Tbone SCT-700

[Tubetec]

Just wondering if anyone has used one of these and what its like .
theres one going cheap near me I've put in an offer on as the basis for a project .

 

[Khron]

Probably good for a body donor? I've "hoarded" a few SCT2000's for modding purposes.

Not much (else) one CAN expect from a K67 and a 12AX7 (Apex 460 circuit, cathode follower & low-ratio transformer)...

 

[RuudNL]

And a 12AX7 soldered on the PCB!

 

[Khron]

The SCT2000's at least have it socketed, and you get a pattern switch already on the power supply

 

[LevinGuitar]

I like them for donor mics, you have most part of a typical tube circuit there, so mods are easy. Head shape, grille and body aren't bad. I always wondered if the chinese U47 head would fit it, does anyone tried it?
Stock it sounds not very pleasant but if you find a low noise one, it can be useful.

 

[Tubetec]

I see Thomann is doing two tube mic psu's , both have the variable patern , one is the same as the standard SCT-2000 power unit, the other has a box a bit like a vintage Neumann NU67 . 50 and 70 euros each .

That is a pain that the tube is soldered into the board , but at least they went to the trouble of adding insulated solder tags in the hi impedence paths unlike WA . I usually prefer to use a ptfe socket on tube mics anyway .

I know those cheap Chinese 12ax7's, they are garbage , I wouldnt even use one on a guitar amp never mind a mic .
transformer is probably crap also , but UTM now does a nice U47 transformer for 30 euros and he can mumetal pot it if you want , Ive found that to be beneficial when used in close proximity to guitar amps .

I have a bunch of 6au6's I might try similar to the Sony or of course the EF86 would also do , Im not a fan of dual triodes with one section unused , I prefer the single tube approach .

The SCT-2000 is definately a better starting point for a project though .

 

[raywisby]

I see Thomann is doing two tube mic psu's , both have the variable patern , one is the same as the standard SCT-2000 power unit, the other has a box a bit like a vintage Neumann NU67 . 50 and 70 euros each .

That is a pain that the tube is soldered into the board , but at least they went to the trouble of adding insulated solder tags in the hi impedence paths unlike WA . I usually prefer to use a ptfe socket on tube mics anyway .

I know those cheap Chinese 12ax7's, they are garbage , I wouldnt even use one on a guitar amp never mind a mic .
transformer is probably crap also , but UTM now does a nice U47 transformer for 30 euros and he can mumetal pot it if you want , Ive found that to be beneficial when used in close proximity to guitar amps .

I have a bunch of 6au6's I might try similar to the Sony or of course the EF86 would also do , Im not a fan of dual triodes with one section unused , I prefer the single tube approach .

The SCT-2000 is definately a better starting point for a project though .

I tried using different 12ax7 tubes in my microphones, mesa, tung sol, electro harmonix! I did not notice any difference!

I also tried the Soviet 6n2p-ev (similar to 12ax7)! sounds harshly in the high mids

 

[kingkorg]

Same circuit as has been for years excepting modern RF proof mods. Two stage cascade, xfmr tertiary NFB to first stage. New mics have 12AX7 instead of 6072.

Standard mic triode self bias>>>>cascade to triode output>>>cap..xfmr...xfmr tertiary NFB to 1st stage is also -10dB/not switch.

Since we are at 12ax7. Here's David Bock describing Manley Ref C. Not sure what he meant here by cascade. Cascode? Cathode follower?

 

[Tubetec]

Maybe you forgot to post the link to the Bock comments Kingkorg .

What sort of ball park money should I be paying for a used SCT-700 anyway .
And the going rate for used SCT-2000 , or did you buy from new Khron ?

I'll probably do it hardwired style but I would like to investigate U47 style cathode bias with a modern tube , that removes the need for two capacitors , one at the grid and the usual cathode bypass .

 

[Khron]

Used SCT2000's go for 120-150e or so (shipped). I briefly considered getting new, but since i was gonna gut them anyway, i took a peek on the used market, and i was pleasantly surprised.

I think i've seen 700's for (even) well under 100 (asking price), but i didn't wanna bother with adding a pattern switch and whatnot.

 

[Tubetec]

Cheers Khron ,

Ive made an offer of 60 plus P&P , I noticed the shockmount is broken , the elastic needs to be replaced. I also asked the seller if their willing to test it and see its all working ok before going ahead with the sale .
If its gone noisey I may be able to negotiate a lower price , even though I'll most likely disgard the Chinese innards entirely , I probably will scavenge/re-use the high value resistors though .

As only the address side of the capsule has gold plate on the SCT-700 ,adding the polar patern would be redundant , however adding a small rotary switch would allow the use of other compatible mics later ,as far as I can tell the PSU is in all other respects identical between the SCT-700,800,1000,2000


Even for the components like the case ,cable,psu, mic body and capsule 60 euros is a hell of a bargain , seeing as I have everything else I need apart from the transformer which IGS will do for me , outlay is less than 100 all up .

SCT-700 case is slightly shorter than the 2000 , 6au6 might fit the bill nicely.
There appears to be two versions of the Tbone SCT700 , the older one with the
ingraved body and the newer one with a painted on logo , not sure what the difference is inside , probably just more cheapness on the newer one if anything .

