Toroidal Iron

[David Root]

I just received a toroidal PT and OT I ordered from Boris Sasic at Trafomatic in NJ. The factory is in Europe, Belgrade I believe, and rfgordon has posted favorably on an OT he bought from them last year. I liked the idea of being able to custom order a single set! I don't know of any other factory offering that deal. Their pricing is less than half Plitron, for example.

I am going to do a "21st Century" build with them, using all modern components (except TOOOOBS of course!) like IXYS silicon carbide FRED bridge rectifier, Black Gate filter caps, PRP 1W metal films from Westlabs, and so on.

The PT is not potted, and the leads come off around almost half the circumference of the tranny. I had planned on top mounting it on the chassis, but the lead spread will look a bit messy if I do that. It is short enough I could mount it inside the chassis, but then I'm not sure I have enough space for the turretboard I need. The design includes a full TUT 1 FX loop so I need a board with about 40 or so turret pairs, which I can get but is 16" long and precludes mounting the PT inside the chassis. Chassis is 19.25 long OD and is one of Allynmey's gromet 60 chassis.

Any suggestions?

I would also like to get a good discussion going regarding merits and demerits of toroids for MI amp applications.

Also, I selected the OT primary at 5k6 and plan to use it on a PP pair of 6550s or KT77s, my math says I can get about 65W from the 6550s and 55W from the KT77s, but I'm not sure what 5k6 might sound like. Obviously it's much higher than the "standard" 3k4 Marshall/Hiwatt number so I would expect it to sound different.

[Jackie Treehorn]

Sounds like an interesting build. To complete the futuristic look, you could make some sort of bracket and mount the transformers on top, on their side instead of flat, then the wires could enter the chassis down the side. I've only used a torroid transformer in a turntable power supply build, but I know what you mean about how the wires exit. Another artsy fartsy method might be to mount them on columns and run the wires underneath the coils into the column then down into the amp.

I'm kind of working on a similar type modern amp. I used the IXYS bridge that parts connection sells for the main B+; something ridiculous like 32 amps at 1200 volts and solen film caps for filtering. 2 KT88s into 5k2, shooting for AB2 direct coupled, with Heyboer iron.

What made you go for the torroids? What are the advantages?

[David Root]

This chassis is going into a Komet 60 sized head cabinet which is a bit smaller, shorter anyway, than a small box Marshall head, so I don't think there will be headroom for a vertical mount. Not sure how roadworthy that arrangement might be too. Thanx for the suggestion anyway, I hadn't thought of that.

If you read TUT 5 you will find a whole chapter on toroids, Kevin O'Connor is nuts about them, but he uses Plitron, which are made in Canada and I'm sure excellent quality. But expensive! Their website says they have factories in Canada, India & China so you'd think they'd have a "low cost" line or two! Anyway it was that chapter that got me thinking it would be nice to put a pair of toroids in a "thoroughly modern amp".

Toroids are about half the volume and half the weight of the equivalent power and bandwidth conventional EI type.

They radiate about half the EMI of EI types.

After that it seems to get a little fuzzier. Toroidal OTs are said to be a bit fussier about DC balance in PP mode, but that isn't a huge negative in a MI design, whereas it might be in an audiophile application.

With Trafomatic, they have an audio division, they market high end audio amps in kit form, seem to have some very competent audio design engineers, one of whom is based in NJ as a way of entering the US/Canada market for boutique MI and audiophile amps. I saw a post last year in this forum about them, with a good tone review, so I decided to take a shot myself. They have two big points to me, 1) they will allow custom speccing on a single unit! and 2) their prices are very reasonable.
They use M6 steel. I have asked about other types, eg M27, but don't have an answer on that yet. One thing you must do if you order them is insist on 120V AC PT design, otherwise you'll get 115V which is too low these days in N. America where it is at least 120V just about everywhere.

My PT is 4.5" OD, 2.5" high and weighs 4.5 lbs. It's a 200VA unit with 320-0-320 at 250mA, plus a 6.5A 6.3V heater tap and a 70V bias tap.
The OT is potted, rated 50W @ 30-20KHz and is 5" OD, 2.25" high and weighs 5 lbs.

For comparison, a Hammond 272JX PT weighs 10.5 lbs (600V CT, 250mA) and a Hammond 1650KP (Potted) OT weighs 11 lbs, according to TUT 5.

[Jackie Treehorn]

Very cool. I will have to check them out sometime. If you're planning on using one of those massive IXYS bridge rectifiers and assuming you're going for around 450 on the plates, it seems like you could have saved even more weight going with a non center tapped power transformer.

[David Root]

Yes, the same IXYS you mentioned, 32 A at 1200V. Takes care of the rectification spikes, and should sound tubier. Yes I'm going to run 450V on the plates, and 447V on the screens, w/~1k 5W screen resistors. No saving on the CT, it's in the base price, but that's OK.

I know 447V on the screens is high in pentode mode, but it's been done, check out the Komet 60 schematic. Mucho NFB. Besides, both the 6550 and KT77 are tuff-tuff-tuff! as Mick Jagger said once (in a different context).