[Hammarlund] SP-600 and Drift

[Charles Ochs]

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