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Post by jcoutu1 on May 15, 2017 9:38:53 GMT -6
I'm interested in hearing more about the PCB's/Projects/Etc. My understanding is that there is an MK-47 and an MKU-47 right? The two mics are the same dual tube layout, but the MKU-47 features the fancy headbasket/pattern switching from Flea right? Are there other differences between these mics? Do they use the same PCB? Also, both of these are Max designs? Where does the Dany PCB fit in? Stam is using a PCB from Dany, not Max right? svart , did you make your own PCB, or were you using one of these? When I ordered this thing, I was thinking it was the same mic that tonycamphd (wish he was back around, honestly) built for Johnkenn , but with a Heiserman cap and without the fancy metalwork. Is this actually a different design? I could stand to be corrected but I don't think the Stam/danny mike is the same as max's 47, like Tony had at first or the follow up mk-u47. It seems to me based on the pics that the psu's are different, as well. Why does the Stam psu have a tube in it ? Is it following the neumann schematic exaclty ? I ask only because I think Max's does ? Not implying there is anything wrong with the Stam 47 mike: just curious to understand the differences. I don't see any tube in the Power Supply. I see a "trimmer" pot. www.vintagemicrophonepcbkit.com/D-47.html |
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Post by svart on May 15, 2017 11:13:57 GMT -6
I also bought a set of random Tung-Sol 6028 tubes to try them as well since my two sets of 408A seem to work OK.
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Post by kcatthedog on May 15, 2017 11:20:59 GMT -6
my bad first time I looked at that link I was on my phone so I though the trim pot was a tube  |
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Post by sll on May 15, 2017 12:38:39 GMT -6
Perhaps it was. I apologize for that. However, I felt that svart was implying that I didn't know what I was talking about. That there couldn't possibly be any issues with these tubes. My experience was completely different. I've built a few microphones and other high impedance devices in my time. I know where the trouble areas can occur. This boiled down to the tubes for me. Maybe I was expecting them to perform better than they did? They are certainly not as quiet as an EF86 or 6072. I'm sorry you thought that. It was not my intention. I'm an engineer, so I'm not real good with the whole communication thing. :\ What I was trying to say is that it seems like the design in general teeters on the edge of stability and with different builds some choices in parts can inadvertently push the system into complete instability. Since tubes are a very loose tolerance device, you're going to have a huge spread of operating conditions, and therefor a huge chance for some tubes to work perfectly well, while others don't work at all. This bell curve can move around depending on the builder's choices. It seems that in your case I think the system is on one end of the curve, so that selection of tubes is necessary to find ones with the right operating attributes to fit the stability of the system. Other folks have had very little trouble with random tubes, which shows that the system has a very large spread of stability. I wasn't trying to point fingers at you personally, just using your experience as a data point in the larger discussion. I still think that needing to select tubes for stability is a symptom of an overall design issue, but the great variation in different builds makes it hard to pinpoint a specific difference that makes things work (or not work). I think that with some experimentation and troubleshooting we could likely bring the success rate with random tubes to much higher level. Currently I think that maybe the bias point of the plate might be iffy depending on B+ and heater currents. The U47 doesn't use a cathode bypass cap, so the AC signal will vary on the cathode and could cause instability here. I'll have to disassemble my mic to see, but I think I ended up using something like 25R in place of the 29R, and then adjusted the B+ until I got a good operating point. In at least *one* Neumann schematic, they show a 68R and 29R in parallel, which would equal ~21R. This in itself could add stability to the design if some tubes had higher GBW sending them off into oscillation. I appreciate this svart. It very well could be instability with the cathode resistor value. It seems to be temperature related. I'm using a 29R. I need to put my mic back together and do some testing. I planned to use a different tube, but have not gotten very far with it. I also don't remember what version of PCB I have, other than it came from Max? I thought about gutting it and do point to point, although all the high impedance parts are basically floating in air off the board. I would also like to check for oscillation when the thing starts acting up. One of the first issues I found was the filament dropping resistor that came with the kit was breaking down thermally and making the thing spit and sputter. I changed that out among most of the other parts and it was better. Still had to sort through lots of tubes to make it useable. |
