I have 3 Radiola 18s, after replacing the volume control rheostat, one works well, so far, and I like the sound. I have 2 RCA 100 speakers that sound muffled. I looked at disassembling these to repair them, but the screws are frozen in the pot metal and have been damaged by someone earlier. I did look inside one speaker enclosure, the speaker mechanism looks interesting, but repairing these will need to wait. I have a nice 2K Ohm primary to 8 Ohm secondary transformer connected between the speaker output jacks on the Radiola 18 and a Jensen K950 PA speaker. In my opinion, the sound is remarkably good and clear for a 1928 radio. I suspect the Jensen speaker is fairly efficient and that this contributes to the sound quality. A few other speakers I tried were not nearly so good.
I've tried the 2 sets that don't work, the power supplies seem to be the problem. Rather than risk the one good set by using its PS to try the other 2 tuners, I've assembled a new power supply to try the two non-working tuners. It isn't wired yet, but is ready to be. This also seemed like a good way to learn about the way these sets work. I'm thinking that it might also be interesting to try building just the audio section of the tuner, the #26 & 71 tubes, and using a CD player, 78RPM turntable or other sources.
I have the Radiola Guy information and various other materials with schematics for the Radiola 18. I think I understand some of the circuit, and have been thinking it might be possible to assemble the section of the schematic including tube number 5(UX-226) & tube number 6(UX-171-A), and connecting the source(CD Player & Etc.) to the grid of the UX-226. I'm certain it's not quite that simple, and that additional circuitry is required. I also would need to know the specifications for the transformer between UX-226 and UX-171-A, and find a source for it. If anyone has suggestions for this project, I'd appreciate hearing these.
If there are other amplifier suggestions using different tubes that are compatible with the Radiola PS, I'm interested in that possibility as well.
The RCA 100 loudspeakers also have a paper cap in series with the coil, so that may cause distortion if it is failing. The power supplies are fairly simple, electrically, but it would probably be a good idea to replace the paper filter caps for reliability, you can find substitutes at Mouser, along with some of the antique radio restoration suppliers. Antique Electronic Supply can furnish replacement audio interstage transformers that will work with the Radiola 17 and 18 type sets, unless you want to try a solid state substitute, The front ends of these are fairly simple, basically just RF coils with some neutralizing capacitors, not sure how reliable the resistors are but that's easy to check.
Regards
Arran
So after reading three paragraphs I think what you are seeking is a transformer to connect the 26 plate to the 71A grid. I would go with this one
https://www.tubesandmore.com/products/tr...tage-10-ma The one you pick is for a particular set which is a collectible hence the $60+ price.
I'm cheep.
Edited 12/21/22
The P-T156 transformers offered by AES *Antique Electronic Supply) do seem to work well for this app. I have a Radiola 16, 17 and 18, along with a 100A loudspeaker. I used the P-T156s on the 16 and 18 successfully. The cabinet for the 100A speaker is pot metal which swells. This is possibly the reason for your not being able to remove the screws. The speaker is a magnetic type very common for the era and not known for being high fidelity. There is a "PI" Filter that acts as a low pass filter. However, they were much less expensive than the GE "Kellog - Rice" Electrodynamic speaker, the archetype of all dynamic speakers that followed. For best fidelity, any good 8" speaker with a 2K Ohm to 8 Ohm output transformer will do. (A transformer from an AA5 AC DC Radio is just what you need.) Your Jensen is just the trick. PA Speakers with a 70V Matching transformer work well. Use the 0.5W 0.75 W or 1.0 W tap
The Radiola 17 and 18 used the new "AC" Tubes, the '26, '27 and '71A. The existing 01A tubes, used on the Radiola 16 and many battery operated radios dating back to 1921, would hum horribly if the filaments were powered by AC.
What are you using as a transformer for the power supply that you are building? The 26s operate on 1.5VAC, the 27 operates on 2.5V AC and the 71A runs on 5V AC or DC.
