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Post by stephens60 on Feb 3, 2020 6:41:24 GMT -7
Hey Everyone from Australia I repair vintage radios from the 30’s till the 50’s most of which use a 6V6G or similar output as a single stage output no push-pull or hi-fi and all use a 3.5 ohm 5” speaker in the mantles. Now my question is many of them have open primary coils on the output transformer which is a common failure in Australia which is normal but sourcing transformers is very hard which leads me to the question, I have come across Hammond transformers which I have been told are one of the best and I have found two which I believe would be suitable but I thought I would ask the experts here.
As mentioned I commonly use a 6V6G And the speaker I deal with regularly is a 3.5 ohm voice coil 5” The two transformers I have found is:
The Hammond 1750C which is 7k primary with a 3.2 ohm Secondary at 5W rating, which I just watched DR.Z’s video on speaker impedance mismatching and I was wondering if this would be close enough to not cause damage to the transformer as changing the speaker is not an option.
The second one I found is also a Hammond 1750AX which has a Primary of 7500k and a 8 ohm secondary with 3.5W rating.
I am tossing up the two as I have been told that the 8 ohm transformer may distort at a lower power level than the 3.2 ohm. Any advice on this one would be greatly appreciated Regards Stephen
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Post by zpilot on Feb 3, 2020 7:48:27 GMT -7
I would choose the 1750C. You did not state the plate voltage which will make some difference, although that is approximate. I assume the 1750AX primary of 7500k is a typo.
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Post by stephens60 on Feb 3, 2020 7:56:11 GMT -7
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Post by zpilot on Feb 3, 2020 8:08:03 GMT -7
The 1750C is a common replacement for Fender Champ amps. I recently used one as an upgrade to a '60's era SE 6V6 Silvertone guitar amp. It will probably be an improvement for your use. I usually get my parts from Antique Electronic Supply.
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Post by stephens60 on Feb 3, 2020 8:13:36 GMT -7
My only concern with the 1750C was that the secondary is slightly lower than the speaker I was worried it may strain the transformer, but considering 3.5 is a old value that’s no longer made I think it’s the next best thing. As you suggest will be an improvement from the 70 year old transformers that I am replacing.
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Post by zpilot on Feb 3, 2020 9:59:03 GMT -7
Speaker impedance rating is a "nominal" value that changes with frequency. You are well within a safe range.
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Post by stephens60 on Feb 3, 2020 18:57:46 GMT -7
Thanks Zpilot Another curiousity I had someone who also restores radios in Aus dealing with the same speaker and output uses a 5W Line Speaker Transformer. They claim that its been proven that the airgap or lack of in the line transformers case doesnt need to be there anyway and you can use this transformer trouble free just like a standard output transformer. They also claim that the 3.5 ohm to 8 ohm secondary isnt a issue either? would like to see anyones input on this i inclued a link to the transformer they use www.jaycar.com.au/5w-line-speaker-transformer/p/MM1900?fbclid=IwAR1S9j5lJQ5YPtp3EFWQCcrimfBpGtpO9xqY74nJXAfhj6aPfsUieWhwGVc
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Post by zpilot on Feb 4, 2020 16:28:28 GMT -7
I think this is a whole different animal than a valve amp output transformer.
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