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 Post subject: 3 tube superhet
PostPosted: Nov Mon 14, 2016 11:42 pm 
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I've been finding all the 100 mA tubes that were produced, with the help of forum members....

viewtopic.php?f=19&t=310619

Sooo....I've been thinking of the idea of building a 3 tube superhet that is just as capable as an AA5 design.

Using an 18FX6A, a 20EQ7 and a 50BM8 along with a solid state rectifier, I think this would work quite well.

I threw together a quick and dirty schematic of the radio. I know it will probably need tweaking depending on what parts I can find.

The dropping resistor for 120VAC would be a 320 ohm resistor dissipating 3.2 watts. A 5 watt would do well, a 10 watt would run extra cool.

-Steve

P.S. the 50BM8 is a triode and a Beam Power tube in one package. I just drew them separated.


Attachments:
100matuberadio.gif
100matuberadio.gif [ 7.98 KiB | Viewed 4937 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 15, 2016 3:01 am 
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You could also use a dropper cap for less heat dissipation. Don't forget a fuse, and maybe an iso tranny!

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 15, 2016 4:44 am 
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Or, replace the silicon diode with a 36AM3B for a nominal heater string voltage of 124 volts. Yes, I know it would then be a 4-tube super, but it'd be all hollow state!

John

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 15, 2016 12:37 pm 
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That is a good idea. Though, I was trying to keep the tube count to a minimum. :wink:

I forgot to mention, the "ground" symbols just mean a common ground. It is not meant to be chassis ground or even earth ground.
The schematic was a thrown together preliminary drawing for concept.

Yes, a fuse would be a great idea and yes, I would probably run it on an isolation transformer. :D

Thanks tor the thoughts and ideas!

-Steve

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 15, 2016 3:29 pm 
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At 88v total one could use a 1N4007 as dropper... I have same in a couple of mine, one has 150ma tubes and other 300ma tubes...


Tom


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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 15, 2016 3:44 pm 
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Keep tubes to a minimum? Blasphemy!

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 15, 2016 3:49 pm 
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35Z5 wrote:
At 88v total one could use a 1N4007 as dropper... I have same in a couple of mine, one has 150ma tubes and other 300ma tubes...


Tom



Yes, that's what I would suggest as well. Takes up the least space of any dropper and creates no heat.

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 15, 2016 4:11 pm 
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dkjones96 wrote:
Keep tubes to a minimum? Blasphemy!
:lol: :lol: :lol: :lol: :lol: :lol:

Since a 50BM8 can handle 250v I'd look at building it with a voltage doubler and PP output...


BTW the 50BM8 was registered by Rogers in May of '58... Their original data sheets show Aug '57 but apparently didn't get around to registering it till early in '58 when they also included 6922(a increased gain 12AU7)...


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 Post subject: Re: 3 tube superhet
PostPosted: Nov Sat 19, 2016 1:06 pm 
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The 50BM8 came in the mail yesterday. Its such a pretty red and blue box!
Tungsram I found out was a Hungarian company, but there is absolutely no indication of origin on the tube or box. In fact, nothing other than Tungsram and UCL82.

Second photo of the three types I selected. 18FX6A, 20EQ7 and 50BM8.
Unfortunately, the 20EQ7 is a dud...

-Steve


Attachments:
50bm8.jpg
50bm8.jpg [ 71.74 KiB | Viewed 4722 times ]
3tubes.jpg
3tubes.jpg [ 80.64 KiB | Viewed 4722 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Sat 19, 2016 2:42 pm 
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Booo that's rotten, I think I have a 6EQ7, definitely no 20...


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 Post subject: Re: 3 tube superhet
PostPosted: Nov Sat 19, 2016 7:16 pm 
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That 20EQ7 has no direct substitute that I can find. If I were building that set I would use 6BM8 audio and 6BA7 converter and either 6LM8 or 6MU8 for I.F. and detector. You simply use the triode as a diode and youre in. This design forces a power transformer on you and gives the safety of line isolation. I would use the 6BA7 instead of the 6BE6 as it is a better tube more gain and it keeps it to all 9 pin tubes to boot. These are my suggestions. If you go with what you have there keep it safe and do not leave out an isolation transformer to save money. 20EQ7 should not be very expensive out here online.


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 Post subject: Re: 3 tube superhet
PostPosted: Nov Mon 25, 2019 3:16 am 
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This project was put on the back burner as I couldn't find a 20EQ7, however some other ideas came to mind.