Do expect hum! The '26 and '71A are touted as an "AC" Tube by the "P T Barnums" at RCA at the time (Read their ads that described reception by RCA radios of this era as being indistinguishable from live performances). However, being that they were both direct heated tubes, there will be hum. The 26 hum was mitigated by the slow responding low voltage high current 1.5V 1A filament fed by a supply with a variable rheostat for hum balance. the 71A Filament differed from the 71 by also having a more heater-like thoriated filament whose emission varied less during the peaks and valleys of the AC than the original 71 tungsten filament did. The real AC Tube in the set is the '27, the first true AC Tube with separate heater and cathode. This is the archetype of every voltage amplifier triode with a heater and sleeve cathode ever since. There was a big push to make what RCA referred to as a "Socket Powered" radio. The first radio in this series was the battery powered Radiola 16 which used 201As ('01As) for the 3 RF amps, detector and 1st AF Amp. The detector used a spring mounted 4 pin socket to minimize "microphonics. The "Power" Amp was a 112A, emitting 0.35 Watts at 135V Plate voltage. The Radiola 17 used the same chassis and almost all parts, except the detector socket and tubes as the Radiola 16. The 27 was used as a detector instead of the 26 as, for some reason, the 26, which is functionally similar to an 01A except for filament, was deemed as unsuitable for use as a detector. Possibly, the AC on the filament would raise the hum level, which was already objectionable by today's standards. The Radiola 18 was similar to the Radiola 17 except for the IF transformers which were "neutrodyne-like" without violating the Hazeltine patent, a slightly different power supply design and the use of "choke and capacitor" coupling instead of the transformer coupling of the Radiola 17 and the direct coupling of the Radiola 16. Note that the Atwater Kent 36 and 40s used the same circuit as the Radiola, while a Zenith of the same era used 27s for all tubes except the output tube. The RCA Radiola 60 Superhet, introduced the same year as the Radiola 18 also used 27s for all tubes except the output tube. This contributed to better fidelity through lesser hum. In addition, the Radiola 60 used a biased "Power Detector" instead of the grid leak detector used by RCA
BTW, Westinghouse and GE built almost all RCA Radios from its founding in 1919 till the acquisition of Victor, (who had started producing radios in 1928) and the introduction of the Radiola 80. Then the tables were turned and RCA built almost all GE and Westinghouse radios from 1930 till between 1934 and 1936.
The 71A has an output power of approximately 0.7W.
I built an RC Coupler with a switch and volume control that subbed for the non-existent radio phono switch. I was not impressed with the level of distortion using this as an AF Amp. The hum likely contributed to that. There are several discussions on the internet on single ended amplifiers and preamps using constant current power supplies, 01A and 71A tubes. (Other than the filament voltage, the '26 and '01A have very similar characteristics and the same base diagram.
Another source of distortion is the "grid leak" detector, which had higher distortion than biased detectors or diode detectors.
Thank you for the responses.
I'll use the less expensive transformer, I like saving money too.
I have a Hammond PT143 transformer with the required voltages except the 1.5V, I thought perhaps the 1.25V from the 2.5V CT might suffice for the #26 tube, or a resistor to drop the 2.5V to 1.5V. Unfortunately I am somewhat new to electronics.
I'm using Radiola Guy's schematic and booklet for component specifications.
I have included a photo of the PS. I think I have all the necessary components, but have yet to solder it all together.
Hi TN,
a wonderful layout on that power supply, beautiful looking.
IMHO, yes, using 1.25 from the 2.5V winding would work, assuming y\that you are installing the "hum balance" rheostats used in the original PS. An alternative, if you have them is to replace the '26s with '01As, get an old computer power supply and use the 5V DC to power all the tubes except the '27. There will be much less hum.
BYW, what is wrong with the original power supplies? These are usually reliable, although sometimes the filter caps (oil / paper style) short. You can test the transformer by simply removing the '80 rectifier and checking voltages. If the transformer is good, you can simply replace the caps and chokes. Do note that that even though they look similar, the Radiola 18 PS is different from the Radiola 17 or Radiola 60 PS.
Below are 2 schematics from N/A, one for the Radiola 18, (similar to the schematic from Radiola Guy). The other is for the Victor 7-11, a radio / phono combo that uses the Radiola 18.
http://www.nostalgiaair.org/PagesByModel...040317.pdf (CAUTION! Do NOT use the voltage chart in this diagram, it is fr the DC Model!)
http://www.nostalgiaair.org/PagesByModel...040265.pdf
These are the same schematic, just presented differently. (RCA did not own Victor at the time). These may be useful. Unfortunately, no specification is given for the rheostats or resistors in either schematic. However, the schematic for the Radiola 33 is similar to the Radiola 18 and does contain resistor values (except for the rheostats), and also a voltage diagram (use the values for "Tubes in tester" and have the tubes installed in the radio. the schematic for the Radiola 33 is below:
http://www.nostalgiaair.org/PagesByModel...040328.pdf
You can measure the rheostat values, but you have to disconnect the wires from the transformer and tube sockets.
Hope this all helps! Best Regards and God's Blessings during this Holiday Christmas and Chanukah season!!