[Hallicrafters] SR-2000 Mod

[Jim Liles]

Hallicrafters SR-2000 Note:

Over time we have found that the SR-2000 generally lacked adequate transmit drive to saturate the 8122's.  Is that a problem?  Absolutely!  If you load the transmitter up with inadequate drive it will be too lightly loaded when full drive is applied causing screen current to soar.  The design of the SR-2000, in tune mode, forces the carrier through the filter somewhere between the -30 and -40db points on the skirt.  If the filter, alignment, and components are close to perfect, you will have adequate drive in tune mode. In most cases you won't.  Full drive will occur when voice/audio is applied because the center 2.1kc of the filter will lose only 6db, not 30 to 40db.  Minimum drive on all bands and all modes must be capable of driving the 8122's grid positive which is required to generate ALC.  

 

Another issue.  The receiver when properly aligned and with reasonable components will deliver a 10db SN/N ratio with ½ watt of audio when .25uv is applied to the antenna lead.  These are good numbers.  Can it be better, yes, With a small easily removable change, a 10db SN/N ratio at .15uv can be had.

 

The SR-2000 was designed for civilian and military mobile use but never left the desktop.  It uses 8122, 7059, and 7056, 13.5v tubes as well as the 6AQ5, 12BA6 and 12AT7.  The 7059 is simply a 13.5 volt 6U8.  The pentode in the 6U8 and 7059 have a mu of 5200.  Was the 6U8 the best choice?  Probably at the time.  We changed V3 and V7 to 6GH8's.  The mu is 7500, a significant gain, wiring them in series.  Re-aligned the 1.65Mc and 6.5Mc systems and now the radio has more than adequate transmit drive on all bands and modes.  Also the receiver now has a 10db SN/N ratio at .15uv and a noise floor close to -140dbm.  You can clearly hear the carrier and sidebands of a .03uv signal modulated at 1Kc AM signal using a calibrated HP 8662A with a 20db coaxial attenuator.  If you want to remove the change install 6U8's rather than 6GH8's, or remove the series wiring and go back to 7059's --- you won't go back. 

 

Kindest regards Jim K9AXN