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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Sat 18, 2012 4:00 am 
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That's why I guessed 1250: it's the lowest impedance (4W) tap on a Bogen 4 Watt line transformer.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Sat 18, 2012 4:48 pm 
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Hello everybody,

Thank you for the replies. I was planning to use a Xicon audio transformer which are available from Mouser. Specifically I had in mind 42TM008-RC which has 800 Ohm (primary) to 8 Ohm (secondary) ratio http://my.mouser.com/ProductDetail/Xico ... 5KTcLQ9KF4.

However this transformer would be appropriate for a single 12K5 circuit. I am still not certain what impedance should the transformer have for the circuit which uses two 12K5 tubes.

Thank you.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Sun 19, 2012 3:08 am 
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Those little 'audio' transformers have lousy frequency response.

As for the 12K5, in theory the impedance would be half for two in parallel but that presumes you can achieve maximum signal amplitude and I don't know whether the radio has enough to swing it.

The SPECO 10 Watt line transformer goes down to 500 ohm, which would be close enough, but the original couldn't have been that low due to being a 70V 4 Watt line transformer so I don't know what the logic in parallel was.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Sun 19, 2012 11:18 pm 
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Thank you. I suspected it should be half of 800 Ohm which is the impedance of a single 12K5, but I was not sure. Thank you for confirming my suspicion.

Yes, you are correct those small transformers are designed for pocket transistor radios and they have lousy frequency response, but they are readily available. What transformer would you recommend that could be freely purchased at a major parts distributor?

Thank you.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Mon 20, 2012 12:21 am 
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The SPECO 5 Watt line transformer: T7025/5. They're like 5 bucks, plus something ridiculous for shipping, no doubt. Well, ebay has them for 5 bucks plus $4 shipping.

I don't know what those things at mouser are for but the specs looks like telephone response.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Mon 20, 2012 1:39 am 
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Thank you.

I could not find a proper datasheet for the SPECO transformer. Looks like it might by made by the CIA - a top secret item.

This is Hammond Manufacturing 117F2 datasheet http://www.hammondmfg.com/pdf/5c0056.pdf

it looks like a right transformer, but it is 10 times bigger than those Xicon transformers.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Mon 20, 2012 6:55 am 
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:lol: Yeah, I've never been able to find anything like a 'spec sheet' either, which means made by either the CIA.... or the Chinese. My bet is China.

As for size, they're 5 watt and the midget things are 200 mW. I'd guess the SPECOs response to be on the order of 100-15kHz, simply because that's seems common for a 'dinky' line transformer, but it might go to 50 hz. Power and 'bass' takes iron.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Mon 20, 2012 9:37 pm 
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Don't worry about the physical size of the transformer. The bigger the better in this instance. PA transformers are not designed to carry DC current, and in that receiver the standing DC current will derate the transformer to some degree due to core saturation. Having a bit of extra "iron" is an advantage.

I'd pick a reasonable PA transformer with various primary taps, and just use the taps that give the best audio level and fidelity from the speaker. That is all that matters, and you aren't designing a high fidelity amplifier.

Pete

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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Tue 21, 2012 2:21 am 
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Thank you, Gentlemen.

I was thinking to use Hammond Manufacturing 117F2 as the audio transformer http://www.hammondmfg.com/pdf/5c0056.pdf and 160H16 as the power transformer http://www.hammondmfg.com/pdf/5c0011-12.pdf. They both are available from Mouser.

Do they pass your smell test?

Thank you.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Tue 21, 2012 3:09 am 
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I'd say the 117F4- the four watt tap is likely to get you closer to the Z the 12K5 wants to see.

Scott Todd


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Tue 21, 2012 6:59 am 
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On a 4 Watt 70V line transformer the 4 Watt tap is 1250 Ohms

You can calculate the impedance by figuring the voltage ratio of the 70.7V (100Vp) primary tap to the rated power into the specified load impedance (which gives voltage) on the secondary. The reflected impedance is then the square of the ratio times the load impedance.

I have the 70V taps pre-calculated because I use these things as 'poor mans' OTPs and, with an 8 ohm secondary, 4 Watt is 1250 ohm, 5 Watt is 1000 ohm, and 10 Watt is 500 ohm. If you want a low impedance with 'small watts' you need a 25V line transformer and, with an 8 ohm secondary, 5 Watt is 125 ohm, 2.5 Watt is 250 ohm, and 1 Watt is 625 ohm.

The 5 Watt SPECO T7025/5 has taps for both 25V and 70V operation (hence the model number) so it can be used from 125 Ohm to 20000 Ohm.

The SPECO is also cheaper than anything Hammond makes.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Tue 21, 2012 4:05 pm 
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Thank you.

I purchased SPECO just to see how it plays in the circuit. I do not like the fact that there is no datasheet for it, however.

Again this is the schematic http://oldradiobuilder.com/12vdiagram.pdf.

I was thinking to use 42IF106 (455 KHz) transformer for OSC http://www.mouser.com/catalog/specsheets/XC-600014.pdf

For the first, second and third IF I thought to use 42IF101, 42IF102, and 42IF103 respectively. They all are 455 KHz transformers http://www.mouser.com/catalog/specsheets/XC-600131.pdf

I did not figure out which transformer to use for the RF stage yet and probably will use the 42IF106.

Does this make sense?

Thank you again.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Tue 21, 2012 4:44 pm 
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Yeah, I know what you mean. I don't like 'no specs' either but in the land of China they haven't discovered what a spec sheet is yet. I actually contacted SPECO about their 20 Watt job, got an answer, and still don't know what it is.

I used that 5 Watt job in my 12AL8 regen. http://flipperhome.dyndns.org/12AL8%20Regen.htm

I also use them, both the 5 Watt and 10 Watt, for push pull amplifiers where the current cancels (and I have a 'secret' [kidding] current balance circuit). With the single ended operation we're counting on the 'iron' to handle the DC so the 'watts' is for that purpose, not actual audio, and I would expect those 200 mW jobs to saturate.

I don't know much about the IF transformers but I'm sure someone else can answer that.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Tue 21, 2012 8:06 pm 
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I will be right back while build norms radio

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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Aug Wed 22, 2012 2:35 am 
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Now I am wondering what would happen if capacitor C1b is eliminated. Would it affect appreciably, or at all, the operation of the receiver? http://oldradiobuilder.com/12vdiagram.pdf

If C1b could be eliminated, then the variable capacitor only has to have two sections and RF transformer does not need to have its tap.


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 Post subject: Re: Norms 12K5 Regen Complete
PostPosted: Feb Mon 13, 2017 7:36 pm 
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I built this radio and never got it to work. I could hear the sidebands, but never the audio. The set also had significant hysteresis. I had changed the tube's resistor to 6.4 Mohms without any change, but when I went to 9 M ohms everything worked perfectly. <just a little tip>

Well, some hysteresis remains, but it is now possible to adjust the regen for BCB AM satisfactorily.


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