[MrFixr55]

The rectifier winding may run hi due to no load, but it seems that the replacement transformer HV secondary voltage is not far out of range considering your no-load measurement 0f 690V vs the 700V rating for the Hammond.

Per the RCA Receiving Tube Manual, the output of a type 80 or 5Y3 tube with an AC input of 350V per plate (what you get with a secondary of 700V with a center tap) varies between 300V at the input of the filter for a 20mA load and 260V for an 80mA load.  Caps for the Radiola 18 PS are only 1.0uF, so I think that the choke values are rather big.  The load on this power supply not including the caps is rather low.  the biggest current draw of an individual tube is the 71A Power Output tube at 20mA at 180V.  At 135V each 26 draws 5.5mA for a total of 22A for all the 26s.  The 27 is used as a grid leak detector, therefore current draw is negligible. Therefore, total B+ current draw for the receiver chassis is about 42mA.  (You can actually touch the tubes in this set without burning your hands.  Measuring the inductance, resistance and calculating the voltage drop may be a good class exercise. (Don't ask me how to calculate it, My Bachelor's Degree was in Life Sciences

Attached below is the info for the Type 80 from the RCA Receiving Tube Manual Version RC10, published in 1930. 

RCA 80 Rect Specs fr RC10 Manual.pdf Size: 429.82 KB  Downloads: 56

Please note that the current curves below this diagram are for RCA 81s, not the 80s.  the graphs above the diagram pertain to the 80
The "typical" full wave power supply circuit is similar to the Radiola 17 which used a voltage divider for the needed voltages. The Radiola 18 differs in that tube current will have a greater influence on plate voltage (It was common for the voltage divider resistor on a Radiola 17 to open.)

Hope this helps, especially for your teaching efforts.

[Chas]

The dried hardened wire insulation can be softened with a heat gun, not SCORCHED, then the wire will be pliable without flaking the insulation while hot. It will become brittle again when cooled.
Since you have the transformer off the chassis the asphalt potting can be melted off in a lab oven set for around 375F. Yes, the asphalt will burn with high temperature heat gun.

The best way is to secure the case with the opening down, place a bread loaf foil tray under the case and allow enough gap for the transformer to slide out. Back up the arrangement on a lower shelf with a large foil roasting tray..

An alternate is to dissolve off the asphalt with odorless mineral spirits. That can take weeks..
The heat gun can be used to extract the transformer by suspending with the wires and warming the can with the heat gun. Once expose CAREFUL playing of the heat gun will get most of the asphalt off then the rest can be dissolved off with the mineral spirits...

Be careful how the melt out is performed, wear cotton gloves that can be discarded.

Keep mineral spirits away from the oven and the heat gun. Spirit vapor will decompose in the hat gun or oven and make nasty toxic fumes and too much fumes are explosive.

FWIW There is one Antique Radio Forum member who re-pots with asphalt. Has no issues doing so.

https://antiqueradios.com/forums/memberl...file&u=514

[TN Allen]

There is so much to take in, all interesting, but also making clear there is much to learn. Thank you both for the sustained information flow.

I doubt there is much reason to unpot the transformer, I need to check the remaining leads, but if their connections and outputs are good, leaving it as it is is probably best. 

The absence of the other RF tubes makes sense to explain the high voltages at terminal strip connections 7 & 8. I had recently read vacuum tubes have high internal resistance and was slowly beginning to understand that the initial readings of ~260V rather than 145V & 165V probably resulted from that absence. Thank you for suggesting this.

Perhaps I can find appropriate resistor values for a voltage divider to drop the terminal strip 7 & 8 voltages from ~260 to 145V & 165V, or perhaps there is a suitable resistor load I can add to the B+ to drop it. I have accumulated quite a few ceramic resistors from audio crossover cricuits I tried but have removed, as well as various other 5+ Watt WW resistors. 

At this point I'd be pleased to hear music through the 2 units, then decide a way forward. Actually, I did hear faint but distinct music through them, which suggests there may be something right, but also wrong in the transfer of the audio signal from the CD player to the #26 tube to the 71-A to the output to the 2K-8 Ohm output transformer. And no shortage of opportunities to learn!

