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 Post subject: Anyone recognize this 3-tube homebrew?
PostPosted: May Mon 23, 2022 7:09 pm 
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I got this home brew 3-tube radio (superhet regen???) in a box of assorted radio parts. It was attached to a home brew power supply. I don't really know what I have here and would like to find a schematic to get the radio chassis functional. I have searched the internet and found some similar projects but nothing with the same tube compliment (2 of 3....one tube missing).

The power supply uses two power transformers and a rectifier for supplying AC & DC voltages. 30 VDC & 6.7 (6.3) VAC. I have tested the power supply and those voltages are what I find at the output terminals.

My main question is about the radio chassis. The tubes present were a 12AT7 and a shielded 6AK5. The output tube (9-pin miniature) was missing. I am not sure what tube was there? I am not sure this radio ever worked as I have found several serious wiring errors so far, including unsoldered wires and electrolytic capacitors grounded on both ends. If I had a schematic I could correct the builder's mistakes. It really appears to be a nice build but things get sloppy under the hood. It doesn't look like an old project but some older capacitors and resistors are used - maybe for the vintage look? Based on the plug-in coils, I think this radio covers the lower SW frequencies. Each 4-pin coil has different windings and could be switched to change frequency. If I can't find any info on this radio project, I will probably just part it out....eventually.

Anyone recognize this 3-tube radio project? Related circuit? Thanks!

Jeff


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Mon 23, 2022 8:47 pm 
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I've seen those photos before, probably this year. Are you sure this doesn't appear elsewhere on ARF?

You'd probably have to make your own schematic, but that's do-able and not too laborious.

Tom

PS -- maybe the missing tube was a triode-pentode like a 6U8A or 6AN8.


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Mon 23, 2022 8:52 pm 
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The unknown 9 pin socket has the filaments wired like they would be for a 12AU7, 12AT7, 12AX7 type. It should work with any of those, but obviously one will work better than the other two.

Those appear to be IF transformers, so based upon that fact the set has to be a superhet. From what is visible below the chassis, there may not be enough components present for it to work. And it would be really unusual to find a set having 6AK5 and 12AT7 tubes with only 30 volts for B+.

Given that really old Sprague electrolytic and the molded plastic caps with red writing, this was most likely built by someone who had a good stock of older parts on hand. The power supply seems like it could even be a couple of decades newer than the radio chassis.

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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Mon 23, 2022 9:38 pm 
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Jameco didn't exist until the seventies.

But that doesn't mean the radio is that recent.

The hobby magazines had a lot of simple receiver projects. Superhets with superregen detectors. Maybe some for HF with a regen detector.


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Tue 24, 2022 1:32 am 
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The VC on the side might be an antenna trimmer. One plug-in coil with antenna coupling for the RF, the other for the LO (with tickler coil FB). On to a 12AT7 mixer plus LO. I say that because double triode mixer-LO front ends were popular in late 1940s and 1950s projects, using 6J6 or 12AT7. The 6AK5 could then be a regenerative IF amp (it seems the 6AK5 plate is connected to one of the IFTs and the front panel pot (regen control?) is connected to the 6AK5 cathode through a 330R resistor). Finally, on to another double triode for audio, note the phone jack.

Why 3 IFTs? Dunno. Simple regen superhets usually had just one. Two is possible. But three?

Do you have a dip meter to measure the resonant frequency of the VC and coils?

I suspect that PS was not intended for that receiver.

This is interesting. I tried to draw a schematic but the details are not quite clear enough, can't see the component values, not everything is shown. It does remind me of some HB receivers I saw in CQ, Radio Electronics, etc but no exact match.

Rob


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Tue 24, 2022 5:19 am 
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I think this is a really nice receiver that has a lot of potential. The 3 IF's are a mystery and the black caps should be updated, but other than that there are a lot, possibly too many, quality components. I would draw a schematic and find out exactly what you've got and then go on to make the necessary changes.

The power supply needs more B+ voltage. Not sure how you're getting 30V from a 12.6V transformer unless that bridge rectifier is somehow configured as a voltage doubler. A replacement transformer is going to be required. Another alternative might be to use a second 6.3V transformer and wire the secondary to the output of the first one. That would give you 115V from which you could add a voltage doubler and come up with a more respectable 200+ volts. Personally, I think this is a keeper.


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Tue 24, 2022 6:51 am 
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Quote:
Not sure how you're getting 30V from a 12.6V transformer


They're not. The Jameco 102112 is a 24VCT transformer. Underchassis wiring appears to feed that transformer to a fullwave rectifier & then large value filter caps. The 30 volts they observed is apparently the unloaded measurement. A floating filter output can rise to 1.4 time the RMS transformer secondary voltage & 1.4 ( 24V) = 33.6V. Looks like they used one 6.3V transformer for filaments (Jameco 102163) and the Jameco 102112 for the low B+.

