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Inexpensive Power Transformers

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Posted on June 10, 2017 at 07:41:50
Triode_Kingdom
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Location: Central Texas
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 Hammond makes a number of transformers with 115/230V primaries and a secondary that's either 115V or 115/230V. Some of these look just right for preamps. One that I'm considering for an upcoming project is the Hammond 186D120. This has a 115/230V primary and a 115V secondary rated for 250 mA. Used in reverse and applied to my line voltage of roughly 120V, it would output about 240V. With a full wave bridge, that translates to a B+ of 315-320V at more than 100 mA.

This transformer is less than $16 from companies like Mouser, even in single quantity. My only concern is whether the lack of a shield might be a problem, especially in a preamp (I want to include a phono stage). Does anyone have any experience using these for this purpose?



 

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RE: Inexpensive Power Transformers, posted on June 10, 2017 at 08:26:16
DAK
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 I guess the phono stage would be susceptible to emi/rfi from an open frame trans. You could buy a small enclosed box and have the PT in it connected by an umbilical.

 

RE: Inexpensive Power Transformers, posted on June 10, 2017 at 10:00:32
Triode_Kingdom
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 The idea of an umbilical and HV connectors has never appealed to me. Maybe I should be looking for a toroidal transformer instead. They generate a much weaker EM field. Mouser has a Triad VPT230-110 with similar specs for about $18. Or there's a Hammond 1182D240 for $44. The preamp I have in mind will enclose everything, including the tubes, so it doesn't matter which type transformer I use. Yep, think I'll go with a toroid for this project.


 

RE: Inexpensive Power Transformers, posted on June 10, 2017 at 10:08:02
DAK
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 I am not going to try to convince you about doing a seperate PS, but you can have the umbilical "hard wired" instead of using connectors. I have a custom preamp made that way and it really is not any hassle.

 

RE: Inexpensive Power Transformers, posted on June 10, 2017 at 10:22:02
Triode_Kingdom
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Location: Central Texas
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 Do you have a photo of your preamp and power supply that also shows the cable? I'd be interested to see what the system looks like externally.

 

" Maybe I should be looking for a toroidal transformer", posted on June 10, 2017 at 11:33:15
1973shovel
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Posts: 10117
Location: Greenville SC
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 I can't be of any direct help to you, as I've never built a preamp using a toroid. But I remember finding the John Curl post linked below, with regard to toroidal use in preamps, and never forgotten it.

I'd be curious if you feel it's a valid observation.

 

RE: Inexpensive Power Transformers, posted on June 10, 2017 at 11:56:25
DAK
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Posts: 2712
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 I have it in storage, so, it will take awhile before i can set it up. IIRC, the power supply wire was 20awg kimber, that was just in heat shrink tubing. It was a big preamp a MFA Luminescense clone with 8 octal tubes. The filament was DC so there was also circuitry for that. So, there was only DC in the umbilical cords. It could be the filament circuit was 18awg. not sure as i am going by memory and i havent used that preamp for over 10 years.

 

RE: Inexpensive Power Transformers, posted on June 10, 2017 at 14:44:25
Steve O
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Posts: 12364
Location: SE MI
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 Not a direct answer to your question but.....IME, 115 VAC primary Hammonds run HOT, both filament and PTs, even unloaded. YMMV

 

RE: Inexpensive Power Transformers, posted on June 10, 2017 at 15:19:33
vinnie2
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Posts: 4481
Location: North Carolina
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 I first tried using a separate chassis for the power supply several years ago and was so happy with the results I do it with almost all of them now. It is fairly easy to build your own umbilicals and it makes moving or working on the two chassis much easier.

 

RE: Inexpensive Power Transformers, posted on June 10, 2017 at 15:26:00
Triode_Kingdom
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Location: Central Texas
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 That's OK, no need to go to the trouble. I think I get the idea. I'm starting to work on the block diagram now, then I'll decide which topology and tubes to use for each of the subcircuits. After that, I'll assemble a list of component dimensions (caps, transformers, etc), and finally design the chassis. So, it will all take some time, and maybe a few delays here and there if I decide prototyping is needed. There's nothing worse than a one-off build that has a layout problem due to insufficient testing. :)

 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 10, 2017 at 15:48:53
JKT
Audiophile

Posts: 612
Location: Midwest
Joined: November 26, 2002
 Lundahl produces quality C core power transformers. Sadly you won't be able to buy any for $15.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: Inexpensive Power Transformers, posted on June 10, 2017 at 16:30:02
HoosierAudio
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Posts: 377
Location: MidWest, USA
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 You may want to look at these.

