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Post by illacov on May 16, 2017 11:04:41 GMT -6
Replacing the resistor helped my mic, but it is not the FINAL solution or fix for the tube noise. Well see there ya have it. Dan's claim was about resistor type but not value. So that's why I'm curious about Svart's build seeing as how he reports to have lots of success with various tubes with it. Even if his changes are considered "slight" for tube circuits, those slight changes can make big changes overall. Cathode resistor on the 251 circuit. Change the value from 1.8k to 2.7k. Obvious shifts in tonality. Mathematically small changes if you go based on percents but the ears don't lie. I see no reason why a circuit like this would be less sensitive to adjustments to operating parameters. Of course the talented folks who built this kit have far more experience with it and a better perspective. However, if the circuit needs mods or changes to perform better (sounds likely) with a broader range of tubes then I'm all for it. Thanks -L.
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ericn
Temp
Balance Engineer
Posts: 14,938
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Post by ericn on May 16, 2017 11:12:40 GMT -6
The last word will be when we convince some Saudi Prince who wants to sound like the Beatles he needs to build a factory that can make VF14m's and do it right ! Someone owns all the old equipment used for manufacturing the VF14m. Anyone here know who that is? I believe Andreas Grosser or Oliver Archut knew who it was, Herr Grosser even knew the individual I believe.... I know at one point Oliver looked into either Equipment for VF14's or AC701's but even at current market it wasn't feasible. The original equipment is probably either some where in Russia tubes were still the basis of most of their elec into the 80's or it was melted down as scrap and I'm driving around in recycled tube manufacturering equipment! As for Grosser he had that FET thing !
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Post by svart on May 16, 2017 12:23:00 GMT -6
Andreas Grosser famously once said that (paraphrasing) "Any tube can substitute for a VF14m, if it is a GOOD tube and you are willing to make other electronic allowances". This thread made me go back to that wonderfully painful thread on groupdiy where they are fighting over who invented the dual 408 VF14 substitution circuit. One comment was made about the sand resistors being replaced with a much better alternative by danderloo. Something about how those resistors were contributing noises that sounded like tube noise, but once replaced with an alternative type, the noise subsided? Thanks -L. Which resistors was the discussion about? Almost certainly anything from back in the day was either wirewound or carbon composition. Looking at the pics, they were wirewound in the U47.
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Post by illacov on May 16, 2017 12:50:20 GMT -6
This thread made me go back to that wonderfully painful thread on groupdiy where they are fighting over who invented the dual 408 VF14 substitution circuit. One comment was made about the sand resistors being replaced with a much better alternative by danderloo. Something about how those resistors were contributing noises that sounded like tube noise, but once replaced with an alternative type, the noise subsided? Thanks -L. Which resistors was the discussion about? Almost certainly anything from back in the day was either wirewound or carbon composition. Looking at the pics, they were wirewound in the U47. Looks like the only sand resistor (on the studio 939 site for the MK47 kit) is the 1K5 5% resistor. The 6 watter. I'm not hip to whats in the power supply? Or elsewhere? dandeurlooCare to clarify which sand resistors in question were causing that issue? Thanks -L.
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Post by EmRR on May 16, 2017 13:21:04 GMT -6
I don't think that filament circuit is anything anyone would do today, and smacks of something that delivered best results in the lab with the VF14M. That does not mean it will at all work well with any other tube having a different internal structure or material composition. It is also a shortcut to get DC filaments out of a single DC voltage, in a time when stand alone DC filaments were not usually capable of besting a well implemented AC supply in any reliable way with regards to production.
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Post by sll on May 16, 2017 15:10:52 GMT -6
Which resistors was the discussion about? Almost certainly anything from back in the day was either wirewound or carbon composition. Looking at the pics, they were wirewound in the U47. Looks like the only sand resistor (on the studio 939 site for the MK47 kit) is the 1K5 5% resistor. The 6 watter. I'm not hip to whats in the power supply? Or elsewhere? dandeurloo Care to clarify which sand resistors in question were causing that issue? Thanks -L. The 1K5 dropping resistor for the filaments came with the kit as the sand resistor. Those broke down thermally and changed values randomly. That was cause for a lot of noise initially. Changed that out to a wire wound chassis mount that's heat sinked to the bell end of the mic. After that, it was just chasing the tube noise. Those were used in the power supply as well, but I changed them all to wire wound. I think they are 10W or even higher? Can't remember now.
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Post by aamicrophones on May 17, 2017 14:10:45 GMT -6
Doug is right, the filament circuit in the original U47 with the VF14 was simply to extend the life of the tube. The plate voltage which in the case of the U47 is also the filament supply is ramped up slowly through the big wirewound resistor at the bottom of the microphone.
