The 5B5 Fender Pro was made from 1951 until 1954. They featured octal pre-amp tubes until roughly the summer of 1953 when they switched over to the newer 9A based Noval pre-amp tubes we still use today.
This particular example, SN 3275 still has octal pre-amp tubes and could be from '52 or early '53. Serial numbers 2500 to 3500 were made in 1952 so that should solve that question. The speaker also has a 1952 date stamp.
It uses a pair of 6L6Gs (G for "glass" ) with cathode bias to make about 18 watts. The cabinet houses a single Jensen P15N. It features 2 volumes and a single tone control. While it can be a bit dark with some guitars, it is perfect with an old Telecaster. And when you consider that the Tele was the only guitar they made at the time, it makes a lot of sense. The Strat was still 2 years away. In the tradition of the day, it has a microphone input so you could accompany yourself.
Also in the old time Fender tradition, the nuts and bolts all have nail polish on them, and the woman who assembled it has her name written on a piece of masking tape on the chassis. This amp was assembled by "Eileen".
SN is 3275. It is also on the chassis.
I performed the original repairs on this amp before it belonged to me. I replaced the coupling caps and electrolytics, all of which were nearly 50 years old at the time. I also had to replace that last input jack.
You can see the nail polish on the heads of the bolts
Someone spot welded the 3rd bracket on by hand.
"PM" (permanent magnet!) was a new thing still; they had recently moved from field coil speakers to permanent magnets using AlNiCo. The P15N's frame still has a flat on it for mounting the output transformer.
This is pretty unusual, the power transformer is using grain oriented steel laminations and they used a 2x2 lacing or interleave pattern. It's a Triad 6516 as specified on the factory schematic. Usually you only see this kind of fancy core in an output transformer.
The output transformer
Eileen's signature on a piece of masking tape.
Factory schematic annotated with actual voltages.
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