HA

 

Virtual Umbilical..., posted on June 11, 2017 at 06:38:14
caffeinator
Audiophile

Posts: 1729
Location: Pacific Northwest
Joined: August 22, 2003
 Hi Triode Kingdom,

I get what you're saying about not being crazy about the idea of an umbilical-type arrangement. A lot of the benefits of that arrangement are the distance of the power transformer from the rest of the preamp, as well as the fact it is in a separate metal enclosure.

You can get just about all of those benefits by using a larger chassis and placing the power trans/supply and preamp circuitry at the other end of the box. The power trans can also be in its own box inside the chassis for shielding. Shield the wires from the power supply, route close to the chassis plate, etc., to minimize radiated fields.

 

Yes, that's probably what I'll do, posted on June 11, 2017 at 08:54:29
Triode_Kingdom
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Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 I've been thinking about the same thing. The chassis for this will be constructed from an aluminum extrusion with a 3" X 8" cross-section. I bought a 4' length, so I can cut it to practically any size. If it's about 14" deep, that would allow positioning the PT and power supply circuitry a reasonable distance from the phono section. I'm planning to shield the phono tubes (all 9-pin), and it might also be possible to add a small mu-metal shield around the PT. I think this will work well. :)



 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 13, 2017 at 04:51:32
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 I will only agree with what John Curl says if it is amended along these lines:

curl: What we forgot was that the windings of a toroid transformer are wound on top of each other, rather than in separate bobbins.

to make it factual: What we forgot was that the windings of a toroid transformer are typically wound on top of each other, rather than in separate bobbins.

I guess my point is that the problem curl mentions is not a function of the toroid but a function of how nearly all of them are wound. There is absolutely no reason why someone cannot wind a toroid with the primary as a semicircle on one half of the core and the secondary on the other half effectively giving the results of a dual bobbin device and I really have a problem with people lumping design choices choices and factual data into the same pile. It is not the fault of the toroid that it is wide bandwidth, it is the fault of the designer of the transformer. Next thing you know we will all be accepting the universal fact that tubes have a mushy vintage sound.

While I am at it, there is another gross overgeneralization used in magnetics wrt C-cores vs. EI cores and that is the following often repeated fact about the superiority of C-cores

The grain direction stays parallel to the winding length for the entire magnetic circuit for C-cores and stamped laminations always have some portion of the magnetic path that runs perpendicular to the grain.

The problem here is what happens when you are using a non-oriented material? As many of you know, I love nickel as a core material and it just so happens that the materials I use are all non-oriented and lets just say that being lectured by customers on the "universal superiority" of c-cores gets tiresome. To close things out I will give the one universal benefit of the tape wound cores is the ability to use much thinner lamination thickness than with stamped lams and the one benefit of stamped lams over C-cores is the possibility of a much smaller airgap.


 

RE: Inexpensive Power Transformers, posted on June 13, 2017 at 05:46:44
GeorgePope
Audiophile

Posts: 102
Location: Havant , UK
Joined: March 15, 2017
 From my experience , do not bother cutting costs or using Hammonds . Hammond power transformers are nothing special and appear to be marginal with a 50Hz line frequancy . Here in Fareham , UK the mains supply voltage leans more towards 250V rather than 240V . Hammonds with 230V primaries over-run heaters and get warm very quickly .

Toroidals are usually ok and run cooler but I would recommend getting some decent transformers wound to spec with M6 cores and an electrostatic screen from a reputable winder .

George

 

RE: Yes, that's probably what I'll do, posted on June 13, 2017 at 07:04:27
caffeinator
Audiophile

Posts: 1729
Location: Pacific Northwest
Joined: August 22, 2003
 Hi Triode Kingdom,

I've had a project on the back burner forever with similar design parameters...a pre-amp I plan to put in a rack-mount case, which is almost 19" wide, I think. Obviously plenty of room to separate the noise-makers from the noise-takers.

Btw, I recently took a class about EMI/RF interference - it was great and went a long way to explaining a lot of typical layout and wiring practices. One fundamental tenet was to, whenever possible, shield or otherwise mitigate noise sources, as opposed to noise 'victims.' I suppose one could always do both, especially if weight and cost aren't huge factors (this was an aerospace-centric class; weight is obviously critical), but often the biggest overall dB/$ payoff will be had when attacking sources.

Distance is a key strategy, since, depending on the type of noise, typical diminution is on the order of 1/(separation distance) or even 1/(separation distance)^2...you get the idea. Shielding can also yield significant benefits, as can good wiring practices.

