On Thu, 28 Sep 2017 06:27:49 -0000 (UTC),
(Hank) wrote:

In article ,
wrote:

One other thing, the dial bulbs are burned out. Bayonette base, wired in
series, two 3.2 volt bulbs. A schematic shows them as Mazda 1490. ( I
never heard of Mazda bulbs). Anyhow, where can I get such bulbs?
The bayonette base is the same as a #47 bulb.

BTW: The 6 volt feeding them comes from the filament center tap in the
35Z5 rectifier tube. How they get 6volts from that center tap evades me,
but that's how it is.....

This looks to be the schematic for the set. It's an AA6, with an RF
amplifier ahead of the converter.
http://www.nostalgiaair.org/pagesbym...6/M0015076.pdf

We used to use schematics of sets like these back in the 1950's and
'60's as interview questions for EE and technician canditates. One very
good one was "talk to me about the rectifier circuit."

Understanding the circuit starts with understanding basic electricity,
particularly Kirchoff's current law and Thévenin's theorem. What you
are calling a "center tap" between pins 2-3 of a 35Z5 is at the 21.5%
point of the heater, not the center. According to the RCA data sheets,
with 35 volts between pins 2 and 7, you'll see 7.5 volts across pins
2-3. Now add a #47 (6.3v 150 ma.) bulb across pins 2 and 3, and feed
the plate from pin 3. RCA claims that the voltage across pins 2-3 (and
the bulb), with a 60 ma. draw on the cathode is now 5.5 volts and
between pins 2-7, 32 volts.

That, of course, doesn't "add up." Assuming RMS values, a 150 ma.
heater plus 60 ma. of DC is only 210 ma. RMS, which should explain the
lower voltage between pins 2-3. However, also consider that current
flow in the plate circuit, with a 40 mike cap connected to the cathode
is a pulse, not a full half sine wave, and that RCA measurements were
made with a VTVM, which isn't a "true RMS instrument." Those pulses,
and the low thermal inertia of the pilot lamp filament(s) explains why
the bulb is at full brilliance when the set is warmed up and playing.

Another "trick" to the series circuit is that during warmup, the series
filaments are a voltage divider that is essentially constant for any
current passing through the heater string. These heaters have a steep
positive temperature (heat/ohms) coefficient, so the startup current is
more like 1.5 amps. If you are going to fuse the radio, you need to
measure the resistance of the filament string cold, and work from there.

That's for starters.

Now, as to the rest of the set, take a look at the B+ circuit and
voltages. The tube data sheets give 100 volts on both plate and screen
as "typical operation" points for AA5 tubes. However, this set says 76
volts. Also note that "typical operation" specs an initial bias (-1
volt for 12SK7), while this set has the cathodes of the RF tubes
grounded.

Yes, that schematic is a very good interview subject.

Don't get foxed by "Mazda." That was a GE trademark from around 1910 to
denote "tungsten filament," and was licensed by other bulb
manufacturers. GE dropped using it in 1945. If you need bulbs, a 1490
is a 1490, readily available today.

If you are going to play the radio without recapping, the critical caps
are the coupling caps between stages, particularly the cap feeding the
audio power amp. Also, check the value of the grid leak resistor(s).
You should see zero volts DC at the grid of the 35L6.

The AA5 (and AA6) are, with the DC-3 airplane, and the GG-1 locomotive,
standout classics of American design. They look "simple" but there's a
lot of thinking that made them robust and reliable without being
complicated.

Hank

Thanks for the detailed explanation. I had a suspicion that the "center
tap" of the rectifier tube was not actually CENTER. The more I work on
these old sets, the more I notice that they are all pretty much the
same, particularly if it's the same brand. One website I looked at, said
that the chassis is a number -xxxx-. (I forget the number, but it was a
4 digit number 10__). I guess that means this same chassis was used in
other RCA radios, not just these 66X_ (1 thru 4) models. But considering
the years they were made, that makes sense. They produced the chassis
and made numerous cabinets to fit around them.

I have to say that I am highly impressed that this 70 year old radio
still works, with it's old caps and all. I have considered recapping it,
or at least the critical caps and the power supply filter caps. But then
again, "if it works, dont fix it". Having it power up with no hum, was
what impressed me most, since the electrolytics are usually bad.

Thanks for explaining that "Mazda". That had me puzzled. In all the
years I have worked on this stuff, that is the first time I saw that
word used. I should have suspected it was a trademark....

That schematic is clearer and better than the one I had, so that will
help too.