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 Post subject: Assistance requested with a superhet build.
PostPosted: Aug Mon 03, 2020 3:22 pm 
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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
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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: 1162
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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Location: Seattle, WA (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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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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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 2285 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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I did some experimenting of my own, and found out that the 12BE6 will do just fine at 12V plate.

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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Apr Wed 07, 2021 3:42 am 
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Location: Albany, NY
I've been mulling over mixer designs and instead of making another thread and adding clutter I would just add to this topic. Considering that, this may be a potentially silly question but is it possible to use a tuned Colpitts oscillator for the oscillation section of the 6BE6? Every instance I see online uses a Hartley oscillator and it'd be nice to not have to deal with figuring out the ideal tap position for the oscillator coil as it seems to be a bit critical when operating at lower voltages.


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Apr Wed 07, 2021 4:40 am 
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Location: Arvada, CO, 80004
I think the answer would be no. It needs a plate supply. However, it might work with an old Heptode/Pentagrid converter, such as a 1A7G/GT, 1A6, 1C6, or a 2A7, or a 6A7, 1D7G etc...

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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: Apr Thu 08, 2021 12:27 am 
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Joined: Dec Sun 14, 2008 3:33 pm
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Location: Tokyo
Compared to the Hartley, the Colpitts has a shorter tuning range (which may or may not be a concern for your needs) and lower output.

Rob


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: May Fri 21, 2021 5:49 pm 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 671
Location: Albany, NY
I've been working creating a 12BE6 superhet that also will work as a shortwave converter as well as a short range broadcaster.

With regards to the superhet configuration it's a 12BE6 being run into a single 455kc IF transformer and then into a MK484 IC for RF amplification and demodulation and then temporarily into an op amp based AF amplifier. Coils used at the moment are the antenna and oscillator coils salvaged from a Knight Star Roamer.

I may or may not go tube for the RF amplification and demodulation. I am designing this to be portable and so space is at a premium.

My question is that I currently have the oscillator and RF front end sections on separate variable capacitors and when I adjust the RF front end variable capacitor in order to bring it into resonance for best sensitivity the frequency shifts slightly. Is there any way to correct this? I've built an NE602 shortwave converter with a similar separate oscillator and RF front end configuration and adjusting the RF front end variable capacitor does not cause the frequency to shift.


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: May Sat 22, 2021 1:15 am 
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Joined: Dec Sun 14, 2008 3:33 pm
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Location: Tokyo
Quote:
My question is that I currently have the oscillator and RF front end sections on separate variable capacitors and when I adjust the RF front end variable capacitor in order to bring it into resonance for best sensitivity the frequency shifts slightly. Is there any way to correct this? I've built an NE602 shortwave converter with a similar separate oscillator and RF front end configuration and adjusting the RF front end variable capacitor does not cause the frequency to shift.


This frequency shift is called 'pulling' and there's no easy way to eliminate it. A separate LO oscillator would certainly help, but that's not really practical in this case, is it? The usual procedure is to use a ganged RF + LO VC plus a small, separate VC, something around 15~25pF. The small VC is peaked for signal or for noise. Or else, eliminate the small VC and live with imperfect RF-LO tracking.

Perhaps the easiest solution is just to tune the LO to the frequency range you want to receive, then peak the RF section for noise, then re-adjust the LO tuning. Not elegant, but it works. Btw, be careful you don't inadvertently tune the RF section to the image frequency, which would be 910KHz above the frequency you want to hear.

Rob


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: May Mon 24, 2021 7:00 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1247
ACORNVALVE wrote:
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.


I am sorry but the links to my previous websites are broken and the URL's don't work. I had to go to a new service provider. But if you hunt around, on the worldphaco.com website, which is now re-hosted & running again, you will find the article on the EF98 & OC16 radio.


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: May Thu 27, 2021 3:17 am 
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Joined: Mar Sat 08, 2008 3:21 am
Posts: 638
Location: hillsborough, nc
There were commercial receivers that didn't use tracking, the Hammarlund Comet Pro being a great one.
There are actually some advantages, like intentionally using the other side for the osc, to reduce interference in some cases, as an RF gain control, and being able to set the frequency range as you want. You can also choose the end of the range with the best tuning rate.
I think this fine and only takes a second longer than ganged tuning, but MUCH simpler!
I have my dad's Comet, which I first remember from about 4 yrs old. I'm 79 now, so it's pretty familiar.
WL


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: May Fri 28, 2021 10:19 am 
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Posts: 1247
928GTS wrote:
I am using a small red coloured transistor radio coil for the oscillator



...ooops.

A transistor oscillator coil is completely unsuited for use in a tube radio circuit. There are multiple reasons in this case.

You require an osc coil suited to tube use. They are available from AES (Antique Electronic Supplies) or Tubesandmore.com


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: May Sat 29, 2021 5:48 am 
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Location: Arvada, CO, 80004
ACORNVALVE wrote:
928GTS wrote:
I am using a small red coloured transistor radio coil for the oscillator



...ooops.

A transistor oscillator coil is completely unsuited for use in a tube radio circuit. There are multiple reasons in this case.

You require an osc coil suited to tube use. They are available from AES (Antique Electronic Supplies) or Tubesandmore.com


I would agree. The reason that comes to mind first is the impedance of the coil is too low.

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


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 Post subject: Re: Assistance requested with a superhet build.
PostPosted: May Sun 30, 2021 3:47 pm 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 671
Location: Albany, NY
ACORNVALVE wrote:
928GTS wrote:
I am using a small red coloured transistor radio coil for the oscillator



...ooops.

A transistor oscillator coil is completely unsuited for use in a tube radio circuit. There are multiple reasons in this case.

You require an osc coil suited to tube use. They are available from AES (Antique Electronic Supplies) or Tubesandmore.com


I've since switched to using either coils salvaged from a parts Knight Star Roamer set or molded leaded inductors. Due to the compact nature of this build I've been experimenting to see if molded leaded inductors have enough Q to allow for reliable oscillator startup when used at low voltage and so far things look pretty promising at least up to 14mc.


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