Help - Toroidal power transformer

[jzucker]

Let me put this another way...

I'm building a 50 watt amp into a 16"x8"x2" chassis. The requirements consist of a SS rectifier, a pair of EL34 tubes, 2 ax7s, 2 at7s, a BF Bassman(ish) output transformer, a power tranny with the following specs: 750VCT (350-0-350) @ 175 ma, 6.3V @ 6A, a choke, and a reverb transformer. The preamp will be based on an AB736 fender reverb (single channel) with no vibrato.

Since space is such an issue, I was considering a toroidal power transformer. I have searched google and was unable to come up with something like what I'm looking for. Are these available and if so, how much and what are the issues?!?

Thanks in advance,

Jaz

 

[John Phillips]

I don't think it will help you - toroids generally have at least as large a footprint as conventional transformers of the same spec I think, although they are less tall usually.

Laney use them, FWIW - not a recommendation IMO, although I can't think of any actual disadvantages they may have...

Space shouldn't be a real issue with that chassis. It's not a lot smaller than a Mark series Boogie, and they managed to get a full 100W amp in those, with two big transformers (mounted at opposite ends of the chassis).

 

[trisonic]

This will shock unquiet/John; I own (but it's in England) one of the first Sessionettes (a 2x10) from the early eighties (don't ask...) that has a toroidal transformer and it is so big Stewart had to mount it on the bottom of the (combo) cabinet.

This amp had one decent sound - somewhat surprisingly it did an excellant Clapton/Beano of the "Have You Heard" variety.

Pete.

 

[John Phillips]

Not so surprising when the Beano sound was done with a Rangemaster Treble Booster (transistor) as well as the Marshall Bluesbreaker.

I'm actually more surprised that it still works. I can't remember how many of those I've fixed - but a lot, anyway.

 

[jzucker]

Thanks John,

I'll bag that idea. Looks like they're too expensive without enough space savings. I'm trying to get 470V on the power tube plates so I have to have a pretty heft PT and then an OT to match so with the 16"x8" chassis, it doesn't leave a lot of space. Boogie uses huge transformers. I should look and see how they do it. Are their trannies on opposite sides of the chassis? Do they use a choke too? I'm just worried about inducing hum into the circuit with that small a footprint but I'd like to stay with the 16" chassis is possible.

 

[John Phillips]

Yes, their transformers are at opposite ends. The PT is behind the power switch, the OT behind the input jacks, leaving space for the power tubes along the back and the speaker magnet in the middle below the preamp tubes! They do use a choke which I think is next to the PT behind the graphic, roughly.

You can eliminate hum even with them all so close together - just make sure each has its windings at right angles to the other two - yes, it can be done with three!

BTW, you have given me an evil idea. I always loved the sound of my AA864 Bassman's 'normal' channel, and I think you could get all the bits into a BF/SF Champ, along with a Celestion Vintage 8 (or even a V10). It would look and operate totally stock from the front too...

 

[jzucker]

Originally posted by unquiet
Yes, their transformers are at opposite ends. The PT is behind the power switch, the OT behind the input jacks, leaving space for the power tubes along the back and the speaker magnet in the middle below the preamp tubes! They do use a choke which I think is next to the PT behind the graphic, roughly.

You can eliminate hum even with them all so close together - just make sure each has its windings at right angles to the other two - yes, it can be done with three!

BTW, you have given me an evil idea. I always loved the sound of my AA864 Bassman's 'normal' channel, and I think you could get all the bits into a BF/SF Champ, along with a Celestion Vintage 8 (or even a V10). It would look and operate totally stock from the front too...

That's kind of my idea though I want to do it in an empty, 16" wide chassis and a 12" speaker!

I want tube reverb too so I need a total of 4 preamp tubes for the AB763 preamp (single channle) with reverb.

Wish I knew how to couple a transistor based reverb driver but I'll do that in a subsequent project. I'm still learning as I go and obviously have a ways to go!

Thanks for your advice. I may take a look at the MK III layout. I'd also like to put a small CPU type fan on the chassis to move some air since it's going to be a combo amp...

 

[Mark Kane]

Alessandro uses 12X8X2" chassis' and his Bloodhound and Redbone run over 480vdc. He mounts the trannies caty-corner and the only long lead you need to worry about is the center tap on the OT.

