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 Post subject: Re: 5 Tube Regen
PostPosted: Jan Sun 21, 2018 12:14 am 
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Joined: Oct Thu 13, 2011 8:26 pm
Posts: 282
Location: Isanti MN
A friend sent an article form Electric Radio, June 2005, beginning on page 10. Very interesting about the German history with specific relation to the regenerative radio. Some interesting references are also mentioned. I notice the use of a 6K7 as an RF amp on the authors version of the regen using the screen grid as the RF gain control

So I tried using a 6AU6 pentode in place of the triode and it seems to work very well. The RF gain control works as expected. The controls work nice and smooth.

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The weather here now is not dry a crisp as it was a week ago but the stations are still coming in loud and clear. Still attempting to get more sensitivity. I'm thinking the 6AU6 plate load resistor should be larger. So next experiment with that. Anybody have any suggestions.

Tried lowering the antenna cap Ca to 15pFd but then only the strongest stations came in. Changing Rk, Cathode resistor, (the 5Kohm) didn't seem to make much difference as long at it wasn't zero, maybe 2K or so. I finally took the 3K pot out and left in the 5K. The relationship between the Rk and Rp must have some control over the stage gain.

Lyn


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 Post subject: Re: 5 Tube Regen
PostPosted: Jan Sun 21, 2018 1:42 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 673
Location: Tokyo
I've tried using an RFC in place of a resistor for the GG amp plate load. It didn't matter very much. The problem is, the internal capacitance within the tube and external capacitance (especially the tube socket) shunt a lot of the signal to ground. At MW and low SW frequencies, you can get a some gain, but you usually don't need it down there. It does help when you have a short antenna (which I do, just some wire run around the ceiling). At higher SW frequencies, most of the gain is lost. Sometimes adding a GG amp will actually cut total signal level, as if it were an attenuator. I've noticed that happening around 15MHz and up.

It's hard to get gain out of an RF amp. That's why the amp plate usually has a tuned LC tank hung on it. The more important functions of the GG amp with a regen are isolating the antenna from the detector and matching a low Z antenna to the high Z detector tank.

Experiment with different values of plate load resistors and try RFCs too. But I think you'll find they don't really make much difference. Once the plate load is 10K or higher, you're getting about as much gain as possible.

The input cap from the antenna is usually 470pF or larger to provide very low reactance to incoming signals. Of course you could switch in different values of C to act as an RF attenuator; I've done that, beginning with no connection at all (just the C of the switch itself and surrounding wiring), then 2pF, 5pF, etc, up to about 500pF. However, it's easier and cheaper just to feed the antenna to a pot (say, something between 1K and 10K), and take the output from the wiper to a 470pF cap and on to the GG amp cathode.

You can vary the GG amp gain on a triode by changing the bias on the cathode. This should be done with a fixed resistor from cathode to ground (1K to 5K), and using a pot in a voltage divider to put a small positive voltage (say, 0 to 15 volts) on the cathode. I think it's easier to just attenuate the incoming RF signal.

Rob


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 Post subject: Re: 5 Tube Regen
PostPosted: Jan Wed 24, 2018 8:12 pm 
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Joined: Oct Thu 13, 2011 8:26 pm
Posts: 282
Location: Isanti MN
Rob, one more time at least. The 6C4 has about the lowest G to H + K capacitance for a triode I could find; 1.8pFd. Searching in my junk I found one. BTW, some of the other mixer tubes have even less capacity.

Is this what you described for bias adjustment? I wired it up this morning and it function well but damp cloudy air again today. Will give it a go again tonight. Made the input capacitor adjustable too.

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Didn't try a choke yet, maybe next. If this works well, I may move on to making a better detector coil with higher Quality. I've been reading Sturley. Don't understand all the math but enough to get the idea.

Lyn


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 Post subject: Re: 5 Tube Regen
PostPosted: Jan Thu 25, 2018 1:05 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 673
Location: Tokyo
Lyn, my description of the bias control was not correct. Sorry, I should have double checked it. It would be best for me to draw it and scan, but the scanner broke and... So we're stuck with words.

