I build simple, low budget, stuff so no fancy input transformer. I'll have to try 's idea of using a 10nf polystyrene bypass. I have a SET breadboarded now that uses 26s as inputs and I've got them biased using 3 AAs, which provide -4.55v. But I didn't give up on battery grid bias. So that amp is in line to be rebuilt with other input tubes (6A6s) and I'll be using cathode bias. Worked great, although I needed more drive to the 6A5G power tubes, which are essentially indirectly heated 2A3s. Using batteries simplifies things too since you don't need to build a separate negative bias supply. You do need to use DC blocking caps on the input, but you can use quality film caps which are much better than electrolytics and the values are much smaller than lytics, so it's a pretty good trade-off. I got tired of having to remove and recharge it so I switched to battery grid bias.Ī non-rechargeable battery is used in such a scheme and battery life is essentially equal to the shelf life, supposedly about 10 years (?) for a lithium. I tried battery cathode bias at first but since the amp sometimes wasn't used for long periods of time, the rechargeable battery would run down. I previously tried this in an amp that used 27s for inputs with various outputs over the years. That said, I'm also using battery grid bias. And I haven't used fixed bias with higher gain tubes in a phono stage, so I'm not sure how useful this would be to you. Fixable if you put a trim pot in there and set the current across the plate resistor I suppose.Ĭlick to expand.I'm not sure I can provide any educated reasoning but it seems as though others have already supplied that. If the tubes are wildly variable you may have to roll a few through to get one that biases and sounds right. You'd need to use one that reasonably matches the tube specs that this was designed for. It would be somewhat sensitive to tube selection though. How much it actually improves performance is debatable but functionally there really isn't any reason this won't work well. If its got ESR problems, that will probably be more obvious at high frequencies so you get HF loss. Too small and the impedance at low frequencies is high, causing a loss of LF response. If the cap is crappy you can get odd effects. In a phono amp you probably need the gain though. That can actually be useful to make the stage more linear. An un-bypassed resistor drops the gain by introducing local feedback. The cap puts it at AC ground, which is common when you need the gain. Probably done that way specifically to eliminate the bypass cap on the cathode, not so much the resistor.
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