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Old 31st Mar 2024, 2:51 pm   #1
ian rose
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Default Output Transformer for Ferguson 208U.

Hello all
I am looking for an output transformer. Approx. 4.5cm X 2.5cm. Primary approx. 310ohms for Ferguson 208U. Size not critical.
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Old 1st Apr 2024, 12:08 pm   #2
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Default Re: Output Transformer for Ferguson 208U.

Update.
Any output transformer will do of approx size; 4.5cm X 2-3cm.
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Old 1st Apr 2024, 1:13 pm   #3
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Default Re: Output Transformer for Ferguson 208U.

If you're stuffed, I do have the winding details to match to an 8 ohm speaker (yes, I have a 208U with an o/c OPT and a duff loudspeaker).

It's a radio that's well worth having running - despite being a short superhet, attention to detail in the front end, and multipoint tracking between RF and oscillator, makes it a cracking little performer!
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Old 2nd Apr 2024, 1:01 pm   #4
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Default Re: Output Transformer for Ferguson 208U.

Thanks Kalee
I have unwound and saved the secondary. winding a new primary might be difficult for me [v thin wire]. But, thank you; will wait a while and come back to you,
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Old 2nd Apr 2024, 2:09 pm   #5
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Default Re: Output Transformer for Ferguson 208U.

I have a smallish transformer marked as SE output
Primary is 337 Ohms. Secondary under 0.5 Ohm
The mounting lugs are approx 5.3CM, the width of the transformer core is approx 4.5cm, depth of the bobbin approx 3cm and it stands just under 4 cm tall excluding the tag board.

is this of any use?
Click image for larger version

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ID:	295748

Adrian
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Old 2nd Apr 2024, 4:04 pm   #6
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Default Re: Output Transformer for Ferguson 208U.

Hi Ian, I see the set uses a UL41 op valve, designed for a load of 3K, so for a 3R speaker you need a ratio of 32 :1. If it uses an 8R speaker as Kalee suggests it should be 20 to 1

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Old 2nd Apr 2024, 5:01 pm   #7
ian rose
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Default Re: Output Transformer for Ferguson 208U.

Quote:
Originally Posted by AdrianH View Post
I have a smallish transformer marked as SE output
Primary is 337 Ohms. Secondary under 0.5 Ohm
The mounting lugs are approx 5.3CM, the width of the transformer core is approx 4.5cm, depth of the bobbin approx 3cm and it stands just under 4 cm tall excluding the tag board.

is this of any use?
Attachment 295747

Attachment 295748

Adrian
Thank you Adrian.
That will be most satisfactory. Manual quotes primary as 310 and 2ndry as 0.7
sounds made for the job. Will send a PM.
Ian
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Old 2nd Apr 2024, 5:10 pm   #8
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Default Re: Output Transformer for Ferguson 208U.

pm returned and as mentioned
I will try and feed the secondary with a small AC voltage and measure the secondary with my DVM and work out the ratio, just to see if it is suitable.

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Old 2nd Apr 2024, 5:15 pm   #9
ian rose
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Default Re: Output Transformer for Ferguson 208U.

Quote:
Originally Posted by Ed_Dinning View Post
Hi Ian, I see the set uses a UL41 op valve, designed for a load of 3K, so for a 3R speaker you need a ratio of 32 :1. If it uses an 8R speaker as Kalee suggests it should be 20 to 1

Ed
Thank you Ed.
Ian
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Old 2nd Apr 2024, 7:49 pm   #10
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Default Re: Output Transformer for Ferguson 208U.

Quote:
Originally Posted by ian rose View Post
Quote:
Originally Posted by AdrianH View Post
I have a smallish transformer marked as SE output
Primary is 337 Ohms. Secondary under 0.5 Ohm
The mounting lugs are approx 5.3CM, the width of the transformer core is approx 4.5cm, depth of the bobbin approx 3cm and it stands just under 4 cm tall excluding the tag board.

is this of any use?
Attachment 295747

Attachment 295748

Adrian
Thank you Adrian.

That will be most satisfactory. Manual quotes primary as 310 and 2ndry as 0.7
sounds made for the job. Will send a PM.
Ian
As an aside, the DC resistance has nothing to do with the actual turns ratio which is what you really need to take into consideration. For instance lets suppose you have a UL41 in a small radiogram that has an output of about 4 watts with reasonable fidelity (more bass). The output transformer will be wound with wire thick enough to take the anode current of the UL41 at this higher power. Ideally the transformer will have a ratio of 32:1. Now another situation, again a UL41 used in a small radio. A smaller transformer wound with thinner wire rated for less current and lower power perhaps only a couple of watts. It will still have (ideally) a ratio of 32:1 but the thinner primary wire will likely have a higher resistance than the one designed to deliver more power. The higher power transformer (for arguments sake) could have a primary resistance of 150 ohms. The smaller transformer might have a primary resistance of 350 ohms. The valve doesn't care because it's the AC impedance that's more important. Both have a ratio of 32:1 but the DC resistances can be totally different.

The DC resistance is sometimes given in the service notes purely as a guide for the actual transformer fitted. It doesn't mean that another transformer that has a similar DC resistance is the correct ratio.

In practice, nearly any transformer will do as a test as long as the primary has adequate current rating.
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Old 2nd Apr 2024, 11:05 pm   #11
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Default Re: Output Transformer for Ferguson 208U.

OK I have just had some time to test the transformer with a 1KHz tone, using both DVM and my HP scope and both with very similar results at different levels.

Feeding a small voltage into the secondary and measuring the primary voltage.
DVM with 0.31 Volts rms across the secondary I got 18.47 Volts rms across the primary.
So that gives a 59.58 ratio.

With the scope at another level 0.72 Volts peak to peak across the secondary gave 44.4 Volts peak to peak across the primary so that gives a 61.666 turn ratio.

So around a 60:1 ratio transformer.

I checked another similar and got around 60:1 again, so nothing down at the 32:1 ratio or 20:1 as suggested.

Perhaps some small transformer for the EL84 as found on eBay could be a better bet even if a bit larger.

I will leave it to you Ian if you still wish to proceed with this transformer I am sure it could work in a fashion but may still leave you wanting a better matched one later.


Adrian

RS used to do universal ones perhaps one of these may show up at some point.
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Last edited by Station X; 3rd Apr 2024 at 5:49 pm. Reason: Please don't use euphemisms for eBay.
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Old 3rd Apr 2024, 5:45 pm   #12
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Default Re: Output Transformer for Ferguson 208U.

Thank you Sideband for that info.
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Old 3rd Apr 2024, 5:58 pm   #13
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Default Re: Output Transformer for Ferguson 208U.

It'll be fine. You can overthink these things.
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Old 5th Apr 2024, 10:36 am   #14
ian rose
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Default Re: Output Transformer for Ferguson 208U.

Paul Sherwin Moderator
Thank you Paul. As far as I am concerned this thread can now be closed.
Regards
Ian Rose
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