Hoax or truth about 6SJ7 (Equiv.: 6J8P/5693/CV3699/VT-116A).. ??? - diyAudio
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Old 5th July 2011, 01:27 AM   #1
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Default Hoax or truth about 6SJ7 (Equiv.: 6J8P/5693/CV3699/VT-116A).. ???

Hi everyone,

Recently I bought a cheap, single-ended 2x 10W class-A Chinese amp. with DAC wich originaly came with: 2x FU-50 (GU-50) high-freq. oscilator power-pentode, 2x 6J8P (6SJ7) pentode, 1x 5Z4P rectifier, all new Shuguang tubes. Not expecting the components being used were top-quality, but I have too say.. Not bad for the kind of money I've spend, blue 1% R's, if I'm not mistaken I even spotted some Wima cap.'s, also the transformers don't sound very wrong..

Unfortunately, when connected to my 32 Ohms Grado headphones at high volume level, the basses were not as "punchy" as I was used to, so I've replaced the 5Z4P rectifier with a 274B (5U4), borrowed from my 300B amp.. BEFORE YOU TRY(!!!): Please check MAXIMUM ratings for the first cap. or your tube won't live long, the 274B is a direct-heating type!.. A bit of overkill, I know.. But the bigger plates (less resistance, right?) was a BIG improvement.. Also replacing the Chinese FU-50 with Russian GU-50's added more 'musical' warmth, maybe becouse of the slightly different shaped top-getters or a finer grid (more turns) for what I could see from the outside.. It all seems to be a matter of taste I guesse, I can only say I liked the sound.. ..! Slowly I was beginning to understand why some call the GU-50 a 'mini' 300B, well.. Sorta.

To taste some other tubes and just for the FUN of tube-rolling, hell I love it.. I was looking to buy some spares. As for the 6J8P pentode drivers I'm in doubt.. I've red so much about it's equivalent 6SJ7, but I don't know what to make of it:


1. Do trad. NOS brands such as RCA/Valvola/Philips/Raytheon etc. SOUND BETTER then newly build Chinese tubes? What brand should I choose for a crisp but above all.. PLEASANT sound? What brands are in general of just lousy quality, or just not fitted for audio purposes?

2. Do glass-envelope types sound better then canned types (6SJ7), if so.. Why?

3. If I don't care much about microphonics, do "J"-grade military tubes have advantages when it comes to the color of sound? Or.. Does a more robust structure means a better / different audio-quality to start with?

4. Does changing the driver tubes a lot to the color of the sound in the first place?

5. I'm not that much of an 'electro'-purist, but what do we know from practice about replacing rectifier-tubes with scilicon diodes? From hear-say: It may cause some problems becouse some tubes need an electron-cloud prior to switching on HV; it needs a delay? Anyone tested this, what are the (dis-) advantages?


Hope an expert can give me some guidance here.. Paying 17,- U$ for a tube is fine, paying another 20,- U$ to ship the single darn (Sorry for cursing.. :P ..!) thing to Europe is, well.. CRAZY! Sooo.. Better be worth the trouble, and yeah.. Audio-phelia is my middle-name.. ^x^ ..! Thanks in advance..

PS: Also THANKS everyone for the great information, I'm learnning a LOT here, this is a GREAT community!
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Old 5th July 2011, 04:37 AM   #2
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1. Do trad. NOS brands such as RCA/Valvola/Philips/Raytheon etc. SOUND BETTER then newly build Chinese tubes? What brand should I choose for a crisp but above all.. PLEASANT sound? What brands are in general of just lousy quality, or just not fitted for audio purposes?

Yes, no, maybe. It depends on the circuit: gNFB or no gNFB? That can make a big difference on whether or not a different tube will sound different. Back in "the day", good, bad, and indifferent VTs rolled off the assembly lines by the tens of thousands every day. You can get some good current makes, and some really horrible NOS types.

2. Do glass-envelope types sound better then canned types (6SJ7), if so.. Why?

Makes no difference. The metal types run hotter, and don't have the glowey bottle factor.

3. If I don't care much about microphonics, do "J"-grade military tubes have advantages when it comes to the color of sound? Or.. Does a more robust structure means a better / different audio-quality to start with?

YMMV

4. Does changing the driver tubes a lot to the color of the sound in the first place?

Another topology dependent factor.

5. I'm not that much of an 'electro'-purist, but what do we know from practice about replacing rectifier-tubes with scilicon diodes? From hear-say: It may cause some problems becouse some tubes need an electron-cloud prior to switching on HV; it needs a delay? Anyone tested this, what are the (dis-) advantages?