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Post by illacov on May 15, 2017 12:56:01 GMT -6
I'm sorry you thought that. It was not my intention. I'm an engineer, so I'm not real good with the whole communication thing. :\ What I was trying to say is that it seems like the design in general teeters on the edge of stability and with different builds some choices in parts can inadvertently push the system into complete instability. Since tubes are a very loose tolerance device, you're going to have a huge spread of operating conditions, and therefor a huge chance for some tubes to work perfectly well, while others don't work at all. This bell curve can move around depending on the builder's choices. It seems that in your case I think the system is on one end of the curve, so that selection of tubes is necessary to find ones with the right operating attributes to fit the stability of the system. Other folks have had very little trouble with random tubes, which shows that the system has a very large spread of stability. I wasn't trying to point fingers at you personally, just using your experience as a data point in the larger discussion. I still think that needing to select tubes for stability is a symptom of an overall design issue, but the great variation in different builds makes it hard to pinpoint a specific difference that makes things work (or not work). I think that with some experimentation and troubleshooting we could likely bring the success rate with random tubes to much higher level. Currently I think that maybe the bias point of the plate might be iffy depending on B+ and heater currents. The U47 doesn't use a cathode bypass cap, so the AC signal will vary on the cathode and could cause instability here. I'll have to disassemble my mic to see, but I think I ended up using something like 25R in place of the 29R, and then adjusted the B+ until I got a good operating point. In at least *one* Neumann schematic, they show a 68R and 29R in parallel, which would equal ~21R. This in itself could add stability to the design if some tubes had higher GBW sending them off into oscillation. I appreciate this svart. It very well could be instability with the cathode resistor value. It seems to be temperature related. I'm using a 29R. I need to put my mic back together and do some testing. I planned to use a different tube, but have not gotten very far with it. I also don't remember what version of PCB I have, other than it came from Max? I thought about gutting it and do point to point, although all the high impedance parts are basically floating in air off the board. I would also like to check for oscillation when the thing starts acting up. One of the first issues I found was the filament dropping resistor that came with the kit was breaking down thermally and making the thing spit and sputter. I changed that out among most of the other parts and it was better. Still had to sort through lots of tubes to make it useable. This is exactly what needs to happen for us to see progress. I figured there may have been some slight changes or adjustments in Svart's build. Of course you may say "Well the difference between 25 and 29 ohms isn't that big." It's 20% difference. Design is crazy like that. I dealt with the same thing designing my device especially with resistor values and how sensitive certain parameters were to slight changes in resistance. I'm not saying this is the Aha moment but this is the start to getting there. Good progress! Thanks -L. |
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Post by svart on May 15, 2017 14:21:24 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. |
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ericn
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Post by ericn on May 15, 2017 17:55:58 GMT -6
I'm interested in hearing more about the PCB's/Projects/Etc. My understanding is that there is an MK-47 and an MKU-47 right? The two mics are the same dual tube layout, but the MKU-47 features the fancy headbasket/pattern switching from Flea right? Are there other differences between these mics? Do they use the same PCB? Also, both of these are Max designs? Where does the Dany PCB fit in? Stam is using a PCB from Dany, not Max right? svart , did you make your own PCB, or were you using one of these? When I ordered this thing, I was thinking it was the same mic that tonycamphd (wish he was back around, honestly) built for Johnkenn , but with a Heiserman cap and without the fancy metalwork. Is this actually a different design? I initially bought a PCB (don't remember who from, but it's blue), but had to abandon a straight build due to the smaller mic body I used. I cut up the PCB and mainly used it to hold the tubes and a few resistors. The rest is either zip-tied or bolted to the metalwork. Yeah I remember yours was more of a home brew! Considering that yours didn't use the board if I spring for one of Danny's PCB you want to see if you can find a fault? |
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Post by svart on May 15, 2017 18:28:14 GMT -6
