Unknown tube mic preamp

Help Support GroupDIY:

rmburrow

Well-known member
Joined
Oct 8, 2013
Messages
192
Attached is a hand drawn schematic of a mic preamp; the manufacturer is unknown.  The tube used is the octal dual triode 6SC7, socket mounted on rubber.  The preamp uses a UTC A-11 input transformer and a UTC A-24 line output transformer.  There is a variable wirewound "T" pad on the output.  The AC power input is through a two prong Amphenol fitting on the chassis.  The audio input and output are through Cinch socket & plug.  These preamps were made for tray mounting since the power transformer primaries are unfused.

The audio response may be lacking, based on the value of the capacitors in the audio path.  The input and output transformers are good quality.

I have photos, but the size of each photo slightly exceeds the attachment size limit.    The unit appears to be of late 1940's vintage.
 

Attachments

  • preamp_5.pdf
    81.7 KB · Views: 69

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,973
Location
NC, USA
Host the photos somewhere and link them.  Or reduce photo size. 

Is the diode a large selenium disc array?  If not, it's not original. 

I had a large plug in Farnsworth preamp that came out of a train car PA system this is reminding me of. 
 

rmburrow

Well-known member
Joined
Oct 8, 2013
Messages
192
Mr. Williams and others, thanks for your interest.  The HT rectifier is a half wave selenium stack.  The capacitors are the old Sprague wax paper ones.  I printed and rescanned to get a couple pictures up here in pdf format.    There are absolutely no markings or other identifying information on the chassis.
 

Attachments

  • Preamp_6.pdf
    1.3 MB · Views: 35

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,973
Location
NC, USA
Would guess a DIY custom console.  I've seen many that look 'manufactured' in quality, and this rings no bells. 
 

mjrippe

Well-known member
Joined
Dec 27, 2011
Messages
2,739
Location
Brooklyn, NY
I'm with Doug on this one.  Custom build in the 1940s makes sense.  Odd to see a per-channel power supply though.  Most racks used a single PSU and distributed the B+ and filaments.  However the design is smart, with the power on one end and audio on the other.  I would replace the wax paper caps and up the value of the one on the output.  With UTC iron, it should be pretty hi-fi.
 

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,973
Location
NC, USA
abbey road d enfer said:
Not really; grid-leak bias, see the 3.3Meg grid resistors.

Yep, I have encountered a number of home made preamps from this era with grid leak bias. 
 
Top