If you're gonna use s SS power supply, best to separate the DC and heater power. That way, you can preheat before turning on the HV. That way, you won't overvolt anything before the heaters are warm. It can be a big problem with DC coupled stages. As for replacing hollow state diodes with silicon, be warned that hollow state diodes have a significantly higher forward voltage, and the output voltage will definitely increase.
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Old 5th July 2011, 06:52 AM   #3
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If you are interested to learn and understand more about tube audio circuits and to find out reasons to the effect of the "good sound", you should get some test equipment and also learn to use those.

For example, frequency response and the amount of distortion ( and it's harmonic content) determines a lot of how the amplifier sounds. Much more than just the tube type used. These two characteristics are just two examples. There are many more.

If you can find difference in the sound between FU50 and GU50, it would be more valuable to know what makes the difference in the measured test results than just to think that soviet tube sounds better than chinese one because of:
Quote:
.....the slightly different shaped top-getters or a finer grid for what I could see from the outside..
Or how do you think about this ?

If you just remain on such "knowledge level" where you randomly change components and search for sound differences this way, you do not really learn much.

Last edited by artosalo; 5th July 2011 at 06:53 AM. Reason: typing error
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Old 5th July 2011, 05:08 PM   #4
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Hi, thanks for your replies,

Yes, I should've known asking these sort of questions would produce some answeres that would even raise more questions, but thanks for a lot of useful information @Miles ..Becouse I'm in the process of choosing, I will never know what the end-result will be until I actualy recieve a tube and plug it in and listen to it's actual sound, this said..

I'm more of a 'scilicon' educated person, if I buy for example a 741 OpAmp from various brands, the difference in sound would be minimal, there are just some absolute max. ratings you'l find in the datasheets, I know some of you won't agree, but for me it is.. As for the GU-50/FU-50, those datasheets looked identical to me. So far I've noticed that with tubes and transformers the sound quality strongly depends on mechanical buildup, different manufacturing processes, strategies, the 'Chinese' way of reducing costs on essential processes, etc. Unfortunately my equipement lacks a real USSR tube-tester, but yes..

I've got and know how to use an osciloscope, LRC-meter, I know how to interprete schematics, I've got a Dutch made 2.700 U$ 300B amp. for comparisson (the 'tube'-sound me I and my partner love the most.) I remember when I selected my amp. I could choose between a 300B and a mesh-plate type, I loved the looks but the rarity of the tubes (COSTS!) and eventualy the sound made me choose for a 300B. What can't be measured however by using an osciloscope is the level of PLEASURE inside a (possibly damaged!) spoiled 36yoa. woman's ear.. This is why I'm speaking in terms of 'pleasand' and 'musical' wich has nothing todo with an amp. being 'audio-phile', yes.. Curves look different, they're never straight, but isn't it the tiniest bit of DISTORTION that makes a tube-sound pleasant? Nooo, don't slap me guys.. @.@

I was curious for some, honnest, well educated feedback about what tubes YOU guys actualy like or would choose if YOU were in my place. Remember, I only interprete datasheets, I just know your hands have seen WAY more tubes then mine! That's where my knowledge ends when it comes to tubes. I also was confused about all gossip found on the net about very common 'Fender Champ' tubes, coming from electro-purists/anarchists, tube-lovers/haters, people who make you pay 100.000 U$ for an amp., people who're trying to de-magnetize stuff with no permanent magnetic properties at all, people who're randomly trying to plug mixed-up triodes and pentodes (I believe it was in this forum someone mixed-up a 6SJ7 and 6SN7.), pretty hard to learn or what to believe.. Funny though how one can learn a LOT from gossip coz, I'd be the first person who's stage-diving in her Philips data-sheet manual to verify, unfortunately it doesn't say a thing about people's experiences, taste, appreciation of sound..

As for the 'cheapo' Chinese amp., it's never been the aim of the game to turn it into a perfect audio-phile instrument, it's more like.. How am I getting as much PLEASURE possible out of 260,- U$, so far my mission has succeeded..

- Replacing DAC, added sym. powersupply instead of 5V USB power.
- Corrected (appearently) insufficient hv.
- Juggling a bit with tubes..

Yes, I know it possibly makes up for lousy output transformers, design, etc.. But finaly it's starting to produce the sound my ears DO appreciate.. So, in the end @Artosalo, it truely HAS been a GREAT learning experience, but you are right.. I should dig deeper into these matters to actualy (audio-phile speaking) improve even further, so..


