"This one is worth taking notice of!!
Brought in to my workshop by a local (Nottingham) gentleman of Irish descent, John hit me with the story of how this Music Angel 300b Hi-Fi amplifier was brand new, just delivered from mainland China, and how the 5Z3 rectifier had given him a "firework display" when first switched on! He had rung me to see if I could possibly help him out, as the amp was now completely dead!
On first bringing the amp into the workshop it took little time to realize what the problem was. First, I had to replace the blown power fuse..............then.................
As the amp had arrived from China with all the valves (tubes) separately wrapped (a good and sensible idea), John of course had inserted the valves himself. OK, no problem in doing this but, as all you "Hi-Fi buffs" out there will already be aware, the 300b tube is a four pin device with two of the four pins being larger than the the other two. The point here being that one could not put the tube in "the wrong way around"............well not at least in theory anyway!
As it turns out, due to the pin diameter in the Chinese tubes used, and also the Chinese ceramic tube bases, the tolerances on them both were sufficiently large for it to be possible to actually inserted the tubes into the bases the wrong way"! Not being technically minded, this indeed is what John had accidentally actually done! The way he had happened to insert them was such that the heaters of the pair of 300b's were actually right across the HT plates supply to virtual ground. This in turn is like a short on the main rectifier tube. It is no wonder that it produced the "firework display"!
Anyway, the two 300b tubes were soon re-inserted the correct way around. Unfortunately a replacement was necessary of the now kaput "coke bottle" 5Z3 with a nice Brimar 5U4GY. This is pretty much the same as the 5Z3, though I only had the GY version which is smaller in size, although John didn't mind this as he was more than happy to get the new amp working. At least the 300b's were fine and certainly "looked the part".
Simple job, simple solution and away we go..............perfect.
Thank you for looking, John.
PS. E-mail received from John, the amps owner, a day or so after the diagnostic result and sort out!
Hi John, Sorry about delay in getting back to you. Yes indeed, the amp works fine thank you and sounds very good. Although it's not that powerful, it drives the Tannoy's OK. It was a pleasure to meet you and to watch you solve the problem with the amp. Great service. I would recommend you to all hi-fi fans who have valve amps that need repairing. Many thanks, John