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FX Audio 6j1 tube preamp - a $31 wonder
- Thread starter Poultrygeist
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FlaCharlie
Super Member
So your output coupling caps are now a combination of film and PIO.I have put 2 pio (K40Y-9) in bypass under the card and i can closed te fx box after.After 2 hours of burning it became better sound... Let's turn and listen
I have understand we can bypass the cathode also someone try or can explain?
Cathode bypass caps are significantly higher in value (capacitance) than coupling caps so electrolytic types are typically used. Most tube amp builders will do whatever they can to eliminate the use of electrolytics.
But the main problem with adding them is that you will increase the gain and the gain in these is already higher than necessary because most amps don't need any gain at all from a preamp in order to produce full power output. One of the effects of the already high gain is that it limits the usable range of the volume control so adding a cathode bypass cap will just make it even more difficult to make small adjustments to volume.
Dr. Ears
Super Member
I believe it’s the 12 volts that matter.Can I substitute this? 5 amp for the 3 am PS 3kx?
FlaCharlie
Super Member
Yes, but the higher amperage rating won't offer any benefits.
Exactly. The amperage rating of a power supply just means that it's capable of supplying that much current but it's the device that's connected to it that determines how much is actually drawn.
A device that draws 1A will still draw 1A whether you power it with a supply that's rated for 1A or one that's rated for 10A.
If you have one of those devices that people use to monitor their electricity usage you can measure the current being drawn. It won't be much. The tube heaters will be responsible for most all of the current being drawn and they only draw a total of 0.35A so at 6.3v they only consume 2.2 watts.
A 12v supply that's rated at 1A can supply up to 12 watts. Just a guess, but I'd be surprised if the total consumption was more than 3w.
Trypower
Member
Well I'm getting bleary eyed trying to find this answer, but I have to admit defeat as I've been thru many pages. Can anyone tell me the values of the two caps I have identified in the photo? I believe I can make out- 1 u J63 -but that's the best I can do. Don't want to guess this means 1uf/63V. Recommendations? And what function do they have in the circuit? I'm ready to update both my 01 and 03, photo is of the Tube 03. Ordering different op amps and caps to give it a go. Thanks in advance for helping the old guy.
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FlaCharlie
Super Member
Besides the higher gain, which is definitely not needed, the other effect of adding a cathode bypass cap will be higher distortion.
That's because an un-bypassed cathode resistor produces what's known as "degenerative negative feedback". Negative feedback is used to make a circuit more "linear", which means lower distortion. If you bypass the resistor you eliminate that negative feedback and distortion will increase.
While it is measurable, I don't know if it would be audible. Frankly, the low voltage used in these results in tube operating points that are so far from ideal that any reduction in linearity might not be particularly noticeable. But distortion will go up and "clean sound" is not something that's associated with increased distortion.
Perhaps you will prefer the sound, though.
FlaCharlie
Super Member
You're correct, those are 1uf 63v film caps that are used as coupling caps. In the schematic I posted above they are shown as C10 and C12.
FlaCharlie
Super Member
Well, I don't own an FX so I may be mistaken. There are two other 1uf caps used on the input to block any DC that might be coming from the output of a source component. Unfortunately, they put them in the wrong place, as I've explained previously.
The two smaller ones may be part of the portion of the power supply that's inside the FX. The schematic that's been floating around seems to be an accurate representation of the audio portion of the -01 and similar preamps. But the power supply portion is not the same. These types of power supplies are not something I'm familiar with. I build tube gear which uses linear power supplies that are much, much simpler.
Some of their parts choices make no sense to me. There is no need to use any part rated at 250v since the voltage in these is nowhere close to that. It's probably just something they were able to purchase very cheaply in bulk.
As I've said, if I owned one of these I would probably just leave it in stock form other than rolling tubes and maybe op amps if it was an -03.
As far as I know there are no schematics of the -03 version. So those two smaller caps may be part of the op amp circuit. If you don't have an accurate schematic and don't know what the parts do it's best to leave them alone.
Now, if you're convinced that they are incredibly poor quality and that a different brand might be better you could swap them out, but I wouldn't suggest changing the values since you have no idea how that affects the circuit.
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Talk about this board is perhaps no possible because there is many different serial (look photo)
I'am now with PIO in input and output and i think "filtering" is better (low noise and more tubey sound) and no problem for the gain ( i used 4700pF), the volume pot is nearly 11H23
I also changed the blue 1uf 63v by wima 1uf 100v mks 4, it's not easy for the place (for my "aop" sparkos) but it's ok. i do it just for doubt about the blue quality AND because i learned with this very long thread
I'am now with PIO in input and output and i think "filtering" is better (low noise and more tubey sound) and no problem for the gain ( i used 4700pF), the volume pot is nearly 11H23
I also changed the blue 1uf 63v by wima 1uf 100v mks 4, it's not easy for the place (for my "aop" sparkos) but it's ok. i do it just for doubt about the blue quality AND because i learned with this very long thread
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Trypower
Member
Thanks fellas. I ordered a few different op amps to try out. And some like value caps plus additional 2.2uf/250V values. I'm curious to see if the 2.2 would make a difference on the output coupling. Seems unlikely, but they're certainly cheap enough for experiment. Parts should be here by mid-late week and I'll report back.
One item of note ... The tubes need to be closely matched or the voltage amplification from channel to channel can be WAY off.