I'm happy to learn from you guys.. ^x^ ..Thanks!
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Old 5th July 2011, 09:14 PM   #5
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Quote:
Originally Posted by Margarine View Post
@Miles ..Becouse I'm in the process of choosing, I will never know what the end-result will be until I actualy recieve a tube and plug it in and listen to it's actual sound, this said..
That's the halibut when it comes to audio design: you never know what it sounds like until you actually listen. In the end, you just have no choice but to try something. That was the big dilemma with one design I did. If I used a cascode LTP, this would give sufficient gain to eliminate a gain stage. The thing is: I couldn't come up with any info concerning the sonic performance. Everything I got back was the usual: cascodes give higher voltage gain, have better high frequency performance, etc. Every audio mention was related to guitar amps, where low distortion isn't so important since the "tone" of a guitar amp is largely distortion.

As for cascoded LTP splitter/gain stages, I found no examples "in the wild", though they are used quite frequently in solid state practice. Did this mean that it wasn't common in hollow state practice because it was "wierd"? Or had it been tried, and found to be a sonic disaster? That left one option: try it. So that's what I did: tried it, and discovered that it was sonically excellent. Now I could put to good use these VHF dual triodes whose spec sheet makes no mention of any audio applications.

Quote:
I'm more of a 'scilicon' educated person, if I buy for example a 741 OpAmp from various brands, the difference in sound would be minimal, there are just some absolute max. ratings you'l find in the datasheets, I know some of you won't agree, but for me it is..
That's how I started out: solid state. With transistors, the gain is so high that circuit operation is almost completely independent of the actual active device. If the transistor can process the signal frequencies, it doesn't much matter what specific type you use.

Quote:
I was curious for some, honnest, well educated feedback about what tubes YOU guys actualy like or would choose if YOU were in my place.
Now you're asking questions that are answerable. I did a design that used PP 807s. This type has an extensive list of audio Q-Points and loads. This being due to the fact that the 807 is the "RF" version of the audio final: 6L6. All they did was repackage the metal 6L6 in a glass bottle with the standard 5AW five pin base, with a top cap connection to the plate for greater ease of construction of RF power amps, and operation at much higher voltages for more RF power out. 807s are more aesthetically pleasing, and are well categorized as to audio performance, so getting excellent sonic performance is NBD, since the work's already been done for you.

For a different project, I selected the 6BQ6GTB -- a TV horizontal deflection final. The spec sheets have no mention of audio use at all. However, I decided to give it a chance since the type was a good match for junkbox OPT and PTX. I came across a Brazilian (Portuguese? It was written in Portuguese) ham magazine from the 1950s that used it as an AM modulator. That seemed an unusual choice, given that the output power would suggest just using 6L6s.

After drawing up some loadlines, this one looked quite promising, and so I went ahead and based a design on it. Though rated like a 6V6 (Pd= 12W for both types) I could get more than double the power.

After building the design, it tested well, since it was mainly making h3, but very little of the nastier, higher order harmonics. Also sounded very good, even before applying any NFB. Given its original design purpose, the ratings also turned out to be quite conservative, and I could increase the no-signal bias to bust that Pd spec (I'm running 'em with Pd= 17.5W and they aren't red plating) for even better sonic performance.

As for small signal types, I used so far:

6J5 -- grounded cathode voltage amp
6SL7 -- high gain dual triode as LTP splitter/gain stage
6SN7 -- medium gain dual triode as cathode follower grid drivers
6BQ7 -- medium gain dual intended as VHF cascode good to 300MHz as cascode LTP splitter/gain stage
6FQ7 -- medium gain dual triode as cathode followers.

6CB6 -- small signal pentode as error amp in voltage regulator
6KE8 -- triode/pentode as error amp
6X8 -- ditto

Other types:

5U4GB -- dual power diode (selected for forward voltage, Isurge)
0A3/VR75 -- gas discharge voltage reference
0C2 -- gas discharge voltage reference

Quote:
Remember, I only interprete datasheets, I just know your hands have seen WAY more tubes then mine! That's where my knowledge ends when it comes to tubes. I also was confused about all gossip found on the net about very common 'Fender Champ' tubes, coming from electro-purists/anarchists, tube-lovers/haters, people who make you pay 100.000 U$ for an amp., people who're trying to de-magnetize stuff with no permanent magnetic properties at all, people who're randomly trying to plug mixed-up triodes and pentodes (I believe it was in this forum someone mixed-up a 6SJ7 and 6SN7.), pretty hard to learn or what to believe..
Audiophoolery Click the image to open in full size.

