[Hammarlund] SP-400X and 210

[Carl]

There are quite a few mods for those sets and many have been shown to offer 
no benefit, kill the AGC or otherwise overload....and worse.
The best SP-200 military BC-forget which one-- update Ive seen had a pair of 
6BA6's, 6BE6 and 6C4 just like the SP-600 plus several other changes all 
done extremely well

Im trying a more measured approach with the goal to be much improved 
sensitivity on the higher bands, improved AVC/AGC, less susceptibility to 
overload, product detector, improved noise limiter, reduced drift.
Ive only made a few changes so far.

Tried various tubes in the RF stages and settled on a 6GM6 for V-1 and a 
6SG7 for V-2. The 6GM6 went into an octal tube base and circuit changes were 
done at the socket to conform to the tubes voltage requirements. I like to 
run it at 125V on the plate and screen and raised the cathode with a 68 Ohm 
resistor. A 6EH7 would be a bit better.  The 6SG7 is an octal and the socket 
just needed some rewiring. Im still experimenting but for the time being I 
limited its gain with a trimpot for a cathode resistor.

The next step is to try a dual triode such as the 6SN7 as the oscillator 
plus buffer, I havent found any reason yet why it wont work. If it does then 
another 6SN7 will be tried as a Pullen mixer if I can get sufficient HFO 
injection. Im trying to stay away from changing sockets but its not 
completely ruled out. There is always the 7F8 which is just a socket change 
without metal work, that tube works good into the FM band as shown by 
Hallicrafters in the SX-42, SX-43, S-47 and SX-62 series.

With a Pullen I dont need much front end gain to establish a low noise 
figure so the 6SG7 stage will be there primarily for image rejection and 
gain limited as in the HRO-60 which is about the same as the R-390A for 
signal handling.
 A 6SB7Y could also be used as a conventional mixer with a noticable 
improvement over the 6L7.

I really want to run the whole radio at 150-175V B+ out of the PS except for 
the PP audio. My 1935 HRO runs at 180V out of the doghouse PS and its quiet, 
sensitive, and stable.
The SP-100 is a one owner/user before me and looks almost new. I got it from 
a guy who had it in a bedroom closet for 25 years and never turned it on, 
got the matching PS also with a very close serial # as well as the original 
manual. Im undecided about just keeping it as is or doing the restuff caps 
and NOS resistor thing like I did with a few other high end 30's radios.

The BC-1004 is in storage in the attic, no plans, it needs work, but for $25 
at Nearfest I couldnt pass it up. I think the SP-400-X was the same, it was 
marked $40 the year before at Nearfest but I believe he took the lower 
offer. That was the same year, same place, I picked up a SX-17 for $25 just 
for the meter and crystal.

Carl
KM1H


----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: <Hammarlund at mailman.qth.net>
Sent: Saturday, July 21, 2012 12:57 PM
Subject: Re: [Hammarlund] SP-400X and 210


> On 21 Jul 2012 at 10:18, Carl wrote:
>
>> The 3 front end tubes are very noisy which will limit sensitivity
>> above about 12-14mc. Ive made some changes to my 400 with more to
>> come.
>
> So what do you use in place of the three normal front-end tubes?
>
> What changes do you contemplate for your SPs?
>
> Ken W7EKB
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