national: FB-7 versus FB-7A

The first ad for the FB-7 in QST was March 1933.  The phone jack, hot with B+ on both the exterior of the jack and the inside, is visible on the left front of the ad pic. Almost immediately after introducing the set, a change was made to IF transformers wound with Litz-wire and with air dielectric trimmer caps to replace the simpler compression types.  Those new IF transformers were also offered as individual components as shown in the full page ad in May 1933 QST. Another change was quickly made to add a second switch on the front panel for the B+ replacing the phone jack, also shown for the first time in the May 1933 issue.  National's ads describe that switch as “for transmitting” which was true since it placed the set in “standby” mode, but the other reason likely had to do with hot-switching the coils. With live B+ in the set, pulling out the oscillator coil which had a grounded aluminum finger-pull handle might release the ground contact before the other contacts.  Holding on to the chassis to keep it from moving while pulling out the coil might cause an unpleasant surprise.  Better to throw that B+ switch first. The hot phone jack migrated to the back panel.  Models with suffix “A” versions of the FB-7 and FBX were introduced shortly afterward with the improved IF coils and that second front panel switch which replaced the front panel phone jack, quickly followed by pictures of a model with the internal BFO knob.   I'm convinced that nearly all surviving FB-7 and FBX sets are the “A” versions.

• First QST ad for FB-7 March 1933 (169 KB)
• Ad for Air dielectric IFs in QST May 1933 (259 KB)

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