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 Replacement audio output transformer
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 Return to top of page · Post #: 1 · Written at 2:23:13 PM on 5 November 2013.
Fredr's Gravatar
 Location: Wahroonga, NSW
 Member since 5 November 2013
 Member #: 1441
 Postcount: 11

Hi, this is my first post on any forum (so please be gentle!). I am repairing an HMV B61B portable and, like most of the old sets I have collected, it has an defunct output transformer (the primary is o/c). I am looking for a replacement but there are no details on the transformer. The set uses a 3V4 audio output and a 2ohm speaker. Valve data shows the load for a 3V4 is 10K with an output power of 250mW. I have searched the web but there are no direct replacement transformers - 10K/2ohm with a power output <5W. I have found a couple of possible substitutes. Hammond Mfg 125ASE multivalued 2.5 – 10K primary and 4 – 32ohm secondary (3W output); a ClassicTone 40-18052 5K/3.2ohm, or ClassicTone 40-18030 8K/4ohm both 5W types. Physical size is a bit of concern for all of them but I guess I will just have to cope with that.

Impedance matching and compatibility has always been a bit of a black art for me and I am not sure how much mismatch is acceptable. For example is the 4 ohm secondary OK for a 2 ohm speaker? Is the 8K primary OK for the 3V4, and what effect would the secondary impedance mismatch have on the primary impedance. (The Hammond has specifications that seems to allows different primary and secondary impedances on the same wires http://www.hammondmfg.com/pdf/5C_125SE.pdf. Not sure how this works!)

So any help in choosing the right replacement transformer would be greatly appreciated. Thanks.


 
 Return to top of page · Post #: 2 · Written at 3:45:04 PM on 5 November 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

If you want it bad enough it would be happening: http://www.evatco.com.au/hamprice.htm.

Someone could chime in with something cheaper, etc.
I can vouch for them as far as you do get what you order.



 
 Return to top of page · Post #: 3 · Written at 3:53:30 PM on 5 November 2013.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6678

Output transformers reflect the secondary impedance into the primary according to the the turns ratio. So, if a transformer is 10Kohm:8ohm, putting a 4 ohm load on the secondary will present a 5Kohm load to the output valve.

If you halve the secondary impedance, you halve the primary impedance, and if you double the secondary you double the primary.

Applying that principle demonstrates how the Hammond 125SE accommodates a variety of impedances.

The 3V4 will transfer maximum power into the load when it's matched to the load, i.e. if it's working into 10Kohm.

If you vary the load impedance up or down, you will vary the power output accordingly. However, if you lower the load resistance too far you'll reach a point where the 3V4 will be working too hard for its own good.

I'm not intimately familiar with the 3V4, but higher voltage power output valves (e.g. 6V6) can usually tolerate a halving of the reflected load impedance without complaining.


 
 Return to top of page · Post #: 4 · Written at 9:45:36 PM on 5 November 2013.
Redxm's avatar
 Location: Tamworth, NSW
 Member since 6 April 2012
 Member #: 1126
 Postcount: 466

I'll have a dig tomorrow. I have a few dead portable that use 3V4's. Might be something usefull in there.

ben


 
 Return to top of page · Post #: 5 · Written at 9:45:04 PM on 6 November 2013.
Fredr's Gravatar
 Location: Wahroonga, NSW
 Member since 5 November 2013
 Member #: 1441
 Postcount: 11

Thanks guiys for your comments. Great stuff, particularly GTC for your explanation.

Am now getting it clearer. I think Smile Hooking up a 8K/4 ohm transformer to a 2 ohm speaker the transformer will see a 2 ohm impedance i.e. the secondary can be considered 2 ohm. This will mean the primary impedance will be 4K (secondary is ½ the rated value means the primary will be ½ the rated value). Going back to the 3V4 with its 10K load and your comment that the 6V6 (and I guess by inference the 3V4) will handle half its rated load, using a 8K/4 ohm transformer – 2ohm speaker means the valve will see only a 4K load. Not enough I think.

Therefore, the Hammond seems to be the best bet. Using the 10K/4ohm configuration hooking up a 2ohm speaker will give a load of 5K on the valve. Not ideal but probably the best. Anyway, I only want to get the set going (thats the fun bit), its not going to be run for any extensive time.

Hey Redxm, any transformers would be really good. Let me know if anything turns up and we can come to some arrangement. Cheers.


 
 Return to top of page · Post #: 6 · Written at 11:04:32 AM on 7 November 2013.
Redxm's avatar
 Location: Tamworth, NSW
 Member since 6 April 2012
 Member #: 1126
 Postcount: 466

I had a look last night. The only one ive found so far is out of philips 168, and it is o/c.
I am certain I have another healing chassis somewhere.

What are the physical dimensions of yours. The crook one I have shouldnt be too hard to rewind.

Ben


 
 Return to top of page · Post #: 7 · Written at 2:17:56 PM on 7 November 2013.
Gandhn's Gravatar
 Location: Cameron Park, NSW
 Member since 5 November 2010
 Member #: 770
 Postcount: 385

Hi Fredr

I have just pulled a transformer from a portable chassis, using a 3V4 output valve, so I'm pretty sure it will be fine for you. Mounting centres are 1 3/4 inch.
Send me an email with your address and I will pop it in the post for you.