If you put a voltmeter on the plate/filament supply you will see it takes about 50 minutes to stabilize which allows the cathode to be heated up to approximately 2000C which yields the optimium amount of free electrons.
Underheating the cathode just means it takes more time for the cathode to get up to its optimum operating temperature. You have to let the U47 warm up each time for 50 minutes before you can listen or measure the internal tube/circuit noise.
The filament of the VF14 draws 40ma and the dropping resistor according to the schematic is 1780 ohms and there is 35 volts reaching the filament which is why it takes 50 minutes to get the cathode up to temperature.
P=E2/R 105v-35v =70v. This means the 1780 ohm resistor must drop 70v. 70x70=4900/1780=2.75 watts 4900/1500=3.2 watts. A 5 watt resistor should be more than adequate.
***** However, this math only works for the VF14 tube and not those with 6 or 12 volt filament. The EF14 draws 470 milliamps, the EF80 draws 300 milliamps and the EF800 295 milliamps. So, this resistor has to be calculated depending on the filament voltage and current.
Today, we use regulated power supplies that bring the cathode temperature up to optimization in less than 2 minutes and the filament supply and plate voltage are pulled up to the correct volatage in milli-seconds after power up.
The last U47 I serviced had a power supply that was running high up at about 121v instead of 105V. Once we changed a resistor in the power supply we got 105v after 50 minutes.
These old resistors will change the values drastically over-time from constant heating. Wirewound resistors are not quite as suscetible to the heat as old carbon resistors unless the voltage or current is increased above their design specs.
There is absolutely no regulation in the U47 supply so if the power from the utility company is higher than normal or lower than normal the U47 supply voltage will follow.
They were wire wound resistors in the U47 for the most part. Carbon resistors back in the early days of tube electronics has 20% tolerance and produced noise. Today we use 1% metal film resistors in the audio part of the circuit and 5% in the power supply.
Resistors don't have a sound other than "noise" they are either the right value to optimize the circuit or not.
Also, the only reason Neumann used fixed bias with the U47 is that back in 1947 a cathode bypass capacitor large enough to provide good low frequency results would be too large to fit inside the body.
Fixed bias does not require a cathode bypass capacitor.
In all our 9 pattern tube microphone designs we use a two stage (dual triode) CCDA circuit. This does not require a cathode bypass capacitor and produces an output impedance 1/10th that of the VF14M from a $10 tube that is exceptionally quiet when selected.
Look at the size of the 1ufd in the original U47 now imagine how big it would be if the capacitance was 100 times larger back in 1947 which is required for HiFi operation of the circuit.
What's important with capacitors is the ESR (internal series resistance). We still used oil filled capacitors today but only at Electrical Power Sub-stations. Oil filled capacitors of the kind of value used in microphone circuits can handle upwards of 10,000V.
However, the maximum voltage in a U47 should be 105v if everything is working properly. We use 2.2ufd metal film WIMA in our microphone builds with 5% tolerance having an ESR 1/100th that of an electrolytic.
If you are using a AMI BV8-08 then you must use a 1ufd capacitors because Neumann used two different winding inductances in each of the dual bobbin construction.
This rolls out the low end at 40hz with the 1ufd if you increase the capacitor value the low end will roll-out earlier. I could figure this out until I read the article on the original BV8 constructon by Oliver.
There is also a low frequency bump in the response because of the transformer's inductance and the capacitance resonating in the very low frequencies much like the low frequency bump back from a tape machine.
That's part of the U47 sound. In our BV8 version we use match bobbin windings and can use a 2.2ufd capacitor to move the resonance down into the sub-sonics and with the CCDA circuit achieve a flat response down to 20hz.
Cheers, Dave
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Post by ChaseUTB on May 17, 2017 20:47:48 GMT -6
I think we all want affordable u47's that sound like 47's and using different frequencies to roll out a mic and different impedance levels with diff parts in the circuit tell my measly brain these things will no doubt affect the sound... I guess what I am saying is no on ever said dam I wish my U47 went flat to 5Hz 😀 If the u47 rolls off at 40Hz that's what I want.. If it takes 50 minutes to heat up and stabilize for the sound that's what I want 😀 I feel all of the " technical improvements " made to the u47 circuits is exactly why no new mics sound like a 47 including the absence of vf14m, m7/ k47, Bvo8 😎
Just want to say this thread has been very educational for me and I am appreciative that other members shared their experience 😀
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Post by svart on May 18, 2017 13:18:27 GMT -6
My 6028 tubes show up tomorrow according to tracking info. I guess we'll see where we stand tomorrow..
