After yesterday’s session at Steve’s and the sheer fabness that were the globe 45s, I got to thinking about whether I could get the STs somewhere near their performance. Could I use the tone of a different driver valve to make up the performance deficit? It turns out that yes, I could. OK, it may seem a quick 24 hour turnaround, but I’ve put a hell of a lot of thought into what I’m about to describe below.
It has to be remembered that I don’t have a lot of money, and simply can’t afford the exotic output transformers that might have made the job of approaching the sound of the globes a lot easier. That’s just the way it is so no point dwelling on it. With expensive OPTs right out, it fell to driver valves to provide the answer.
I whittled it down to two choices: ECC81 or ECC88. ECC88s are nice and linear, but in the past, I’ve found them a bit sterile sounding and besides, my HT is a bit too high for them. I could have used 6n1P, to ameliorate that, but I’m not that keen on them either; a bit dark for my liking. So almost by default it became the ECC81 as driver.
I can hear the groans now, but that’s all I have available. They looked reasonable with a 15K load. So I proposed an LED biased scheme running at 200Va, 2Vk and 7.5mA current. I’ve used one half per channel, which means that there is one valve less on the top deck. I also decided to shorten significantly, the signal paths, by putting the noval socket directly between the two 45s and turning one of the UX4 sockets round so that the filament pins go on the outsides and the plate and grid pins go to the insides. Everything is then able to connect directly using just the component leads, which go point-to-point and are as short as possible.
I’ve used my prized GEC A2900 as the driver valve. Although this valve has very similar electrical characteristics to an ECC81, it is actually an industrial computer type valve like the 5687, but with a gain of 60. It is more linear than the ECC81, is rare as hen’s teeth, goes for £200 NOS and guitarists kill for them as reverb drivers.
The result of the work is a much closer approach to the sound of the globe 45 than I had yesterday. The A2900 is great with the ST45, giving a big soundstage and a clear, crisp and clean sound with just the right amount of warmth and space to allow an ST to achieve 95% of the sound of a globe.
Next job is to find something to use on the front part of the top plate, to cover up all the holes. For now though, I’m well chuffed.
The older you get, the earlier it gets late.