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  • Cartrige Load Missing ?

    I'm currently looking into a Fisher 400C preamp for restoration.
    I have the schematic for the preamp. and it seems to show a loading resistor of 110 K ohm for the phono input ?

    I thought that 47 K was the standard value.

    Was there a number of different values in the past ?

    The Fisher 400C was made in the early 60's (I think).

    Marc
    Last edited by Craig Maier; 06-04-2019, 10:51 AM.

  • #2
    Hi,

    47,000 Ohms was the standard, but there were some unusual cartridges that used other values too. When you look at the circuit, are you sure that there is not another resistor "downstream" somewhere and in parallel with the 110 KOhm value? Anyway, if a cartridge designed to be loaded with 47 KOhms is under-loaded, it will tend to be a bit "shrill" on the top end (it will peak up somewhere between 7,000 Hz and 15,000 Hz).

    Craig

    ps - I remember that pre-amp - it is all 12AX7 based as I recall. Sometimes, there is a grid resistor located right near the first amplification stage (wired between the grid and the circuit common) having a value of around 100 KOhms. 110 K // 100 K ~ 52 K which is pretty close to the desired value.
    Last edited by Craig Maier; 04-30-2012, 08:32 PM.
    "Who put orange juice in my orange juice?" - - - William Claude Dukenfield

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    • #3
      I'm holding the schematic in my, formally nicotine stained hands, at this moment.
      The Fisher 400C uses ECC83 and 12AT7 tubes according to the schematic, although the one I have may have some 12AX7's instead.

      The schematic shows the phono input going to a 10K and then a 100K resistor to ground. The junction of the 10K and 100 K is connected to the grid of a ECC83. The cathode has various R's and C's for different compensation.

      I have been looking for another 100 K resistor to be in parallel with the one shown, but so far no go.

      The schematic was obtained from the internet and as such could be wrong.

      Marc
      Last edited by Marc Hildebrant; 05-01-2012, 08:05 AM.

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      • #4
        Hi,

        hmmm - well, first off, one can not properly substitute a 12AT7 for a 12AX7 or visa versa and obtain proper circuit performance. The bias values are very different (you can see and compare the family of curves in the RCA tube manual). The 12AT7 was optimized for RF mixer service and is thus quite non-linear and the load lines are quite different from its audio frequency cousin the 12AX7. So, I would look into that aspect of things and see what is going on there and put that right first.

        Next, a technique that I recall from those days was sometimes to provide two paralleled RCA jacks on the phono input(s). Both would have the higher value termination (like for cartridges that needed 100 KOhm loading). When using a 47 kOhm cartridge, the user would be required to wire up another 100 KOhm resistor into a male RCA plug and plug that into the adjacent RCA input jack, creating a parallel combination = to the desired value. Is that perhaps what they are doing there?

        Unfortunately, I do not recall the exact circuit configuration that was used. I do recall that it was a fairly good preamp, but not quite as good as the Marantz model 7 or the McIntosh C series in terms of overall performance and sound quality. I repaired all of those models when I was a kid working at the local high-end audio shop - Stuarts Audio, Westfield, NJ (used to be in Cranford back then).

        Craig
        Last edited by Craig Maier; 04-30-2012, 09:49 PM.
        "Who put orange juice in my orange juice?" - - - William Claude Dukenfield

        Comment


        • #5
          Well....it's a mystery. If I had the owner's manual it would help. As it is, I have just page 3 of a Sam's Photofax schematic.

          With regard to the tubes....Yes, the 12AT7 is much different than the 12AX7. I was surprised to see the 12AT7 used in the audio chain, but they did for this model.

          I wonder when the 47 K load became a standard ?


          Marc

          Comment


          • #6
            Hi,

            They could have used 12AT7 because they are more non-linear and thus produce a stronger "tube warmth" sound. The comparison is very easy to do in the Virtual Valve Amplifier - - - just click between 12AX7 and 12AT7 and you will hear the difference right away. As for 47 K being standard, I have an Eico amplifier that was made in 1958 that has 47 kOhms as the phono input termination.