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Post by EmRR on May 18, 2017 14:12:35 GMT -6
Back to the DHT rabbit hole, I poked around a little bit, can't find any examples among the hi-fi nuts of attempting to create DHT's out of indirectly heated tubes (what we've used since 1940), nor even a similar example of one side of a filament tied to cathode. If you found an example, it'd be in some space/weight challenged place like aeronautics, with no relationship to sound quality. I will not ascribe any positive sonic benefits to this approach, only to the specific tube itself which proved to be immune from related problems. I would agree if you could actually come up with a detailed set of test results based on the very small pool of accepted V14M's, and could match another tube with the full set of parameters, you'd likely have the same sound. You couldn't skip a single parameter. And someone would still say it sounds different. Then we can all probably point to an instance in which we did an overdub punch with a completely different type of mic out of necessity or convenience, and couldn't hear that either.
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Post by svart on May 18, 2017 14:46:21 GMT -6
Back to the DHT rabbit hole, I poked around a little bit, can't find any examples among the hi-fi nuts of attempting to create DHT's out of indirectly heated tubes (what we've used since 1940), nor even a similar example of one side of a filament tied to cathode. If you found an example, it'd be in some space/weight challenged place like aeronautics, with no relationship to sound quality. I will not ascribe any positive sonic benefits to this approach, only to the specific tube itself which proved to be immune from related problems. I would agree if you could actually come up with a detailed set of test results based on the very small pool of accepted V14M's, and could match another tube with the full set of parameters, you'd likely have the same sound. You couldn't skip a single parameter. And someone would still say it sounds different. Then we can all probably point to an instance in which we did an overdub punch with a completely different type of mic out of necessity or convenience, and couldn't hear that either. I've done that, using a different mic. It's not elegant, but it can work in some instances if you know the sound you get from your normal mics.
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Post by EmRR on May 18, 2017 14:48:40 GMT -6
I've had it happen with a condenser punching on a ribbon.
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Post by dandeurloo on May 19, 2017 7:49:02 GMT -6
ok, some quick testing: If you look at this schematic: I have added my measurements from my mic in case anyone is interested. One thing is that I used 27R instead of 29R. The 1500 is a metal wound resistor with heatsink I screwed to the body of the mic. EDIT: one more thing. Instead of 10nF, I used 13nF (old russian film cap). EDIT 2: Those that have issues, try turning down the B+ some. It's not especially necessary it be exactly 105V. A few percent high or low should be OK. See if going up or down makes it more stable. I have been running my B+ down a few volts as well to see if that would help. I feel like it has helped a touch, not a lot but every little thing I can do to keep these happy I have been doing. I also, have had to replace a few resistors that came with the kits. Scott and I both swapped out the 1500 with a metal wound that we heat sinked to the body. That did help forsure. I have also had a few other resistors go bad inside the kits with time. It seems a few things can act up over time with these. Still my biggest issue has been in the tubes. BTW, I have tried a number of different PSU designs. Regulated, unregulated, Chokes, no chokes. Seems like the PSU design doesn't have much effect on noise as long as it is built well to begin with. My 2 cents
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Post by svart on May 19, 2017 8:58:18 GMT -6
ok, some quick testing: If you look at this schematic: I have added my measurements from my mic in case anyone is interested. One thing is that I used 27R instead of 29R. The 1500 is a metal wound resistor with heatsink I screwed to the body of the mic. EDIT: one more thing. Instead of 10nF, I used 13nF (old russian film cap). EDIT 2: Those that have issues, try turning down the B+ some. It's not especially necessary it be exactly 105V. A few percent high or low should be OK. See if going up or down makes it more stable. I have been running my B+ down a few volts as well to see if that would help. I feel like it has helped a touch, not a lot but every little thing I can do to keep these happy I have been doing. I also, have had to replace a few resistors that came with the kits. Scott and I both swapped out the 1500 with a metal wound that we heat sinked to the body. That did help forsure. I have also had a few other resistors go bad inside the kits with time. It seems a few things can act up over time with these. Still my biggest issue has been in the tubes. BTW, I have tried a number of different PSU designs. Regulated, unregulated, Chokes, no chokes. Seems like the PSU design doesn't have much effect on noise as long as it is built well to begin with. My 2 cents Interesting data points. I built my power supply on the PCB that came with the mic PCB, but I'm positive I didn't build it exactly like the BOM. I remember using a much lower output voltage transformer, and then scaling up the resistor values as they didn't need to dissipate so much voltage. I pretty much just tried a few values I had in my large resistor bin until I had what I needed. I probably also used whatever caps I had available in HV too, so another hodgepodge. I'm really stoked to see what the 6028 tubes do. They are NOS untested, so who knows how they'll work. If anyone is game, I'd try lowering B+ *and* lowering the 29R to something like 25-27R and see what happens for you.
