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

[Perry Sandeen]

Well there are always poor mods disasters, even from the OEM. Collins filter blocking cap and SP 600's BBOD's
The R-390A from ER magazine had proven results.
In the last 50 years there hasn't been a tube radio that I couldn't make better.
The simple explanation is that all these radios were built to a price point.  Now with the modern components it is even easier to do. 
For example:
On the SP 600's that I'm re-maning, I'm removing the  two original filter chokes  wired as a capacitor input configuration to a 1.5Hy Dynaco replacement followed by a 390uF/450V Nichicon rated for 10K hours at 105C. The following choke and cap are the same.  
The on line calculator said each cap and choke combo had 48 Db of suppression.
The cost was about $60 but I wanted a bullet proof filter.
One can also replace the original steel tube shields with IREC's if you can find them and are willing to pay the price.
What's so good about this list is there various methods and opinions for making the BA's better.
The best comment I heard was " There always better way of making anything except babies"
Regards,
Perrier 

    On Thursday, April 6, 2017 4:06 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
 

 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> 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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