mende: Mende 248WK; exciting coil resistance.

Hello, 

I bought this radio but unfortunately without loudspeaker. I have a few loudspeakers of that kind, but I wonder, what's resistance of the exctitation coil so I could fit the best loudspeaker?

Regards,

Piotr

To thank the Author because you find the post helpful or well done.

Dear Piotr,

if you look at the schematic, you will find 400V at the cathode of the rectifier RGN1064. Behind the choke of the speaker there are 260V, which can be found at the screen grid of the output tube RENS1374d.

This is a difference of 140V.

The currents drawn by the tubes can be estimated as follows. The anode current of the RENS1374d is 26mA. Its screen current is 10mA, which can be seen from its data sheet. The estimatet current of the rest of the set is about 6mA. So the total current is about 42mA.

Therefore the resistance of the field coil of the speaker is about 140V/42mA = 3.3kΩ.

When you test your speakers, please do not forget to check/replace C₇ = 5000cm ≈ 5nF first. This is the coupling condenser from the audion RENS1284 to the grid of the RENS1374d. These condensors are usually no longer good, and the RENS1374d will terefore draw to much current and will be overloaded.

Regards,

Dietmar

To thank the Author because you find the post helpful or well done.

Thank you for your answer, I didn't see this "400V" writing, otherwise I'd calculate that myself :)

But also I have question, as I have more experience in radios from 50s... This is strange superheterodyne/reaction type of radio. Does this reaction works only in Long and Middle Waves and short waves are in superheterodyne circuit, or reaction is used as additional amplification of IF?

Regards,

Piotr

To thank the Author because you find the post helpful or well done.

As can be seen from the fact that the 248W was offered simpy with MW and LW only (248W) as well as with LW, MW and SW (248WK) , the SW is an add-on to the simple version of this set. And also the case of the 248WK is different from that of the 248W.

The 248W as well as the 248WK has for LW and MW a cathode mixer which was common in those days when no special mixing tube was available yet. Both types only have one IF stage, and since RF diodes were not yet available, the IF stage is constructed as an audion. It has a fixed regeneration for the IF frequency and needs not to be changed.

Because the SW of the 248WK is an add-on, the operation of the set is here totally different. For SW the first stage operates as an audion with regeneration. Its detected audio signal is coupled to the grid of the output stage via C32.

The way to process SW was usual at that time. Similar solutions are found in sets of different makes, e.g. Siemens 36WLK .

Regards,

Dietmar

To thank the Author because you find the post helpful or well done.