Radiola 18 Amplifier & PS - Printable Version
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RE: Radiola 18 Amplifier & PS - Radioroslyn - 10-13-2024
Radiola 16 is a battery set using 01As not ac tubes.
RE: Radiola 18 Amplifier & PS - morzh - 10-13-2024
I guess I looked at the header that said 16 and 17.
The 17 was the one I meant to address.
RE: Radiola 18 Amplifier & PS - TN Allen - 10-19-2024
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.
RE: Radiola 18 Amplifier & PS - TN Allen - 10-19-2024
I have the 2.25V and 2.5V information confused, and must look more carefully at the schematic and Hammond transformer information.
RE: Radiola 18 Amplifier & PS - MrFixr55 - 10-19-2024
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.
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.
RE: Radiola 18 Amplifier & PS - Chas - 10-20-2024
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
RE: Radiola 18 Amplifier & PS - TN Allen - 10-21-2024
So much to learn! Thank you for the infomation, I'm beginning to appreciate the complexity and how much was accomplished in early radio and electronics.
I've been thinking about the first commercial radio braodcasts preceding the Radiola 18 by only a few years. I also recently remembered a dentist in Jamestown, NY showing me an early radio receiver and antenna he built to receive the first commercial station broadcasts. He had saved it for decades as he showed it to me around 1972.
Small progress, I've soldered 3 pairs of 30 Ohm resistors in series for the cathode bias with a 400 Ohm center tap between one pair, a 2000 Ohm between a second pair, and no resistor between the 3rd pair. I had bought 30 Ohm wire wound as these were specified on the Radiola Guy schematic. Hopefully these will work temporarily, I can order 20 Ohm later. All of the above are 5 Watt. I've attached a photo. I can plug this assembly into the 0-30 V power supplies using stacked banana plugs to connect to the audio section terminal strip. I'll also figure out a connection to the far right
"Output" shown on the right side of the schematic, which connects to the 1875 Ohm center tap resistor shown on the Radiola Guy schematic that connects to the 5V PS.
I used a 2000 Ohm resistor rather than 1875 Ohm as this is what I could find to purchase.
I'll check the tubes, I think I bought the tubes as tested New-Old-Stock from Tube Depot. I have several others as well from old Radiolas, if none work, I can probably order another.
Thank you.
RE: Radiola 18 Amplifier & PS - Chas - 10-21-2024
I may have missed this early on....
It appears that the 18 is being setup as a teaching aid rather than a restoration.. None the less my responses are still accurate...
I encountered my first 18 at about 9 years old. Seems a friend had the 18 retired from a family member in the back work room behind their garage. It was humming madly and I knowing nothing at the time had hoped it was a bad connection I poke circuits with a stick until I slipped and hit B+. That ended my learning curve on 18's for about 8 years.
All of the RCA's in this series of split chassis do not have a means to accurately align the RF sections. Any damage, repositioning of the RF coils can change the tracking and not in a linear manner. I have used extra turns on the coils as well as reverse turns to alter inductance, cocking the stators of the tuning condensers. RCA did not arrange the rotors that are external to the stators, meaning bending the rotors will not work to compensate for tracking errors. The same procedure as well as changing certain fixed condensers applies to the Radiola 60 Superhetrodyne...
Good Luck!
Chas
RE: Radiola 18 Amplifier & PS - MrFixr55 - 10-22-2024
I assume that the yellow wire will be cut and will be the center tap for the 2.5V line. This all looks good electrically, and extremely good mechanically.
RE: Radiola 18 Amplifier & PS - TN Allen - 10-23-2024
Yes, the yellow wire is to be cut and connected to a 3rd DC PS to power the R-18 radio section. I don't need that voltage though for the audio section #26 and #71-A tube filaments so I'll leave it out of the circuit. I'll cut the yellow wire before connecting the new audio section and insulate the ends.
I'll also connect a wire between one output/speaker banana jack/post on the PS to the junction of the blue/gray 2000 Ohm resistor and the black 30 Ohm resistors either side of it. This appears to be the correct location for that connection, looking at the schematic.
