I recently got an RCA R7A, which uses two 47 tubes in a push-pull output, and at the same time an RCA R7, which uses two 45s in a push-pull output stage. Neither one is working right now, so I can't compare. So far I've seen a lot of old radios that use 45 output tubes, but this is the first I have that uses 47s.

Why is that? Why did the 45 end up being the favorite and not the 47? The 47 seems like it should be an improvement, being a higher power pentode. Just curious. Gratuitous tube picture below. Oh for the days you could buy tubes at Kresge's (and for less than a buck)!

Number 45 is the older tube and was being used in 1929. Number 47 being a pentode has more gain but wasn't available until July 1931.

The 47 was quite popular in larger, console radios of the early '30s. Then as now, many consumers did not appreciate the finer points of technology, and the radio that played the loudest was often judged to be the best!

Interestingly enough, the 47 also turns up in some of the cheapest four tube table radios which appeared during the bottom of the Great Depression as a "power detector."

And then there was the 59 which came out officially in 12/32 but likely in production well before that. Its a tube you rarely see used, the only set I own with one is a National FB-XA from 1933-34. Maybe the 7 pin socket cost too much.

It seems the 47 and 59 were overshadowed by the 42/2A5.

Carl

And dont dorget the 46. I have plugged a 47 into a 46 socket and it works just fine. One of those (46, 47) was called dual grid. I just forgot which one.

Well 47's are cheaper now. That can be helpful. And someone was once selling base adapters to allow you to use a 47 as a triode to replace 45's. I don't know if the adapter was anything more than something to adapt connections, not if anything else was inside. My early Philco 70 uses the 47 and I expect it was one of the first models to use that tube, maybe. Plays pretty loud with just one of them.

In a nutshell:

45s were cheaper, and they were "the only game in town" for a couple of years, so naturally lots of them were used.

The 46, 53, and 59 were developed for class B audio, which had a short vogue due to higher distortion and power-supply rquirements.

The 47 was popular for a while but had distortion problems too. At the trade show where it was introduced, one engineer was reported to have exclaimed "Absolutely wonderful! Here we have 30 per cent distortion in a bottle."

In any event, 2.5V tubes were the creation of RCA, while Philco (who made a lot more radios than RCA) settled on 6.3V. Philco won. The 42 was a 6V tube.

Hey, maybe the push-pull arrangement brings it down to just 15% distortion in a bottle?

Bob

The 42 was the 6-Pin father of the 6F6 and the 6V equivalent of the 2A5. From the 42 page of the HB-3, "For additional data refer to Type 6F6; and to Types 6F6 and 2A5 for additional curves."

The 46 is an interesting tube. It is a triode with two first grids. Connect them together and you get one value of transconductance. Connect one of them to the plate and you get a different transconductance. The purpose was to cut down on the number of types of spare tubes that had to be kept in stock, since the same type could now be used in a class-A driver stage and a class-B output stage. Didn't catch on with broadcast radio manufacturers to any great extent, but the military and industry loved it. 53 was a twin triode which also could be used as a driver or an output stage. It reappeared in octal form as the 6N7.

RCA's original purpose in bringing out 2.5-volt tubes was to control heater-induced hum in low level AC powered amplifiers. Type 27 was the first of the series. With better cathode design, higher heater voltages could be accommodated. 6.3 volts was a logical choice, since it is the nominal voltage of the three-cell lead-acid batteries used in cars and many other things at that time.

National Radio kept to the 2.5V tubes well into the 30's for the HRO when he was satisfied the problem was solved as the CE claimed they had too much hum for AC sets which were designed starting in 1933. The 6V equivalents could be ordered or swapped in for battery use.

Carl