I initially bought a PCB (don't remember who from, but it's blue), but had to abandon a straight build due to the smaller mic body I used. I cut up the PCB and mainly used it to hold the tubes and a few resistors. The rest is either zip-tied or bolted to the metalwork. Yeah I remember yours was more of a home brew! Considering that yours didn't use the board if I spring for one of Danny's PCB you want to see if you can find a fault? I'm not sure it would be an outright *error* of the PCB, but more of an issue where maybe the 29R needs to be 25R in some mics, or maybe the B+ needs to be higher or lower depending on the tubes used, if you get my meaning. Since this mic is operating without bypassing, it's going to be easy to get it to oscillate to some degree. Doing a cathode bypass would help the stability (if that is what is causing the issues) but it would completely change the way the mic sounds, and we don't want that. I'm thinking that it might end up being a pot on the board for a resistor so the end user can "calibrate" the tube to the mic. Some folks might not remember, but there used to be a time when a TV repairman would have to replace tubes and then "adjust" the TV's alignment with pots and stuff because the new tubes wouldn't be exactly the same. Radios too! |
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ericn
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Post by ericn on May 15, 2017 18:39:16 GMT -6
Yeah I remember yours was more of a home brew! Considering that yours didn't use the board if I spring for one of Danny's PCB you want to see if you can find a fault? I'm not sure it would be an outright *error* of the PCB, but more of an issue where maybe the 29R needs to be 25R in some mics, or maybe the B+ needs to be higher or lower depending on the tubes used, if you get my meaning. Since this mic is operating without bypassing, it's going to be easy to get it to oscillate to some degree. Doing a cathode bypass would help the stability (if that is what is causing the issues) but it would completely change the way the mic sounds, and we don't want that. I'm thinking that it might end up being a pot on the board for a resistor so the end user can "calibrate" the tube to the mic. Some folks might not remember, but there used to be a time when a TV repairman would have to replace tubes and then "adjust" the TV's alignment with pots and stuff because the new tubes wouldn't be exactly the same. Radios too! Yeah I get it I'm just thinking if we can eliminate a variable in this, I contribute my literal 2 cents. I don't know why but I assumed there was a pot or substitute this resistor if... in this design for that very reason. |
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Post by EmRR on May 15, 2017 19:36:16 GMT -6
I'm not sure it would be an outright *error* of the PCB, but more of an issue where maybe the 29R needs to be 25R in some mics, or maybe the B+ needs to be higher or lower depending on the tubes used, if you get my meaning. Since this mic is operating without bypassing, it's going to be easy to get it to oscillate to some degree. Those still seem like awfully small moves for tube circuits, to make enough change to either cause or fix problems. Is this exacerbated by the use of grid R values like this? You average tube preamp has no problem swinging down to even 25% of specified B+ and sometimes up to 150%, so long as filament is stable and higher voltage doesn't affect cap voltage or transformer current ratings. I'm just now seeing filament voltage is DC series with cathodes tied to filament, in effect making it a (partial?) DHT (directly heated triode)? The modern answer in DHT world is to use a filament PSU which acts as a voltage source at DC with the output resistance near 0R, then near 5Hz rising to some kilo- ohms, behaving like a current source. But....that's a veer from the answers we seek at the moment, and a redesign. It would seem at least that 1K5 series R with the filaments might need trimming with B+ or cathode R variations. Wildly, I wonder about use of an inductance in place of or in series with the 1K5. |
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Post by stratboy on May 15, 2017 19:58:16 GMT -6
I could stand to be corrected but I don't think the Stam/danny mike is the same as max's 47, like Tony had at first or the follow up mk-u47. It seems to me based on the pics that the psu's are different, as well. Why does the Stam psu have a tube in it ? Is it following the neumann schematic exaclty ? I ask only because I think Max's does ? Not implying there is anything wrong with the Stam 47 mike: just curious to understand the differences. I don't see any tube in the Power Supply. I see a "trimmer" pot. www.vintagemicrophonepcbkit.com/D-47.htmlI think the MK-47 is the single tube predecessor to the dual tube MKU-47. I have a MK-47 with a Flea capsule I got from Yotonic last year. He told me it was built by Toby Foster, with PS mods by Scott Hampton. I have no idea what tweaks they made, but it sure sounds good. No noise issues, although it's not quite the same mic as what's being discussed here. I offer this because it may support Svart's point that little variations in the published design might make the mic quieter and more stable, accepting a wider range of tubes. |
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Post by drbill on May 15, 2017 20:23:34 GMT -6
MK-47 has TWO tubes.
My understanding was that the differences between the MK and MKU were the body, head basket, etc..
