11-10-2017, 11:23 AM
I also just completed a small stereo pre-amp. It uses 2 12AT7s which may be replaced with two 12AX7s for more gain. One triode drives the second, but a switch allows the injection of a signal at the grid of the 2nd triode if less gain is needed.
It has its own PS with B+ at 180V and the filaments running in parallel at 6.3V. It measures 7 1/2 x 3 x 3" including the veneered wooden cabinet. I tested it with a HK audio generator (a very cool device in it's own right) and it can pass signals from 1hz (no gain - probably some loss) up to 50,000Hz (and beyond). It is +/- 3db between 20hz to better than 20khz depending somewhat on input level and the gain adjustment which is a 650K stereo pot between the triode sections ( wanted a 1M pot but didn't have one). Two sets of RCA in (one high level, one low) and stereo RCA out.
It can also be used as a mixer for stereo into mono.
P.S. If not using the 1st triode - ground the input. No sense in having it sitting there howling, even if you can't hear it.
I have added both of these devices to my work bench. Now I won't have to go dig up an amp when needed to test a project. It has already paid off in testing a Fisher receiver last evening.
It has its own PS with B+ at 180V and the filaments running in parallel at 6.3V. It measures 7 1/2 x 3 x 3" including the veneered wooden cabinet. I tested it with a HK audio generator (a very cool device in it's own right) and it can pass signals from 1hz (no gain - probably some loss) up to 50,000Hz (and beyond). It is +/- 3db between 20hz to better than 20khz depending somewhat on input level and the gain adjustment which is a 650K stereo pot between the triode sections ( wanted a 1M pot but didn't have one). Two sets of RCA in (one high level, one low) and stereo RCA out.
It can also be used as a mixer for stereo into mono.
P.S. If not using the 1st triode - ground the input. No sense in having it sitting there howling, even if you can't hear it.
I have added both of these devices to my work bench. Now I won't have to go dig up an amp when needed to test a project. It has already paid off in testing a Fisher receiver last evening.
