FX Audio 6j1 tube preamp - a $31 wonder

drjohnnyfeva said:
The Voshkods are great when you want or need a little more punch. They seem to have a little more gain too, as they are marginally louder than any other brand I've used. But that's not a bad thing; in fact, I prefer them with my Sony AV receiver that I use for music and movies in our living room. The RCA's are smooth and very clear. They don't add as much punch, but everything is very musical and real sounding. They are easier to "control" than the Voshkods. The Voshkods can make some of the music too punchy - distorted - with some material. It's almost like a distortion pedal and you need to either back off the source material or, for my system, dial back the tone controls. Since I don't like futzing with the tone on my office system, just tending to leave it set and forget about it, the RCA's are better for that. They are more forgiving with all of the source material, even streamed music. And they sound just great.

The Voshkods are great in the one system for movies and tv, tho. Voices are more forward and since I don't use a center speaker in there, that's a bonus. I had Motorola's in there that during conversation in movies, I was jacking the volume to hear, only to have to quickly dial it back when action began.

And therein lies the secret with having experimented with so many tubes: you remember the sound and what some did better than others and use them to their strength and minimize their weakness in each system they're in. RCA's allow me to forget the pre's are even in the line for my office while putting out crystal clear music and while the Voshkods are real good for music, they really do better with movies and tv for me.

Of course, I recently finally found some inexpensive Western Electric 403a's, so they are in my office system now. And I happened upon an Ericsson set of 403a's from Sweden in the same search, so I'm testing those out too. Very nice so far, not sure they overcome the RCA's for my tastes, but we'll see. The search continues. The 403's were also a bit more expensive than the RCA's when I bought them, but lots of folks have now found the RCA's and prices have gone up to or beyond the $25 I paid for each set of the 403a's.

The last thing I'll add is that I may be the only one on this thread, but I hated the Mullard M8100's. The only tubes I've ever bought that I couldn't find something redeeming about. The whole bottom of the soundstage was gone. It was almost linear sonicly, like being in a pit and the sound going over you. The most bizarre experience. Took them out, reseated them, same weird phenomenon. Have only put them in one other unit and it was the same there too. Don't appear damaged or fake, but who knows. Just terrible. They were also the second most expensive ones I've purchased and that taught me to be more frugal when getting things to try. I think they were like $45 for the pair like 3 years ago on ebay, so since then I've only approached $30 on any pair with most coming in under $10 for a pair. 6AK5's are still plentiful if not the Western Electric or even TungSol's from the states. I had a pair of TungSol's in my cart, but $42 was just too rich for my blood. Maybe I'll find a couple singles for cheaper that aren't matched. Who knows? Anyway, hope that helps.
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Thank you DJF, just what I was curious about!
 
So when I ordered the original Tube01 from Ali recently, I mentioned that I also got a pair of opamps for my Tube03 that I happened upon. They actually looked like the old Burson opamps I considered buying long ago.

https://www.bursonaudio.com/wp-content/uploads/2018/12/V5i-S1.jpg

These are a little different, but the price is right and I wondered if maybe they were meant to be less expensive alternatives...

https://www.aliexpress.us/item/3256...t_main.11.786e1802k9ReBd&gatewayAdapt=glo2usa

Anyway, the Oracle II's are in and they sound ok. I've only had them in a couple days and that's after the MUSES8920's in for a year or more - which I was really happy with. I'm uncertain if I "like" the Oracle II's. Nothing jumped out sound-wise as was the case when I changed from the stock opamps to the MUSES8920's. I've also tested some LME's, 1656's, and a few others. The MUSES were a very noticeable improvement immediately and more with time, so I'm willing to give the Oracle II's some time, but I'm leaning toward going back to the MUSES already.

I was really hopeful that the Oracle II's were going to be another noticeable improvement, but so far, I feel let down, tbh. Which is OK for $4.50/ea, but since the expectation was for a cheap Burson - that others have raved about, I'm just kind of meh. They don't deliver the bass and the music just doesn't seem/sound/appear as crisp and natural... and that is at all volume levels. I'll leave them in for now, but I'm sort of fighting myself to do so, so I don't imagine them lasting much longer unless "burn-in" with opamps is a real thing, which has not been my experience, per se.

Who knows. Anyway, thought I'd report that for those still swapping opamps in the Tube03. Experience is education with this stuff.
 
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...so just a quick update...

