13-05-2020, 03:28 PM
I "repaired" one modern valve amp that had an EL84 and a similar Octal valve (forget which KTxx Beam Tetrode) and what looked like a too small audio transformer. An EL84 is described as Pentode, but actually it also is a beam tetrode. The g3 isn't a grid like on an EF80, it's beam forming plates.
"Do not operate valve changeover switch while power is on."
I explained that either valve could sound like the other just with slight adjustment of the cathode resistor. That it was a gimmick.
One ECC82 or ECC82 grid was earthed via the output transducer on the springline reverb. Putting a 470k at the valve base grid to 0V fixed the hum.
Additional problem was it was a painted chassis and the earths on most of the input sockets were not actually earthing due to clearance and thickness of paint. Unused earth tags were wired.
"Do not operate valve changeover switch while power is on."
I explained that either valve could sound like the other just with slight adjustment of the cathode resistor. That it was a gimmick.
One ECC82 or ECC82 grid was earthed via the output transducer on the springline reverb. Putting a 470k at the valve base grid to 0V fixed the hum.
Additional problem was it was a painted chassis and the earths on most of the input sockets were not actually earthing due to clearance and thickness of paint. Unused earth tags were wired.
