Low cost transformer for valve projects. - UK Vintage Radio Repair and Restoration Discussion Forum

Top Cap · 24th Jul 2019, 9:57 pm

Found this, might be useful for that preamp or phono amp project.
Split Primary for 115V or 230V and a high voltage secondary of 220V at 68mA. Also 14V at 1A for feeding a 12V regulator. I have ordered a pair for trial with my Lesbox phono amp.

https://www.ebay.co.uk/i/27392585750...fqY8oDt5NaiHww

Mike. Watterson · 25th Jul 2019, 1:18 pm

Or you can use a Shaver transformer for up to about 15W to 20W HT. Suit 2 rectifier 125V DC or four rectifier 250V DC fullwave.
LT via a separate transformer.
A bit cheaper, also no postage (local electrical DIY or wholesale).

ionburn · 25th Jul 2019, 2:16 pm

It will be interesting to hear how they go as the appearance is (to me) somewhat unusual for power transformers, and the site gives no indication of efficiency. One thing that oftens catches me out with small transformers is the drop under load.

Mike, I have had a quick look but cannot see which you are meaning. The ones I came across were somewhat more expensive than the one in the above link although around 20W.

GrimJosef · 25th Jul 2019, 2:53 pm

Quote:

Originally Posted by ionburn

... One thing that oftens catches me out with small transformers is the drop under load ...

The thing that has caught me out occasionally is that mains transformers not specified for audio use can hum/buzz annoyingly, especially when you've gone to some trouble to get the hum through the speakers down below -80dB.

Cheers,

GJ

daviddeakin · 25th Jul 2019, 3:08 pm

Quote:

Originally Posted by Mike. Watterson

Or you can use a Shaver transformer.

I have often wondered why you can't find shaver transformers for sale by themselves (i.e. not as part of an actual bathroom fitting) from the usual transformer manufacturers. It's like they're reserved for the bathroom fitting market and no one else is allowed to buy them!

GrimJosef · 25th Jul 2019, 3:18 pm

I suspect the people who make the bathroom units bulk buy the transformers in custom lots. They don't use off-the-shelf components. So few people build their own shaver supplies that it wouldn't be worth anyone's while making the specialised transformers, and certifying them for compliance, on the off chance a passer-by might want one.

Cheers,

GJ

MrBungle · 25th Jul 2019, 3:44 pm

Just in case anyone hasn't seen them, there are some reasonably priced transformers available here: http://primarywindings.com/product/s...nsformer-copy/

Made in UK and decent quality. No "dubious" stuff.

Mike. Watterson · 25th Jul 2019, 7:36 pm

Quote:

Originally Posted by daviddeakin

Quote:

Originally Posted by Mike. Watterson

Or you can use a Shaver transformer.

I have often wondered why you can't find shaver transformers for sale by themselves (i.e. not as part of an actual bathroom fitting) from the usual transformer manufacturers. It's like they're reserved for the bathroom fitting market and no one else is allowed to buy them!

You don't have to keep the plate. I've paid as little as €12 INC VAT @ 23% and no shipping.

Some are inherently current limiting due to lamination design.

Top Cap · 26th Jul 2019, 5:40 pm

Quote:

Originally Posted by Mike. Watterson

Or you can use a Shaver transformer for up to about 15W to 20W HT. Suit 2 rectifier 125V DC or four rectifier 250V DC fullwave.
LT via a separate transformer.
A bit cheaper, also no postage (local electrical DIY or wholesale).

I currently use cheap back to back 15v transformers which work very well but take up a fair bit of space. Are shaver transformers full isolated or small auto transformers? Also are they made to fit into a plastic moulding and not designed for say chassis mounting?

Originally Posted by ionburn

"One thing that often catches me out with small transformers is the drop under load ...

For a phono amplifier the load is quite low and fairly constant.

I have two on order so when they arrive I may be able to give you more info about them. Incidentally, a quote for manufacture in the UK was over £80 each

G6Tanuki · 26th Jul 2019, 7:07 pm

I've played-around using "shaver transformers" - specifically as a 110V HT-supply for a "Larkspur" R209 receiver and 120V for a similar-era spy-transceiver that usually ran from a pair of primary-batteries.

