Hammarlund receivers are some of the nicest AM receivers out there. They sound great right out of the box. You can generally find them for reasonable prices even on Ebay. They have nice wide IF stages and are very simple to use. They all have some means of providing selectivity, either a Q-Multiplier or crystal filtering as in the HQ-145, and while they are not a substitute for a 75A4 for selectivity, they do well in all but the worst band conditions.
You will notice that I do not mention the HQ-170 or HQ-180. The additional selectivity provided by these two receivers has a trade off and that is audio quality. I have an HQ-170 and would not recommend this mod for them because it accomplishes nothing. They are good receivers in their own way but cannot provide the audio quality that the 100, 110, 145, and 160 can because the low IF's are too restrictive. There has been some work done to improve the audio quality by stagger tuning the low IF's (not found in the HQ receivers covered by this article) with some success. But this is a subject for another article.
These receivers were all sold with a feature that Hammarlund called "Auto-Response". It basically is a limited negative feedback circuit that allows for good fidelity at lower volume control settings and more communication type audio as you advance the AF gain control. While it sounds like a great feature, it's a JS idea in operation. This mod straightens out the audio in these receivers. It is by no means a substitute for pulling audio off of the detector and feeding a big audio amp and speaker system but it will provide quite pleasant audio. I have also found that the S-100 and S-200 Hammarlund speakers are pretty nice. They are not like a big speaker system but good speaker for the HQ receivers and the ham shack.
(1) The way this all works is that a portion of audio from the secondary of the audio output transformer is fed back to the bottom end of the volume control, through a 100 Ohm resistor, which is held above ground with a 47 Ohm resistor, a very simple form of negative feedback. First remove both resistors and connect the bottom of the AF gain pot directly to ground. Now you have communication audio all of the time!
(2) The 0.01 uF cap that comes from the AF gain pot to the grid of the first audio amp is changed to a 0.1 uF cap. I used a 400 volt cap with success. But I would think a smaller voltage rating would be fine here.
(3) You will also note a 0.005 uF cap between the plate of the 6AQ5 and the screen. I removed that also.
(4) The remaining part of the problem is the couplate that goes between the plate of the 12AX7 first audio and the grid of the 6AQ5 audio output tube. It functions as a decoupling resistor for the first audio stage and a grid resistor for the 6AQ5. The value of each is 500k Ohm. It also has a 0.01 uF capacitor coupling audio from the grid to the plate. There are two other capacitors in the couplate that tend to alter the frequency response of the unit.
Note where each pin of the couplate is connected. Simply connect a 470k Ohm 1/2 Watt resistor from pin 6 of the 12AX7, where you removed pin 2 of the couplate, to B+ which is where pin 1 of the couplate was connected. Connect another 470k Ohm 1/2 Watt resistor from the grid pin 1 of the 6AQ5 to ground. The last step is to put a 0.1 uF 400 volt cap between pin 6 of the 12AX7 and pin 1 of the 6AQ5.
(5) Distortion was a little higher than I preferred at that point. At 1 kHz, it was nearly nine percent at one Watt output. This is certainly in line with other receivers of this design. The 75A4, for instance, has nearly the same distortion figure. I put a 0.01 uF cap and a 1M Ohm 1/2 Watt resistor in series and connected it between the plate of the 6AQ5 and the cathode of the 12AX7. This provided enough feedback to lower the distortion below three percent. I used my signal generator and swept the receiver. The recovered audio was reasonably flat from below 40 Hz to about 9 kHz, and very pleasing to the ears!
(2) 470k Ohm 1/2 Watt resistors
(2) 0.1 uF 400 volt caps
(1) 1M Ohm 1/2 Watt resistor
(1) 0.01 uF 600 volt cap
If you have any question, email me at firstname.lastname@example.org