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Old 19th October 2012, 03:01 PM   #1
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Default eico st70 input impedance question

hello everyone, my name is matt and i am somewhat new to the wonderful world of tubes. i just bought a very clean eico st70 from a reputable audio dealer in new york. (amp is currently on the way to my doorstep) it has been gone through and verified to be a great working unit. this amp will be pushing a pair of sansui sp1700 or a pair of sp2500 speakers, i figure i will choose the pair best matched for the amp..

i have read a lot on modding and such and i'm familiar with my way around a soldering iron. looks like they have already done the "loudness switch" mod, so that's a bonus... but to get to my real question, i have been reading the manual and noticed there are several different input impedance levels for certain inputs:

input sensitivity input impedance
phono a 4Mv 100K ohm
phono b 4Mv 47K ohm
tape head 3 3/4 1.4Mv 1M ohm
tape head 7 1/2 2.15Mv 1M ohm
fm 480Mv 500K ohm
am 480Mv 500K ohm
Muiltiplex 480Mv 500K ohm
tape 480Mv 500K ohm


so, me being new to tubes and impedance levels.. i really don't want to mess anything up.. I have figured out by doing some investigative research that my technics 1200 with an audio technica at120e will match nicely with the "phono b" input, but what inputs would be safe to hook my tv, or a cd player to? would any of those inputs be safe or would i need to modify something?

thank you very much for your help!
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Old 19th October 2012, 04:06 PM   #2
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FM, AM, Multiplex, Tape (NOT "Tape Head") are all equivalent and usable for TV, CD, DVD sound input. They all go directly to the volume control, so there is no chance of overload if they exceed the 400mV nominal level - though the volume control may lose a bit of resolution as it will be turned down quite a ways. But that's normal with vintage equipment - most was more sensitive than really needed.
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Old 19th October 2012, 04:15 PM   #3
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excellent!!! i really appreciate your help!

i'm so exceited to get this thing hooked up and listening to tubes!
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Old 19th October 2012, 05:31 PM   #4
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so, pondering a bit more.. was the "tape head" input an option to add a direct link from the actual tape head to the amp? bypassing any internal tape machine pre amplification?
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Old 19th October 2012, 11:39 PM   #5
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yes, the dynaco PAS2 had one, too. I have never seen the tape machines that required this. Wollensak's, the early consumer tape recorder, had its own amps. However, I saw a tube era Ampex 1/2" machine, and it had channel amps in a little rack like the backsplash on a washing machine.
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Old 19th October 2012, 11:56 PM   #6
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There were a number of playback only decks that had stereo playback heads in the very early 1960s. I've seen a few mostly British made machines like the Collaro decks: Collaro Tape Decks - not sure whether these were sold in the U.S. market or not, probably a few Japanese decks made it to the U.S as well.

Not very common I suspect and most seem to have perished over the intervening years.

The ReVox G36 was pretty popular around here, this was a full blown and expensive machine at the time. (I have a nice one, and am only the second owner..)
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