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What Audio Transformer To Buy? - Printable Version

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What Audio Transformer To Buy? - Pepperoni - 09-27-2011

I have a Philco 40-190 and the audio output transformer is DOA. I looked at all my parts sets and no match. AES, Radio Daze etc. have audio output transformers but I honestly don't know which one to buy. Could someone give me a little guidance as to which one will work?
http://www.nostalgiaair.org/PagesByModel/339/M0013339.pdf
Thanks,
Pepper

Re: What Audio Transformer To Buy? - codefox1 - 09-27-2011

I don't think it's that critical, somewhere between 5 and 10 K primary impedence and 4 or 8 ohm secondary should work out fine. Can't do any harm trying out something you may have in your stash provided that it is not shorted or open. Do find out why the old one burnt out before going further. Transformers are pretty sturdy untill crossed.

Re: What Audio Transformer To Buy? - Pepperoni - 09-27-2011

Thanks Fox! I look at Radio Daze again. There's just such a huge selection there I wasn't sure which to choose. As for why it went open on the primary side I just have no idea at this time. I'm disassembling the whole radio because of all the bad wiring and I'm finding all kinds of problems that could have caused it. The wiring is so bad and I do see lots of areas where shorting took place. Burnt areas on the chassis etc.

Pepper

Re: What Audio Transformer To Buy? - John R - 09-27-2011

Hi,
According to RCA receiving tube manual RC-13, 1937, the load impedance of a 41 tube operating at a plate voltage of about 180 volts, where your 40-190 is at, should ideally be about 9,000 ohms. A push-pull 41 operation would be about twice that or about 18k ohms. A Hammond universal type push-pull center tapped primary audio output transformer gives you taps to select the pimary and secondary impedances you wish to match up to. See the Hammond catalog on the Radio Daze site. A Hammond 125A at 3 watts or a 125B at 5 watts should work for you. The 40-190 is rated at 2 watts audio output power per the schematics. You can measure the speaker voice coil DC resistance with a ohmeter to determine it's approximate impedance with the secondary of the audio output transformer disconnected. Multiply the ohmeter reading by 1.25 to get the impedance. This is then the secondary impedance you wish to match. Use the universal impedance chart supplied to match to the pimary and secondary impedances ideally needed for good match. I personally would go with the 125B for current handling headroom. Icon_smile Good luck, John

Re: What Audio Transformer To Buy? - Pepperoni - 10-14-2011

I'd just like to clarify the connections on this Hammond 125B audio transformer I just put into this 40-190. Primary side's pretty simple but I want to make sure that I have my secondary side correct. My voice coil reads about 4 ohms so I'm using pins 4 & 5 of the transformers secondary. Pin 4 is connected to chassis ground and pin 5 to my speaker. Is this correct?
http://www.nostalgiaair.org/PagesByModel/339/M0013339.pdf
http://www.hammondmfg.com/pdf/5c0054.pdf
Thanks,
Pepper

Re: What Audio Transformer To Buy? - John R - 10-14-2011

Pepper,
You may try different secondary connections to get the best operation...loudness and quality. With a 4 ohm secondary impedance you go down the 4 ohm vertical table to get a match close to the theoritical primary tube load impedance. In this case around 18kohm.
So it looks like connections to secondary pins 1 & 3 with one being the ground would give you about 15kohms. Pins 5 & 6 with 5 being ground would give you around 22kohms primary load impedance. Icon_biggrin See what works best for you. Icon_clap
John

Re: What Audio Transformer To Buy? - Pepperoni - 10-14-2011

Thanks John I'll experiment with it. I powered the set up and I have the local AM station coming in faintly but tuning the radio makes no difference. I still get the local station no matter where I'm at on the dial. So I've got some bugs to work out.

Pepper

Re: What Audio Transformer To Buy? - exray - 10-15-2011

Pepperoni Wrote:Thanks John I'll experiment with it. I powered the set up and I have the local AM station coming in faintly but tuning the radio makes no difference. I still get the local station no matter where I'm at on the dial. So I've got some bugs to work out.

Pepper

That's a symptom of a dead local oscillator.

-Bill