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Tube Amplifier Circuit3/22/2021
Not only does this reduce the magnitude of the voltage on each wire.A valve heater is simply a length of tungsten wire which gets red hot when you shove current through it, so its not much different from a light bulb.The valves used in guitar amps are indirectly heated (except for some rectifier valves discussed shortly).This means they can be supplied with AC voltage straight from a transformer.
The ECC83 12AX7 has a 9A pinout, which has three connections to the two heaters. Tube Amplifier Circuit Series From AThis means they can either be run in series from a 12.6V supply or in parallel from a 6.3V supply. The ECC81 12AT7, ECC82 12AU7, and 12AY7, also have the same pinout. Many other noval valves such as the ECC88 6DJ8 and 6N2P have a 9AJ pinout which only allows the heaters to be operated in parallel from 6.3V (pin 9 is connected to internal shield). If youre into tube rolling then you can use a double-throw switch to select between one pinout or the other. Most power transformers designed for valve amps have a 6.3V heater winding, and all the heaters are run in parallel from it. The transformer must have enough current capability to manage the total current draw of all the heaters. For example, the ECC83 data sheet quotes 300mA at 6.3V, so three bottles would together amount to 900mA. If the transformer doesnt have enough current rating then you can use a small, separate transformer, just for supplying extra heaters. If that is the case then youre free to use 12.6V (a 12V transformer is close enough) if you want to, or whatever suits the design. Traditionally, an indicator lamp was often run off the heater supply too. However, LEDs are not built to withstand much reverse voltage. This diverts reverse current around the LED and keeps the reverse voltage across it to one diode drop. The magnetic field modulates the electron stream inside the valve, leading to hum. ![]() You might think that the stray capacitance would be too small to couple low-frequency hum, and youd be right. The peaks are usually clipped off by rectifier action, and who knows what industrial motors pollute the supply with hash. These ugly corners and spikes contain high-frequency enegry which couples quite easily through even tiny capacitances. This may be a direct connection or an elevating circuit (see shortly). Leaving the heater supply floating will result in almighty hum due to primary-to-secondary transformer leakage current. AC heater supplies should also be balanced to suppress the EM field.
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