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Post by svart on May 19, 2017 19:34:27 GMT -6
So I got the 6028 tubes today. They work just fine. They have a higher noise floor than the WE408A, but it seems like they have more gain too. After about an hour of burn-in the noise floor dropped a little, I listened for any strange noises, but all I got was regular tube hiss.
So 3 random sets of tubes that work fine in my mic.
I'll double down on my theory that there is some variable in the builds that seems to make these tubes work or not, and that tube selection is just a band-aid fix.
Maybe next I'll get another set of PCBs to mess with so that I don't need to disassemble mine. That way I can test some theories without worrying about screwing my mic up!
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Post by Guitar on May 19, 2017 19:52:14 GMT -6
svart I appreciate your enthusiasm on this subject!
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ericn
Temp
Balance Engineer
Posts: 14,938
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Post by ericn on May 19, 2017 23:30:04 GMT -6
So I got the 6028 tubes today. They work just fine. They have a higher noise floor than the WE408A, but it seems like they have more gain too. After about an hour of burn-in the noise floor dropped a little, I listened for any strange noises, but all I got was regular tube hiss. So 3 random sets of tubes that work fine in my mic. I'll double down on my theory that there is some variable in the builds that seems to make these tubes work or not, and that tube selection is just a band-aid fix. Maybe next I'll get another set of PCBs to mess with so that I don't need to disassemble mine. That way I can test some theories without worrying about screwing my mic up! OK your starting to convince me.
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Post by Ward on May 20, 2017 8:36:58 GMT -6
So I got the 6028 tubes today. They work just fine. They have a higher noise floor than the WE408A, but it seems like they have more gain too. After about an hour of burn-in the noise floor dropped a little, I listened for any strange noises, but all I got was regular tube hiss. So 3 random sets of tubes that work fine in my mic. I'll double down on my theory that there is some variable in the builds that seems to make these tubes work or not, and that tube selection is just a band-aid fix. Maybe next I'll get another set of PCBs to mess with so that I don't need to disassemble mine. That way I can test some theories without worrying about screwing my mic up! I love your dedication to this!
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Post by EmRR on May 20, 2017 9:55:35 GMT -6
It reminds me of the problems people have had with the PM660 limiter kits, a substantial number of the 5687 tubes out there were drawing double the expected current and burning down the PSU's. Shouldn't happen, but there's a funky batch.
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Post by jakeharris on May 21, 2017 11:00:26 GMT -6
So I got the 6028 tubes today. They work just fine. They have a higher noise floor than the WE408A, but it seems like they have more gain too. After about an hour of burn-in the noise floor dropped a little, I listened for any strange noises, but all I got was regular tube hiss. So 3 random sets of tubes that work fine in my mic. I'll double down on my theory that there is some variable in the builds that seems to make these tubes work or not, and that tube selection is just a band-aid fix. Maybe next I'll get another set of PCBs to mess with so that I don't need to disassemble mine. That way I can test some theories without worrying about screwing my mic up! Can you post a picture of your build?
