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 Post subject: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 1:07 am 
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Joined: Apr Mon 22, 2019 5:29 pm
Posts: 7
Location: Plymouth, Massachusetts
I picked up this receiver at the MIT flea market in Cambridge, MA on Sunday. It uses uses a 455 kHz I.F. with milsurp filter cans, it has 3 bands, 80 & 40M plus what seems to be above 40M that was probably used for CHU. 7 tubes not counting the rectifier and regulator.
Here's the tube line up:
12AX7
12AT7
6BA6 X 2
6J6
6C4
6AK6
6X4
OB2

The seller was the son of the builder who is now a SK and did not know anything about it.
Does anyone have any ideas, ring any bells?

Thank you,
Jon

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20190818_094215.jpg [ 181.73 KiB | Viewed 1970 times ]

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20190818_094236.jpg [ 218.63 KiB | Viewed 1970 times ]


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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 10:57 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 997
Location: Tokyo
Just some ideas. A while back, I read through radio mags from the 1950s into the 1960s (TV-Radio News, CQ, QST, etc). I didn't see anything exactly like this but I did notice many HB receivers used double triodes for RF amps, mixers, and LOs. The 6J6 in particular was very popular.

So, it appears to me that the 6BA6 is possibly an RF amp (impedance coupled to the mixer?), the 12AT7 (noting its position next to the IFT) the mixer, the 6J6 the LO. The 6C4 maybe an AM detector. 1/2 12AX7 BFO. 1/2 12AX7 and 6AK6 for the audio. The 6X4 and OB2 are obviously for the PS.

Now, the two xtals marked Channel 47 24.7Mc and Channel 329 32.9Mc. I guess that there are actually 455KHz xtals inside to provide a IF filter, and for some reason, those xtal cases had to be used.

Do you a 12AX7 so that you can test the receiver?

Rob

For reference, here are two circuits from QST 7/1955:


Attachments:
MIXER 6J6.png
MIXER 6J6.png [ 54.05 KiB | Viewed 1921 times ]
LO 6J6 .png
LO 6J6 .png [ 46.16 KiB | Viewed 1921 times ]
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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 11:04 am 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 28210
Location: Annapolis, MD
Homebuilt...

You probably need to start by drawing up a schematic

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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 11:49 am 
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Joined: Apr Mon 22, 2019 5:29 pm
Posts: 7
Location: Plymouth, Massachusetts
Rob, Thanks for your thoughts. I have tested the receiver and it works. It receives 80 and 40 meters. I don't have any old Handbooks but I do have access to some.


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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 11:52 am 
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Joined: Apr Mon 22, 2019 5:29 pm
Posts: 7
Location: Plymouth, Massachusetts
pixellany wrote:
Homebuilt...

You probably need to start by drawing up a schematic

It's definitely homebrew. I posted my original message in the Homebrew category but it looks like a Mod moved it here.
Ya, before I started drawing I hoped someone would recognize it and save me some time.


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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 3:15 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3555
Location: Monterey California USA
Regarding the crystals, those are WWII tank radio crystals that have a fundamental in the 455-456 kHz range. They were popular for making receiver filters. In the original application they were multiplied all the way up to the 25-40 MHz range in an FM transmitter and were stored in a drawer in the transmitters in enough quantity to cover an entire band. The channel number marked on them is the end product. There is a list of fundamental frequencies for those in one of the surplus handbooks.

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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 4:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 8666
Location: Beaver Falls, PA. USA
See if there are any date codes on the major components; that will give you an approximate year of construction. Then, look through the QST issues before that time. Offhand, I would say it's late 1950s or 1960s vintage.

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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 4:58 pm 
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Posts: 7239
I agree with Tim about the time era of this design. There were a lot of two band receiver designs of the era using an IF and crystals in the 1700 Khz. range through "band imaging" but I don't recall seeing any limited coverage receivers just like yours with another band and a 455 Khz. IF using the FT-241 style channel crystals. I don't know what conversion scheme is used in that one but is there any chance that third band is intended to provide 20 meter coverage and an image/misalignment issue is causing it to cover the upper end of the 40 meter range?

