t.bone SCT2000 (Apex 460) - U47 mod with both halves of the tube in parallel

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sirko

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Dec 1, 2011
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14
Hello,

I plan to mod an t.bone SCT2000 (same inner life as Nady TCM 1050, Alctron HSGT-2B) to be a more U47 alike.
For now I decided to do the following:

Change the capsule to a microphone-parts.com CK-47.
Change the tube to a NOS 6072a.
Change the transformer to a TAB-Funkenwerk T-14 or a T-11 or a T-47, because the Cinemag seems to sound too clean for my taste.
Remove RF filtering caps C9 and C10.
Replace C8.
Remove cathode follower.
Change zener diodes D5 and D6 in the power supply to 56V.

On Fox Audio Reearch they seem to connect both halves of the tube in parallel to get a more VF14 like behaviour:
http://foxaudioresearch.ca/The47.htm  (Look for topic "New Circuit ...")

When I wanna do that too, what capacitors do I have to change to half their values afterwards? C8? Which capacity is right? Any others?
What pins of the tube do I have to connect with?
Are there any other changes nessecary for this mod?

Any help is appreciated.

Sirko
 

gandhalf3

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Apr 25, 2010
Messages
60
When your mod will be done it could be great to share some picture of your work.

I would like to mod my SCT2000 too.

Regards.
 

sirko

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Dec 1, 2011
Messages
14
Hi,

Hmm, the link seems to have a referer check. Here is the thread (about removal of cathode follower) where the picture of the circuit is linked from:
http://studioforums.com/eve/forums/a/tpc/f/1526095781/m/2111089522/p/1 (Its a post from craigmorris74, more at the bottom)

Meanwhile, Brian Fox from Fox Audio Research has answered me (Thanks!). To connect the tube halves in parallel, connect the tube as follows:
-----
                Valve 1          Valve 2
Cathode      8        ---->    3
Grid              7        ---->    2
Plate            6        ---->      1

The filament or Heater circuit are pins 4 and 5 connected together and ground pin 9 to operate the heater on 6.3 volts.

If you are building your own power supply, you also have the option of running on the heater on 12.6 volts by connecting only pins 4 and 5 (12.6 V and ground) and NO connection to pin 9.  But 6.3 volts is more common. The tube doesn't care as long as the little heater is glowing orange.

If possible use PTFE (teflon, American term) insulated wire. Or if you don't have that put a piece of Teflon tubing around the little wires that you connect the triodes together with.  It will work without this, but the output (plate( can feed back into the input (grid) and reduce the amplifier gain a little. The heater wiring does not need this teflon.
-----
Since the tube halves in parallel have an output impedance of 10000 ohms, the output transformer needs at least a ratio of 10:1, right?
I hope, the T49 fits in the body: http://tab-funkenwerk.com/id82.html

Regards
Sirko
 

vp47

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Joined
Nov 30, 2011
Messages
9
Let us know how it goes. I have one of these that stays in a drawer unused. I'd be interested to see what you come up with.
 

okgb

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Oct 18, 2004
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5,850
Location
Winnipeg Mb Canada
Might as well go for authentic
c12schem.gif
 

mattrico

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Jan 27, 2021
Messages
12
Sorry to dig up an old thread but I'm wanting to turn my SoundArt STM5000 (same as Alctron GT-2B) into a U47 style mic, and after doing some reading there's some stuff I'd like to try, however being somewhat of a noob I'm a bit perplexed (I just finished modding an HST11A into a c12/251 style but want to go a bit crazier with this u47 build). So, questions:
a) What's the trick with removing the cathode follower? My tube is soldered, not socketed. Not sure if this will be a problem.
b) Once I've done that, is it simply a case of just running new wires from pins 8-3, 7-2, and 6-1?
c) Brian Fox mentions running a 6922 tube instead of a 6072, for a bunch of benefits, including giving the preamp stage a fairly flat response from 20hz-20khz.
The 6922 Tube I'm keen to try this. He says it just requires 2 minor changes, though doesn't mention what they are, and since my tube is socketed I might not be able to do the changes anyway. I've attached some pics so you can see how it's all hooked up.
I've already got a 3U Audio 47 capsule to go in, plus some diodes and capacitors left over from the C12/251 build, but don't want to buy a tube and transformer until I'm sure of how I'm going to do the build.
Thanks in advance for any help!
Matt
 

