It isn't the gain but the Internal noise.? ?One can have too much gain.? The internal noise of a tube is roughly proportional to its Gm.? A 6SG7 has about four times the Gm of a 6SK7.? One problem with just plugging a "better" tube in is that the load resistances and other characteristics will be different.? For the better tube to be optimum the associated RF coils will have to be designed to match.??

While the higher Gm tube will be quieter it may not make s lot of difference at communications frequencies where atmospheric noise often exceeds receiver noise by many times.? ?

Another factor at least for these two tubes is the range of control grid voltages.? A 6SK7 requires about 40 volts to be cut off,? 20V will cut off a 6SG7.? A 6BA6,? which is si.ilar to the 6SG7, also takes about 40 to 50V to cut off.? So the AVC and gain budget must be designed for the particular tube.? Minimising

IMD also requires the correct voltages and transformer design. It's not a trivial problem.

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-------- Original message --------

From: don Root <drootofallevil@...>

Date: 10/27/22 3:59 PM (GMT-08:00)

To: [email protected]

Subject: Re: [HallicraftersRadios] SX-110 2nd IF Circuit Design

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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,

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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