hallicraft: S72R

The original version had an ANL on/off switch above the earphone socket (Automatic Noise Limiter). The wood under the brass grill still has the hole on later S72R models. The S72R appears to have been sold till 1959.

The S72R seems to have several revisions and early versions seem the same as S72. Later versions have a second electrolytic multicap mounted in the hole beside the main one. This fouls any earphone headset on rear panel, unless it's narrow or soft head strap.

A variety of large 7.5V/90V (and 7.5V/9V) battery packs are compatible. It has two paralleled series filament chains, so a filament failure when on AC mains can cause a second failure.

The mains plug is docked on chassis to switch over to battery power. A box with 60 x AA cells and 5 Alkaline D cells is quite economic to power it and can be in a replica case.

The mixer and oscillator are two separate tubes, allowing reliable operation to 30MHz at end of LT. The BFO on/off (for CW, or possibly SSB) is also RF gain when  the BFO is on. There are two 1U4 for the IF amp. A second IU5 is the BFO.

Performance may be better than the Zentith Transoceanic and the RCA and Hallicrafter versions of it, but at expense of half the battery life! The electrical band spread allows easy tuning even though it's continious coverage from about 540KHz to 30MHz in only four bands that slightly overlap..

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The earliest version band spread seems to have only tuned the local oscillator.

The earlier version had a tone switch (capacitor to 0V on 1U5 preamp anode) as well as the ANL switch (the diode part of the 1U5 used for BFO connected to  3V4 grid via a capacitor).

The 1U5 BFO is wired as a triode (g2 to anode) and seems to couple to the IF purely by leakage.

AGC is only via g1 of the 1T4. Manual RF gain control is by varying g2 on the pair of 1U4 (sharp cutoff)  used for IF amp from  0V to 90V. This method does vary the gain a lot on any sharp cut off pentode and Russian rod tubes that I've tested.

The AC/DC 110V power consumption is quite high due to the filament series dropper having 270 Ohms to 0V adding about 30mA or about 3.5W consumption adding to the twice as much as normal LT and HT currents, the other factor is HT current. A typical  four tube set might take 12mA @90V HT. This set takes about 110mA LT @ 7.5V and 30mA HT @90V on battery, so about 110 + 30 + 30 = 170mA on 115V DC mains, about 20W, though the manual says 25W on AC mains.  The battery packs will thus last about half of normal usage of 55mA LT and 15mA HT in something like RCA Globetrotter (5 tube set).

 

Edit:

On the earlier versions with a tone control switch, the switch is on the bandspread control.

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