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Post by yotonic on May 15, 2017 20:44:18 GMT -6
I think the MK-47 is the single tube predecessor to the dual tube MKU-47. I have a MK-47 with a Flea capsule I got from Yotonic last year. He told me it was built by Toby Foster, with PS mods by Scott Hampton. I have no idea what tweaks they made, but it sure sounds good. No noise issues, although it's not quite the same mic as what's being discussed here. I offer this because it may support Svart's point that little variations in the published design might make the mic quieter and more stable, accepting a wider range of tubes. Hey Spence you are correct, Toby and Scott made a number of modifications to Max's "design". Toby has worked on my vintage Neumann U47 as well as many of other people's U47s. Not only did the power supply require some modifications by Scott, but Toby made some tweaks where he noticed potential issues. That mic you have (which I want back!) ranks right up there with my favorite 47s - Long body Neumann and Voxorama. Guys like Toby Foster and Andreas Grosser have so much experience working with mics and are so fairly priced in today's market that I would never do anything without them. |
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Post by stratboy on May 15, 2017 20:46:00 GMT -6
MK-47 has TWO tubes. My understanding was that the differences between the MK and MKU were the body, head basket, etc.. To quote Mr. Zappa, "I could be totally wrong..." Yotonic could weigh in here. |
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Post by drbill on May 15, 2017 20:51:15 GMT -6
MK-47 has TWO tubes. My understanding was that the differences between the MK and MKU were the body, head basket, etc.. To quote Mr. Zappa, "I could be totally wrong..." Yotonic could weigh in here. I know for sure I can count to two, cause I got that many fingers on one hand. So, you can go to the bank with that. Beyond that, these are all hand built. Who knows what they got in em. For sure there were "mistakes" that were fixed and substituted along the way. |
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Post by johneppstein on May 16, 2017 2:58:02 GMT -6
It would be great if svart found something that myself and others have missed. It still does not explain why I had to go through 50-60 tubes to find a few pairs that were useable. If something was wrong with my build, no pairs of tubes would end up being quiet. Meaning the problem would follow the mic regardless of what was put into it. I do find some of the tone of this thread to be reminiscent of GS. Makes me feel like I should take a break from this forum. I think you're forgetting a key factor in the stated hypothesis.................. INTERACTION In this case, interaction between the tubes, the circuit, and stray capacitance and inductance between traces in the layout. EDIT: I see the discussion has got well past this in the last page..... oh, well................. |
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Post by stratboy on May 16, 2017 3:53:28 GMT -6
To quote Mr. Zappa, "I could be totally wrong..." Yotonic could weigh in here. I know for sure I can count to two, cause I got that many fingers on one hand. So, you can go to the bank with that. Beyond that, these are all hand built. Who knows what they got in em. For sure there were "mistakes" that were fixed and substituted along the way. Totally cool with that. Looking forward to Svart's tube tests. Substitution and elimination is a very good troubleshooting technique. |
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Post by Ward on May 16, 2017 5:48:14 GMT -6
Just one question for the engineers in this thread...
Is the 6028/408a tube pair the last word in a viable VF14m replacement or are there other things to try?
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ericn
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Post by ericn on May 16, 2017 6:25:37 GMT -6
Just one question for the engineers in this thread... Is the 6028/408a tube pair the last word in a viable VF14m replacement or are there other things to try? What day of the week is it? Who's talking ? Is it raining? Are you wearing black or white socks? You know this is a moving target and everybody has their own Idea and in many cases that changes with the weather! Or are you trying to start something 😎 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 ! |
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Post by Ward on May 16, 2017 9:11:41 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.... |
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Post by gevermil on May 16, 2017 9:42:46 GMT -6
I have a box of 100 Phillips 6028 tubes
All I have pulled out have been quiet
I'll gladly donate some to who ever is doing some
Valid testing . I have a few other kinds that I'll donate as well
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Post by illacov on May 16, 2017 9:48:41 GMT -6
Just one question for the engineers in this thread... Is the 6028/408a tube pair the last word in a viable VF14m replacement or are there other things to try? Fox Audio has a mic that's based on the ECC88/6922 tube. I've heard some clips with it and I was very impressed with it. Very old school, smooth top end, very solid and big proximity effect. I don't know who did this sort of thing with the 6922 before, if at all, but that's one tube that I really liked and its still in production. I know at one point he was doing a dual 6922 version as well. Thanks -L. |
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Post by Ward on May 16, 2017 9:50:06 GMT -6
Just one question for the engineers in this thread... Is the 6028/408a tube pair the last word in a viable VF14m replacement or are there other things to try? Fox Audio has a mic that's based on the ECC88/6922 tube. I've heard some clips with it and I was very impressed with it. Very old school, smooth top end, very solid and big proximity effect. I don't know who did this sort of thing with the 6922 before, if at all, but that's one tube that I really liked and its still in production. I know at one point he was doing a dual 6922 version as well. Thanks -L. 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". |
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Post by illacov on May 16, 2017 10:05:50 GMT -6
Fox Audio has a mic that's based on the ECC88/6922 tube. I've heard some clips with it and I was very impressed with it. Very old school, smooth top end, very solid and big proximity effect. I don't know who did this sort of thing with the 6922 before, if at all, but that's one tube that I really liked and its still in production. I know at one point he was doing a dual 6922 version as well. Thanks -L. 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. |
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Post by drbill on May 16, 2017 10:11:14 GMT -6
Replacing the resistor helped my mic, but it is not the FINAL solution or fix for the tube noise.
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