1066/1073 OT questions

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Alex1238

New member
Joined
Jan 15, 2022
Messages
4
Location
Stockholm
I recently got my hands on a decent handful of BC107s, 108s and a few 109s, all C-bin although i haven't gone through and measured the β on any of them, i'd like to build one or two 73 style pres with the 109s and maybe a couple 66s with the 108s, but i've run into a bit of a roadblock regarding transformers, OT's specifically, i'm looking at both Lundahl's lineup as well as OEP at the moment as they're the most readily available to me, but i can't really seem to find a decent match for the elusive LO1166, and i frankly can't find a whole lot of info on it, it's also not particularly well specified how it is applied in the schematics i've been able to find, anyone here have any experience with the things? I've found CJs notes on them that give plenty of info on the specs, but i can't necessarily find a good match, what's common to use in modern clones, i've also stumbled upon the Carnhill VTB1847 but those seem to be almost equally well documented/available, would i have to roll my own, so to speak?
Also what's with the gap? I don't know much about transformers but from the little i know i've read about gapping being used to increase current handling before saturation, does this serve the same purpose or is it something else, and is it critical? I'm a bit on the broke side at the moment so any advice about how i could proceed without spending hundreds on iron would be highly appreciated.
 
Carnhill. Check out the EZ1290 project. The Carnhills are on the cheaper end and sound great.
You need a gapped OT for the Neve output stage because it is class A. The bias current for the transistor flows through the transformer. It a transformer has current flowing in it, you want a gapped wind. You can try a ungapped if you are curious to see why (I believe it wrecks the low end or increases distortion)
 
Thanks for the info, i had totally blanked on the Class A part, that makes a lot more sense now, the Carnhills seem to float around $45 for the OT and $80 or so for the Input transformer, does that seem right? I've been reading up a little and the OEP A262A2E seems to be a suitable input one? As for the output it's still a bit on the pricy end for my liking but I'm starting to see that it is quite a special piece of iron, i might just have to eat it after all. I wouldn't be totally opposed to winding my own if i could source lams and some good literature on the matter, i've managed to wind my own ribbon mic transformers quite successfully but this feels like a whole other playing field
 
I’ve used the carnhill VTB1148 in Don classics 1073 builds. In mine and others opinions are very authentic sounding. You can pick them up at AML in the UK as posted above.
 
You can look at UTM transformers also. No experience with them, but many members here seem to be happy with them. Less expensive than Carnhill, significantly so for the input tx. I believe they’re supposed to use the same winding scheme as the originals.
 
You can look at UTM transformers also. No experience with them, but many members here seem to be happy with them. Less expensive than Carnhill, significantly so for the input tx. I believe they’re supposed to use the same winding scheme as the originals.
These guys look great, never heard of em, will definitely investigate, thanks for the tip!
 
is there anything in particular wrong with them?
If you do try them, make sure they come with shielding or you add a shield. The input transformer will pick up hum if it has no shielding.
 
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