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 Post subject: 38 as regenerative detector
PostPosted: May Tue 11, 2021 12:34 am 
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Location: WA 98407
Here’s a novel but truly excellent receiver using a 38, an early 1930s, 5-pin tube, as a regenerative detector. The 6C8 twin triode serves as a grounded grid RF amp and a first audio. The 41 provides the power to drive a speaker. This relatively easy project resulted in the hands-down, no-argument, prove-me-wrong, no-comparison best SW regenerative receiver I’ve ever had the pleasure to operate. I claim no credit for it—I simply built it from information posted by the very knowledgeable Dave Newkirk W9BRD on the Radioboard website. This receiver hears everything my SX-28 hears. Drives a speaker with good volume, too.
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The regeneration control is as smooth as the finish on a Steinway. No click, thud or plop when it goes into oscillation and there are no hand effects whatsoever. Note two departures from traditional regens: the absence of an RF choke in the detector plate circuit and the presence of a 47K resistor in series with the audio coupling cap. These are contributions from W9BRD’s experience and wisdom. I’m no expert, but I suspect that all-metal construction also contributes to the receiver’s superior characteristics. My bench supply provides a B+ of 160vdc, but I think anything from 100 to 200 volts would do just fine.
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The 5:1 reduction drive and dial are what I’d call “hobbyist grade.” They show up on ebay from time to time (several variants). It’s adequate for SWL use but not really suitable for ham use unless the LC components in the front end were configured to tune a much narrower frequency range. One last thing—the meter is there only because it was mounted in the panel I re-used from another project.


Last edited by allthumbs on May Fri 14, 2021 3:33 pm, edited 2 times in total.

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 Post subject: Re: 38 as regenerative detector
PostPosted: May Tue 11, 2021 1:20 am 
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Joined: May Sun 16, 2010 3:22 am
Posts: 305
Location: Scotia, NY 12302
Very nice job building. I enjoy the simple 1, 2 and 3 tube sets. I might have to build one
myself. I've got more than enough parts in stock.

Al

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 Post subject: Re: 38 as regenerative detector
PostPosted: May Tue 11, 2021 1:53 am 
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Location: Equinunk PA 18417
I also like the build, not show-offy except to show how much care was taken. Brass hardware. Your dial looks great.

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 Post subject: Re: 38 as regenerative detector
PostPosted: May Thu 13, 2021 12:13 am 
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For the necessary higher AF voltage gain to drive the 41 I recall that we also increased the plate load R of the 6C8 AF voltage amp from 47 k to 247 k by putting two seriesed 100-k Rs in series with the 47 k part. 8.2 k for the cathode R of that stage looks right.

Any of the AF power pentodes from that era should work well as a detector. I tried the 38, 41/6K6GT, 89, and 12A5 -- all with excellent results. (Hmm, but I haven't tried a 59... :-) ) My particular hookup is a Hartley operating at 3 to 3.1 MHz (for ham-band coverage with an external converter) so allthumbs's cool build also proves the 38 in the Armstrong configuration.


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 Post subject: Re: 38 as regenerative detector
PostPosted: May Thu 13, 2021 3:37 am 
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Yes, I increased the first audio plate load as W9BRD recommended but instead of putting resistors in series, I cheated and substituted a single 220K for the original 47K. Works great! All the audio you could want, drives a speaker with volume to spare. :D

Tom


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 Post subject: Re: 38 as regenerative detector
PostPosted: May Fri 14, 2021 11:11 am 
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Joined: Nov Sun 22, 2015 4:51 pm
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Close enough! :D


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 Post subject: Re: 38 as regenerative detector
PostPosted: May Sun 16, 2021 5:31 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Cambridge MN USA
How much better selectivity would it have putting a tuned circuit in the cathode of the first half of the 6C8?

Scott Todd


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 Post subject: Re: 38 as regenerative detector
PostPosted: May Sun 16, 2021 5:56 pm 
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Joined: Jan Thu 07, 2016 11:39 pm
Posts: 195
Location: Poulsbo, WA
What are the details on the detector coil and tickler?

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 Post subject: Re: 38 as regenerative detector
PostPosted: May Tue 18, 2021 12:31 am 
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N0BST: I don't know, but am willing to bet that you'd gain nothing but troubles if you did that. After all, this is a regenerative receiver and if built and operated properly will have selectivity suitable for CW reception.

bcascisa: 49 meter band coil: 15 turn primary, 5 turn tickler:
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My coils are combined with a 35 pf variable capacitor paralleled with a 52 pf fixed capacitor (note: schematic incorrectly shows 82 pf). For other bands, wind coils accordingly. E.g., my 31 meter coil is 9 turns of #24 spaced over 7/8" and a 5-turn close-wound tickler. So, experiment. Things are not as critical as some of the literature might lead you to believe.

Another thing: If you built yours as a replica of mine, you'd only approximate the same coverage. There'll be too much in the way of stray capacitances and inductances in your build to get identical coverage, one receiver to the next. I choose the LC components in the detector tuning cct. so that the tuning range for each coil I wind is quite narrow. You can see the results if you look closely at the dial scale in my original photos. It took a fair bit of experimenting to get the coverage I settled for. But that's half the fun. :D

Tom


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 Post subject: Re: 38 as regenerative detector
PostPosted: May Wed 19, 2021 5:54 pm 
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Location: Poulsbo, WA
Much and many Thanks :D

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 Post subject: Re: 38 as regenerative detector
PostPosted: May Thu 20, 2021 12:35 am 
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bcascisa: You're welcome. If you can't find those old-time plug-in coil forms, plastic pipe from Lowe's/Home Depot will work just as well. You'd just have to figure out how to mount them.

Tom


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 Post subject: Re: 38 as regenerative detector
PostPosted: May Thu 20, 2021 6:22 am 
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Location: Poulsbo, WA
Fortunately, I have a box full of the Bud & Hammarlund coil forms.

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 Post subject: Re: 38 as regenerative detector
PostPosted: May Thu 20, 2021 6:58 pm 
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Then you're in good shape. Do post pics once you've built it! :D

Tom


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 Post subject: Re: 38 as regenerative detector
PostPosted: Jul Mon 26, 2021 7:56 pm 
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Joined: Mar Wed 26, 2008 3:06 pm
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Location: Cave Springs, AR
What kind of antenna are you using with it?

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