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Old 22nd October 2009, 06:29 PM   #1
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Question Using Toroidal PTs as tube OTs - methods and sources

Hello all --

I am trying to get my arms around using Toroidal dual pri/dual sec type power transformers as OTs for tube amps. Here are the main questions I have:

1. How do you figure out which primary/secondary values you need? How do VA ratings for the xfmr translate to wattage? Are there formuli or decoder rings somewhere?

2. What topologies are best suited for these xfmrs? Any tricks to broaden frequency response?

3. What are some good USA toroid suppliers? Shipping costs for European suppliers are BRUTAL. I am aware of Antek - but they do not offer PTs (>25VA and <300VA) with the specs you most often seen mentioned as usable (115v/115v/6v/6v, 115v115v/18v/18v). Antek offers a full line of PTs with 12v secondaries -- could these be used?? which VA ratings are usable??

I have looked around in several threads but have not found definitive info - I am hoping to pull this info into a single thread so it is useful to the community at large. Thanks for the help!!!!
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Old 22nd October 2009, 06:50 PM   #2
Fenris is offline Fenris  United States
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If I'm wrong, somebody please correct me, but as I see it secondary values are calculated as follows:
Find your target primary impedance from the tube data sheets, let's say 4500 ohms as an example. Now take your output impedance, let's say 8 ohms, and divide. 4500/8 = 562. The impedance ratio is the square of the voltage ratio, so take the square root of your previous result (562) and you get 23.7. If your primary is designed for 230 volts, then you would look for a transformer with 230/23.7 = 9.7v secondary.

Best topology for a toroid is parafeed, which completely eliminates DC. Next best is Push-pull, but the OPT has to be very well balanced to null DC or else it will saturate. Single Ended is out due to the lack of an air gap.

I'm in the same boat, I'm looking at toroids for an OPT. Looking at Antek specs, it seems thier bigger toroids have larger core losses, so bigger might not be better. Also, while toroids may be wide bandwidth, that bandwidth may not be linear, i.e. peaks and dips at some frequencies. I've heard of success stories with toroid OPTs, but I've also been warned off of them by experts like John Broskie. When I finish my o-scope I plan on testing the distortion of some toroids to see how suitable they are.
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Old 22nd October 2009, 07:30 PM   #3
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I have been thinking about building Crowhurst's twin-coupled amp using toroids. This is similar to McIntosh' unity coupled circuit but with TWO push-pull transformers, one in the cathodes, one in the plates. Since the output is split across both, it's a lower ratio and requires much less inductance.
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Old 22nd October 2009, 08:55 PM   #4
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shoog has the bully on toroidal opts and the tricks. George (tubelab) and others are also fans of this approach. My next build is a parafeed SE which I want ot try toroids in. Calculations are the same as for any opt (square of this, ratio of that... you know the drill) but it does pay to Mcdonald the VA rating by a factor of around 50% to avoid saturation effects and ensure bandwidth. So, where you might think a 60W toroid will work fine, opt for the next size up...
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Old 22nd October 2009, 09:41 PM   #5
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Saturation depends on core area, core material, number of turns, and voltage. For most of these power toroids, it will occur at rated voltage and 40 Hz or so regardless of size. So if you want lows, you'll need to have less voltage across it, or a toroid with a greater rated voltage. These are all rated 120 / 240... and a push-pull amp with 300V power supply might put 1000V p-p (350VRMS) across the primary. It will saturate at full power around 60 Hz.

So low voltage tubes, twin-coupled amp, or maybe find a 240:240V transformer and add your own voice coil winding...
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Old 22nd October 2009, 10:34 PM   #6
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I'm somewhat stymied by this use of power transformers, toroids or otherwise, as output transformers. Are some DIYers really this frugal that they resort to this. Perhaps for voice only PA systems, but not for quality home audio. The output transformer is the most important part of a tube amplifier. (Not counting OTLs of course.) This is no place to be "el-cheapo". Why not just save up your shekels and buy the best "real thing" that you can?
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Old 22nd October 2009, 11:07 PM   #7
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the opt is ONE part of the tube amp. It can be the weakest link, but so can the driver circuitry, feedback, power supply etc etc.

There is a looooong history of repurposing components in diy, often successfully. F'rinstance, sweep tubes as outputs, G2 screen drive, even u/l connections in reality are an adaptation of a pre-existing component's design.

THe only abiding question should be does it work?
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Old 22nd October 2009, 11:27 PM   #8
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Quote:
Originally Posted by Tom Bavis View Post
I have been thinking about building Crowhurst's twin-coupled amp using toroids.
This sound way cool to me - where could I find a schematic to look at?

Do both OTs connect to the same speaker???? In parallel ?? series??? In which case each OT needs to be half the impedence? So to drive a 4 ohm output you need two OTLs with 2 ohm secondaries -- is that right??

thx much for the post!!
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Old 23rd October 2009, 01:14 AM   #9
Matt BH is offline Matt BH  United Kingdom
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Toroids work fine. I am using a pair of 34VA clunges in my workshop amp. 6080 push pull using CCS.
You really do need to be very careful about DC imbalance. Use some CCS. Also its not completely correct (it is in part) to use a bigger transformer. The smaller ones have more turns per volt, which equates to more inductance per unit volume (if that makes sense). Also they seem to have less leakage inductance and capacitance. This does make sense, since a larger toroid has way more windings in contact with each other. Also most run of the mill toroids are scatter wound, take a look at some, they really are bad. Crossed over windings are just asking for trouble.
I bought a whole box of these little trannys, I have not tested them as far as frequency response, leakage inductance etc. but I do have the kit to do it. So if any one is interested I could test them out as typical of the breed small toroids. I will have to repair my old HP 334A though (I have a Fluke true RMS meter but it gets sketchy above 15Khz)
Anyway I think the use of mains toroids as output trannys to be a great idea. The thing is to use valves/tubes with a low Anode R. Ie 6080/6532/6C33CB(the supply voltage is then quite low eg 180-200V). Then chose toroids with low secondary voltage like 6V or even 3V to get the required ratio. Or even use a cathode follower output stage(insane problems driving it though).
Nice.
Matt.
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Old 23rd October 2009, 03:24 AM   #10
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re Crowhurst's design - you could start with this thread... Crowhurst's Twin Coupled Amplifier
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