            Craig
            "Who put orange juice in my orange juice?" - - - William Claude Dukenfield

            Comment


            • #7
              Hear and measure the difference between a 12AX7 and a 12AT7 triode. Here is an interesting experiment to do with Diamond Cut - using the VVA and the Spectrum Analyzer and its Distortion Analyzer:

              Create a 1 kHz sine wave using the Make Wave Generator at -10 dB, 44.1 kHz, 16 bits

              Bring up the spectrum Analyzer and set it up to measure Total Harmonic Distortion (Options Menu - further details are in the users guide)

              Bring up the VVA, bring up the Advanced Controls, and set it for "Sweet" and then Preview using the 12AX7 while noting the %THD. Adjust the Operating Point for the minima which came out to be around 0.216% (depends on where the drive, mix and detail controls are set too, but just choose a fixed setting for those and proceed as it is all relative).

              Stop the preview and then switch over to 12AT7 and adjust the Operating Point for the %THD minima. In my test, the %THD came out to be around 1.0 %

              And, these differences are quite audible too.

              Craig
              Last edited by Craig Maier; 05-01-2012, 10:52 AM.
              "Who put orange juice in my orange juice?" - - - William Claude Dukenfield

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              • #8
                Craig,

                Eico amp ??? Gad....we used to say "Eico means Stinko"

                Marc

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                • #9
                  LOL - I've never heard that one before - - - at least it has the right input termination resistance. Actually, this one (an HF-20) has a Williamson (ultra-linear) P-P output circuit and produces a flat response (as measured on my workbench some time back) from 20 Hz to 50,000 Hz at 20 Watts. Not too shabby. The output tubes are supposed to be 5881, but I switched them over to KT-66s.

                  Craig
                  "Who put orange juice in my orange juice?" - - - William Claude Dukenfield

                  Comment


                  • #10
                    Originally posted by Marc Hildebrant View Post
                    it seems to show a loading resistor of 110 K ohm for the phono input ?

                    I thought that 47 K was the standard value.
                    The value of such resistor doesn't mean the input impedance of the amplifier.
                    It depands on the rest of the circuit.
                    Also consider the difference between resistance and impedance.

                    Comment


                    • #11
                      ric29,

                      You are correct that the rest of the circuit needs to be taken into account. In the case of this circuit, the input circuit is a 10K resistor to a 100K with the other end of the 100k to ground. The junction of the 100k/10k goes to the grid of the 12AX7 tube. With the tube operated in a common cathode arrangement, the impedance of the tube is very high...thus the phono input see's 110 K of resistance. Now, to be accurate, there is some imaginary term to the complex impedance but at audio , it's really small. Thus, the impedance/resistance is pretty much the same and is dominated by the resistors.

                      I could benifit from having an owners manual for the preamp to read what the instructions said about phono loads...

                      I'm still checking out the preamp to see if I have the correct schematic.

                      Marc

                      Comment


                      • #12
                        Hi folks,

                        The input termination Resistance for phono cartridges are not defined in terms of complex impedance. It is strictly the desired resistance termination term that is called out by the manufacturers of the phono cartridges. Furthermore, the input Z of the grid circuit of a High-mu triode is very high - - - greater than 100 MOhms (since the {control grid} is biased negatively with respect to the cathode, that pathway is basically a reverse-biased diode) so 47 kOhms in parallel with that value has no pracitcal effect on the first stages input equivelant circuit.

                        Craig
                        Last edited by Craig Maier; 05-07-2012, 12:14 PM.
                        "Who put orange juice in my orange juice?" - - - William Claude Dukenfield

                        Comment


                        • #13
                          I have found a circuit diagram for a late 1950s phono preamp which has an input termination of 102 KOhms. It says in the manual that this input is designed for GE Variable Reluctance Cartridges, Pickering, Fairchile and Recoton cartridges. The amplifier is an Eico HF 20. So, perhaps around 100 KOhms was more standard back in the early days of high performance audio.

                          Craig
                          "Who put orange juice in my orange juice?" - - - William Claude Dukenfield

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