Remember:

* If it sounds too good to be true, it probably is.
* Extraordinary claims demand extraordinary evidence.

When you see extravagant claims, get a grip on your wallet.
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Old 5th July 2011, 10:01 PM   #6
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I don't understand, is it a hoax, or a joke, to drive 32-Ohm headphones by GU-50?
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Old 6th July 2011, 03:09 AM   #7
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Quote:
Originally Posted by Wavebourn View Post
I don't understand, is it a hoax, or a joke, to drive 32-Ohm headphones by GU-50?
Well, according to the Chinese manufacturer the design as 'was' SHOULD've been able to drive 32 Ohms, well it works, but not idealy.. But what I've found was that the hv. became instable at higher volume levels so I've replaced the rectifier 5Z4P with a 274B type AFTER having double checked (among some other things, the first cap. needs to be LESS then 4uF(!) wich is also a spec. for a 5Z4P), done some startup current measurements, all within limits.. Seems like it's working like a charm.. Now my 32 Ohm headphones are capable of blowing my head off, producing rich and balanced sound at modest levels, just with a little more punch.. Maybe I could have solved it by changing the power-stage design, but that would need an expert, wich I am NOT..

The hoax thing was about the claims ppl. make about sudden designs, usage of tubes. Looking for some spare 6SJ7's, I was just wondering wether or not I should pay a LOT for rare glass-envelope types, if 1 micro-meter of gold plating is going to improve my sound quality, you know.. Stuff you won't find in datasheets..

@Miles: Omg., a friend of a friend who's done a lot of tube guitar-amp designs and repair, who had to end his carreer becouse of his age / bad eye sight just offered me a nice collection of 6JS7's, various brands all tested, NOS/NIB even matched.. So, I'll be testing next few days.. ^x^ ! As for design extra-vaganza..

A concept can't be too 'weird' if the idear behind it is good.. I got intrigued by this design, the idear behind it is quiete intresting (Sorry, Dutch only, but I think the schematics speak for them selves) :

Scylla
Prypjat (Припять)

Don't expect to find matched pairs of those rarities at reasonable prices, but don't they just look AWSOME!
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Old 6th July 2011, 03:57 AM   #8
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I still don't understand why 40W tube is needed to drive headphones that need miliwatts only. It is not the best tube for headphones. Much better one is 4P1L, it is an ideal headphone driver, like small 300B. 4P1L in triode mode is much more linear than GU-50.
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Old 6th July 2011, 06:29 AM   #9
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@Wavebourn: Ehh, 40 Watts??? Maybe being used as HF oscilator as intended during WW2 in communication apperture, but I'm only getting 10-12W tru my speakers, the GU-50 is being used in triode-mode, single-ended class-A..

But THANKS for reminding me to take schematic notes for the power-stage, I only did that for the hv. rectifier filter part but.. *blush* .. To be honnest, I realy don't know where the headphone output's coming from actualy, could it be tapped directly from the 6JS7 driver tubes? There's a switch, switching between phones and speakers but it doesn't turn one of 'em off, more like an impedance switch, so.. I suspect also the headphones are using the output transformers.. After entirely replacing the DAC + powersupply, after fixing the 'wobbly' hv. line, I got all excited to put it to the test and forgot all about the power-stage..

Still not sure btw. what could've caused the lack of 'punch' hv. power issue, a faulty 5Z4P? Transformers not performing well? I only had a spare 274B available.. As it is now, the sound has improved dramatically for the speakers as well as for the headphones output!

But yeah.. I agree, ie. the Mini-Dot MK2/3 is a WAY better headphone amp., my 300B is a WAY better speaker amp.. This Chinese build is just a little bit of both, economy style.. But at least I dare to heat-up a soldering iron inside, if my 300B needs fine-tuning I'd hire a professional.. :P ..! So..

The Chinese just indeed have "Created Creativity" (Wavebourn), heheh.. ^x^

Last edited by Margarine; 6th July 2011 at 06:33 AM.
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Old 6th July 2011, 06:54 PM   #10
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40 Watts is specified maximum of power dissipation on anode, no matter are they used in RF amplifiers, or AF amplifiers. But I have an experience, sometimes during breadboarding they dissipate more that 100W on anode, and after that are alive and well. When the chassis is upside-down (like on attached picture) they are invisible, but smell of hot iron tells that something is wrong...

http://wavebourn.com/forum/download.php?id=508&f=7

It is one of last my prototypes that uses GU-50 tubes. 80W per channel stereo amp.
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