[Hammarlund] SP-600 and Drift

[Richard Knoppow]

----- Original Message ----- 
From: "Charles Ochs" <chuckochs at hotmail.com>
To: <hammarlund at mailman.qth.net>
Sent: Saturday, July 06, 2013 8:26 AM
Subject: [Hammarlund] SP-600 and Drift


> Of course all these old radios will suffer from changes in 
> AC input voltage to some extent.The R-390 (dreaded 
> 3TF7)and the HRO-60 (dreaded 4H4-C) included current 
> regulator "ballast" tubes in an attempt to minimize this 
> problem. The HRO-50 series did NOT include this feature.
> Lately, I have been thinking of installing a "global" AC 
> line regulator to run all the gear in the shack. A large 
> Sola unit is on the wish list.
> Audio quality of the SP-600 is very good, although it 
> lacks the power of the previous SPs in that it uses a 
> single ended circuit instead of the PP audio. The earlier 
> versions conformed to the then-standard practice among 
> high-end radio manufacturers toward "high-fidelity" 
> operation, which assumed operation through some big 
> honking speaker.I run all of my receivers through simple 
> attenuators to drop the output to line level, and then 
> into the switching matrix of my audio console, which feeds 
> the two speakers in the corners of the shack. Works like a 
> charm, and I get total audio consistency. In this setting, 
> the SP-600 has outstanding audio. Works great on the R-390 
> also, and I can still use the front panel audio level 
> control.Chuck N1LNH

     If you are feeding line level you are still going 
through the receiver's power amp. Try feeding your external 
amplifier from the diode load, you will hear a major 
difference.  Single ended pentode amplifiers of the sort 
used in many communications receivers and tube home radios 
have quite high distortion. Its typical to give the power 
spec as the 10% point.  The distortion is quite audible 
especially when compared directly to a non-distorted output. 
Beside the high harmonic distortion there is also a lot of 
intermodulation distortion which tends to exagerate the 
effect of noise even on CW. Receivers that have push-pull 
amplifiers get rid of some of the distortion; the balanced 
amplifier will little even harmonic distortion although the 
odd harmonic, which makes the sound harsh, will still be 
there.  Pentodes tend to have more odd harmonic than triodes 
but some feedback will help either type.  Another problem 
with the sigle ended amp is the DC in the output 
transformer. This tends to cause core saturation and a 
reducion in primary inductance with consequent loss of low 
frequency responce.  In a push-pull amplifier the DC is 
balanced out and has no effect on frequency responce.
     The SP-600 and many other receivers have problems with 
distortion caused by the AVC time constants being too fast. 
This causes a sort of intermodulation distortion from the 
low frequencies of the incoming signal modulating the AVC 
level. You can see it on the SP-600 signal strength meter 
since it reads carrier level at the detector. On signals 
with heavy modulation it tends to dip downward on modulation 
peaks. This is an actual reduction of the signal at the 
diode from the AVC trying to follow the modulation envelope. 
For high fidelity reception of AM this effect can be 
eliminated by turning off the AVC and setting RF gain 
manually.  A longer time constant in the AVC will also help. 
You can change it without modifying the receiver by putting 
a capacitor across the AVC terminals on the back. The audio 
from the detector is very good.
     The first advertising for the SP-600 gave a quite 
different tube line up than the production receivers had 
including a push-pull audio stage.  I think Hammarlund found 
they just could not fit everything on one chassis.  I have 
never seen any detail of the original design. The ad 
features a drawing of the receiver, which looks pretty much 
like the production model but not quite identical. 
According to the ad it had calibrated bandspread for the ham 
bands.  I don't quite know how this was accomplished but 
think there may have been a movable pointer on both dials. 
In any case, this proposed receiver was never produced.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com