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RE: Ceramic Phono Input

Posted by Interstage Tranny (A) on February 11, 2017 at 07:54:08

In Reply to: Ceramic Phono Input posted by mark.korda@myfairpoint.net on February 11, 2017 at 07:36:37:

Sometimes, there is a small capacitor installed between the input jack and tube grid. Also, there maybe a large inline resistor to increase the input impedance, which can end up decreasing the level of the input signal. The small input coupling caps were fine with the high impedance crystal and ceramic carts of the day, yet not needed with an input which is of low impedance.

So, do you have a schematic of this fine little sounder ? If not, simply trace out the signal path from the ceramic input jacks. You will see a large (Ohmic value) resistor to ground at the jack or on the selector switch. You might also see, or only see another high value (in Ohms) resistor from the ceramic input jack to the mag phono input jack or wiring. There may be a small ceramic capacitor across that R or simply a small cap from one jack to the next, as was an inexpensive wiring method back then. Simply "bypass" the "inline" gain robbing (resistor) and frequency dependent(small capacitance value cap) parts with wire, for your newfound application.

While you are carefully looking "inside" the amp, are there any other high gain inputs, like a Mic or Tape Head input ? With some serious studying and realizing the selector switch wiring pinouts, we can "convert" those typically unused high gain inputs into extra phono inputs. Of course, we would need to know the R-C values to apply and install the phono EQ, but everyone should have two turntables nearby and ready to play, right ?

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