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Post by svart on May 21, 2017 11:03:29 GMT -6
So I got the 6028 tubes today. They work just fine. They have a higher noise floor than the WE408A, but it seems like they have more gain too. After about an hour of burn-in the noise floor dropped a little, I listened for any strange noises, but all I got was regular tube hiss. So 3 random sets of tubes that work fine in my mic. I'll double down on my theory that there is some variable in the builds that seems to make these tubes work or not, and that tube selection is just a band-aid fix. Maybe next I'll get another set of PCBs to mess with so that I don't need to disassemble mine. That way I can test some theories without worrying about screwing my mic up! Can you post a picture of your build? Ok, I'll get some pics
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Post by jakeharris on May 21, 2017 13:00:24 GMT -6
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Post by category5 on May 22, 2017 9:22:35 GMT -6
Kcat just directed me over here and it's too long a thread to get totally caught up but I can possibly add a few points to the discussion. There are plenty of tubes that have been used in U47 inspired mics, and clones. The unique thing about the 408 is that it can happily run off of 1/2 the heater voltage provided by the unmodified U47 circuit. Wiring two 408a tubes in parallel, with their heaters wired in series solves two problems with finding a suitable VF14 replacement. 1 - the 36V heater voltage is divided, so each tube gets roughly 18V to the heater which is close enough to the specified 20V to work just fine. 2 - The output of the tubes running in parallel provides enough current to sufficiently drive the large output transformer. Another interesting point is that since the tubes are run in parallel, there is no specific need to match them in any way. In fact, two different brands can be mixed without issue, maybe even to tailor the sound to a finer degree (I have threatened to experiment but never found the time). As for noise, I do not believe the problem lies solely in the tubes. Why? I have built a couple of mics that seemed to be very picky about which tubes they perform well with, only to find those very tubes perform just fine in another mic. I even swore off WE tubes early on, only to buy a box of 50 for a good price and find that pretty much every one I've pulled out worked fine in the mics I have in my possession still. I genuinely think the problem lies around the quality and type of the dropper resistor used to derive the heater voltage. The sand-bar type resistors were problematic most of the time while higher quality, sinked resistors seem to resolve the problem. Getting more contact between the resistor and the mic bell may help too, by drawing more heat out of the resistor (they get ridiculously hot). Certainly, in my experience I have found most tubes to work just fine in my mics, free of the crackling and pulsing being discussed here. WRT noise floor, 408a isn't the quietest tube, and running 2 in parallel can compound the noise issue, but in my experience noise is pretty acceptable for most applications, and those where noise is a problem probably beg for a more modern mic design anyway. One of the reasons Neumann started using emphasis at the capsule and de-emphasis in the circuit was for the 6dB HF noise advantage. Vintage circuits will carry some noise penalty no matter what. Also, the 408a wasn't designed for microphone applications, which doesn't mean it can't work great, it just means you have to accept any variables that may find their way into the equation, and more relaxed QC is obviously one of those. I'd also be glad to try someone else's noisy tubes to see how the translate from mic to mic. It may end up revealing some other important details. If STAM is releasing a commercial product based on the dual 408a design I guarantee he is off Max's Christmas list.
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Post by kcatthedog on May 22, 2017 18:33:49 GMT -6
Appreciate all the expertise here but always good to have cat5 chime in. He's probably built more of these than most.
He had mentioned to me in the past about being able to mix tubes and I ran a gold tipped nos rca and an Erickson for a while and you do hear good differences, so you can tailor the sound of the mike by varying the tubes.
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Post by Johnkenn on May 23, 2017 17:57:04 GMT -6
Kcat just directed me over here and it's too long a thread to get totally caught up but I can possibly add a few points to the discussion. There are plenty of tubes that have been used in U47 inspired mics, and clones. The unique thing about the 408 is that it can happily run off of 1/2 the heater voltage provided by the unmodified U47 circuit. Wiring two 408a tubes in parallel, with their heaters wired in series solves two problems with finding a suitable VF14 replacement. 1 - the 36V heater voltage is divided, so each tube gets roughly 18V to the heater which is close enough to the specified 20V to work just fine. 2 - The output of the tubes running in parallel provides enough current to sufficiently drive the large output transformer. Another interesting point is that since the tubes are run in parallel, there is no specific need to match them in any way. In fact, two different brands can be mixed without issue, maybe even to tailor the sound to a finer degree (I have threatened to experiment but never found the time). As for noise, I do not believe the problem lies solely in the tubes. Why? I have built a couple of mics that seemed to be very picky about which tubes they perform well with, only to find those very tubes perform just fine in another mic. I even swore off WE tubes early on, only to buy a box of 50 for a good price and find that pretty much every one I've pulled out worked fine in the mics I have in my possession still. I genuinely think the problem lies around the quality and type of the dropper resistor used to derive the heater voltage. The sand-bar type resistors were problematic most of the time while higher quality, sinked resistors seem to resolve the problem. Getting more contact between the resistor and the mic bell may help too, by drawing more heat out of the resistor (they get ridiculously hot). Certainly, in my experience I have found most tubes to work just fine in my mics, free of the crackling and pulsing being discussed here. WRT noise floor, 408a isn't the quietest tube, and running 2 in parallel can compound the noise issue, but in my experience noise is pretty acceptable for most applications, and those where noise is a problem probably beg for a more modern mic design anyway. One of the reasons Neumann started using emphasis at the capsule and de-emphasis in the circuit was for the 6dB HF noise advantage. Vintage circuits will carry some noise penalty no matter what. Also, the 408a wasn't designed for microphone applications, which doesn't mean it can't work great, it just means you have to accept any variables that may find their way into the equation, and more relaxed QC is obviously one of those. I'd also be glad to try someone else's noisy tubes to see how the translate from mic to mic. It may end up revealing some other important details. If STAM is releasing a commercial product based on the dual 408a design I guarantee he is off Max's Christmas list. Good to see you here, Shane...
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