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mattrico

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Jan 27, 2021
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12
Anyone? Would love to start modding this while I have some down time but don't want to make a mess of it.
 

xeawr

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Dec 14, 2016
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Germany / Austria
Sorry to dig up an old thread but I'm wanting to turn my SoundArt STM5000 (same as Alctron GT-2B) into a U47 style mic, and after doing some reading there's some stuff I'd like to try, however being somewhat of a noob I'm a bit perplexed (I just finished modding an HST11A into a c12/251 style but want to go a bit crazier with this u47 build). So, questions:
a) What's the trick with removing the cathode follower? My tube is soldered, not socketed. Not sure if this will be a problem.
b) Once I've done that, is it simply a case of just running new wires from pins 8-3, 7-2, and 6-1?
c) Brian Fox mentions running a 6922 tube instead of a 6072, for a bunch of benefits, including giving the preamp stage a fairly flat response from 20hz-20khz.
The 6922 Tube I'm keen to try this. He says it just requires 2 minor changes, though doesn't mention what they are, and since my tube is socketed I might not be able to do the changes anyway. I've attached some pics so you can see how it's all hooked up.
I've already got a 3U Audio 47 capsule to go in, plus some diodes and capacitors left over from the C12/251 build, but don't want to buy a tube and transformer until I'm sure of how I'm going to do the build.
Thanks in advance for any help!
Matt
Dear Matt,

how can I help?! Let me see. First of all, I'm an electronics noob myself and have read this thread a thousand times wondering how to put this marvelous sounding mod into practice. I have worked a bit with chinese tube microphones, many of them being Alctron mics.

Here's what I can contribute:
a) removing cathode follower: From my own experience (take it with a grain of salt) the cathode follower is cleaner / less distortion and more headroom than without it. Is it bad? That's hard to tell. As far as I know the WA47 has a cathode follower and it sounds pretty darn good, relaxed, big but still clean. It does remind me of a Neumann a bit.

I don't think the soldered tube will be a problem.

b) I have wondered about this myself, because elsewhere I have read that one has to adjust the cathode resistor as well. Conflicting statements - maybe someone else can chime in?

c) 12AT7 (ECC81) and 12AX7 (ECC83) are the most used variants in dual triode designs. They worked for AKG, Manley etc. Given the availability of them I wouldn't consider going the extra mile.

I think this endeavor is suited to get a "nice sounding mic", but I think it's impossible to get anywhere close to a 47-style sound.

From my experience the biggest factors are (in that order):
1. Capsule (70%)
2. Tube (15%)
3. Caps / Transformer (5%)
4. Rest (Power Supply etc.)
given that there is no apparent ringing going on with the mic body and the caps/transformer are not microphonic.


I would appreciate it as well, if one of the pros could chime in and enlighten us in regard to b) .

Last note: From what I read about using both halves is that the sound gets creamier or more smeary, which didn't sound like something I wanted from a mic. I always want as much resolution and detail as possible!

Cheers!
 
Last edited:

mattrico

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Jan 27, 2021
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You are right about a few details, though essentially it comes down to a matter of taste, and intended sound. There's nothing "wrong" with the cathode follower, Brian Fox sings its praise, essentially it's a really great way of implementing the other side of the tube to resolve impedance issues. And in terms of sound quality, definitely just swapping out some components + tube, capsule and transformer, you can achieve a great sounding mic.
The thing is that it's not always desirable to achieve a "high definition" sound, the clearest sound isn't always the best (look at how popular neve 1073 style preamps are, for example). The idea of running both tube plates in parallel is to aim for more of the vintage sound. If I were to build it the same as my other HST-11a clones but with a different capsule, yeah it's gonna sound different but it won't be much of a different beast overall, it'll be more of the same but with a different EQ. So that's pretty much why I thought about going down a different path with this particular mic, hence my questions. The whole thing is definitely still eluding me and at the moment I have the mic sitting in front of me all pulled apart, just waiting for someone to jump in with the details I need to proceed with the rebuild.
 

xeawr

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Dec 14, 2016
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Germany / Austria
Hey Matt,
thanks for your response. I understand what you're saying. As for the 1073 keep in mind that in this audio realm distortion and frequency response has nothing to do with the level of detail you hear. The 1073 are extremely precise in regard to detail, although they are not "distortion free" and don't have a linear frequency response. This is exactly what keeps me from trying the "both triode halves"-thing: you basically split the signal and sum it together again which will result in phase-issues, comb-filter effects etc.
This is not desirable regardless of vintage or modern mics. We tend to see very short and simple circuit paths in most (vintage) microphones.

