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Thanks, Don.? I’m admittedly a bit fussy about a couple of things – one is that I try to restore as true to the original as possible.? This can be particularly challenging when a radio has been – shall we say – “molested” by a mad scientist.? The other thing is that I enjoy writing articles about my restoration adventures and try to gather as much technical and historical information as possible.? The use of the 6SK7 as the 2^nd IF caught my attention because what I’m used to seeing in most IF chains is a lineup of identical tubes.? For example, the HQ-180A is 6BA6’s.? The HRO-60 has a train of three 6SG7’s.? The Hallicrafters S-40 has two 6SK7’s in a row.? The NC-98 has two 6BD6’s in a row.? The use of a 6SG7 followed by a 6SK7 is very interesting!

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73 – Steve, KW4H

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Steve

I agree with your comments, which I believe, apply to various big old consoles as well as communications receivers.?

Your original comment “6SK7 was used for the 2^nd IF, while a 6SG7 is used for the 1^st IF.” Is quite an observation of info that really does not stand-out.

Hope your restoration proves interesting and successful.

Don VA3DRL

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Don,

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Very interesting!? Well, as we all know a lot of this old equipment was designed for a specific price point and target audience.? There were receivers with weak front ends that had awesome audio (particularly the ones with auxiliary input), and there were radios with great front ends, but audio sections that were maybe one step above a handheld transistor radio.? A few were spectacular on both ends of the spectrum, but not many.? One example is my HRO-60, which has fantastic push-pull audio and (for the day, anyway) a highly sensitive front end (if you align it correctly and can deal with the coil drawer design). ?A Hammarlund HQ-180A, by comparison had only so-so audio, but fairly sensitive and selective reception.? Hallicrafters had them all over the scale as well.?

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I’ve never owned a SX-110 before and am admittedly fairly new to Hallicrafters designs (never had a Hallicrafters in my younger years), but am learning fast.? My impression so far is that the SX-110 (and its twin the SX-99) were midrange.? The audio isn’t push-pull, but it’s good enough.? The reception – I’m not sure about yet – I’m still working my way through the restoration, but have been working backwards from the audio output.

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73 – Steve, KW4H

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Just thinking.. if the intent of the receiver was to function only with weak signals? present, the extra gain of the 6SG7 would be preferable, but if? there were strong in-band signals, the 6SK7 might be preferred. Seems to me ?there must be trade-offs depending on the rf gain[s], the AVC and any bypassing of an Rf amp ?for lower frequencies.. so 6SK7 or 6SG7 isnt a one size fits all. Intermod {IMD} must be in the designer’s mind.

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Reminds me; at about grade 10 in school, and in the pursuit of getting more weak signals from Heathkit AR-3, ?I somehow connected an If output from the AR-3 into the early IF stage of an old console with a good AF amp and speaker; and man oh man; did I get stations… all over the dial from 540 kilocycles to 3o MCees. ?I had not yet discovered resonance, let alone intermod!

Don VA3DRL

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Thanks, Mike!? In contrast, National often used all 6SG7’s in their IF design (such as in my HRO-60).? The difference is an interesting point.

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73 – Steve, KW4H

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Hello Steve!

Excellent question!

The 6SG7 has higher gain than the 6SK7.

The 6SK7 can handle stronger signals.

And the 6SK7 provides better AGC control since it’s a remote cutoff tube.? The 6SG7 is semi-remote cutoff.

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I hope this helps!

Mike/
K5MGR
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All,

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A curiosity – I’m restoring a SX-110 at the moment and see that a 6SK7 was used for the 2^nd IF, while a 6SG7 is used for the 1^st IF.? I’m trying to understand why that is – why not use two 6SG7’s?? What was magical about the 6SK7?

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73 – Steve, KW4H

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