Below is Tbones vintage mic psu , they even went to the trouble of replacating the style of knobs found on old German gear .

Added a couple of SCT 700 gut shots , note in the final pic he's had to modify the assembly to accomodate the tube socket .

 

[Tubetec]

Even as is the quality of the mic kicks the arse off WA's attempt at a tube mic , and probably equals many branded products ,like 'Tele' going for ten times the price.

Maybe Ian Bells PCB valve mount methodology could be beneficial in mics especially where were caught for space.

 

[Aleguitarpro]

My experience with sct700 to make a sort of u47 clone is removing the metal bell that contain output transformer and soldering a socket for the tube. I stay with a triode (half used) but I don't remember what I used. Installed a k47 and a decent transformer. I sold the mic but was in ballpark with my ef800 based u47 clone. Power supply can be modified adding variable pattern control but space is limited.

 

[Tubetec]

This showed up on search ,
M5 appears to be a similar circuit ,but without the built in psu found in the domestic version .

http://xaudiaelektrik.blogspot.com/2013/02/thiele-microphones-schematics-and.html

 

[kingkorg]

Maybe you forgot to post the link to the Bock comments Kingkorg .

What sort of ball park money should I be paying for a used SCT-700 anyway .
And the going rate for used SCT-2000 , or did you buy from new Khron ?

I'll probably do it hardwired style but I would like to investigate U47 style cathode bias with a modern tube , that removes the need for two capacitors , one at the grid and the usual cathode bypass .

https://groupdiy.com/threads/what-if-you-opened-up-a-manley-reference-c-and.76279/post-981033
https://groupdiy.com/threads/what-if-you-opened-up-a-manley-reference-c-and.76279/post-983912
https://groupdiy.com/threads/what-if-you-opened-up-a-manley-reference-c-and.76279/post-980986

 

[Tubetec]

https://www.ebay.ie/itm/392352100825
Thats only 19mm wide , might fit in the SCT-700 psu for remote patern switching .

Thanks for including the link back to the other mic topic Kingkorg , where did you find the Manley style case again ? The mark up branded on branded tube mics is truely astounding .

 

[Tubetec]

I was reading up the details on the Sony C800g , it seems like the tube is mounted up on its own vibration damped floating sub assembly . I can hear the groans because of the mention of the Sony ,but Im not remotely interested in the Peltier B%^&*it . Its like the designer was a fan of the Daleks from Dr Who , it looks scarey ,even to the trained eye .

There are plenty of nice small anti vibration mountings around these days, mini drone /steadicam platforms might be the most appropriate size for a tube mic , they cost around 10 $ .
These typically provide excellent damping , their designed to isolate the gyros and acelerometers in drones from motor vibrations , so they should do just the right job to isolate a tube from any vibrations in the mic case . The basic element is a rubber grometed ball which acts like a cushion , a PCB could be drilled to accomodate the mounts easily .

Ive used mini rubber motor mounts to isolate tubes before , typically though the motor mounts are designed to support a few ounces of weight , the tube , socket and PCB is probably less than 30 grams all up . The softer suspension on the drone mounts will probably gobble up much more vibration than the relatively stiff motor mounts .

below:
solid rubber motor mounts
example of modern grommet style drone/camera mount with frame .

As far as bias goes , I found the Neumann Thermo , its as simple as it gets really , the cathode is maintained above ground by a low value resistor and some extra voltage is provided on the heater supply to account for the difference . Id probably go with the usual U47 config by applying the polarising voltage to the back plate and save on having to mount up a cap in series with the grid .

I was wondering if anyone had tried bulk foil resistors as anode load for a tube mic. 0.5W is adequate in the 6AU6/Sony design , the nice thing about minimising the number of components is theres no problem spending a few euros here or there . The stability of the bulk foils is probably way above what matters in this situation but when we only need one or two ,why not . Low ohm values are around too so using bulk foil as heater series resistor is a possibility .

Doing the calculation for the U47 style heater/ cathode bias resistor on the Sony 6au6 circuit the series resistor also comes out at 5 ohms same as the thermobottle . An LT supply of 7.1 volts would be needed to leave the tube with the required 5.7 volts on the heater and the cathode with 1.4v .

I had contact back from the seller of the mic earlier , there having a long weekend away for the St Patricks festival , I'll hear from them next week , but the purchase of the mic is more or less a done deal .

 

[Tubetec]

I got the mic for 60 + postage in the end , be a few days before it arrives .

 

[Tubetec]

I found a neat drone anti vibration mount that looks around the right proportions for use inside a condenser microphone . 72mm wide but it could easily be trimmed at the edges to fit almost any wide body mic .
Looks like a good fit for mounting up a tube socket on the board with the hole in the middle .
Should also be easy to mount up turret tags or PTFE insulated pins as required depending on the pitch of the component leads , I dont really even see the need for copper tracks as everything could be hardwired , but maybe a good ground plane scheme wouldnt do any harm . Im not sure if the black Epoxy glass is carbon loaded or not , perhaps theres a better material from an insulation standpoint .

 

[Tubetec]

https://www.leiton.de/special-technologies-black-fr4-printed-circuits.html
link to details about black FRP there , seems its equivalent in every way to normal opaque FRP , but may help sheild sensitive components from ambient light .