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The maximum dissipation recommended by Mullard for the E元4 is 25W. To work out the dissipation of the valve in watts, one multiplies the plate current by the voltage at the plate minus the voltage at the cathode. To work out the current at the plate, one subtracts the screen current from the cathode: 92-21 = 71mA. So 92mA at the cathode and 21mA on the screens. The screen current is calculated in a similar way - by taking the voltage drop at screens, in this case 20v (the 430v of the power supply minus the 410v actually on the screens) and dividing it by resistance "seen" by each screen (2 x 470R = 940). The current at the cathode is calculated by taking the voltage at the cathode (25v) and dividing it by the value of the cathode resistor (270ohms). To calculate the power dissipation of the E元4s, one first has to work out how much current flows at the cathode and screen. The voltage drop across the cathode resistors is 25v. The 100ohm plate resistor drops the supply voltage down from 430v. The figures *in outline* are:Īround 425v is supplied to the plates and 410v to the screens. In fact, the circuit at idle exceeds the maximum power dissipation recommended by Mullard for both plates AND screens. The schematic - a detail below - reveals the intention to squeeze as much power as possible from the valves. But the power section - four cathode-biased E元4s - was new. The preamp of the AC80/100, for instance, took elements from the top boost circuit of the AC30. When Denney and Underdown came to design the AC80/100 in the late summer of 1963, some of the ground-work had already been laid. Nonetheless a 100uf cathode bypass capacitor was generally the recommended value. The mathematics and physics are incredibly complex.
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The resistor was often bypassed by a capacitor as a means (a) of limiting distortion and (b) enhancing frequency response, though no-one at the time could quite agree on what was happening. Interesting to note though that Westrex later assembled AC80/100 chassis under contract for Vox.įor many hi-fi manufacturers and circuit designers (including Mullard), cathode bias was a tried and tested topology - normally, for higher power amplifiers, two E元4s or KT66s in push-pull, with a resistor at the cathode to govern the amount of power dissipated by each valve. Guitar and hi-fi amps (the latter mostly based around KT88 valves) rarely ran in the UK to anything more than 40-50 watts and the large cabinet amplifiers designed and built by Vortexion and Westrex for use in cinemas and for public address generally had circuits unsuited to anything else. There had been nothing like this amp before. Derek was instrumental in designing and setting up the means of producing dozens of Vox amps and keyboards, the AC100 included. Sad news - Derek Underdown, chief engineer of JMI, passed away on 16th April, aged 92. The chassis currently in the box is a later grey panel AC100. The number is stamped 0220 - the "0" being an occasional prefix on early plates (0185, 0221, and a few others).
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The box of AC80/100 serial number 220 survives: thick-edged, black cloth, corner protectors, white warning plaque. I discovered that most of their circuits were borrowed and sometimes modified to suit." It was at this time that the "works" moved to Erith. Not sure how many we made over a probable 2 years. An investigation showed that the circuit that they "copied" from a Japanese mixer was wrongly drawn. We just copied the sample supplied and I pointed this out to Derek. Subsequently I found that there was a capacitor fitted the wrong way round that caused the problem. We had problems as the channels had to be selected to have the same gain and noise levels.
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I then went full time with them and we made the later mixers in a grey metal case and a proper printed circuit board this time for each channel. I got involved building/repairing the electronics for them. I built these for a company called EvT who had a camera shop in Bexleyheath and they also did film striping and made, under license, a film sound adapter for adding sound to 8mm film. "The parts were "cheapest they could find" with germanium transistors at the time. I think I made about 50 but may have been less as they wanted to test the market." Don't know who made the chassis and brackets. The front panels were made of Traffolyte engraved by Dartford Engraving Company who had a small workshop unit behind a house almost opposite the Jennings music shop. May have been Glocks in Crayford that made the wooden boxes. Jennings supplied all the parts but not the case which they had made by their usual case makers and fitted at their works. I first made these in my front room at home and they were built on small veroboard panels for each channel. "The first mixers I made were like the one in the brochure as picture 2.