I think I have everything assembled and correctly soldered, I should try connecting it all together and providing power, though I must admit to some reluctance.
I have one other question regarding the center tap on the 700 Volt Hammond PS transformer, does it really connect to the chassis ground? I've soldered it to the ground lug on the 3 prong AC connector, which is connected to lug 9 on the terminal strip and to the chassis. In other words, the 700 Volt CT wire, and the ground lugs on the terminal strip and the AC input are all connected to each other and to the 1/4" Aluminum chassis plate.
Regarding using the new PS and audio units for instruction. The new campus radio club faculty advisor and I looked at the schematic yesterday. He knows about as much about the circuit as I know, but finds it interesting, the students may also. I've offered use of both if they do want to use them, as well as two Radiola 18s that need work. I'd lend them the one working R-18 I have as well. I suspect that if and when I have the new sections working and safely assembled they will go to the campus radio club and be used to explore tube technology. As I mentioned earlier I teach at Maine Maritime Academy, our Engineering Dept. students are working toward a Coast Guard license. They have varied interests and may appreciate an opportunity to work with tube radios.
RE: Radiola 18 Amplifier & PS - Chas - 10-23-2024
Do not connect the center tap of the high voltage winding to the “U” ground of power. All the transformer iron can be connected to the “U” ground. Note the 18 schematic for reference of the antenna/ground.
Chas
RE: Radiola 18 Amplifier & PS - TN Allen - 10-25-2024
A few days ago I connected the various components. In so far as I know after checking the cicuits against the RadiolaGuy schematic, I have it all correct.
But the B+ voltages are way off, about 260 V between terminal connections 7 & 9, and same between 8 & 9.
I'm wondering if the suggestions his schematic provides for different inductor and capacitor values may be off. These are for a 10mH inductor followed by 2 5 mH inductors and 4uF capacitors replacing the 1 uF capacitors. These are suggested if "swinging choke" is not used. I assume the choke/inductor I bought is not a "swinging choke."
On the positive side, I was able to hear very faint but distinct music playing, the filaments all heated and there was no smoke.
I'm tempted to remove and replace the 4uF capacitors with 1uF capacitors if this might drop the voltages to the 145V & 165V values for the 7-9 & 8-9 terminal strop connections.
RE: Radiola 18 Amplifier & PS - TN Allen - 10-25-2024
It occurred to me that the 700VCT Hammond PS transformer I'm using is way too high. I assumed that it was an appropriate substitute because it was described as such. But, knowing what I have recently learned doing this project, I think that assumption was erroneous.
Does anyone know what the high voltage CT coil output in the Radiola 18 PS should be?
RE: Radiola 18 Amplifier & PS - TN Allen - 10-26-2024
I've looked at the two extra Radiola 18 power supplies I have. Much of the point-to-point insulation on one is crumbling but the transformer wires seem safe. Perhaps I can carefully cut these leaving as much length as practical extending from the transformer then shrink tubing over the wires carefully. Perhaps the transformer is still OK, and re-starting this project with it may produce better results.
RE: Radiola 18 Amplifier & PS - TN Allen - 10-26-2024
The transformer leads are deteriorated, but seem sufficiently intact and are easily reinforced with the shrink tubing, which has a heat setting adhesive that may make the old leads sufficiently robust. This would not however be safe enough for use on campus.
I fpund the 2 leads that have highest resistance for the 110/120V AC input, ~3.4 Ohms vs. 4.1 Ohms. I'm using the 4.1 Ohm pair for input. I also tested the high voltage output at 699 VAC, and the #80 tube filament voltage at 6VAC. I need to try the centertap lead, and I have yet to test the remaining leads, but they are now as safely covered as I can make them.
The insulation on the old leads had broken down where the wire was bent around parts of the chassis and/or near connections. I suspect it was potentially deadly left as it was. It had crumbled and fallen iff in several places.
Applying heat near the "tar-like" substance in th
I'll remove the inductor unit next and check it out. Perhaps the shrink tubing can also save it.
Any suggestions will be greatly appreciated, thank you.