thorens: PR24 (RP-24); Stereo-Verstärker output trans impedance

Hi

I have a THorens PR-24 which is unfortunately missing an output transformer . My tech is having a little trouble with the circuit diagram and cannot locate the Primary or seconday impedance details for the output transformer ( ultralinear push pull) . I am keen to get the amp up and running but require these datils so I can get a repalcement pair ( preferably orig) . Any assistance is greatly appreciated. I never thought I would see this amp in Australia...I only dreamed of having it to match my TD -124.So I have it now its looking pretty sad at present....thanks from Australia...Kevin

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Dear Kevin,

the out impedance can be easily derived from the diagram. There are two 8-ohm windings, that can be series or parallel connected to have 4, 8 or 16 ohms. The primary impedance could be obtained from the good transformer still existing on the other channel, removing the tubes from their socket and measuring with a bridge the impedance from plate to plate. Probably it is measured at 1 KHz: anyway you can check if this is the proper frequency, just reading that each secondary winding is 8 ohms with the same bridge.

Best wishes for your amplifier,

Emilio

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Thanks Emilio...I shall pass this on to my technical expert....

best regards......kevin

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Dear Kevin

For the Impedance, they visit Frank's electron tube data-sheets for 6973 Tube -Type. The bias-Voltage is -22 Volt.

The PR 24-Amp. is not a easy device to restore. they must examine all insulations well replace all shielded AF-Lines.

Caution: The ciruit operates at very high operating voltages.

Joerg Sigg

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Dear Joerg,

I fully agree on the need for a careful check of leakage, especially that of the 0.22 uF coupling capacitors. I am not sure of the grid biasing voltage, since there are two summing bias sources. The first one, coming from the -22 volts sources across a voltage divider (about one half the 50K trimmer and a 27K resistor)should be around 11,5 volts. The second bias source comes from the cathode resistors, 470 ohms, and looks to be around 17 volts: not sure, but I read 17V. This means that the tubes are operated with zero-signal plate current around 35 mA and probably with a negative grid bias of about 28 volts.

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Dear Kevin,

my early answer was too hasty. Here is the proper way to measure the primary impedance from the still existing transformer.

You should connect the secondary windings for 16 ohms impedance, jumper between the two terminals in the middle, maybe V to NR. Then connect a 16 ohms resistive load across the extreme terminals, probably JR and N on the diagram, not sure due to its poor quality. You need a precise and non-inductive resistor: a reries/parallel combination of Allen Bradley carbon composition resistors or of metal film resistors could perform well. Keep wires as short as possible.

Now you can measure the plate-to-plate primary impedance at 1KHz. To minimize errors, repeat the measure for a single plate to center and check that its value is just one half of the previous reading.

Emilio

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Thanks again....I will call the technical man asap!!...regards to all ..........Kevin

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Hi

Just looking a bit further on with this project, can anyone suggest a pair of suitable output transformers for the Thorens Amplifier....I cant see too much hope in finding another Thornes mathcing pair or even one...so any suggestions for a replacement are appreciated..

Thanks

Kevin

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Hi Joerg

Thanks for that info...I will put it together with Emilio's points and give it to my expert...

regards

Kevin

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Kevin,

once measured the primary impedance, you can contact:

Antique Electronic Supply

Ask Jan First

and ask them for a suitable couple of ultralinear replacement types.

By the way, I guess that the bias feeding trimmer, 50 Kohms linear, should be adjusted for zero volts between the two cathodes, pin 7, of each output stage.

Best wishes,

Emilio

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