 

[jzucker]

Originally posted by Mark Kane
Alessandro uses 12X8X2" chassis' and his Bloodhound and Redbone run over 480vdc. He mounts the trannies caty-corner and the only long lead you need to worry about is the center tap on the OT.

So they are on extreme diagonal ends of the chassis?

 

[Mark Kane]

Yes!

 

[jzucker]

Originally posted by Mark Kane
Yes!

Do you know of a picture of the bottom of his chassis? I'd be interested to see his layout.

 

[Mark Kane]

I just emailed you a pic. Let me know if you need any help figuring it out.

 

[jzucker]

Originally posted by Mark Kane
I just emailed you a pic. Let me know if you need any help figuring it out.

Thanks. I think I got it figured out. It looks like 3 gain stages and then the phase invertor unless there's another tube under the perfboard that I can't see. Interesting way to lay the chassis out. I wish my XX amp was laid out like that. It'd probably be quieter...

Jaz

 

[JamesPeters]

Do the "headphone positioning trick". It'll help ensure you get no hum induced from the PT to the OT.

You hook up the primary of the PT to the wall (with nothing on the secondary connected--insulate all the ends of the secondary). Then with the OT, hook up headphones to the secondary (leaving the primary disconnected/insulated). Wear the headphones, and move the trannies around on the chassis until you hear no hum.

Generally, placing the OT and PT with the cores at 90 degrees is a good idea. However, sometimes you'll find a certain amount of space between them improves things a lot, and aligning them both in the center of the chassis helps too. And, that the placement and orientation of the choke can also "carry" the hum from the PT to the OT. You don't have to hook the choke up to anything for this test--just position it on the chassis and listen for the change in hum in the OT.

Works like a charm.

 

[jzucker]

Thanks James. There's no choke on this amp so that's not an issue. I'll try your suggestion. Great tip!

 

[JamesPeters]

One of the many useful nuggets of info I found on AX84, that was. Good luck!

 

[jzucker]

Thanks. I'm bagging the torroidal idea by the way.

 

[nomadh]

I just ran across this and torodial transformers have interested me from way back. I was dealing with transformers as computer surge suppressors. I had been using standard transformers that were 30+ pounds, ran hot and even made a constant buzzing noise. They were pretty expensive and irritated the customers and were obviously wasting alot of the power as heat. One day I ran across a company selling an isolation "torodial core" transformer. It was much smaller, quieter, lighter and cooler. I assumed it wasn't as good. So the salesman had me hook up my scope plugged light with a dimmer switch into the wall and I was stunned. That dimmer was generating 500-700 volt spikes in the line. My old transformer was passable limiting the spike to <70 volts but the torodial was holding the spike to <7v. I truly have no specific knowledge about any advantage a torodial core may have in audio applications but it seems most of the advantages I saw should carry over. It seems to me the waveform should be cleaner, the heat and size less, and cheaper to mfg. I just thought I'd throw this down and see if it sparks any interest in continuing or info that its a dead end.

 

[nomadh]

From wikipedia:

In electronics, a toroid is a circular magnetic core made of iron powder or ferrite, around which wire is coiled to make an inductor. Toroidal coils are used in a broad range of applications, such as high frequency coils and transformers. Toroidal inductors can have higher Q factors, higher inductance, and carry more current than similarly constructed solenoid coils. This is due largely to the smaller number of turns required in coils with cores of material of high magnetic permeability, which reduces resistance. The magnetic flux in a toroid is confined to the core, preventing its energy from being absorbed by nearby objects, making toroidal cores essentially self-shielding.
In the geometry of torus-shaped magnetic fields, the poloidal flux direction threads the "donut hole" in the center of the torus, while the toroidal flux direction is parallel the core of the torus.

 

[trisonic]

Well, apart from my Sessionette (still going John - though Eric did not use a Rangemaster but I digress) I now have a second amp (this time downstairs) a Maven Peal Ganesha that has a toroidal Tranny.
It's big but then the amp produces a big sound too, I love it!

I know nothing of magnetic fluxes and the like but Dave Zimmermann does.

Best, Pete.