[Hammarlund] SP-400X and 210

Sun Jul 22 20:24:29 EDT 2012

Sounds like you've got quite a collection there, Carl.  Now you just need to get in there and get them all going.  I decided I wanted to do original restorations- I've got the R-390A if I want performance.  

The SP-400 I got was not nearly as inexpensive as yours, but it was in excellent condition, and just needed cleaning up and recapping.  It included the power supply, speaker and cable and covers.  I only had two significant problems: the knobs and the bandspread:
It came with National knobs on the two tuning shafts.  I couldn't find anyone with the original small Hammarlund knobs.  I searched ebay for months, until an obscure offering for a box of knobs.  It looked promising, and I asked the seller to take some more photos.  In the end, I got my two knobs for $6.
The other strange thing was that the bandspread worked on the broadcast band, but not on the 14-30 band.  I did a lot of head scratching over what might be wrong, and finally opened up the tuning covers.  Right there I could see that the bandspread wires had been clipped and moved- it took about 5 minutes to return to original.  I still have no idea why the original owner would have done that

The BC-779 I bought was significantly modified with 6SL7s in the RF, mixer and oscillator.  The noise limiter 6N7 socket had been stripped, and was missing two lugs.  And, there were a variety of modifications to the detector and AVC circuits.  I fortunately found the CQ article describing the changes, and was able to track down and remove all of them.  I wound up rechecking the circuit against the schematic line by line.  I think the radio works pretty well for a 1935 design, and was quite a leap forward at the time, but I never tested it in its modified form.

Ed  W2EMN

-----Original Message-----
From: Carl <km1h at jeremy.mv.com>
To: kgordon2006 <kgordon2006 at frontier.com>; Hammarlund <Hammarlund at mailman.qth.net>
Sent: Sun, Jul 22, 2012 6:42 pm
Subject: Re: [Hammarlund] SP-400X and 210

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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