 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 13, 2017 at 19:14:02
Triode_Kingdom
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 " There is absolutely no reason why someone cannot wind a toroid with the primary as a semicircle on one half of the core and the secondary on the other half "

Dave, have you ever wound a transformer like that? My experiments with this technique at RF frequencies indicate that coupling degrades and core losses increase significantly. Perhaps things are different at the mains frequency, or maybe I did something wrong. I tried it only on one occasion using various cores and several hundred watts of RF. I quickly abandoned the idea in favor of isolating the primary and secondary using an insulating layer of fiberglass tape between them.







 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 14, 2017 at 04:30:02
mah
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Posts: 87
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 If you reverse connect a transformer you are running it outside of its design parameters. Give it a try with cheap transformers but bear in mind that winding current and voltage ratings may be compromised as may B-H characteristics.

 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 14, 2017 at 04:31:13
dave slagle
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Posts: 5430
Location: NYC
Joined: April 27, 2001
 yes I have. wound in that manner and as you say the leakage inductance goes up which means regulation goes down. I was actually rather surprised on how much of a difference the pri-sec proximity effected both the output voltage and regulation at line voltages and I was using *gasp* Stamped M6 Laminations so I did have the cross grain issue which may have made things a bit worse. Of course this is a function of all transformers so anything you do to reduce winding to winding capacitance increases leakage so you just have to pick your poison so to speak.

Along the line of the toroids... Antek has a 50VA unit that will fit your needs for $28 and the single secondary winding will be fine for your bridge. I dropped a note to John @ antek about doing a 50VA version with dual secondaries for FWCT use to match his 100VA and 200VA units which are just too big for linestage / phono use.

dave

 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 14, 2017 at 06:24:33
cpotl
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Posts: 1002
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 "If you reverse connect a transformer you are running it outside of its design parameters."

Hmm... I'm not sure why you say that as a blanket statement. Of course with any transformer, whether reverse connected or connected in the standard way, it is always possible to run it outside of its design parameters. But if the transformer is reverse connected and the normal secondary (now being run as the primary) is not asked to pass any more current than it would when running as the secondary in the usual configuration, is there any reason why it would be "outside the design parameters"? (And assuming the voltages on the windings are essentially within the design voltages.)

Would the transformer even "know" that it was running in reverse-connected mode? All it knows is that a primary current within its design parameters is flowing, and a secondary current within its design parameters is flowing. Why would the transformer care?

Again, I am assuming both primary and secondary currents and voltages are kept adequately within design parameters. I think this was the case in TK's original example. (120V on a nominal 115V winding is within acceptable tolerances, I think.)

Chris

 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 14, 2017 at 08:38:35
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 For power transformers I agree since we are just talking voltage and current however if we expand this to audio transformers then we also have to take into account bandwidth and frequency response doesn't always work as expected when you change connections.

dave


 

RE: Inexpensive Power Transformers, posted on June 15, 2017 at 14:55:17
Audio Pete
Manufacturer

Posts: 152
Location: Long Island,NY
Joined: May 22, 2015
 Just did the same thing myself. I bought an industrial step up transformer that can supply 500ma @240v. Adds up to 340v. Not much voltage but plenty of current. Plus, it's mounted (actually welded) inside a heavy, steel box. Then, for the heater supply, I am going to use a $20 SMPS of 12v,5A. The whole thing will become a 25w "rat rod" amp.

 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 16, 2017 at 10:33:37
Triode_Kingdom
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Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 "Antek has a 50VA unit that will fit your needs for $28"

Antek has several products that would work for this, including the 160V AS-05T160. I just wish these weren't made in China. Even Hammond is doing that now for some of their line. I'm having to source more and more NOS to be sure of getting Western-made components.



 

RE: " Maybe I should be looking for a toroidal transformer", posted on June 18, 2017 at 19:31:29
Tom Bavis
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Posts: 961
Location: Upstate NY
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 I have a few of those Hammonds (single primary version) and the secondary has a higher resistance than the primary, about 10%. I would guess that they're designed with about 10% step-up (about 132V unloaded) to give 120V output at the rated 0.25A load. In reverse, the voltage would be 10% low unloaded, dropping another 10% at full load. These could be used in the usual direction with a voltage doubler - that's how I expect to use them.

I used a 1:1 toroid to power a small amp (w/ voltage doubler), and wound an additional 6V secondary on it for heaters.

 

Thanks Tom, good info! (nt), posted on June 19, 2017 at 23:03:31
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 nada aqui

 

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