[Hammarlund] Old Hammarlunds - modifications.

[Kenneth G. Gordon]

On 14 Apr 2011 at 10:54, Richard Knoppow wrote:

>     The Geissler article sounds familiar but I think the one 
> I mentioned was earlier.

That could be: Geissler changed the RF stages (both of them) to some sort 
of cathode-coupled dual-triode circuit, cascode I think (2nd triode is 
grounded-grid), and both the mixer and HFO to dual-triodes. He also 
changed the AGC and Noise Limiter circuits, but as far as I can see, did 
nothing about the audio stages.

I think I can understand his reasons for the mixer mod, as the 6L7 is very 
noisy, although I certainly would not have chosen the replacement circuit he 
did, but the RF stage mods and even his HFO mod I would not have 
done...or at least not the way he did them. Much too invasive and "messy".

Also, although the 6L7 is a very noisy mixer, I believe the overall gain from 
the 2 properly designed RF stages make that noise problem a "non-issue".

At least it sure appeared that way to me after I installed the simple triode PD 
in my original BC-779 back over 40 years ago. The receiver was so quiet and 
sensitive that it impressed me.

Perhaps he changed the mixer because his RF stage mods produced 
insufficient gain and he needed a much lower-noise mixer to compensate.

> This article changed the RF and 
> mixer to dual triodes and changed the audio amp from triode 
> connection to pentode connection with feedback.

Commander Lee did the audio mod. He also added a 6BE6 as a 
combination BFO/product detector....a circuit which I have never liked. Using 
the 6BE6 as a product detector alone is all right, although it is not one of my 
favorites, but I would most certainly not include the BFO in the same 
envelope. IMHO, the 6BE6 is a POS anyway...

I've always thoroughly disliked pentagrid mixers: their only real purpose was 
to save a tube in the AA5.

> I don't 
> think these are good mods.

I completely agree with you.

> The only mod I suggest is adding 
> a VR tube for the LO.

That makes sense. I'd do the same. I think I would also add that 3.3 pfd 
negative temp coefficent cap to the oscillator tuning. THAT one was 
suggested by a Hammarlund engineer in "QST" in 1957, page 58, so I trust it 
a bit more.

> In general, I don't like wholesale 
> modification.

Neither do I.

As I have repeatedly insisted, although we HAVE learned a lot about certain 
particular circuits since 1935 (like AGCs and detectors), and our operating 
conditions have changed, nonetheless, the Old Time engineers were no 
dummies either. 

Before making any subtantial changes to anything "ancient" we should 
understand WHY they did what they did, not just WHAT they did.

IMHO, most ham mods are not worth the effort and usually result in a piece 
of equipment that works worse than it did originally.

At the very least, any modification should be 1) NECESSARY, to meet 
changing operating conditions, and 2) LOOK and WORK as though it was 
factory designed and installed.

And I would NEVER attack a museum-quality, historically important, piece of 
equipment.

Fortunately, there are a lot of junkers around for us to play with.

Ken W7EKB