REVERSED CONNECTIONS

Evening Post, Volume CX, Issue 50, 27 August 1925, Page 14

REVERSED CONNECTIONS

Experience is the be3t school; one . learns most from mistakes, nnd if other people's mistakes are available to learn by,' so much the hotter. That is c.na ■ good- reason lov belonging tr> a wireless rluli. ■ All tho m'emlwjfs make- mistakes at. times, and it is surprising to find how . oFle'n their difficult ies have lieen common ti.spei'ieiifiea. .... . | A writer in a Southern paper recently." ( described ;vn inquest, upon a neiitrodyi-ifc , *■>) that w-mildn't work iiflw- tinine of the

Hiring hail been reiil'rungod. The Coroner's verdict, in aecirvcliiiicu with tins

ii'dlifiil evidence, was that tlio grid re turn of- the lifoh-frequeiicy valves bad I heeii made in llio positive instead of the ' lunntivo side of tlio filauii'nt battery. It happened tliat the method of wiring used ■• made if very difficult to trace this prrnr, so tli.it the set was lor a time qiiitp a pnzzV. j "A someivFiaf rUiu'liir difficulty was ex- j lifi-ipiK-eJ rrc-Rntly \\y « friend of Ihe prflicnt writer. TTe had a Iwo-sta^o ,-i^rli'ifir<-(iicriry anijilifipr spparatc from lii« rocoivev. and tlii«, though it* had l-. ? 'n pivina excellent service, suddenly iv.^rif on rt-ilso. l! produced sounds,

t-Tit it didn't amplify them. Tha first ttage madt a alignt increaw ia volum*

over the input, the second stage addod no appreciable strength. Burnt-o-jt transformer windings were suspected, but tests showed that they were all right. As no alteration in wiring had been made, there was no use in hunting through the amplifier itself. It turned out that the owner had made one 'if those mistakes that can't be made —he had reversed his filament oattery con-nections—grid-return to positive again. Correction of this worked an instant cure.

The reason why this amplifier would not amplify with positive bias on the grid was simple. Dry-cell tubes were being used, with a battery giving under three volts. The positive grid voltage threw the operative portion of the valve's curve past the upper bend, which was low down owing to the filament being at a rather low temperature, and'so the changes in grid voltage made little or no change in the plate current.