They're generally not designed for 'continuous duty' at their supposed-rated power - meaning poor regulation [not-good when you're not crystal-controlled] and getting-hot-when-run-on-continuous-load.

Mike. Watterson · 27th Jul 2019, 11:33 am

If claimed to be 20W, that's a limiter spec. They might only be good for 10W to 15W.
A clue is the size.
Barely enough for HT only of a mains set, unless no RF amp and modest O/P. No good to power USA Mains sets. Some USA Battery/Mains sets take too much power (5 valve or more 50mA series filaments and dropper). No use at all for my battery / 115VAC Hallicrafter S72 (100mA filament current approx), too much voltage drop, so BFO won't run.

Mike. Watterson · 27th Jul 2019, 11:41 am

Quote:

Originally Posted by Top Cap

Are shaver transformers full isolated or small auto transformers? Also are they made to fit into a plastic moulding and not designed for say chassis mounting

They have to pass 2KV or 4KV isolation, I forget which. They are for naked people in wet bathrooms!

The secondary is isolated and centre tapped. Some have an additional winding for an LED. The shutter then operates a switch connecting the primary. I connect a 2nF 2KV XY rated ceramic capacitor from the secondary centre tap to mains earth, which dramatically reduces mains borne noise on Battery/Mains valve radios. I have a couple mounted in surface cooker switch boxes with a hole drilled in the panel near the edge on the 230V side to allow a 3 pin 5A plug. I modify the 230V socket so the USA 2 blade plug can't fit. The 115 outlet is always wired between CT and one of the 230V outlet connections.

All the models I've taken apart used screws/bolts to mount to the panel. Some have a series thermistor in series with the primary against the laminations. That can be removed. None of the ones I've got had an embedded thermistor like on some tiny transformers in clock radios or modern portable mains/battery radios.

Most have a current limiting lamination design so no fuse is needed. A user replaceable fuse would be a shock hazard in a bathroom / washroom.

julie_m · 27th Jul 2019, 12:41 pm

Shaver transformers are wound on a split bobbin to provide full isolation. That way, you cannot get a shock from coming into contact with just one side of the output, since the transformer winding is not connected to Earth except through the tiny parasitic capacitance between the windings -- an important consideration given that in a bathroom, your impedance to Earth is likely to be much lower if you are in contact with Earthed pipework directly or via water .....

However, these transformers do run warm, and really are only designed for use for as long as it takes to shave. (The placement next to a washbasin helps ensure that even in a busy communal environment such as a leisure centre changing room or a campsite facility block, there will be a cooling-down period while users complete the rest of their ablutions.) The one I used once for a temporary HT supply during some development work has a self-resetting thermal cutout. Cheaper, modern ones might have just a single-shot thermal fuse.

McMurdo · 27th Jul 2019, 3:56 pm

The ebay transformers look like R-Core transformers, I first came across them in indian-made equipment but I've seen them in old american computer printers from the 1970's so they aren't all that new. Maybe a transformer expert will elaborate!

turretslug · 27th Jul 2019, 5:55 pm

I'm intrigued, too- is the R for ribbon or round (finished profile) or somesuch? They remind me of C-core types, which I believe have a long strip of steel wound into a shaped doughnut, then sawn into two C-shapes to fit the winding bobbins. If the C-core represents a stepping-stone towards the full toroidal type, then R-core looks like another step on from that. There was a lot of noise (of the marketing variety) about R-core transformers in the hi-fi world at one point.

24th Jul 2019, 9:57 pm	#1
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Low cost transformer for valve projects. Found this, might be useful for that preamp or phono amp project. Split Primary for 115V or 230V and a high voltage secondary of 220V at 68mA. Also 14V at 1A for feeding a 12V regulator. I have ordered a pair for trial with my Lesbox phono amp. https://www.ebay.co.uk/i/27392585750...fqY8oDt5NaiHww __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

25th Jul 2019, 1:18 pm	#2
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Low cost transformer for valve projects. Or you can use a Shaver transformer for up to about 15W to 20W HT. Suit 2 rectifier 125V DC or four rectifier 250V DC fullwave. LT via a separate transformer. A bit cheaper, also no postage (local electrical DIY or wholesale).

