadly the 2nd gamble always plays 2 tracks at the same time doubling the signal and distorting so can't do the test.

Tried ABX comparator 11/16 first time and 9/20 second time.

Output:
Default : Primary Sound Driver
Crossfading: NO

17:27:35 : Test started.
17:29:59 : Test restarted.
17:29:59 : 01/01
17:31:32 : Test restarted.
17:31:32 : 02/02
17:31:52 : Test restarted.
17:31:52 : 02/03
17:32:57 : Test restarted.
17:32:57 : 02/04
17:33:16 : Test restarted.
17:33:16 : 02/05
17:33:34 : Test restarted.
17:33:34 : 02/06
17:33:53 : Test restarted.
17:33:53 : 02/07
17:34:24 : Test restarted.
17:34:24 : 03/08
17:34:51 : Test restarted.
17:34:51 : 03/09
17:35:22 : Test restarted.
17:35:22 : 03/10
17:35:41 : Test restarted.
17:35:41 : 04/11
17:36:05 : Test restarted.
17:36:05 : 05/12
17:36:21 : Test restarted.
17:36:21 : 06/13
17:36:36 : Test restarted.
17:36:36 : 06/14
17:36:54 : Test restarted.
17:36:54 : 06/15
17:37:22 : Test restarted.
17:37:22 : 06/16
17:37:43 : Test restarted.
17:37:43 : 07/17
17:38:14 : Test restarted.
17:38:14 : 08/18
17:38:28 : Test restarted.
17:38:28 : 08/19
17:38:50 : Test restarted.
17:38:50 : 09/20
17:38:50 : Test finished.

----------
Total: 9/20
p-value: 0.7483 (74.83%)

-- signature --

Yep I can't hear this particular 3% distortion near the top of a signal in this particular piece of music.
I thought I heard something but its all in my head.

 

[pma]

Thanks for the new online results as well!

Shall we say that a miserable tube preamp is more difficult to tell, in the ABX, then super duper SOTA DACs?

 

[fpitas]

Immense numbers of 12AX7 tubes have been made, too. The difference is that there was/is a continuing demand for them.
Many/most of the TV tubes (especially of the 1960s, and especially those with odd heater voltages for use in series-string, power transformerless, "killer" chassis designs) are essentially worthless.

There has been a cottage industry of repurposing plentiful, truly cheap NOS (new old stock) for "hifi"/audio. Some of those efforts make more sense than others. The problem is that, successful designs can stress the supply of the tube type(s) required, driving up prices.

I'm surprised we aren't seeing the car radio tubes that were once popular. I'd think the low anode voltage required would be attractive for this hobbyist sort of stuff.

 

[mhardy6647]

I'm surprised we aren't seeing the car radio tubes that were once popular. I'd think the low anode voltage required would be attractive for this hobbyist sort of stuff.

Space-charge tubes! I am perversely fascinated by them!

Space Charge and Other Low-Voltage Tubes

Tube projects don't always require lethal voltages! Some tubes were designed to run at 12V. Others were not but work well anyway at B+ down to four volts.

www.junkbox.com

FWIW, I blame Pete Millett.

Low-voltage Hybrid headphone amp

Because I am... what I am I think sometimes about building amplifiers based on a space charge "power amplifier" tetrode!
Y'know... like this, maybe?

Twenty blistering milliwatts of awesome, throbbing output power! Just the thing for techno or heavy metal, with appropriate loudspeakers, of course.

These were "really" meant to be used as drivers for a transistor power amplifier... but, you know, if the loudspeaker's sensitive enough...

tube data above are from https://www.worldradiohistory.com/B...ooks/RCA-Receiving-Tube-Manual-1961-RC-21.pdf

The space charge tubes definitely yield low grade fever hifi components!

 

[fpitas]

Space-charge tubes! I am perversely fascinated by them!

Space Charge and Other Low-Voltage Tubes

Tube projects don't always require lethal voltages! Some tubes were designed to run at 12V. Others were not but work well anyway at B+ down to four volts.

www.junkbox.com

FWIW, I blame Pete Millett.

Low-voltage Hybrid headphone amp

View attachment 552378

Because I am... what I am I think sometimes about building amplifiers based on a space charge "power amplifier" tetrode!
Y'know... like this, maybe?

View attachment 552379

These were "really" meant to be used as drivers for a transistor power amplifier... but, you know, if the loudspeaker's sensitive enough...

tube data above are from https://www.worldradiohistory.com/B...ooks/RCA-Receiving-Tube-Manual-1961-RC-21.pdf

The space charge tubes definitely yield low grade fever hifi components!

Yup! I found them in my RCA tube manual. Had no idea they made such things.

 

[mhardy6647]

They met an interesting but transient need. The space age ushered in the glamour of solid state consumer electronics, and, of course, the notion of auto radios that could run off of the native low-voltage (DC) power available in a vehicle rather than an inefficient and noisy (electrically speaking) vibrator type* power supply was attractive to the auto industry, too.