I can take some of this to the electronics lab. for explanations and assistance, which is probably a good idea!

Thanks again.

[MrFixr55]

One way of dealing with this is to "reverse engineer" the Radiola 18 and PS.  With the "good" unit al hooked up, CAREFULLY measure your B+ voltages, both at the plates and cathode of the 80 Rectifier, then at the output of the filter, thereby determining the voltage drop across the inductors.  Even though these sets do not draw a lot of current, the "unloaded" output voltage of a power supply can be quite a bit higher than the output when the radio is connected to the power supply.  Another consideration is that although the no-load rectifier plate winding value of the original Radiola transformer and your new transformer are the same, the AC voltage for the rectifier winding on the Radiola transformer may drop when the various filament windings are under their normal load.

Another way of dealing with this is to determine the total current and specified B+ output. Test your power supply using a resistor across the B + to ground that will duplicate that load and measure your power supply's loaded output voltage.  Compare that to what you get for the values that you get for the complete original Radiola 18.  If my calculations were correct in my previous, the total B+ current load is 42mA.  So if E= IR and R= E/I, then R= 165 / 0.042= 3,929 Ohms.  The current capacity of the resistor needs to be sufficient.  So if P=EI, then 165 X 0.042 = 6.9W.  You may find that your supply puts out much more than 165V, even under this load. you may need to vary the resistance to find out the true output under a load of 0.042A

Next, subtract 165V from your measured "under load" output.  For example, if the under-load output of your power supply is 250V and the desired voltage is 165V and the current load is 0.042A (42mA), then the dropping resister needed is based on the following calculations:
250V (New PS output voltage) - 165V (Voltage required by Radiola 18 chassis) = 85V (dropping resistor voltage).  If E= IR then R= E/I and if the voltage drop is 85V and the current (which is constant anywhere in a series circuit) is 42mA or 0.042A, so 85/0.042 = 2,023 Ohm, or 2KOhm.  to further complicate things, it is good to put a filter cap after the dropping resistor.  This may increase current.  I will leave it up to the engineers to determine the additional current added by a cap.  I would assume that for a cap as low as 1uF, it would be low.  For pure DC, assuming leakage of the cap being none, then after charge, the current is 0A.  However, this is the real world, not the theoretical world.

When filtering pulsating DC, the lower the size of the filter capacitors, the lower the output but the higher the hum.  The inductors must be of a higher inductance (and possibly resistance) to compensate for this.  Early AC radios had capacitors of lower value than newer sets because electrolytic caps were not in use at the time.  Note that there are 3 inductors in the power supply.  2 are for B+ and the third is for the plate of the output tube.

In addition to the plate voltages for the various stages, check the "bias voltages", especially for the output, and verify the correct speaker wiring (see further below).

Here is the schematic of the output and power supply stages of the Radiola 18 (The 60 would be similar but not exactly the same):
   

In my first experiments with a Radiola 60 chassis and power supply found when I was 14 at the town landfill (since lost with the sale of the family home), I did not have a schematic, so I merely traced the wiring and connected an AA5 speaker between the 71A's plate and the output of the power supply.  The radio played loud but with objectionable hum.  I did not know that the 2 pin jacks on the power supply were for the speaker, since why would the speaker jacks be on the power supply chassis?  I did not have access to the Radiola 60 schematic? (There was no internet then and it was not in the local library.)  And, of course, the capacitive coupling would have a rather high cutoff at 60 Hz (the frequency of the filament-based ripple), and the bass response of the 100A and 103 speakers was not large to begin with.  These factors would mitigate hum.

Since the output stage is a "Class A", and since the speaker is capacitively coupled, I don't see a lot of harm done in operating the set without the speaker. However, if you are using a "more modern" speaker scavenged from a late 1940s to early 1960s 5 tube AM radio (that uses a 35L6, 50L6, 50C5, etc.), ensure that you are using the output transformer from that set.  Ensure that the "blue" lead is connected to the output coupling capacitor (which should be at least 0.5uF).  Ensure that the red lead is connected to the center tap of the resistor across the 5V filament supply to the 71A output tube.  If it is connected to B+ or if a 2K Ohm output transformer is not being used, then volume will be very low.