Radiola 18 Amplifier & PS - Printable Version

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RE: Radiola 18 Amplifier & PS - MrFixr55 - 10-26-2024

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. 


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.

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RE: Radiola 18 Amplifier & PS - Chas - 10-26-2024

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/memberlist.php?mode=viewprofile&u=514

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RE: Radiola 18 Amplifier & PS - TN Allen - 10-27-2024

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.

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