I removed the Oracle II's today. I actually had a pair of MUSES8820's leftover from a Mouser order, so I used them to replace the Oracle II's. I also went back to the RCA tubes over the Ericcson 403b's for now. I was using the MUSES8920's in this unit, but a while back I compared two Tube03 units in my system, one with the 8820's and the other with the 8920's, and wound up preferring the 8820's. So when I found the extra pair of the 8820's I was happy to put those in.

Anyway, the Oracle's just weren't doing it for me. All things being equal, with the RCA's back in, there was some added sibilance with the Oracle's that I couldn't take. The MUSES are really clear and crisp and don't distort vocals with the /s/ sound. I really like that instead of when is gets over done and sounds like the hiss of a snake with lyrics ending in s.

So the MUSES are back in and all is right with the world again. lol.
 
drjohnnyfeva said:
...so just a quick update...

I removed the Oracle II's today. I actually had a pair of MUSES8820's leftover from a Mouser order, so I used them to replace the Oracle II's. I also went back to the RCA tubes over the Ericcson 403b's for now. I was using the MUSES8920's in this unit, but a while back I compared two Tube03 units in my system, one with the 8820's and the other with the 8920's, and wound up preferring the 8820's. So when I found the extra pair of the 8820's I was happy to put those in.

Anyway, the Oracle's just weren't doing it for me. All things being equal, with the RCA's back in, there was some added sibilance with the Oracle's that I couldn't take. The MUSES are really clear and crisp and don't distort vocals with the /s/ sound. I really like that instead of when is gets over done and sounds like the hiss of a snake with lyrics ending in s.

So the MUSES are back in and all is right with the world again. lol.
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Happy to know these still have plenty of mileage with the right op-amps (and capacitors!). I simply don't have the time to look into working the latter, but it's an option for the future.

For the past month or so, I've run my FX-Audio Tube 03 with Voskhods and no op-amps in a daisy-chain with the Nobsound E6 with the Russian tubes. I've had no real problem with this set-up! It sounds great. Treble sings like crazy to the point that I use the tone control on my amp (NAD 3020i) to tame it. No sibilance thankfully. Although I have noticed that the dynamics that I got just from the E6 have reduced, along with the increased noise floor from using two tube pre-amps + one SS pre-amp (I know ppl here will crucify me for this, but I just like working with what I got.)

Also to note: I definitely prefer the sound of my Tube-03 without any op-amps. It might just be the ones I used (OPA2134s) that didn't vibe with me over time, so maybe the MUSES8820s will sound even better!

Definitely thinking of reverting back to my E6 only. Will take notes when I do go back to that and see if my experience with it alone is the same as before. Not sure what I'll do with my Tube-03 if I prefer this but I know I definitely won't sell it/give it away :)
 
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I have a modded Aiyima Tube-A3, Siemens 5654W valves, no op-amps, input and output coupling capacitor have been changed to Clarity Caps CSA 4.7uF, make a better match for SS amp, 12v 50W linear power supply. Upstream I have e1da 9038d portable USB-C DAC/headphone amp and downstream Arcam rHead headphone amp & HiFIMAN Sundaras...
 
drjohnnyfeva said:
The Voshkods are great when you want or need a little more punch. They seem to have a little more gain too, as they are marginally louder than any other brand I've used. But that's not a bad thing; in fact, I prefer them with my Sony AV receiver that I use for music and movies in our living room. The RCA's are smooth and very clear. They don't add as much punch, but everything is very musical and real sounding. They are easier to "control" than the Voshkods. The Voshkods can make some of the music too punchy - distorted - with some material. It's almost like a distortion pedal and you need to either back off the source material or, for my system, dial back the tone controls. Since I don't like futzing with the tone on my office system, just tending to leave it set and forget about it, the RCA's are better for that. They are more forgiving with all of the source material, even streamed music. And they sound just great.

The Voshkods are great in the one system for movies and tv, tho. Voices are more forward and since I don't use a center speaker in there, that's a bonus. I had Motorola's in there that during conversation in movies, I was jacking the volume to hear, only to have to quickly dial it back when action began.

And therein lies the secret with having experimented with so many tubes: you remember the sound and what some did better than others and use them to their strength and minimize their weakness in each system they're in. RCA's allow me to forget the pre's are even in the line for my office while putting out crystal clear music and while the Voshkods are real good for music, they really do better with movies and tv for me.

