Sowter doesn't have the inductors but read on...
Passed onto me from somebody who definately knows what they are talking about on Helios' - inc;uding the original Veroboard prototypes....
D'oh! I just typed out a long post, then managed to crash my pc and lost it all!
Anyway - in short, from a phone chat I had this afternoon:
The inductor value is 12H (12 Henrys) and it's the bass control so it's a good idea to get a good one. For such a high value you'll probably need to go with a ferrite cored unit. (The originals were ferrite cored.) The original inductors were hand made and the ones used in the first full production modules were produced by a company that no longer exists. As far as my friend knows, Sowter made some of these for fitting in the modern copies sold by Vintage King in the US (for rather a lot of money!) and they might be able to help you out with them.
Ther might be some more info on this as the question prompted some rummaging in the "junk" cupboard which yielded one of the two original-original prototypes of these units, made on veroboard! My friend describes it as the Mk 0 design that never went into production whereas the Mk1 production unit is the Mk2 design. The two Mk0 design/Mk1 production units that my friend has are what's with Raindirk and there shoufl be commercially available copies available very soon - at a price.
A few further comments to pass on about the circuit and Helios console in general:
What goes before and after the EQ circuit is critical in getting it to sound right.
If you build any of this Helios stuff you need to use germanium transistors and get the right transformers as without these you have no hope of getting it to sound right.
The original console was noisy, had "crosstalk like you wouldn't believe, way up into RF", had little headroom and ran very low levels internally.
The famed sound of the console and EQ came from these problems and the fact that they lead to the circuits distorting very easily. Distortion in the EQ, and most of the other circuits, is "badly asymmetrical" and the settling time from saturation to full recovery after removal of the signal is "about a weekend"! The engineers using the console frequently (almost routinely) drove it into (and kept it in) distortion by keeping the gain high and the fader low to get a warm sound and use the natural compression of the circuit saturation.
OK, I thought I'd left a bit out so I was just checking what I wrote and that fourth paragraph is gibberish! I got my Mk0/Mk1/Mk2 mixed up. I wrote it off the notes I made during the phone call but somewhere it all went wrong. It seemed so lucid at the time! I've called and checked this morning that what I now have makes sense. So here's a new version, now that I'm awake and with coffee ! There's a bit added that I forgot yesterday too.
Quote:
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Me, in my last post above:
Ther might be some more info on this as the question prompted some rummaging in the "junk" cupboard which yielded one of the two original-original prototypes of these units, made on veroboard! My friend describes it as the Mk 0 design that never went into production whereas the Mk1 production unit is the Mk2 design. The two Mk0 design/Mk1 production units that my friend has are what's with Raindirk and there shoufl be commercially available copies available very soon - at a price.
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This makes no sense; it should've read:
There might be some more info coming later on this as the question prompted some rummaging in the "junk" cupboard which yielded one of two original-original prototypes of these units, made on veroboard! My friend describes it as the Mk0 design that sounded best but never went into production; the Mk1 production unit is apparently a small revision of this design. The two production units that my friend has are what's with Raindirk and there should be a commercially available modern version very soon - but at a price." He's also got a couple of the further slightly modified EQs from the console in Olympic Studio 2. These he describes as "technically a bit better; much more stable; almost identical to the first production units but not quite as warm sounding - not enough distortion".