The SX-100 is a very useful receiver. It is dual conversion on all frequencies and has three IFs. It also has fairly deep skirted bandpass filters from 500Hz to 5 KHz due to the low second 50 KHz IF. The same receiver design is in the S-76, SX-88 as well as this SX-100. It is an early designed departure from standard receivers of the same era with the low second IF 50 KHz that was picked up by the later Drake receivers. Next to a crystal or mechanical filter the selectivity is second best and far superior to the standard 455 KHz IF radios. The S-76 was marketed as a cheap SX-71, but clearly outperformed it.
I can hear about everyone clearly on the AM Ham nets. First, the audio is processed thru a AM-COMM DNR speaker. I do use a 8 inch school house speaker vs the internal AM-COM speaker and the 8" speaker is far superior for clarity. This AM-COMM DNR speaker removes most static, but not all static such as static crashes. It does lower static crashes though. It also does not alter the received audio like most DNRs or sound watery. The SX-100 noise limiter removes or lowers to a minimal level the static crashes and other band noise without distorting the audio. I read the SX-100 MK 2 series noise limiter is better than earlier models for lowest distortion. The SX-100 apparently can receive either the upper or lower AM sideband or both. I use both sidebands. The RF gain is set about 6 to help remove band noise and the volume is set about 4. Finally I use the notch filter to remove the last of the noise and peak the higher frequencies my old ears prefers. Beyond the occasional static burst that gets thru and does not interfere with the receive audio, it sounds like FM. This SX-100 is a very useful radio and the notch filter is the 'icing on the cake'. I am not sure if other general coverage tube radios have a notch filter. The Ham band only National NC-303 has a notch filter. Some state the SX-100 drifts, but I do not find any drift after 1 minute warm up. Not even sure if there is drift during warm up, but I always warm up one minute. I think some Hams think of the SX-100 as more of a toy vs a serious receiver, but it is not a Collins R-390A or 51J-4. It is a reasonably good receiver. Do note my SX-100 was rebuilt and aligned by the Hallicrafters Guy 15 years ago. An old stock SX-100 may not perform as well as my receiver. The 50khz IF is noted to need re-alignment on these receivers, but not to the point it will not receive reasonably well.
An interesting test of old tube receivers is at this site:
https://w1vd.com/BAreceivertest.htmlThis SX-100 is a very close design match of the tested SX-88 receiver up to the audio output design. Performance would be close to the same, but the low frequency audio distortion at 100 Hz would be about 3% higher in the SX-100 due to the SX-88 audio amp PP vs single ended audio as in the SX-100. It is shown the AGC in the SX-88 and SX-100 does not add a lot of distortion and is superior to many receivers. All tube receivers have AGC distortion issues, some much worst than others though. Another interesting parameter is the Recovered AN Audio S/N measurement. Author states 45 is good the SX-88 (and SX-100) is 50.
So, if you cannot afford the Collins 51J-4 with the AM mechanical filter, this SX-100 is a gem. I owned a 51J-4 years ago and it could hear anything. The Collins easily outperformed a rebuilt and upgraded Hallicrafters SX-71 that was no slouch. I should have not sold the Collins. But, IMO the $300 (some less) SX-100 is almost as good reception wise, but less the Collins outstanding mechanical construction for about 1/4 the cost. Best receiver? The 46lb Collins 51J-4 of course ($1K+) or the 85lb Collins R-390A ($600+) and 52lb Collins R-392 ($300+).