Quote:
A replacement transformer is going to be required.


Probably not, unless they want to reconfigure the power supply for something those two transformers can't deliver.


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Tue 24, 2022 3:19 pm 
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allthumbs wrote:
I've seen those photos before, probably this year. Are you sure this doesn't appear elsewhere on ARF?

You'd probably have to make your own schematic, but that's do-able and not too laborious.

Tom

PS -- maybe the missing tube was a triode-pentode like a 6U8A or 6AN8.


Tom,

Maybe this receiver appeared here in the past but I got the receiver about a week ago and took the photos a couple days ago. I looked through all related threads in the "Homebrew" category of ARF for the past year and did not see anything.

Yes, I will likely need to draw out a schematic.

Jeff


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Tue 24, 2022 7:05 pm 
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Thanks to everyone who commented on this receiver, much appreciated. I will keep this radio & power supply and continue researching for a similar circuit. Hopefully I can draw out a schematic that would explain how the 3 IF's are used. Would the low B+ voltage be fine with high impedance headphones? That I am not sure of.

I am just guessing based on condition of aluminum chassis (like new), but I think this whole build is in the last 10 years or so. The vintage components were used intentionally to probably match the period. Maybe something from a 1950's - 60's radio construction book or magazine?
I need to spend some time on world radio history......

Jeff


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Wed 25, 2022 4:37 am 
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Radio News, May 1947. Page 48ff.

https://worldradiohistory.com/Archive-R ... 7-05-R.pdf

This is an example of what I've seen in magazines from late 40s, early 50s. It's not yours but there is a resemblance. 6J6 and 6AK5 were available from war surplus; lots of projects used them. I was thinking the front end would be the same, but using a 12AT7 instead of a 6J6. Then a 6AK5 regenerative IF detector, and another double triode for the audio. But the 3 IFTs are a puzzle. Possibly a BFO? As you say, the chassis looks too clean for a 70 year old homebrew.

Draw up the schematic. We can make this work!

Rob


Attachments:
6J6 schematic.png
6J6 schematic.png [ 61.71 KiB | Viewed 1900 times ]
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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Thu 26, 2022 3:08 pm 
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First, I apologize for the amateur drawn schematic. I am really good at reading them, just not so good at drawing them. I am a retired biologist who restores vintage radios (tube & transistor) and builds simple radios (crystal, regen, TRF) as a hobby. I always had an interest in electronics, but I chose another career path in life. Now I have the time to enjoy this hobby and learn along the way.

Hopefully the drawn schematic is legible, as it is accurate and based on what I have. I have highlighted the IF transformer leads (pri/sec) from each of the 3. I don't know what type they are so the schematic symbol for them is general. I should have split the 12AT7 tube and relocated the filament string to avoid the clutter but I realized that late in the game.

I also attached another close-up photo of the under-chassis - hopefully a sharper image.

I have been looking through the 1947 and later radio magazines on WRH but found nothing close yet. If anyone has a recommendation for the unknown 9-pin output tube (pin-out similar to the 12AT7), I am tempted to try this receiver with the attached power supply. If I can get this operational, I plan to replace all the vintage capacitors with new replacements.

Jeff


Attachments:
DrawnSchematic.JPG
DrawnSchematic.JPG [ 867.68 KiB | Viewed 1827 times ]
ChassisDetail.JPG
ChassisDetail.JPG [ 744.58 KiB | Viewed 1827 times ]
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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Thu 26, 2022 10:40 pm 
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I didn't see any sign of an oscillator coil in the pics, so I'm wondering is one of the IF transformers is the oscillator coil.


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Thu 26, 2022 11:51 pm 
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Just a couple of quick observations -- you are showing the cathode of the 6AK5 going to +30 vdc. I don't think that can be right (but I might be wrong. I often am :oops: ).

I think you could make better sense of your wiring diagram if you were to redraw it so that you don't show the tube base pins in their physical positions and locations, i.e., 1 through 9 in a clockwise pattern. And for less clutter leave out the filament and its pin numbers -- we already know that almost all 7-pin miniatures use pins 3 and 4 for the filament and most 9-pin tubes use 4 and 5 (and sometimes 9, e.g., a 12AT7). Draw it like a typical schematic, then add the pin numbers accordingly. Example:

Attachment:
schem.JPG
schem.JPG [ 15.73 KiB | Viewed 1782 times ]

Hoping this might help you,

Tom


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Fri 27, 2022 12:10 am 
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Yes. When I trace something out, kt's very much a component by component trace, to get it down. Redrawing it turns it into a real schematic, one that makes sense.


