[Hammarlund] 6AH6 vs 6C4

[Kenneth G. Gordon]

On 9 Jun 2011 at 14:10, Richard Knoppow wrote:

>      Be aware that the output level of the ECO is likely to be 
> different than from the 6C4

Yes. I am aware of that, thanks. :-)

> plus its circuit loading will 
> likely be different which _may_ affect dial calibration.

There is no doubt about that at all, as a matter of fact. However, I suspect 
that problem, and the previous one, can be handled with a bit if ingenuity. 
We'll see.

> The 
> stability of an ECO depends on finding the right balance of 
> screen and plate currents.

Yup. 

> When the correct ratio is used 
> the circuit is essentially immune to voltage variation over 
> a very wide range. This also confers some immunity to 
> filament voltage variation which I find is a serious 
> contributor to drift in some receivers.

Again, yes. This last is one reason that National (and others) incorporated 
filament current regulation (the 4H4C in the HRO-60) in the oscillator circuits.

The Collins R-390 and 390A are other examples.

Anyway, I am going to do some experimentation.

My goals are MINIMAL circuit and mechanical changes, coupled with greatly 
enhanced stability.

As I have repeatedly stated, one of my basic principles is "KISS"...

BTW, Electric Radio magazine recently published a very short article on a 
solid-state plug-in replacement for the 4H4C which almost exactly mimics 
that tube's characteristics, using an LM-317 in current regulator mode.

I am sure a similar device would be easily built to replace the filament current 
regulator in the R-390(*)

I might add that in the case of an ECO in the Hammarlunds, another 
possible solution is a dual-triode configuration: triode oscillator followed by a 
triode buffer of some sort, like a cathode-follower, perhaps.

However, I suspect this would require changing the socket, and I want to 
avoid that. I don't know of any dual, high-mu triode in a 7 pin miniature. Not 
enough pins...

In any case, in the HQ-110, I think the simplest, at this point, will be the 
6AH6 ECO, since that completely precludes requiring any mechanical 
changes.

I'll try that first.

Ken W7EKB