Paralleling both halves supposedly reduces plate resistance, which possibly makes better use of the cheap sounding stock transformer. So it's always a sacrifice as long as you're working with cheap parts. This is the very reason I've reduced my mic modding to a minimum last year, because it's just throwing money at average sounding/looking mics.

Don't want to derail this thread but let me add this bit: A $300 capsule + $100 transformer plus $50 parts is the price of a TLM102 or a used AT4050 or something similar. There are many microphones that perform very well and sound good out of the box (and can be sold again).

If your mic is open on the bench and waiting for the both-halves-of-the-tube-mod, then by all means do it and report back your findings. Just connect the two halves, each with a small wire.

BTW I also found this. An interesting read:
 

mattrico

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Jan 27, 2021
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Yeah I get what you're saying. Essentially in trying to chase a vintage sound, it could end up just muddying up the sound across the board. Perhaps I'll abandon the idea and just look into using the 6922 tube, which Brian says is a nice improvement on the 6072, and can be found in NOS versions much more easily than 6072's. Still, in doing this I'm not entirely sure which 2 mods he did in order to get it to work. Anyone know? I might have to message him and hope he gets back to me.
In terms of cost to benefit ratio, I would much rather have something I built than a TLM102, those things are way overpriced considering the sound. I definitely didn't spend that much on my capsule either, it's a 3u Audio capsule and was a couple of hundred landed. But yeah I get where you're coming from. It starts to not make a lot of sense once you add it all up. It all ends up coming down to which mods add the most bang for your buck. And whether you actually enjoy sitting around soldering :)
 

xeawr

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Germany / Austria
Check out the Vintage King Neumann shoot out. I downloaded the files and did multiple blind A-B comparisons (male and female) and to my huge surprise the TLM102 was always second. First was always ... u guess ... the U47.

After that I bought two TLM102s, and they proved to be awesome really. These along with the other Neumanns (U87A, U89, and TLM103) have treated me better than all the cheap modded mics (with only one exception).

But I know; I often had to admit to myself that I'm very picky. So the modded Alctron might totally work for you.
 
Last edited:

xeawr

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Dec 14, 2016
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Keep in mind, that your tube is soldered. You don't want to get a nice 6922 and then mess up the soldering job. Test the tube first in another mic. But then again - the 6922 is not a drop in replacement for the 12A*7-family (heater and specs are different). So you probably don't have another mic around where you could test the tube. Why not get the 5751 instead? It's a drop in replacement, sounds nice and is being used in the WA47. No its not a VF14, but hey - I like it a lot.

If I were you I would just solder the two triodes in parallel with the current tube - just 'cause, what the heck, you can - and see if you like the results.

Consider replacing the electrolytic caps with quality ones, at least the cathode bypass - and maybe use a polystyrene cap for the coupling position. That one does make a difference.

All the best!
 
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mhelin

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I put parallelled 6922 with an unknown K47 (guess it was 3u) into friends sct2000 where it sounded quite cool and vintage. Had to use lower (47k) plate resistor though.
 

mattrico

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Jan 27, 2021
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I had read about the 5751 in other threads so that could be a good option. In terms of running them in parallel, do I need to solder in that resistor Mhelin mentioned or is that only needed for the 6922?

Do I need to cut or bridge any of the lines on the PCB before I run them in parallel? I was assuming I would have to, at least to eliminate the cathode follower.

On the TLM102 front, I just can't understand the love for it. The TLM103 is a better mic, but the 102 is nothing special in my opinion. But then if you can use it and get good results from it, then opinions don't really matter. It just comes down to whether you're happy to spend that amount of money on that mic, which I wouldn't be. For the same money as one of those here in Australia, I bought a Beesneez U87 clone, which is a great sounding mic. So I guess that's what it would come down to in the end for me.
 

mattrico

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Jan 27, 2021
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Was doing some more reading and apparently some mics with this circuit come with a 12AT7, and there are some good tested NOS 12AT7's I can source locally, so I think I'll go down that path (the one I've found apparently has balanced sections too which would help if I go down the parallel tube path). I shouldn't have to touch the transformer then (Brian Fox says the stock transformer is a decent offering so I may as well try it first in this configuration before resigning it to the bin). Then I can just swap out some caps, the diodes in the PSU, then swap the capsule, and should be in business.
 
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