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Fri 27, 2022 12:18 am 
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The plug-in coil in the upper left is the antenna coil, the other plug-in coil is the local oscillator coil. The left section of the 12AT7 is the mixer and the right section is the local oscillator. T1 is the input IF transformer. The 6AK5 is the IF amplifier and the pot is the gain control (the bottom of the pot should be grounded). T3 is the output IF transformer. T2 is the BFO coil and the right half of the missing tube is the oscillator that goes with it. The left half of the missing tube is the detector.

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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Fri 27, 2022 12:50 am 
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allthumbs wrote:
Just a couple of quick observations -- you are showing the cathode of the 6AK5 going to +30 vdc. I don't think that can be right (but I might be wrong. I often am :oops: ).

I think you could make better sense of your wiring diagram if you were to redraw it so that you don't show the tube base pins in their physical positions and locations, i.e., 1 through 9 in a clockwise pattern. And for less clutter leave out the filament and its pin numbers -- we already know that almost all 7-pin miniatures use pins 3 and 4 for the filament and most 9-pin tubes use 4 and 5 (and sometimes 9, e.g., a 12AT7). Draw it like a typical schematic, then add the pin numbers accordingly. Example:

Attachment:
schem.JPG

Hoping this might help you,

Tom

Thanks Tom. I started out showing all pins #'s in order and the complete internal tube layout for my own clarification. Once I got going, I realized that doing so was just making the drawn schematic cluttered and confusing. Appreciate the help!

Jeff


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Fri 27, 2022 1:02 am 
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Yes, I think Jim's got it. The double triode mixer/LO front end, a popular late 40s/50s circuit. 6AK5 IF amp. Another double triode for the BFO and a stage of audio amplification. But no AM detector. So it was a CW only receiver. The earlier HB receiver designs usually had a IF regenerative detector, not a separate BFO.

I wonder why the builder chose a 30VDC B+. It's not impossible to get it work on such a low B+, but it makes life difficult. He could have used two back to back low voltage transformers to power the heaters and provide 100~120VDC or so for the B+.

If you ground the unconnected end of the 2.5K pot, plug in 2K headphones, it might even work.

Rob


Last edited by shinkuukan on May Fri 27, 2022 1:21 am, edited 1 time in total.

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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Fri 27, 2022 1:20 am 
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Something else: the tuning range. The LO VC is 210pF. Assume a minimum C of about 30pF in circuit (stray C, etc). 210/30. a 7:1 ratio. The square root of 7 is 2.6. 2.6 is the tuning ratio. Eg, 6 to 15MHz. That's rather a wide range for a CW only receiver without a bandspread VC. Perhaps he intended to add caps to the plug in coils to narrow the tuning range.

Rob


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Fri 27, 2022 1:21 am 
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Jim Mueller wrote:
The plug-in coil in the upper left is the antenna coil, the other plug-in coil is the local oscillator coil. The left section of the 12AT7 is the mixer and the right section is the local oscillator. T1 is the input IF transformer. The 6AK5 is the IF amplifier and the pot is the gain control (the bottom of the pot should be grounded). T3 is the output IF transformer. T2 is the BFO coil and the right half of the missing tube is the oscillator that goes with it. The left half of the missing tube is the detector.

Thanks Jim!
I think your analysis may be correct. The LEFT plug-in coil has a very small "A" written in marker on the bottom. The RIGHT plug-in coil has a sloppy drawn "O" or "C" on the bottom. Now going back and looking, it looks more like an "O". (Sorry, in hindsight that was an important clue).
Do you have an idea of what the missing tube was? A 9-pin dual triode ideal for OSC/DET?
Thanks for taking the time to look it over. Now this homebrew is getting interesting.....if it can actually work.

Jeff


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 Post subject: Re: Anyone recognize this 3-tube homebrew?
PostPosted: May Fri 27, 2022 1:27 am 
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shinkuukan wrote:
Yes, I think Jim's got it. The double triode mixer/LO front end, a popular late 40s/50s circuit. 6AK5 IF amp. Another double triode for the BFO and a stage of audio amplification. But no AM detector. So it was a CW only receiver. The earlier HB receiver designs usually had a IF regenerative detector, not a separate BFO.

I wonder why the builder chose a 30VDC B+. It's not impossible to get it work on such a low B+, but it makes life difficult. He could have used two back to back low voltage transformers to power the heaters and provide 100~120VDC or so for the B+.

If you ground the unconnected end of the 2.5K pot, plug in 2K headphones, it might even work.

Rob

Thanks Rob!
I was looking forward to your comments. It is starting to make sense now. The next question is what tube would be ideal for the open socket? Jim mentions oscillator function and detector in each half of the tube.

Jeff


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