How about a 70L7, 12EQ7 and a 19HV8?

Using the 19HV8 as a converter using the same scheme GE used with a 12AU6 as a converter with the triode section as an audio amp.
12EQ7 as the IF and detector
Then the 70L7 as a rectifier and audio output.

All 150 mA tubes instead of 100 mA.

AVC can be applied to the 12EQ7, but not the converter stage.

Hopefully, it will work well enough. Those GEs that used it seem to work well too.

Just need to find the right osc coil and pair it with the matching variable cap and an antenna loop or loopstick.

I placed an ad in the Classifieds for what I want, now to see how things go. :D

-Steve


Attachments:
begin.jpg
begin.jpg [ 164.91 KiB | Viewed 1792 times ]
trio.jpg
trio.jpg [ 301.1 KiB | Viewed 1792 times ]
70l7ch.jpg
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GE 861.JPG
GE 861.JPG [ 186.16 KiB | Viewed 1792 times ]
100matuberadio.gif
100matuberadio.gif [ 9.18 KiB | Viewed 1689 times ]

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Last edited by azenithnut on Nov Tue 26, 2019 1:49 pm, edited 1 time in total.
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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 26, 2019 6:23 am 
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I like your new tube lineup and the schematic, except I don't get the use of the power tube also as a rectifier.
The original silicon diode was a better choice, no?
I have to be convinced that you can have both a bias voltage and B+ on the cathode of the 70L6 and not have trouble as well. :lol:

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Tue 26, 2019 11:03 am 
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Thanks! I could easily find 12EQ7s all day long, but the 20EQ7 is an enigma.
I even offered tube sellers $20 if they could find me a new one.

The 70L7 is a two section tube.

It contains both a rectifier and a Beam Power output stage.

I could reinstate the solid state rectifier, that is why I left it in the image if I decided to put it back.

I've updated the schematic to show the two sections of the 70L7

-Steve


Attachments:
70l7.jpg
70l7.jpg [ 17.37 KiB | Viewed 1696 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Wed 27, 2019 7:54 am 
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You could also use a 12SF7 for the IF amp and detector.

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Wed 27, 2019 10:22 pm 
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Yes, a 12SF7 would work just fine, but I decided on the 12EQ7. Does a 12SF7GT exist?

I poked the chassis full of holes today and dry fitted some of the parts. Well the transformer is a fake, just a choke, but is the same size as the one I am getting.
Also needing some IF transformers and osc coil.

The variable cap is from an old GE I had, and I think will do fine.

-Steve

EDIT: I added a simulation of the complete look to the top with tubes.


Attachments:
holeychassis.jpg
holeychassis.jpg [ 187.81 KiB | Viewed 1610 times ]
tempback.jpg
tempback.jpg [ 193.82 KiB | Viewed 1610 times ]
tempfront.jpg
tempfront.jpg [ 238.93 KiB | Viewed 1610 times ]
top-temp.jpg
top-temp.jpg [ 179.16 KiB | Viewed 1585 times ]

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Fri 29, 2019 1:31 am 
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Steve, I think you can build a 2 tube set using a compactron with 2 separate pentodes, such as were used in some TV IF strips back in the day those tubes were popular. That would give you the converter and IF in one tube if you used the 12AU6 type circuit, and your second tube could be a triode pentode audio amplifier. I don't think there were any 150 ma compactron IF tubes though, so you might need to use a filament transformer. Two diodes, one for rectifier and one as detector would complete the set.

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Fri 29, 2019 2:07 am 
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Like.

Norm

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 Post subject: Re: 3 tube superhet
PostPosted: Nov Fri 29, 2019 4:01 pm 
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I think I read on the TCA Forum that there is an RCA publication on the design of a three tube set using the 100 ma tubes... I have a production RCA set using 4 tubes.
The same article said that RCA never manufactured a 3 tube version. And it also said that there never any AM broadcast band sets manufactured with Compactrons although they did make it into some FM tuner designs..

Robert.


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 Post subject: Re: 3 tube superhet
PostPosted: Nov Fri 29, 2019 4:33 pm 
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Yes, a 6AR11 paired up with a 6T9 or even possibly a 6T10 could work if a solid state diode is used for a detector along with a solid state rectifier.
I thought about the idea, but I will continue with my three tubes here.

John, Jim, Dennis, Robert and Norm, thank you for the comments. I think I may have secured the parts I need to build this little set.
Waiting on a delivery from an ebay and an AES purchase.

-Steve

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