Thanks, Daithi. I know about those backplates, and the man is John Sleep, 'The Gramophone Man', at West Pentire, i believe. I've bought something from him years ago, maybe any parts, or records... But the backplates he's made have a different back collar, as it's made so it fits directly in the tonearm, with 'no need of that perishable rubber collar'...! This I believe might be a fatal error... The rubber collar is depicted as a disturbance from which these new plates get rid off... Indeed it is a fundamental part of the mechanical impedance chain, and must not be ignored, absolutely! The compliance at the back of the soundbox is needed, together with the SB body mass, to decay the vibrations transmitted from the needlebar support to the SB body, then through its body mass and the collar compliance, to the tonearm body. These vibratioins are indeed a leak of energy from the main source (needle) to the SB body instead of going to the diaphragm, and this is a part of the circuit (I suspect it was never represented in the Percy Wilson's circuit schemes in Gramophones and Electrical Reproducers, 1929) which derives part of the needle energy out of the diaphragm. So the impedances there, as said, must not be changed. Probably the difference of having a rubber collar or not are only noticeable to the finest ears, but there will be a difference, of course. Think that fine-eared gramophonists and mechanical tinkers use to test rubber collars of different compliances, finding different SB responses when changed. I believe the bass transmission to the diaphragm is what suffers most in sound production if the joint at the abck of the SB is stiff. Besides that, part of the vibration energy migrating through this path, in absence of the rubber, is reflected back to the needle point, causing more wear to the grooves. Then annoying resononances at some notes can reach the needle, wearing or breaking the grooves very fast if the sympathetic note is being reproduced. So IMHO, these new backs, precious and very good otherwise, are marred because of this happy idea of supressing the space for the rubber collar. They should be repeated but with their true dimensions, as the originals. I'm sorry for John, otherwise a very kind man and helpful, but I would never install one of those backplates as they are... They need some work at the back collar to open the inner diameter to the dimensions of the rubber collar. With a lathe at hand, this would be a very easy modification to do...
Inigo
|