Of course, I recently finally found some inexpensive Western Electric 403a's, so they are in my office system now. And I happened upon an Ericsson set of 403a's from Sweden in the same search, so I'm testing those out too. Very nice so far, not sure they overcome the RCA's for my tastes, but we'll see. The search continues. The 403's were also a bit more expensive than the RCA's when I bought them, but lots of folks have now found the RCA's and prices have gone up to or beyond the $25 I paid for each set of the 403a's.

The last thing I'll add is that I may be the only one on this thread, but I hated the Mullard M8100's. The only tubes I've ever bought that I couldn't find something redeeming about. The whole bottom of the soundstage was gone. It was almost linear sonicly, like being in a pit and the sound going over you. The most bizarre experience. Took them out, reseated them, same weird phenomenon. Have only put them in one other unit and it was the same there too. Don't appear damaged or fake, but who knows. Just terrible. They were also the second most expensive ones I've purchased and that taught me to be more frugal when getting things to try. I think they were like $45 for the pair like 3 years ago on ebay, so since then I've only approached $30 on any pair with most coming in under $10 for a pair. 6AK5's are still plentiful if not the Western Electric or even TungSol's from the states. I had a pair of TungSol's in my cart, but $42 was just too rich for my blood. Maybe I'll find a couple singles for cheaper that aren't matched. Who knows? Anyway, hope that helps.
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See post #4617.(Yes, it appears that I posted to myself!)
 
Pike72 said:
Is there any info/compatibility on 5726/6AL5 tubes that you know of?
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The short answer is that I don't know. The heater voltage is the same as many of the others like the 6AK5 - which is the standard tube for this thing. The problem is comparing the tube stat numbers of the 6AK5 with others, at least that's what I did. I have not seen the 6AL5 mentioned in the thread, but I may have missed it. Nevertheless, from first appraisal, the 6AL5 has the same heater voltage, but the amps being drawn on the heater are double the value of the 150mA of the 6AK5. This is also true of the 6AU6. So my answer is MAYBE.

I plugged a pair of Tung Sol 6UA6's into the Tube03 many Moons ago. They worked, but I didn't keep them in very long. the unit was definitely warmer, but, like I said, I didn't have them in long to get too much hotter. I was concerned about frying the unit and FlaCharlie has commented on the heater values being double that of the 6AK5's and how units are often spec'd to perform a little above what is stock, but maybe NOT double.

I took the chance to try the 6AU6's - which might be a sister to the 6AL5's, but that will have to be a decision you have to make. Here's a link to the Tube sheets. Pull up the 6AK5's and one of the PDF's of that one - any one, they are the same. In another window, find the 6AL5's and compare those spec values and see what you think. It is mentioned as an RCA HB3 - I don't know what that mean, but the 6AU6 is too, which makes me think they might be related.

I didn't get too into it. You should look at them closely. Here's the link to the page with all of the ones mentioned in this post.

https://frank.pocnet.net/sheets62.html
 
drjohnnyfeva said:
The short answer is that I don't know. The heater voltage is the same as many of the others like the 6AK5 - which is the standard tube for this thing. The problem is comparing the tube stat numbers of the 6AK5 with others, at least that's what I did. I have not seen the 6AL5 mentioned in the thread, but I may have missed it. Nevertheless, from first appraisal, the 6AL5 has the same heater voltage, but the amps being drawn on the heater are double the value of the 150mA of the 6AK5. This is also true of the 6AU6. So my answer is MAYBE.

I plugged a pair of Tung Sol 6UA6's into the Tube03 many Moons ago. They worked, but I didn't keep them in very long. the unit was definitely warmer, but, like I said, I didn't have them in long to get too much hotter. I was concerned about frying the unit and FlaCharlie has commented on the heater values being double that of the 6AK5's and how units are often spec'd to perform a little above what is stock, but maybe NOT double.

I took the chance to try the 6AU6's - which might be a sister to the 6AL5's, but that will have to be a decision you have to make. Here's a link to the Tube sheets. Pull up the 6AK5's and one of the PDF's of that one - any one, they are the same. In another window, find the 6AL5's and compare those spec values and see what you think. It is mentioned as an RCA HB3 - I don't know what that mean, but the 6AU6 is too, which makes me think they might be related.

I didn't get too into it. You should look at them closely. Here's the link to the page with all of the ones mentioned in this post.

https://frank.pocnet.net/sheets62.html
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Thanks again DJF for the info! I'll let you know when I find out more.
 