25th Jul 2019, 2:16 pm	#3
ionburn Heptode Join Date: Dec 2014 Location: Lincoln, Lincolnshire, UK. Posts: 583	Re: Low cost transformer for valve projects. It will be interesting to hear how they go as the appearance is (to me) somewhat unusual for power transformers, and the site gives no indication of efficiency. One thing that oftens catches me out with small transformers is the drop under load. Mike, I have had a quick look but cannot see which you are meaning. The ones I came across were somewhat more expensive than the one in the above link although around 20W.

25th Jul 2019, 3:18 pm	#6
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: Low cost transformer for valve projects. I suspect the people who make the bathroom units bulk buy the transformers in custom lots. They don't use off-the-shelf components. So few people build their own shaver supplies that it wouldn't be worth anyone's while making the specialised transformers, and certifying them for compliance, on the off chance a passer-by might want one. Cheers, GJ __________________ http://www.ampregen.com

25th Jul 2019, 3:44 pm	#7
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Low cost transformer for valve projects. Just in case anyone hasn't seen them, there are some reasonably priced transformers available here: http://primarywindings.com/product/s...nsformer-copy/ Made in UK and decent quality. No "dubious" stuff.

26th Jul 2019, 7:07 pm	#10
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Re: Low cost transformer for valve projects. I've played-around using "shaver transformers" - specifically as a 110V HT-supply for a "Larkspur" R209 receiver and 120V for a similar-era spy-transceiver that usually ran from a pair of primary-batteries. They're generally not designed for 'continuous duty' at their supposed-rated power - meaning poor regulation [not-good when you're not crystal-controlled] and getting-hot-when-run-on-continuous-load.

27th Jul 2019, 11:33 am	#11
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Low cost transformer for valve projects. If claimed to be 20W, that's a limiter spec. They might only be good for 10W to 15W. A clue is the size. Barely enough for HT only of a mains set, unless no RF amp and modest O/P. No good to power USA Mains sets. Some USA Battery/Mains sets take too much power (5 valve or more 50mA series filaments and dropper). No use at all for my battery / 115VAC Hallicrafter S72 (100mA filament current approx), too much voltage drop, so BFO won't run.

27th Jul 2019, 12:41 pm	#13
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Low cost transformer for valve projects. Shaver transformers are wound on a split bobbin to provide full isolation. That way, you cannot get a shock from coming into contact with just one side of the output, since the transformer winding is not connected to Earth except through the tiny parasitic capacitance between the windings -- an important consideration given that in a bathroom, your impedance to Earth is likely to be much lower if you are in contact with Earthed pipework directly or via water ..... However, these transformers do run warm, and really are only designed for use for as long as it takes to shave. (The placement next to a washbasin helps ensure that even in a busy communal environment such as a leisure centre changing room or a campsite facility block, there will be a cooling-down period while users complete the rest of their ablutions.) The one I used once for a temporary HT supply during some development work has a self-resetting thermal cutout. Cheaper, modern ones might have just a single-shot thermal fuse. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

27th Jul 2019, 3:56 pm	#14
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,269	Re: Low cost transformer for valve projects. The ebay transformers look like R-Core transformers, I first came across them in indian-made equipment but I've seen them in old american computer printers from the 1970's so they aren't all that new. Maybe a transformer expert will elaborate! __________________ Kevin

27th Jul 2019, 5:55 pm	#15
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Low cost transformer for valve projects. I'm intrigued, too- is the R for ribbon or round (finished profile) or somesuch? They remind me of C-core types, which I believe have a long strip of steel wound into a shaped doughnut, then sawn into two C-shapes to fit the winding bobbins. If the C-core represents a stepping-stone towards the full toroidal type, then R-core looks like another step on from that. There was a lot of noise (of the marketing variety) about R-core transformers in the hi-fi world at one point.