I glanced through some of the West Coast/Orr handbooks of the era but didn't see anything like your receiver.

The good news is these circuits are simple enough that even if you cannot find an original construction article it should still be easy to trace out. Even if you find the original article, don't be surprised to find changes as builders modified circuits to incorporate their preferences while also modifying to make use of components they had on hand.

It looks like the original builder spent some time with this one. If you decide to expand its range, consider an outboard converter and designs for these showed up in the handbooks. That setup would provide good sensitivity, stability, and image rejection for higher band coverage and although crystal control provides better stability building a mechanically and thermally stable fixed frequency L/C oscillator is far simpler than its variable counterpart so don't let the lack/expense of suitable crystals deter you from building a converter if your interests lead you in that direction.

Rodger WQ9E


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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 8:40 pm 
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Joined: Sep Tue 30, 2014 6:08 am
Posts: 5554
Location: Norfolk, VA
What's the power transformer part number and maker? With that, a search at ARH or the IA with the part number is possible to find a project that used that transformer. It's worked well for me - I could stand at the sellers table, and using both sites, I was able to find the exact article in a 73 Magazine issue. It also helps to identify any missing parts that may be unobtainable or deviations from the project layout.

The same technique works well with Meissner or Miller can coil and transformer part numbers - search on the part number, and it's a simple process of elimination after that.

The only setbacks are when it's a QST or ARRL-only project, as the search is hampered by a dearth of that info online.

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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 10:27 pm 
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Joined: Oct Tue 30, 2018 5:51 pm
Posts: 9
Don't overlook the possibility that this was not copied from one article but is an original or partially original design!

This is almost the same as the (80/40) band imaging design but with a switched local oscillator range provided by the panel switch (with the trimmers going to the oscillator coil).

John, KU6X


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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Wed 21, 2019 11:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3498
Location: Seattle WA US
FYI: The crystal marked Channel 47 should have a fundamental freq of 457kc. Channel 329 fundamental is 456kc.
-Chuck


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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Thu 22, 2019 12:53 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 997
Location: Tokyo
What's the tuning range for each band?

Something that puzzles me: there seem to be just two RF coils (on the left side of the chassis, last photo), but three LO coils. And that dual section VC topside: that is RF tuning, I presume, but with possibly only one section used?

As I mentioned, I did go through CQ, QST, Radio TV News/Electronics World, and Radio Electronics, looking for and saving articles on HB receiver construction. From late 40s to early 60s. I didn't see anything quite like this one. Makes me think it was probably one of a kind, using what the builder/designer had on hand.

Could you draw out the mixer and LO circuits? That would be the heart of the radio.

Rob


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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Thu 22, 2019 10:32 pm 
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Joined: Jul Tue 21, 2009 1:38 pm
Posts: 1004
Location: SW WA 98565
WS1K wrote:
I don't have any old Handbooks but I do have access to some.


Jon,

Nice find! Envy your access to the MIT Hamfest! I haven't been to one in decades... The older handbooks (ARRL, ORR, RSGB, + antennas) are a gold mine of information.
Likewise the various 73, SAMS, and TAB book publications. You can't have enough reference material.
Some stuff you see for maybe a few years, then you never see it again for decades, or even at all.
Best of Luck with your receiver!

73, -Tom N1BEC/7


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 Post subject: Re: 7 Tube Ham Receiver
PostPosted: Aug Fri 23, 2019 4:01 am 
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Posts: 2102
Location: Costa Mesa, California
It looks like the RF coils are for the antenna and RF plate (6BA6). These are tuned with the ganged capacitor separate from the LO tuning. Their range must be broad enough for both bands or even three bands. The LO then does not need a dual capacitor for tuning, and tracking is not an issue--as there is no need to tune two different frequencies (455 KHz, or the IF, apart) with one control. This tuning scheme would be awkward for a commercial unit, but for a homebrew, where the operator understands the scheme, it is functional. The third variable capacitor under the chassis is for the BFO. I agree with others that drawing the schematic is the first step.

Norm

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