6AK5 is also a small signal pentode tube like the 6AU6, except it has different operating points (transconductance, gain, etc.) along with the extra heater current drawn like mentioned.

Both those tube being pentodes have five elements. A 6AL5 is double diode, which means it's a dual section two element tube.
Sort of like a rectifier tube (diode), but the 6AL5 was used for detection duty in FM receivers...or AM...sumthin' like that...
 
Mike Stehr said:
6AK5 is also a small signal pentode tube like the 6AU6, except it has different operating points (transconductance, gain, etc.) along with the extra heater current drawn like mentioned.

Both those tube being pentodes have five elements. A 6AL5 is double diode, which means it's a dual section two element tube.
Sort of like a rectifier tube (diode), but the 6AL5 was used for detection duty in FM receivers...or AM...sumthin' like that...
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Thanks for this info. Do you have any experience with how the 6AL5 might perform in the FX Audio units? I have no real understanding of the various tube types other than looking at their specs from the sheets.

I mentioned that I have inserted and played the 6AU6's in the Tube03 very briefly. The unit ran a bit warmer than with the 6AK5's, but that I didn't keep them in for a prolonged period of time. I know that the 6AK5's were not "sound" or output tubes in their real life applications, but that they and the 6AU6's are both RF pentodes seemed like a possible at working in this unit.

The 6AL5's, otoh, being double diode is something I have no familiarity with. If they are wired completely differently and potentially carry current differently than 6AK5's that would seem to be a bad match. Can you shed some light on this if you are able?
 
The 6AK5 and the 6AL5 serve completely different functions and are not compatible with each other- as Mike Stehr mentioned. Both tubes have the heaters on pins 3&4, other than that there are no similarities.
 
drjohnnyfeva said:
Thanks for this info. Do you have any experience with how the 6AL5 might perform in the FX Audio units? I have no real understanding of the various tube types other than looking at their specs from the sheets.
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No, I have no experience with the FX audio Tube03 unit.

One thing to pay attention to with regard to tube data or the actual tube schematic/symbol, is how many elements are in the tube.
Once understood, you can clearly see the 6AL5 dual diode is not compatible.
drjohnnyfeva said:
I mentioned that I have inserted and played the 6AU6's in the Tube03 very briefly.
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What tubes are originally used in the Tube03?
 
Mike Stehr said:
No, I have no experience with the FX audio Tube03 unit.

One thing to pay attention to with regard to tube data or the actual tube schematic/symbol, is how many elements are in the tube.
Once understood, you can clearly see the 6AL5 dual diode is not compatible.

What tubes are originally used in the Tube03?
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Thanks for clarifying. I understood from your post that the 6AL5 wasn't an equivalent of the 6AK5, but was attempting to understand why because "double diode" is something never mentioned in this thread.

So the original unit, the one that prompted this great thread, the FX Audio Tube01 unit, came with the 6J1 tube and the Tube03 came with the 6K4 tube. Both of those tubes list the 6AK5, specifically, but many more as equivalents. I've found more listed as equivalents using the European numbers EF95. Both have also been reported to function with tubes listed as EF93 and EF94. Among some of the listed compatibilities with the EF93's are the 6BA6, which, if I recall was also a potential equivalent on RadioMuseum... EF94 you get the 6AU6.

Anyway, bunches of tubes have come up on this thread. I've tried a bunch myself. I've tried to stick to specific equivalents of the 6AK5, but have tried others to limited success. The 6AL5 was one that I had never heard and it's clear that it is NOT a tube for these units.
 
From my experience .... I'd be leery of the (edit- Chinese) 6K4. I wrote this back in post #4432:

"I rechecked all my tubes for gain as circuit provides for in the Tube03. With 1Vac in and volume preamp control at max here's the results: The 6K4's that came with the preamp, no markings of any kind- 1.87 and 0.97 (!) ; the Chinese 6J1's that came with the Tube 01- 3.27 and 3.28 ; and the (4) GE 5654W bought new elsewhere online- 3.52 , 3.48, 2.56, 2.43."

I've since sold the 03 and kept the 01 so I can't follow up. But if you end up with a balance issue then swap the tubes side to side and see if the problem follows the tubes. I gave the subsequent owner matching pairs of 6J1 and 5654W along with those totally mismatched 6K4's. The only reason I sold the 03 was because I felt the tone controls were unecessary- for my purposes.
 
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drjohnnyfeva said:
Thanks for clarifying. I understood from your post that the 6AL5 wasn't an equivalent of the 6AK5, but was attempting to understand why because "double diode" is something never mentioned in this thread.

