Uploaded June 2017 | Updated September 2026, 18 hours ago
It’s easy for an amateur engineer to elaborate on scenarios due to lack of understanding, with ‘overdrive’ in amplifiers. One person with assumed electronics authority explained to the Hoofbag that The Beatles used to set their Vox AC30s to the 110 volt setting, running them off Europe’s 230 volt mains supply, to get more power.
In this event, initially the electrolytic capacitors would burst, spilling their contents within the amp, apart from the power transformers going up in smoke. Sorry, no increase in output power, just a destroyed amp! Overdrive, in guitar amplification, means pushing into the instrument socket a higher than usual signal voltage, and harmlessly forcing the amp into maximum output power where a signal waveform become clipped. In all the years that The Hoofbag has been doing valve amps, HB can categorically state that the only part of a valve that should glow red is the cathode, via its heater. A glowing anode: the grey or silver part of a valve/tube that is seen through the glass tube, infers trouble: there’s too much current flowing through the device. Soon, the device will stop working and be destroyed. An EL84 pentode has five electrodes: cathode, control grid, screen grid suppressor grid and finally anode. A glowing screen grid is sure to be a sign of faulty design or a reduction in the resistor feeding it. In the Vox AC4TV, the screen’s HT + resistor is 470 ohm where it should be in the region of 5k6 ohms. Consequently, the screen grid is dissipating about 4 times the amount of electrical NRG than it is designed to, so it’s glowing orange. Increasing the grid dissipation wouldn’t increase the EL84’s output, proven by past experimentation. Maybe Vox wants to sell more EL84s? If there’s anyone who can enlighten Hoofbags why Vox should want to do this, please leave a comment. Here's a link to the schematic for further information:
website.lineone.net/~m0dzo/AC4TVschem.pdf
It’s easy for an amateur engineer to elaborate on scenarios due to lack of understanding, with ‘overdrive’ in amplifiers. One person with assumed electronics authority explained to the Hoofbag that The Beatles used to set their Vox AC30s to the 110 volt setting, running them off Europe’s 230 volt mains supply, to get more power.
In this event, initially the electrolytic capacitors would burst, spilling their contents within the amp, apart from the power transformers going up in smoke. Sorry, no increase in output power, just a destroyed amp! Overdrive, in guitar amplification, means pushing into the instrument socket a higher than usual signal voltage, and harmlessly forcing the amp into maximum output power where a signal waveform become clipped. In all the years that The Hoofbag has been doing valve amps, HB can categorically state that the only part of a valve that should glow red is the cathode, via its heater. A glowing anode: the grey or silver part of a valve/tube that is seen through the glass tube, infers trouble: there’s too much current flowing through the device. Soon, the device will stop working and be destroyed. An EL84 pentode has five electrodes: cathode, control grid, screen grid suppressor grid and finally anode. A glowing screen grid is sure to be a sign of faulty design or a reduction in the resistor feeding it. In the Vox AC4TV, the screen’s HT + resistor is 470 ohm where it should be in the region of 5k6 ohms. Consequently, the screen grid is dissipating about 4 times the amount of electrical NRG than it is designed to, so it’s glowing orange. Increasing the grid dissipation wouldn’t increase the EL84’s output, proven by past experimentation. Maybe Vox wants to sell more EL84s? If there’s anyone who can enlighten Hoofbags why Vox should want to do this, please leave a comment. Here's a link to the schematic for further information:
website.lineone.net/~m0dzo/AC4TVschem.pdf