Good luck with the restoration

Harold


 
 Return to top of page · Post #: 8 · Written at 5:51:09 PM on 7 November 2013.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1253

"The crook one I have shouldnt be too hard to rewind."

There is an article in Electronics Australia March 1991 on how to go about rewinding an output transformer, including how to calculate the turns ratio from the speaker voice coil DC resistance and the output valve impedance.

Shows how to make a bobbin from fibreboard or formica. Only a hand drill required, plus a spool holder made from a piece of dowel and some chip board.


 
 Return to top of page · Post #: 9 · Written at 10:11:17 AM on 8 November 2013.
Scraps's Gravatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

"The crook one I have shouldn't be too hard to rewind"

I've looked at a couple of crook OP transformers from AWA's and they're full of some sort of black tar. I've kept them to pull apart on a rainy day but has anyone ever found a way to get that black stuff out? I did manage to repair one but the broken wire was just near the end. Even chipping out a very small channel in that stuff was a chore.


 
 Return to top of page · Post #: 10 · Written at 1:04:24 PM on 10 November 2013.
Fredr's Gravatar
 Location: Wahroonga, NSW
 Member since 5 November 2013
 Member #: 1441
 Postcount: 11

Thanks guys. Much appreciated. Gandhn I've emailed you.

The transformer has been dipped in what appears to be pitch, a black tarry like substance (I remember my old man melting pitch to seal up cracks in an old car battery). It will probably melt and drain away if you leave it in a hotish oven overnight (I might try this if I can persuade my wife to let me use the oven...). It has numbers?8881 on the top

Thanks STC830 for the reference to the EA article. I'd like to try it but not too sure about my skills Smile But will look it up and give it a go. It could be useful for a couple of blown o/p trannies I have.


 
 Return to top of page · Post #: 11 · Written at 2:38:57 PM on 10 November 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

Some of it is hard and brittle and chips away like glass,
and further in, it's a bit gooey/tarish... might be from drying
out on the exposed surface of the tar?
but then up toward the inside it's also brittle like glass again.

Lucky, there is a slime at the bottom of the dish preventing
it sticking, but you'd still be fighting a vacuum trying to pull it
out without drilling into it at the corners.

There are a couple of inserts around the transformer to keep
it suspended in the centre of the housing while the tar is poured in,
so it should be a safe bet to drill down around all the inside edges of the housing.

I used a drill bit about the size to drill a small rivet hole,
but I wasn't trying to save the transformer, just the housing.
I would use a very small bit if I were trying to save the
transformer, but still, I only nicked it a few times! Smile

Image Link

Image Link


 
 Return to top of page · Post #: 12 · Written at 3:51:06 PM on 10 November 2013.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1253

"I'd like to try it but not too sure about my skills"

One of the arguments of the EA article encouraging the DIYer is that the enamel on modern wire is much more robust than that used 70-80 years ago. This means that strictly parallel and neat turns and inter-layer insulation (to minimise the voltage between turns and layers of the winding) is no longer needed. (Insulation between primary & secondary windings is still recommended).

Another argument is that absolute accuracy in the number of turns is not necessary. The reason for this is that the nominal impedance of a winding is for a quite a low frequency (400Hz I think), and varies considerably for other frequencies. Valve impedance would vary too.

Unfortunately my eyes have deteriorated in recent years so decided in the end not to rewind the transformer I had in mind - just too fiddly having picked up a reasonably priced replacement.

re the tar- if it is melted in, couldn't it be melted out, as long as it is thermoplastic and not thermosetting like epoxy?


 
 Return to top of page · Post #: 13 · Written at 4:03:03 PM on 10 November 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

I didn't try melting it. Maybe that would work,
but to start with, I thought it was some kind of resin.
It's like glass, and it can smash into powder.

I think that whatever you reset it with,
a layer of black Sikaflex automotive sealant would
make a really authentic substitute.


 
 Return to top of page · Post #: 14 · Written at 4:47:21 PM on 10 November 2013.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1253

Could try a small piece of it and see if it melts.

Being brittle cold sure sounds like a tar. If it was cracked maybe something has evaporated out of it causing it to shrink & crack. If that something is not vital to the tar's function, should be able to use it again.

The idea of encapsulation is to exclude moisture by completely filling the void around the transformer.
Don't know Sikaflex but if it doesn't attack the winding insulation should be OK. But would need to fill the space - if liquid should be OK. Wouldn't use Sikaflex if it is water based.

It is likely that the original encapsulation was done in a vacuum to pull out the moisture, which can't be done at home. But in a factory they need speed hence the vacuum - a good bake in air around 100C should suffice especially with modern wire which shouldn't be affected by the heat.


 
 Return to top of page · Post #: 15 · Written at 5:04:17 PM on 10 November 2013.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 896

If I get an open one, I was thinking of just using the case
for originality. Just Sikaflex in a layer at the bottom,
not for the filling whole thing,
or you'd never get it out ever again, that I'm sure of! Smile

If you're ever tossing one out, I just found that the goop at the
bottom is wax, in relatively massive proportion, and extremely clean, and well preserved.
It would be good for the waxed parts in your nicest radio for sure.








 
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