[Radioroslyn]

Radiola 16 is a battery set using 01As not ac tubes.

[morzh]

I guess I looked at the header that said 16 and 17.
The 17 was the one I meant to address.

[TN Allen]

Thank you for the information, it all helps, although understanding some of it takes time. One reason for doing the PS and audio units is to learn about the electronics, and I am learning, but slowly.

I have both units soldered with the exception of the Hammond replacement transformer center taps on the 700 Volt coil and the 2.25V coil. The original RCA schematic indicates the 700 V coil CT is connected to ground. I assume this would be the chassis ground, but as it is 700V it seems like something to be very certain of. Perhaps someone can confirm this connection to chassis ground?

The Hammond replacement transformer also has a center tap wire for the 2.25V winding, I won't be using this winding as the #26 and #71A tube filaments are 1.5V and 5V respectively, however, I also need to do something with the 2.25V CT wire end, perhaps it should also be connected to the chassis ground?
I've soldered the 2.25V wires to the terminal strip, although these are superfluous given a 2.5V filament voltage is required rather than 2.25V.

Regarding the audio unit filament voltages, I have two new adjustable 0-30V 10 Ampere DC power supplies which I intend to use for the #26 and #71A filament voltages. This seems like a simple solution to having only the 5V filament transformer winding in the Hammond transformer that connects to the #80 filament. Although an expensive solution, these 2 PS units will also be useful in a motion control project I have underway.

Once the soldering/assembly is complete, I plan to use a Variac to slowly power the connected PS and audio units up without the tubes initially. If nothing seems wrong I can then disconnect the 120 VAC, install the tubes, power the units up slowly again using the Variac, and check the 145 voltage at the #7 terminal strip connection, and the 165 voltage at the #8 terminal strip connection.

I assume I should connect a speaker/load across the output before turning any of it on and have a 2K & 4K Ohm primary with an 8 Ohm secondary to connect between the output and speaker. I also have an old CD player to use for input via the RCA jack. Hopefully the CD output voltage will be adequate for the input on the #26 tube. 

Thank you for the assistance.

[TN Allen]

I have the 2.25V and 2.5V information confused, and must look more carefully at the schematic and Hammond transformer information.

[MrFixr55]

If the 2.25V winding is not being used, the center tap wire can be insulated by taping off.  If the 5V filament winding has a center tap, DO NOT GROUND THAT.  Remember that the Rectifier filament is at B+ level (180-300V) above ground.

Remember also that the 71A and 26 units must be powered by separate power supplies whose DC outputs are ISOLATED from chassis ground.  Ideally, each DC output should have a center tap by putting 2 20 Ohm 5 Watt resisters in series and placing this resistor pair across each of the 2 DC outputs.  The junction between these 2 resistors is the "center tap" to connect the cathode bypass resistor for the 71 (on the 5V supply) and the 26s (on the 1.5V supply).  This duplicates what was done on the original Radiola 18 Power Supply.  However, since these are DC supplies, the cathode bypass resistor can be connected to one leg of the supply.  Per the RCA Receiving Tube Manual Version RC10 (1932?), this should be the negative leg of the supply for both the '26 and the '71A. Refer to the diagram below.  This is from the Radiola 18.  The rest of the radio circuit was removed for clarity.

RCA Radiola 18 PS and Audio.pdf Size: 90.57 KB  Downloads: 11

The Radiola 18 transformer is likely a holdover from the Radiola 17.  The Radiola 17 used "hum balance" rheostats that were likely deemed to not be worth the small amount of hum that can be eliminated by this adjustment.  The fixed resistors were cheaper than either the rheostats or redesigning the transformer with center taps.  This is why there are no center taps on the filament windings of either the Radiola 17 or 18 transformers.

[Chas]

The earlier "S" (balloon) bulb 226 could be "damaged" if it was dropped and did not break. The elements, because of being secured only at the press could move side to side thus the relationship of the filament to the rest of the tube structure can cause an out of balance condition and exacerbate the likelihood of hum. Meaning, the use of 226 could still have a hum issues despite balancing. Therefor, to minimize the chance of hum the use of the 26 "ST" envelope that has support devices fastened to the tube inner structure that wedge against the shoulder of the glass envelope keeping the elements aligned. The changeover to ST bulbs also included changes to the oxide filament coatings to improve emission..

Note that a new power transformer will not likely develop the same B+ under load that the original did, the respective resistors connected to the center of the hum balance resistors are for bias. Insufficient or excessive bias will be a problem. The tube will have excessive plate current thus further dropping the B+ or the tube will be near cutoff causing a clipping distortion. Those values of resistors are not called out in the Riders printing of the schematic. If that resistor is still intact in an OEM SPU it should be measured to get a starting value for the bias...

Note too, to little bias in the RF stages will lead to excessive gain and oscillation of the RF stages... The RF stages are not shielded for high gain. In fact alignment of a normal receiver must be performed on a wooden bench or some 4" from a metal bench to prevent interaction with the RF coils...

GL

Chas