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I have replaced a 12AX7 tube with ECC85 in my guitar amp. Seems to work great and sound great. The part that is questioning me is whether the sound is being at its best because it takes a good 20-30 seconds before the volume reaches a good level. This is probably because original heating gives 6.3v at only 300ma and the tube needs about 400ma. It works but the question... will it sound any different? better? How will it affect the tone of the vacuum tube?

In other words. Having lower voltage than recommenden on heaters - good or bad?

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  • \$\begingroup\$ When you apply 6.3 volts to the tube heater, it will draw whatever current it is designed to use, regardless of the current that the originally specified tube used (unless that makes the total heater current for the whole amp more than the power supply can produce). \$\endgroup\$ – Peter Bennett Feb 13 '16 at 0:14
  • \$\begingroup\$ Keep in mind that even with your original tube the AMP took some time to temperature stabilize. It is the nature of tubes. \$\endgroup\$ – Michael Karas Feb 13 '16 at 0:40
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Both the 12AX7 (ECC83) and ECC85 heaters are designed to run on 6.3V, but the ECC85 heaters draw more current because they have to heat up a larger cathode area. This increase in current draw shouldn't significantly affect the heater voltage in a tube amp, since it is only a small proportion of the total transformer loading.

The Cathodes in the ECC85 have to be larger because this tube has much higher conductance than the 12AX7. It might take a bit longer to heat up due to the increased mass of its cathode assembly. However 20-30 seconds seems a bit too long.

Is it new? Tubes gradually lose emission as they age. Another possibility is that the circuit was not designed for a tube with such high transconductance, so it takes longer to reach a satisfactory bias voltage.

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The problem is also one of the tube's lifetime. Moderate underheating a tube is more damaging than moderate overheating since the electrons leaving the cathode don't reliably reach exit energy and rip out ions more often. The ions take a lot of energy compared to their charge. The metal ends up in arbitrary places. Metal dendrites on anode and gate and metallic shades on the glass eventually cause problems.

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