[R-390] R-390A front end tubes

[Bob kb8tq]

Hi

Well, I can show you examples (used and possibly abused) from both those companies that are pretty 
useless for really low level sensitivity measurements on a poorly shielded radio. 

Bob

> On Apr 6, 2017, at 6:46 PM, Perry Sandeen <sandeenpa at yahoo.com> wrote:
> 
> Hi Bob,
> 
> All good points. 
> 
>  I think my HP and Tek signal generators will have enough filters to make their readings valid.
> 
> The attached is what's on the back burner until I get a bunch of SP 600's done.
> 
> 
> On Thursday, April 6, 2017 4:42 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> 
> Hi
> 
> I started doing this sort of thing on various radios back in the 1960’s. While sensitivity is fairly easy
> to measure at the output of the radio, it’s not always as easy to measure at the *input* of the radio. 
> Shielding on the radio’s input often is less than perfect (at least on the radios I’ve been able to afford). 
> The same is true of signal generator shielding. The gotcha there is that 0.01 uv probably is 0.5 
> uv of leakage past the shielding and 0.01 uv on the generator dial. 
> 
> The next issue is overload performance. As you run up the gain on the front end of a radio, you hit
> the later stages a lot harder. Things like mixers don’t like that very much. Measuring that sort of thing
> was way beyond my abilities in the 60’s. For that matter it was way beyond my understanding. A decade
> or two later it was indeed all explained to me in a series of courses. It also became pretty easy to measure
> with the sort of gear I then had at work. The bottom line is that while it is not easy to measure, the impact
> is very real. 
> 
> None of this says “don’t do it”. It’s simply something to be careful of. You want to do it in a reversible 
> fashion. It’s also well worth checking the antenna system and possibly improving or repairing it. A preamp that 
> properly matches the antenna (very high Z or very low Z) often is a reasonable alternative to modifying the radio.
> 
> Bob
> 
> > On Apr 6, 2017, at 3:00 AM, Perry Sandeen via R-390 <r-390 at mailman.qth.net <mailto:r-390 at mailman.qth.net>> wrote:
> > 
> >  Hi List and Larry,
> > Thanks for the extensive article on tubes for the R 390A front end.
> > From what I can discern, the RF section is set up for sharp cutoff pentodes.
> > I wonder if the "A" would be any better by using a 6AK5 in the front end as Collins did on the R-388?
> > FWIW, Ray Osterwald (sp) ER radio editor created what he called "a competition grade" "A" using IIRC a 6HA5 triode in the front end but did have to change the bias.
> > I have a copy of the article.  If any wants one please email me off list and I send you a copy. IIRC its also in the Y2KR3 manual.
> > Regards,
> > Perrier
> 
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> 
> <Working Part 1_F.tif><Working Part_2_F.tif><comp_grade1_A.pdf>