One item of note ... The tubes need to be closely matched or the voltage amplification from channel to channel can be WAY off.
Pike72
New Member
I've been pokin around here since '20 when I got a tube03. Tubes have been rolled(Voshkods now), ready to tackle op-amps.
Ordered a set of BurrBrown OPA2134PAs from Amazon, $20 for a set. Wow, what a change. Now, I have a new op-amp rabbit hole to go down!
Pike
Be carefull Pike i like to catch them (and release)
You need to change the blue capacitor by some wyma (the red) every one who do it can said it's better
I tryed Voskod Mullard Rca Sylvania Cifte GE ... (for french people or around i can make some sale) many aop and i'm now on Sparkos Lab SS3602 for my life...
Today i test derating resistor on output!
You need to change the blue capacitor by some wyma (the red) every one who do it can said it's better
I tryed Voskod Mullard Rca Sylvania Cifte GE ... (for french people or around i can make some sale) many aop and i'm now on Sparkos Lab SS3602 for my life...
Today i test derating resistor on output!
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lazylistener
Member
I got the OPA2134PAs too and the difference was surprising! Can't imagine how the more expensive ones would fare.
Been running with my FX Audio Tube-03 for the past few months and can't complain. The sonic differences mainly come through via headphones but it sounds great over speakers as well. I've since upgraded my DAC to a Topping E30II. The general wisdom I've picked up around here is that sonic differences between DACs are slight, but there was an immediate jump going into this from a Fiio E10K. So much so, it became fatiguing listening to it for several hours, whereas the E10K was smooth and easy. Thankfully the E30II is adjustable so I run it at -20db with my Tube-03 at max gain. Now it sounds great!!
Also, does anyone have any leads to get a pair of Philips EF95s? The Holland ones that people here rave about. Seems harder to come by now.
Been running with my FX Audio Tube-03 for the past few months and can't complain. The sonic differences mainly come through via headphones but it sounds great over speakers as well. I've since upgraded my DAC to a Topping E30II. The general wisdom I've picked up around here is that sonic differences between DACs are slight, but there was an immediate jump going into this from a Fiio E10K. So much so, it became fatiguing listening to it for several hours, whereas the E10K was smooth and easy. Thankfully the E30II is adjustable so I run it at -20db with my Tube-03 at max gain. Now it sounds great!!
Also, does anyone have any leads to get a pair of Philips EF95s? The Holland ones that people here rave about. Seems harder to come by now.
Trypower
Member
Oh, didn't know about the Burr Brown. A couple of days ago I ordered the OPA 2134PA from Mouser, but they're Texas Instrument. Is there likely to be a difference between the two? Are there any other makers of this op amp, thought the number was proprietary to the manufacturer? New to solid state gear, been fooling with vacuum tube stuff for years.
drjohnnyfeva
Well-Known Member
There are some expensive ones in that list. The Burstons, Sparkos, Muses, and even the OPA627's are all more expensive than the unit itself. For me, that excluded them from my trials. I might buy a pair of Burstons sometime, but only because I have other units now that I can experiment with them. I got an Aiyima A07 a few weeks ago that I took out the stock NE5532 opamps and replaced them with the LME49720NA's. I've used that same opamp in the 03 and liked it better than the stock ones there - which also happen to be the NE5532's. Right now I have three Tube 03's. One has a set of MUSES8920's, one has the ebay ordered OPA627AU's, and the other has a pair of LME49720NA's. I like each of them. The only one I'm uncertain about it's authenticity are the 627's, but they still sound great. I think I prefer them altho the MUSES8920's are close and not too different than the LME's.
The opamps you ordered are available on Mouser. I've been programmed by reading this thread that anything ordered outside of Mouser or Digikey has a high-liklihood of being counterfeit. Same number at $6.15/ea. plus shipping, which is cheap. Digikey has free shipping, but I have an account at Mouser. They are out of MUSES8920's but they do have the 01, 02, and 03 the last time I checked. There is supposedly a new chip for the 8920, which is listed as end of life. If they are comparably priced, I'll probably give them a go when they become available - Feb '23, maybe.
There's another opa1656 that was recommended by rob43 some time back in this thread that I also oredered. It is a smaller dual chip that needed to be mounted on a a dip-8 socket. I got it on ebay also. It also sounded better than the stock. I haven't put those back into any of the three Tube03's I have to compare them. They were replaced by the MUSES8920's in the unit I continue to enjoy in our living/movie room system. I liked the MUSES enough that I didn't go back.
I might have to order the 2134's just to give 'em a spin.
FlaCharlie
Super Member
I'm curious what you mean by de-rating the resistor. Are you talking about the 100k resistor going to ground from the output jack?
The term "de-rating" is used to describe the relationship between the wattage rating and actual wattage that the resistor is dissipating. Good engineering practice is to use a resistor that's rated at 3x the actual dissipation. When I build stuff I prefer to de-rate by 5x whenever possible.
In order for it to dissipate wattage there has to be a voltage drop across the resistor and current flow. Of course, there should be no DC voltage on that side of the coupling cap, only AC (the music signal).
If you take some measurements and use Ohm's Law to calculate it, you'll find that the actual dissipation is essentially zero so the wattage rating is not critical and increasing it will have no effect. That resistor isn't in the signal path. Apparently you don't understand its function.