sirko

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


sirko

I'll try to answer myself. When I have a look on this schematics for replacing C8 and removing cathode follower:

http://www.angelfire.com/ky3/craigorris74/a460schemCFremoved.JPG

it seems I have to connect tube pins 1 with 6, 2 with 7 and 3 with 8 and 4 with 5. Is this correct?

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.

pucho812

I'll try to answer myself. When I have a look on this schematics for replacing C8 and removing cathode follower:

http://www.angelfire.com/ky3/craigorris74/a460schemCFremoved.JPG

it seems I have to connect tube pins 1 with 6, 2 with 7 and 3 with 8 and 4 with 5. Is this correct?


link does not work...
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

sirko

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

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

Might as well go for authentic
GKB Audio / Greg Boboski


 

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