EL84 Cathode Fusing method used on Brian May’s handmade Fryer AC30s and on the ‘We Will Rock You’ productions handmade Fryer AC30s:
On Brian’s 3 Fryer AC30s the 2 cathode fuseholders and 2 LEDs are mounted on the top control panel. Each push-pull pair of EL84s has a 200ma slo blo fuse. If an EL84 begins to fail and starts pulling a lot of current, the fuse trips and the amp will still work fine on the remaining push-pull EL84 pair. The remaining EL84 pair will of course not have the correct impedance relationship to the mains transformer’s secondary HT winding and the amp might have some additional distortion and lower volume as a result, but this will probably not be very noticeable in most situations.
Simon Croft who was one of the guitarists in several of the international ‘We Will Rock You’ productions, told me about when an EL84 cathode fuse blew during one of Tokyo 2006 shows. Simon said that he was surprised to look back at the amp head panel and see one of the cathode fuse LEDs lit up, because he had not noticed any change to the AC30 sound or the amp volume.
My amp tech friend Dave Peach suggested this method of cathode fusing in the early 1980s when I was playing my AC30s regularly in semi pro bands around Sydney. Later in the early 1990s when the Matchless amps began to be imported into Australia by Charlie Bevk of ‘Guitar Crazy’ in Sydney, Dave drew up for me the method of individual EL84 cathode fusing. Kevin O’Connor from Canada’s London Power has also written about similar methods of fusing push-pull pairs of EL84s.
In 2003 as my WWRY AC30s began to be used with the Australian Melbourne production, we used trial and error to establish what the best fuse value was – 200ma worked out to be a good compromise.
Schematic and further info coming