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martedì 11 novembre 2014

A band switch for my tube regen receiver

Recently I added a band switch to my tube regen receiver, allowing to select an HF band and the lower part of MW broadcast band, in addition to the existing tuning range, which initially covered from about 1300 kHz to about 3400 kHz.

The construction of the receiver was described in some of my previous posts (in Italian), listed below starting with the most recent ones:
Ricevitore a reazione finito
Alimentatore per ricevitore a reazione
Alimentiamo il ricevitore rigenerativo a valvole
Il mio primo ricevitore a reazione

The simple modifications for adding the band switch was suggested to me by Bernd, one of the members of the great regenrx group on Yahoo. They consisted, for the HF band, in reducing the number of turns of the tuning coil from 50 to 40. For the MW band, I added a 330 uH Neosid inductor in series with the 50 turns of the tuning coil (so raising its total inductance) and added a regeneration tap at 10 turns from the ground side of the coil (for the other two bands, the tap is at 3 turns from the ground side). A 2-way, 3-position rotary switch was added on the front panel to allow an easy selection of the desired tuning range, as depicted in pictures below.

For my initial tests of the new setup, I did not check the frequency limits of the two new bands precisely (may be I will try to do this in the near future, with the help of an RF generator).
I simply went around with the tuning knobs trying to figure out how the set worked after the modification. Results are not fully satisfactory at present, in my opinion. It seems to me that the receiver in not very sensitive, both in the HF band and in the MW band. Only strong local signals produce a good audio level.
So, there is still some work to do to improve this little radio further. For the moment, I captured some moments of my tests in a couple of videos on YouTube:

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