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 Post subject: Assistance requested with a superhet build.
PostPosted: Aug Mon 03, 2020 3:22 pm 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 656
Location: Albany, NY
I've been trying my hand at building a superheterodyne circuit after spending much of my time experimenting with regenerative type receivers. I wanted to start with something simple and so I chose this circuit:

http://www.peeblesoriginals.com/radioarticles/images/am-superhet3a.jpg

I constructed the circuit using a 12BE6 and 12AT6 and, with a B+ of 70v, have the circuit working pretty well. I am using a small red coloured transistor radio coil for the oscillator and the 455khz IF transformer taken from a Knight Kit Star Roamer parts unit. My goal is to be able to use this circuit with space charge tubes so that I may run it off of 12vdc. I am using the 12AD6 for the mixer and the 12AE6 for the diode detector/first audio. When I change the tubes over to the space charge tubes and reduce the B+ to 12vdc I am having trouble having the set function. I can hear the oscillator start up as the tubes start to warm up but then after a certain point in warm up the oscillator drops out and refuses to come back to life.

I'm quite new to the topologies of superheterodyne designs, are there any component values that I should adjust in the above circuit to better accommodate it to 12vdc operation?


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Aug Mon 03, 2020 3:40 pm 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 171
Try to change the 22k resistor at the frequency changer's G1 into a 47k . Perhaps it will help to keep the oscillator running.
Also, the plate resistor of the audio amplifier is too large for 12V operation. I took a look at an ECKO car radio schematic and is using a 10K resistor.


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Aug Tue 04, 2020 12:34 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 1067
Location: Tokyo
I suspect the tap on the osc coil does not provide enough feedback at 12V. I think I'd use a string of 9V batteries in series for the osc B+ (18V, 27V, etc) to determine the minimum voltage needed. That transistor radio LO coil you have might not work at 12V with vacuum tubes. A standard AA5 type, if you can get one, might work better.

I notice the detector circuit has no RF filtering. At the very least, I'd add a 47p or 100p cap to the 'BLACK' IFT secondary, going to ground, across that 560K resistor.

BTW, strictly speaking, the 12AD6 and 12AE6 are not 'space charge' tubes. Low voltage space charge operation places a positive voltage on the first grid (ie, the control grid), then puts the signal on what is usually the screen grid. Obviously, a triode cannot be used in this low voltage space charge mode. The 12K5 power tetrode is, I think, the only space charge tube designed specifically for 12V car radio use.

Take a look here at a homebrew 12V superhet using tubes. The notes should be hepful:

http://members.iinet.net.au/~cool386/12 ... erhet.html

Rob


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Aug Tue 04, 2020 1:26 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
I would also try omitting the 22k resistor and the 47/50p capacitor and connect the 1st grid directly to the top of the coil of the oscillator LC tank.


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Aug Thu 06, 2020 2:02 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 768
Here is another home brew radio using 12V tubes, the EF98 and the ECH83 converter. Of note I found that the ECH83 is not much chop above about 12MHz for the oscillator (triode) section. But its perfectly fine for the MW band and up to 10MHz shortwave.

http://worldphaco.com/uploads/THE_EF98-OC16_RADIO.pdf

The EF98 is a great low voltage pentode. With a (23:1)^2 impedance matching transformer and the transistor's output choke, it creates an amplifier with only two active devices, which has an input impedance of 10M Ohm and an output impedance of 4 Ohms and only requires a couple of volts drive for full output power. It is one of those examples when "old combines with new" for a good result.


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Aug Fri 07, 2020 12:55 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3900
Location: Lexington, KY USA
How did you wire the oscillator coil?

Attachment:
Screenshot from 2020-08-06 19-51-12.png
Screenshot from 2020-08-06 19-51-12.png [ 49.76 KiB | Viewed 522 times ]


https://www.mouser.com/datasheet/2/449/XC-600135-1212399.pdf


The transistor ones usually provide both a tap on the main winding and a separate primary winding, so there may be a way to make the circuit work.

Ted


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Aug Thu 13, 2020 6:13 am 
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Joined: Mar Sun 22, 2020 5:56 am
Posts: 801
Location: Arvada, CO
I did some experimenting of my own, and found out that the 12BE6 will do just fine at 12V plate.

_________________
Electronics are filled with smoke. It’s my job to put the smoke back in when they fail.


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