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 Post subject: Re: 3 tube superhet
PostPosted: Nov Fri 29, 2019 9:57 pm 
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I decided to take a piece of copper clad board and make a ground substrate which is isolated from the chassis and makes it easy to solder to.
It will be hot while the chassis will be "not-so-hot" :wink:

I didn't do my best work carving it out, but it will still do. I glued it in with clear RTV.

-Steve


Attachments:
copper-clad-board.jpg
copper-clad-board.jpg [ 197.3 KiB | Viewed 1640 times ]
board-marked.jpg
board-marked.jpg [ 118.01 KiB | Viewed 1640 times ]
ground-plane.jpg
ground-plane.jpg [ 185.45 KiB | Viewed 1640 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Sat 30, 2019 10:16 pm 
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Hmm, I thought I would get some interest in my copper clad board idea. :lol: I guess its not unique. :wink:

Anyhow a few more pics.

-Steve


Attachments:
chassis-top-populated.jpg
chassis-top-populated.jpg [ 249.38 KiB | Viewed 1610 times ]
chassis-bot.jpg
chassis-bot.jpg [ 198.09 KiB | Viewed 1610 times ]
heater-wire.jpg
heater-wire.jpg [ 261.64 KiB | Viewed 1610 times ]
tube-glow.jpg
tube-glow.jpg [ 181.56 KiB | Viewed 1610 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Sat 30, 2019 11:50 pm 
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I decided to rewire the heater string with a twisted pair to try to avoid inducing any hum.

-Steve


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heater-wiring.jpg
heater-wiring.jpg [ 201.64 KiB | Viewed 1602 times ]
undch1.jpg
undch1.jpg [ 212.89 KiB | Viewed 1602 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Sun 01, 2019 12:49 am 
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azenithnut wrote:
Hmm, I thought I would get some interest in my copper clad board idea. :lol: I guess its not unique. :wink:

We were too awestruck to reply :P

Very nice neat job so far. Looks really good.


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 Post subject: Re: 3 tube superhet
PostPosted: Dec Mon 02, 2019 3:33 pm 
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Thank you for the compliment Bob! :D

I managed to get it to the point the power supply and audio amplifier is working. I played some music through it and it sounded pretty good,
However, I think I will convert the 1st audio amp to contact bias over cathode bias.

It appears the triode section of the 19HV8 is pretty much the same as a 12AT6 triode.
As most radios I've looked up using the 12AT6 use it with contact bias, i'll give it a try.

Anyone feel I'm making the wrong move?

-Steve

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Tue 03, 2019 11:33 pm 
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Here is an update.

The audio amplifier works. I started wiring the IF and Detector section.
I decided to use more copper clad board "islands" for the B+ buss as well as connection points.
They are also glued down with RTV.

The oscillator coil does not have a way to mount it, so I came up with the idea of a piece of K&S brass tubing from an RC hobby store.
The coil fits into it nice and snug, so I soldered a 1/4" length to the copper and inserted the coil.

The original Miller 70-OSC coil had a clip for snapping it into a hole.

-Steve


Attachments:
undch2.jpg
undch2.jpg [ 217.76 KiB | Viewed 1538 times ]
70-osc.jpg
70-osc.jpg [ 223.69 KiB | Viewed 1538 times ]
k-s.jpg
k-s.jpg [ 261.14 KiB | Viewed 1538 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Tue 10, 2019 7:12 pm 
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I powered it up yesterday, and did an IF alignment at 455 KC, using Lou Haskell's "COMPULIGN" generator.

https://antiqueradios.com/forums/viewto ... 2&t=315218

It took a little tweaking of the caps to finally hear a signal, but it peaked up nicely! :D
However, no signal. I used an antenna loop from a junk Belmont Bunny, but nothing...
I discovered the LO was not oscillating, so I tried reversing connections on the "tickler" coil and the LO sprang to life!

However, no signals, just odd spurious responses. I finally connected a longwire to the loop and I immediately found the local AM station 1570 WPTW which came in solid, however, I had to turn it up full to listen to it at room filling volume...

Then I noticed..... The IF amplifier 12EQ7 did NOT have B+ wired to it yet! Holy cow! I can't believe it even worked THAT well without power! :shock:

Once that was properly connected it REALLY came to life! So much so it is unstable and drops into oscillation and motorboats at the slightest touch! I think I may need to add tube shields.

The front end is way out of alignment, but I'll deal with that once I have a proper loop antenna built up.

But, its getting there! :D

-Steve

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Wed 11, 2019 11:02 am 
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Congrats!

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Sun 15, 2019 12:49 am 
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Great project, Steve! I like your idea of an isolated ground plane. Keep us posted.

Someday, I plan on reconstructing the derelict "Plant A" chassis that I found at a hamfest. Someone made a CW monitor out of it, and did a sloppy job. I probably have all the parts on hand; just need the time.

Another idea is a three tube set, using a 1A7, 3A8, and 117L7. GE made a three way portable with that tube lineup, and it wouldn't be hard to redesign it into an AC only version. The filament circuit of the 1A7 and 3A8 is the cathode resistor of the pentode section of the 117L7, so no dropping resistor is needed.

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Sun 15, 2019 11:38 am 
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Thanks Tim, I had to look up the GE you mention. It is the model HB-412.
Intriguing circuit to power the two battery tube heaters through the audio output cathode.

http://www.nostalgiaair.org/pagesbymode ... 008020.pdf

Better make sure C17 isn't leaky or you may blow filaments!

-Steve

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Tue 17, 2019 11:34 pm 
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I started on making a loop antenna for this little radio.

I found some old Vectorbord and cut it into two pieces, then glued in some wooden dowels to wrap wire around.
The bolts will not stay, they are only there to hold things together while the epoxy cured.

Not there yet.

-Steve


Attachments:
vectorboard.jpg
vectorboard.jpg [ 414.27 KiB | Viewed 1271 times ]
vectorbord.jpg
vectorbord.jpg [ 116.42 KiB | Viewed 1271 times ]
loop-frame1.jpg
loop-frame1.jpg [ 199.06 KiB | Viewed 1271 times ]
loop-frame2.jpg
loop-frame2.jpg [ 206.89 KiB | Viewed 1271 times ]
antennaframe.jpg
antennaframe.jpg [ 496.03 KiB | Viewed 1244 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Wed 25, 2019 3:32 pm 
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I had an accident while trying to wind the loop antenna.

It slipped from my grasp and fell on the floor, and shattered into many pieces!
I had no idea the phenolic perf board was so fragile! :shock:

I found some fiberglass board and rebuilt it. No windings yet...

-Steve


Attachments:
antenna-frame-fg.jpg
antenna-frame-fg.jpg [ 179.7 KiB | Viewed 1182 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Thu 26, 2019 6:40 pm 
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Wound 33 turns onto loop frame and dipped it.

Too much inductance. i need to unwind a few turns.

-Steve


Attachments:
loop-antenna.jpg
loop-antenna.jpg [ 354.24 KiB | Viewed 1135 times ]
megacycle-meter.jpg
megacycle-meter.jpg [ 735.67 KiB | Viewed 1135 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Fri 27, 2019 2:02 am 
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You've got a Bona-fide Mega-Cycle Meter? Wow, that little radio was born lucky, :lol:

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 Post subject: Re: 3 tube superhet
PostPosted: Dec Fri 27, 2019 1:47 pm 
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I guess I am a sucker for a good quality GDO. :D

The 59LF is the only one I know of specifically made for frequencies below 1.8 MC.
Many other GDOs have additional coils available for lower frequencies, but their main function is in the HF to VHF range.

-Steve

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