The only problem was that practical and cost-effective transistors suitable for high frequency use came late to the party. Therefore, for a brief, shining (in the thermionic sense!) moment, hybrid radios with low voltage vacuum tubes in the front end and transistors for the final audio amplification stage (or stages) were a thing. Economically speaking, they arrived at a propitious time: there was a pretty signifiant recession as the 50s gave way to the 60s that led to sea change in what Americans were willing to accept from an automobile. Purchase price and (gasp!) fuel economy became important to Mom & Pop Americans!

Interestingly, Chrysler's original intent was to introduce Valiant as a brand, not unlike Ford's current approach to the Bronco nameplate (don't get me started on that one)... but I digress...

EDIT:
___________________
* Re: vibrators... get your mind(s) outta the gutter! For those not old enough to have experienced them, this kind of vibrator was an electromechanical nightmare gizmo that made choppy, square wave AC out of DC. Nasty and brutish, but at least it allowed one to use transformers to get some high voltage into the old Delco radio!
Yup. vibrators and trannys. Sounds like something best not researched on the internet unless SafeSearch is "on".

 

[fpitas]

They met an interesting but transient need. The space age ushered in the glamour of solid state consumer electronics, and, of course, the notion of auto radios that could run off of the native low-voltage (DC) power available in a vehicle rather than an inefficient and noisy (electrically speaking) vibrator type* power supply was attractive to the auto industry, too.

The only problem was that practical and cost-effective transistors suitable for high frequency use came late to the party. Therefore, for a brief, shining (in the thermionic sense!) moment, hybrid radios with low voltage vacuum tubes in the front end and transistors for the final audio amplification stage (or stages) were a thing. Economically speaking, they arrived at a propitious time: there was a pretty signifiant recession as the 50s gave way to the 60s that led to sea change in what Americans were willing to accept from an automobile. Purchase price and (gasp!) fuel economy became important to Mom & Pop Americans!

View attachment 552382

View attachment 552383

View attachment 552384
Interestingly, Chrysler's original intent was to introduce Valiant as a brand, not unlike Ford's current approach to the Bronco nameplate (don't get me started on that one)... but I digress...

EDIT:
___________________
* Re: vibrators... get your mind(s) outta the gutter! For those not old enough to have experienced them, this kind of vibrator was an electromechanical nightmare gizmo that made choppy, square wave AC out of DC. Nasty and brutish, but at least it allowed one to use transformers to get some high voltage into the old Delco radio!
Yup. vibrators and trannys. Sounds like something best not researched on the internet unless SafeSearch is "on".

Yup, vibrator supplies were elementary switchers. They briefly made transistorized vibrator replacements, as I recall.

Going by the graphs, those low voltage tubes work a lot better if you can give them more like 30V.

 

[Salt]

Thanks for the new online results as well!

View attachment 552377

Shall we say that a miserable tube preamp is more difficult to tell, in the ABX, then super duper SOTA DACs?

Sophisticated question.

 

[solderdude]

Next test NOS 44.1 R2R versus DS DAC ?

 

[danadam]

I tried and failed to find the test files I created a long time ago with added distortion.

I made some here:

Harmonic and Intermodulation Distortion

Somehow the first post in this thread, on harmonic distortion, disappeared through the magic of moving the thread around. I have added it to the first post so the title is now correct (Harmonic and Intermodulation Distortion). Btw this is Don being “super polite” and not blaming the proper...

www.audiosciencereview.com

 

[DonH56]

Yup, vibrator supplies were elementary switchers. They briefly made transistorized vibrator replacements, as I recall.

Going by the graphs, those low voltage tubes work a lot better if you can give them more like 30V.

I really wanted to keep a couple of dad's old CB radios that had vibrators in them but just couldn't justify more "stuff" in the house I knew I would not use. One of them I fixed with a shiny modern electronic transistor vibrator many years ago. It blew up after a few month's use so I went back to mechanical. I could have rolled my own and had it last longer but life moved on.

 

[DonH56]

I made some here:

Harmonic and Intermodulation Distortion

Somehow the first post in this thread, on harmonic distortion, disappeared through the magic of moving the thread around. I have added it to the first post so the title is now correct (Harmonic and Intermodulation Distortion). Btw this is Don being “super polite” and not blaming the proper...

www.audiosciencereview.com

Thanks, I have not looked at that thread in a long time, lot of new posts. Often enough I only look when I am explicitly tagged so get a notification about it, otherwise I tend to ignore my old (and new) threads.

 

[pma]

A lot of witty posts here. I understand that myth debunking is painful.

 

[Bob from Florida]

Next test NOS 44.1 R2R versus DS DAC ?

Currently doing that sighted with a Gustard Audalytic D70N versus a Schiit Modius. I can't even convince myself with sighted testing that there is a difference.

 

[pma]

Currently doing that sighted with a Gustard Audalytic D70N versus a Schiit Modius. I can't even convince myself with sighted testing that there is a difference.

Thank you Bob. By now, we have no positive abx result that would discern a mediocre tube preamp from the DAC/ADC loop. Audiophilia is a strange disease.

 

[solderdude]

Perhaps more of an anxiety than a disease