So the original unit, the one that prompted this great thread, the FX Audio Tube01 unit, came with the 6J1 tube and the Tube03 came with the 6K4 tube. Both of those tubes list the 6AK5, specifically, but many more as equivalents. I've found more listed as equivalents using the European numbers EF95. Both have also been reported to function with tubes listed as EF93 and EF94. Among some of the listed compatibilities with the EF93's are the 6BA6, which, if I recall was also a potential equivalent on RadioMuseum... EF94 you get the 6AU6.

Anyway, bunches of tubes have come up on this thread. I've tried a bunch myself. I've tried to stick to specific equivalents of the 6AK5, but have tried others to limited success. The 6AL5 was one that I had never heard and it's clear that it is NOT a tube for these units.
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Thank you, DJF! Your investigation saved me from possible disaster.
(we are not worthy)
 
Trypower said:
From my experience .... I'd be leery of the 6K4. I wrote this back in post #4432:

"I rechecked all my tubes for gain as circuit provides for in the Tube03. With 1Vac in and volume preamp control at max here's the results: The 6K4's that came with the preamp, no markings of any kind- 1.87 and 0.97 (!) ; the Chinese 6J1's that came with the Tube 01- 3.27 and 3.28 ; and the (4) GE 5654W bought new elsewhere online- 3.52 , 3.48, 2.56, 2.43."

I've since sold the 03 and kept the 01 so I can't follow up. But if you end up with a balance issue then swap the tubes side to side and see if the problem follows the tubes. I gave the subsequent owner matching pairs of 6J1 and 5654W along with those totally mismatched 6K4's. The only reason I sold the 03 was because I felt the tone controls were unecessary- for my purposes.
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Yeah, I remember that post vaguely. So with just the brief bit of looking I did to refresh my memory on some of this, I think part of the problem is some of the suffixes added to the tubes like W's at the end of 5654W, etc. There are 6K4P tubes that I think are completely different than 6K4. I haven't looked into it, but 6K4P tubes are equivalents to the 6BA6. I think I found this by a Tesla 6X31 tube that was listed as an equivalent to the 6AK5. Basically, I just went from radio museum, the tube sheets, and what @NZVinylFan had commented on and tried.

Anyway, I tend to agree that the 6K4 tubes are odd. I have not used that tube other than the ones supplied, but I have used the Russian 6k4p-ev tubes, that I'm not even sure are "6K4's" anymore. Nevertheless, the Russian 6k4p-ev's are nice. 6BH6 is another that @NZVinylFan suggested that he used in the Tube01, but that I tried in the Tube03 and had a failure. NZ stopped rolling tubes when he found the 6BH6 for his Tube01 and I have not retried them in the Tube03 after the failure. I use them in another tube buffer I have. They are in that one for good and I appreciate NZ for suggesting them.

There are a lot of super close number letter combinations on these and the user had best beware of tube dyslexia, it's a real thing. Use caution and always recheck.
 
I did edit my post to say CHINESE 6K4. I see this statement from the Valve Museum- "The 6K4 is a sub miniature VHF/UHF amplifier triode that Sylvania introduced in 1947 some three years before Mullard launched the equivalent EC70. So these tubes are obviously not a substitute for the Chinese 6K4 pentode. I also have some 6J1P-EV Voskhod will seem to be quite fine, like the GE 5654W.

This is an embarrasing story. Back when, I used to spend A LOT of time with tube audio gear, both hi-fi and guitar amps. So I cloned a complete 1959 Fender Super amplifier. A friend has a nice collection of these old Fender amps, so I borrowed his original to compare. Back and forth, switching cable between the two amps at my feet with a Fender Stratocaster for hours. And No, I'm not a real guitar player by the way. Now, at one point long into my comparison, I reached down and turned the treble up slightly on THE amp and thought, Oh yes, that's more like the OTHER amp. So let's unplug now and go back to the ... other ... one. Well, horrors! It turned out I wasn't even plugged into that amp where I turned up the treble and heard a difference, it was of course- the other one. Yep, time to turn everything off and walk away. Lesson learned. I expected to hear that treble difference, I did hear it, but yet it wasn't really there. FWIW I'm just passing this along. My suggestion for audio gear it always- let the device come to you. There's no hurry to make a judgement. It will speak to you down the road. Just ignore it. Might be a day, might be a week. Time tells all of course.
 
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