I know that use of the 5751 tube in place of conventional 12AX7 tubes is a popular swap. In many pieces of Fisher equipment however, its use can really alter the intended operating parameters for whatever stage it is installed in -- and others as well:
1. As is generally understood, the 5751 only produces about 70% of the gain that a 12AX7 does. In stages that depend heavily on NFB to produce the correct response, this reduced "open loop" gain will then change the intended response the NFB is designed to produce. Most notably, use of this tube in the phono preamp positions will notably alter the LF response characteristics of the intended RIAA response curve (reducing bass response). In units with active tone control circuits (400C/CX/CX-2, X-202/B, X-1000, and 202T), use of the 5751 in the tone control position will reduce flat response, and the maximum boost/cut range available from these controls. In units with passive tone control circuits (all stereo receivers and lower tier integrated amplifiers), the greatest effect from using this tube in the tone control/filter position is a reduction in input sensitivity, along with slightly reduced filter effectiveness.
2. The other virtually never addressed concern for using this tube, is that the heater of the 5751 draws nearly 17% more current than the heater of the 12AX7 does. This then has two other impacts on Fisher gear:
A. If all the phono preamp, filter and tone control 12AX7 tubes are replaced with 5751 tubes, then there is now 17% more current being drawn from the DC heater/bias supply for those units using this topology (all FM Stereo MPX receivers, preamps, and top tier integrated amplifiers), with the result being that the heater voltage to the series tube circuits is now reduced by over a volt, and output stage current draw is notably increased. For lower tiered integrated amplifiers that power the heaters of these stages by way of direct output stage current draw, current flow through through the output stage is notably increased as well with the use of 5751 tubes, because of the change in bias voltage they produce. Of course, the increased current draw has a direct impact on the heat produced by the power supply components, which now needs to supply this increased current on a continuous basis.
B. Because Fisher equipment invariably uses heater string configurations to power the heaters of the tubes used in the stages being discussed, use of an odd lot of 5751 tubes can produce different types of unintended consequences. For example, the popular notion of using these tubes in the phono preamp position by themselves not only leaves them starved for heater voltage, but also then potentially over-voltages the heaters of other 12AX7 (or 12AT7) tubes that these 5751 tubes are in series with (in some units), leading to a notably shortened life for those tubes. Depending on the way the heater strings are wired, the use of an odd lot number of these tubes will then alter the sound of either one or both channels, and in unexpected/unanticipated ways.
I hardly wish to dump on the fun of tube rolling. But if anything, this shows how the sonic "goodness" assigned to some tubes in reality is primarily the result of their interaction with the circuit they are installed in, rather than coming from some intrinsic quality they possess over more common types. Besides basic make up and characteristics, there can be differences between various tube types to be sure, such as in consistency of characteristics, tendency (or not) towards microphony, linearity, etc. But in high fidelity equipment, the greatest sonic impact from the use of one tube type over another will primarily be due to the interaction that the tube has with the circuit at hand, as the above example shows. Therefore, when you casually change between tube types in your tube rolling exercises, just be sure the account for the collateral issues that such changes can make.
Dave
