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 Post subject: Re: High Fidelity AM Tuner Project - mulling over possibilit
PostPosted: Feb Fri 15, 2019 2:25 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 997
Location: Wayside, NJ Monmouth
I proto typed this a year ago. Works very well. With the Loop in my basement in a Aluminum clade house, I was able to pull staions in from PA and the east end of Long Island. Local stations and the NYC stations really boomed in. I liked the Regen circuit, worked very smooth. I will be feeding the audio into a small amp, driving a 15' full range speaker. So the fourth tube won't be used. Working on the wooden / metal box for it now. Hope to have it done by spring.


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 Post subject: Re: High Fidelity AM Tuner Project - mulling over possibilit
PostPosted: Feb Fri 15, 2019 10:52 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 794
Location: Tokyo
I really like Franklin regens for LW and MW. No need for a tap or tickler coil. The very small value tank to control grid cap (typically just 1 or 2pF) imposes very little load. However, the upshot is the bandwidth near oscillation onset is very sharp, sharp enough to make speech difficult to understand. Perhaps not the best choice for a HiFi AM radio. But for DX'ing with a loop antenna, that is a great combination.

Getting back to the mixer stage. Why not use a pentagrid mixer with a separate LO instead of a pentode? Back in the 1940s, that was the standard method. Pentodes are quieter but that doesn't matter for MW. Pentodes are more subject to pulling (you have to connect the LO to either the control grid or the cathode). Pentagrids provide the better isolation of electron coupling. Many top quality communications receivers used one or two RF stages, followed by a pentagrid mixer, and that was for SW.

Interesting thing about fullwave and infinite impedance detectors: because they don't load the IFT as much as an ordinary diode detector, the bandwidth is narrower. Not necessarily a good thing for a HiFi AM tuner. OTOH, the distortion is much less, and I think that is the more important factor.

Rob


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 Post subject: Re: High Fidelity AM Tuner Project - mulling over possibilit
PostPosted: Feb Fri 15, 2019 6:29 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 38
Location: Texas, U.S.A.
...
I like that antenna, N2LXM.

To Rob's point about effect of load impedance on bandwidth of the last I.F. transformer, if the bandwidth does turn out to be too narrow, perhaps a suitable resistance could be put across the secondary.


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 Post subject: Re: High Fidelity AM Tuner Project - mulling over possibilit
PostPosted: Feb Sat 16, 2019 7:44 am 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 3091
Location: Tucson, Arizona U.S.A.
Using a pentode as the local oscillator in an electron coupled circuit will also improve the isolation between the oscillator and the incoming signal.

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Jim Mueller


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 Post subject: Re: High Fidelity AM Tuner Project - mulling over possibilit
PostPosted: Feb Mon 18, 2019 5:33 pm 
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Joined: Apr Tue 17, 2012 7:58 pm
Posts: 59
Location: Metamora MI, 48455
I happened across an eBay seller that had a large stock of NOS Meissner coils. I prefer the build quality of the Meissner's over the Millers, and I don't think I could piece together a complete receiver using what I have on hand, so I bought the Meissner coils and now plan to use them instead. This has also altered my plans a bit.

I now have Meissner part numbers 14-1067, 14-1068, and 14-1069 for the front end. These coils are "Tri-Banders" that cover 535 kHz up to 19 MHz in three bands. One is the antenna coil, one is the RF stage coil, and finally one is the oscillator coil. Does anyone have the datasheet for the Meissner 14-1069? I'd like to determine if the coil is intended for the 6SA7 family (with the tapped osc. coils) or if it is the traditional type of osc. coil used with the 6K8, 7A8, 6A8, or 6J8 etc. The Meissner literature I can find is somewhat contradictory.

So, now that I'll be covering shortwave was well, I really do need to keep the noise figure of the front end down. A 6SG7 should still work adequately here, but a 6AB7 might be better yet. Or, perhaps a 6AC7 with a manual gain control. I have options for the RF stage. The mixer stage I'm still unsure about.

The seller also had the Meissner variable selectivity/bandwidth coils, types 17-4700 and 17-7412, one of which is an input IF transformer, the other of which is the output. These coils are iron cored like the Millers for higher Q, but offer three bandwidth positions as opposed to just 2 which is nice. One affords extremely sharp selectivity (more like one would find/want in a communications receiver), the widest position is "high-fidelity" and affords a fairly wide bandwidth at the expense of selectivity of course ("suitable for reception of local stations" according to Meissner literature), and the middle position is somewhere in between, and according to the Meissner literature, is "comparable to a typical home radio" (read AA5).

I've also made up my mind to ditch the full-wave detector in favor of a simple infinite impedance detector with a 6J5 or some similar triode. The low distortion of the infinite impedance detector can't be beat, and the full-wave diode detector has a disadvantage in that you get half the output voltage for a given input from the IF stage when compared to the normal half-wave diode detector or infinite impedance detector. What's the point in selecting a high-gm pentode for the IF stage if you're just going to throw some of that gain away with an inferior detector?

Once I nail down what type of oscillator coil(s) I have, then I can decide on a mixer/osc. or converter stage and begin drawing an actual schematic and calculating component values.


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 Post subject: Re: High Fidelity AM Tuner Project - mulling over possibilit
PostPosted: Feb Tue 19, 2019 2:43 am 
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Joined: Sep Mon 16, 2013 2:42 am
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Location: Tucson, Arizona U.S.A.
You can determine which type of oscillator coil you have with an ohmmeter.

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Jim Mueller


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 Post subject: Re: High Fidelity AM Tuner Project - mulling over possibilit
PostPosted: Feb Wed 20, 2019 2:22 am 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 8007
Location: Ohio 45177
I was looking in an old radio mag from a link in another old ARF posting about something else. I had to scroll thru the magazine download to get where I wanted, but on the way I saw something about a "hi-fi" tuner, and it was all tube. The catch is, it was a totally TRF radio instead of superhet. This in late postwar magazine. Remember the Miller HI FI AM tuner was a crystal radio!


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 Post subject: Re: High Fidelity AM Tuner Project - mulling over possibilit
PostPosted: Feb Sun 24, 2019 9:48 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1241
Location: Champaign IL 61822
Dale H. Cook wrote:
benman94 wrote:
I consider the 10 kHz theoretical limit for any US station ...

There is nothing "theoretical" about it. [quote]

It is very much "theoretical" in that lots of stations don't even go out that far.
On the flip side, other stations reach within 20 Hz of it with a seriously
rising frequency response as they approach it.


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