Hello all. This Trace-Elliot "Velocette" baby 15 watt combo,
owned by a gentleman named Jim, was originally brought to me about two years ago via a second-hand music shop in Mansfield,
Nottinghamshire with two faults. Firstly it had a crackle, and secondly, when it wasn't crackling, it still sounded pretty
awful! Nevertheless Jim loved the portability of his amp and wanted to get to the bottom of the
On close examination, the crackle turned out to be one of the two ECC83 tube bases. This was promptly replaced and this eliminated the first problem.
Now, why was it sounding so duff? The test gear confirmed that indeed it should have sounded rough! The wave-form on the scope was showing bad signs of this and yet everything seemed OK in all ways such as voltages etc. I also noticed that on full drive the two screen grids (grid two's) of the pair of EL84 output tubes were glowing immensely. Through experience this tells me that there is something wrong with the plate load. I tried a pair of "good 'ole Mullards" just in case but, although marginally better, the problem still existed. Things were looking and pointing to the output transformer. This amp has an 8 ohm internal speaker (Celestion Vintage Re-issue) and an external 16 ohm jack socket which, if used, turns off the internal 8 ohm speaker. I decided to check the amp again with a 16 ohm load on this external socket and not surprisingly it performed much better!
I now realized that I was right and that this was an output transformer issue......but
what? I spent time now looking into this with the following conclusions:- Firstly, it is clearly marked on the circuit board which colours relate to the 3 different impedance's. 4 - 8 & 16 ohms (though the 4 ohm is not actually wired and "tied back"). After a lot of scrutinizing and testing/checking it turned out that the company that manufactured the output transformer had actually marked the coloured lead-outs the wrong way
round! Please let me explain:-
The start of the secondary winding was coloured as the end, and the end was coloured as the start! This wouldn't matter if one used the whole winding (as in the full 16 ohms) but the 4 ohm taping (though not used) and the 8 ohm tap (certainly used on the internal speaker) would be absolutely wrong. Hence the problem. If you have one of these amplifiers you also may well have this same manufacturing/design problem!
As the output transformer on these is hideously small anyway, I simply decided to design and make a new one of double the size. This at least raises the "saturation point" considerably. Jim (the owner) has since never looked back and tells the shop manager that....."the little Velocette is absolutely wonderful now and I wouldn't change it for the world". It is perfect in both weight/size, portability and especially on sound quality now too! Yet another satisfied customer!
The replaced output transformer.