Hamptone LCMP tube micpre question

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seavote

Well-known member
Joined
May 31, 2006
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Location
Long Beach,New York
i'm considering an LCMP tube pre build:
http://www.hamptone.com/figure1.htm
http://www.hamptone.com/tape_op_art.htm

the metal work is no problem. i'll use the recommended power supplies from mouser. this leaves the audio board to DIY. My problem is that i dont understand how to hook up the jensen jt115
http://www.jensen-transformers.com/datashts/115kepc.pdf

to the LCMP audio PCB
http://www.hamptone.com/LCMP_CV.htm
4 pins on each side of the jt115. 6 pins on the pcb where the input transfomer is connected. what am i missing?
 
Hummm...
Looking at the PCB, I count 12 pins for the input transformer (XT in), and 8 for the output transformer (XT out).
On XT out, 2 pins are soldered to the ground so at first sight it may look like 6 pins... But the JT115 is an input transformer, right? Not an output one.

On the JT115 you need to use only 6 of the 8 pins
1 is input hot
4 is input cold
5 is output hot
6, 7, and 8 to the ground


For XT in, only four of the twelwe pins are usefull on the PCB, they are labelled 4,5,6, and 7.
4 is output hot connect to 5 on the JT115
5 is output ground connect to 8 on the JT115
6 is input hot connect to 1 on the JT115
7 is output cold connect to 4 on the JT115

you can use the two pins on both side of the one labelled 5 on the PCB to connect pins 6 and 7 from the transformer, or tie 6, 7, and 8 on the JT115.

Happy new year!
Axel
 
Yes jt115 is an input transformer. i havent decided on an out put transformer yet. thanks for your help. perhaps you can indulge another question. the LCMP uses 50.4 VDC to supply the 4 tubes heaters(wired in series). am i right in thinking that if i build one channel (that's my plan)
and add another when i can swing the $$$$. i must add some type of "dummy load" to the heater circuit? if so, would i calculate it the following way? 2 tubes are used in each channel:1 6072 and 1 12au7. each tube wired in series requires 12. vdc@150ma. so 300ma (two tubes in a channel) divided by 25.2vdc equals 11.904 ohms.
can i use a 12 ohm resistor ,to ground,for a dummy load? i hope so as this will mean i'm starting to understand what i'm doing better. :wink:
 
almost there! But be carefull, a 12 Ohms resistor would draw way too much current!

You assumed that R = I/U that's your error.

R = U/I wich means R = 25,2V divided by 0,300A = 82 Ohms

Now that you have the value for your dummy load resistor, you should worry about the power dissipated in it...

P = U X I = 25,2 X 0,3 = 7,56 Watts. For safety margin let's use some 15 Watts. So you need a 82 Ohms/15W resistor... Not so common, and not so cheap...

Since you plan to build the second channel someday you could go another route;
Wire the heater circuit on 2 more Noval sockets, and plug in a couple of whatever old 12xx7 you may have in your junk boxe... Here is your perfect dummy load...

Axel
 
mad ax, thanks. the best solution is the simplest.i'm ordering double caps and resistors anyway so i will make 2 pcbs and populate them. trannies,XLRs and switches on the second channel will come later. i have a bunch of 12au7s (uos) ,im sure i can find 2 extras that work.does anyone know what output transformer came with the LCMP kit. the jensen 4:1 output transformer is about $125 and i'm looking to go cheaper.
 
I don't know what came in the original kit, but I used an Edcor WSM15k/600 in my PTP LCMP and it sounds great. $10USD. Maybe Edcor makes a version that would work with a pcb. Also, if you email Scott Hampton, I'm sure he'd be glad to give you the info you're looking for.

best of luck.
 

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