The "Crystal and Valve"

Radio Record, Volume II, Issue 25, 4 January 1929, Page 9

The "Crystal and Valve"

Correspondents. in Difficulty

N the issue of the "Record," dated November 380, 1928, was published a crystal and yalve circuit with claims of superiority. over the usual crystal and single-yalve amplifier. Two or three letters have come to "Pentode" in which’ the constructors seem to be having a little trouble with the valve. Apparently it is not functioning properly in their cases, and one or two hints for their use muy provide a basis for experiment for others who have constructed this sét. The writer explained the construction of ‘a set from which he has had good results but in this, und ‘in all ‘cases wiere a valve is ealled upon to function as high and low frequency amplifier, care has to be exercised to see that the amplifying valve does not rectify. A valve cannot rectify and amplify at high and low frequencies * together. Assume the reader has the set in front of him; take out the crystal and listen with the phones. If music is heard at all -the valve is rectifying. Instead of explaining why the valve is detecting, a few suggestions will be given to remedy this trouble. First increase the B battery supply, and at the same time give the correct amount of grid bias as specified by the manufacturers of the valye in use. This bias is inserted between the F- terminal on the on the secondary of the transformer, and A-. . The 4-meg. grid leak can be replaced by one of 100,000 ohms or better still a high frequency choke can be substituted. The construction of a suitable choke is given in the "Radio Record" dated December 21. To repeat the hint regarding the type of valve to use. A high frequency or resistance capacity audio valve -is quite unsuitable. Preference should be given to one of the 201 A type. In general, a valve with an impedance between 6,000 to 10,000 ohms should be. used, and with 90 volts B battery a 44 VC battery can be used. Give the grid a negative bias by connecting the. F- terminal to C- on the bias battery. _By applying these few tests to the receiver, the results will well repay the little trouble and expense incurred. Results Too Weak. T WOULD like to know some _par- _ tieulars about the crystal valve set you described last week in the "Radio Record." I made it up, and followed the diagram closely, made all the wiring as ‘short as possible, but the result was a failure. T could get 2YA all right with it, but then it was nowhere near as good as the crystal and valve amplifier; in fact, it seemed sort of half smothered and thick. As for 8YA and 4YA, all I could hear of them was the faint whistle and no more. _I can’t understand why the same outfit changed from one to the other should be so unsatisfactory. I really, expected to hear boisterous volume from the local station for I get plenty of volume at present, using the crystal and amplifie.. I ean pick up ‘he Australian. stations anc Christchurch ‘weak speaker strength, so this

district cannot really be blamed for the poor result. My aerial is about 180 feet long, and 40 feet high, and well out in the open. I would very much like to make your set up, if you can tell me how and where I made a mistake; if any. I used the exact materials you specified. This set would oscillate, and the reaction was good, but-?-(C.D. MecG., Wellington). TRY tapping the aerial coil and give fewer turns, say 15 to 25; then test the valve to find whether it is Working as an audio amplifier alone. Do this by disconnecting the .0001 m.f.d. condenser, short-circuiting the 4}-meg. grid leak and turning reaction to zero. If 2YA doesn't come in at speaker strength then the trouble is either transformer or crystal. Test the erystal by listening with the phones across the P and B of primary with valve turned out. Across the secontlary winding of the transformer the signal strength should be slightly less. Assuming everything is OK, slip in the grid leak and listen. If the music is distorted or there is a great loss of signal strength, try a new leak, or, better still, substitute an H.I. transformer. ‘Now slip in the .0001 condenser, and if the trouble recurs the valve is rectifying instead of amplifying. There are one or two remedies for this which have been dealt with above. Which Circuit is the Better? 66 ALTHOUGH I get fairly good results from my crystal set with valve amplifier, I feel impelled to construct one of the sets described recently in the ‘Radio Record.’ I would use the same components for the amplifier, but would ask which will give the better results-the set described in the isstie of 30/11/28, using reaction, or the double tuning system, and connected to amplifier in the usual way?" Although the circuit employing two erystals is an improvement on the usual single detector one, the results do not equal those of a circuit in whieh a single detector is used with the addition of reaction. In this case probably the set employing regeneration is to he preferred as a general receiver. But if, on the other hand, the only station desired to be received comes in powerful enough to operate the detector successfully, then the full wave set is to he preferred, and will give considerably more volume from the local staiton.