Here's the standard circuit: take the ground end of your 3.9K cathode resistor and connect it to the wiper of your 38K pot (btw, you have a 38K pot? Odd value). One end of that pot goes to ground, the other end to your 40VDC. You can put a cap from the wiper to ground; if you hear a scratchy noise when turning the pot, put a larger cap on the wiper, say, .1uF. However, the way you have drawn it should work and it does. Fine. Don't change it.

I'm not sure about that 140pF VC in parallel with the 330pF cap. I suspect adjusting that doesn't make too much difference. If you removed the 330pF, I think you would get much greater control over the incoming signal level.

As for suitable GG tubes, I've tried a bunch: 6C4; 6AB4; the triode which come with 6JN8, 6EA8 and 6U8 pentode/triode combinations; 6AK5 (triode connected), etc. And I've tried RFCs, and varied the value of the plate resistor and the cathode resistor. I didn't find much difference. I did notice at high frequencies GG amps sometimes attenuate the signal rather than amplifying it. Sometimes when I controlled the gain by bias adjustment (as you are doing now), the signal would sneak around the GG amp and make it to the detector, rendering the GG amp pointless. These are common problems with GG amps above 15MHz or so. Not a problem for MW.

A higher Q coil will improve selectivity. Good for SW, maybe not so good for MW BC.

I certainly do not understand all the math in Sturley. I generally just look for the 'good parts'.

Rob


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 Post subject: Re: 5 Tube Regen
PostPosted: Jan Thu 25, 2018 4:59 pm 
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Joined: Oct Thu 13, 2011 8:26 pm
Posts: 282
Location: Isanti MN
Thanks for all the input. I now understand your circuit does the same thing as mine with less parts, simpler. We used to null (zero) the output of op-amps with a summation circuit similar to what I made and my mind jumped to that circuit. Anyway, the circuit does work; even with the heavy fog laden air that we are experiencing now. The old circuit, without RF amp and with some simpler amps, wouldn't work in conditions like this.

The 38K pot? From the junk box, it measures 38K, probably a cheap 40K on the low end of tolerance. I wired it with the top end clockwise. That reduced the gain. Oops, that was funny at first. The cathode voltage goes up to about 10 volt max, at which point the plate voltage gets to +100, the tube is cut off. The audio becomes a hiss, then quiet. I'll keep the suggested tubes in mind. I'm sure I have some in the junk box; Recognize some of the numbers.

Right the 140p trimmer in parallel has no effect. Next will try only the 140p.

The radio sensitivity is really good. However, it is difficult to separate adjacent stations. They overlap at times. It seems that once a strong local is tuned in, it stays even when the variable tuning cap is turned quite a ways off. Getting way off gets rid of it, then a weaker station can be found by "sneaking" up on it. Guess that is where the skill comes in. Not sure why that is happening.

Maybe it is time to try different coils. The socket is a 4-pin and I have a couple 4-pin tube bases. Also might try some SW bands. Any suggestions? Not familiar with that so much.

Thank you for your input, Pretty darn good RF amp. Lyn


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 Post subject: Re: 5 Tube Regen
PostPosted: Jan Fri 26, 2018 9:01 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 673
Location: Tokyo
Those tubes were just the ones I had on hand and so I tried them. Not really a recommendation. The 6AB4 triode (=1/2 12AT7), though, was designed for VHF work (mixer, GG amp) so I thought it might be a good choice. I don't think the choice of tube is very critical in this application. Almost anything should work well enough. I wouldn't use a triode with high heater current, eg, power triodes, etc, because they get hot. In principle, I wouldn't use a high mu triode, eg, 12AX7, but I wouldn't be surprised if it worked all right. The 6C4 you have should be as good as anything, and it draws only 150mA for the heater.

Separating strong stations: I usually begin by reducing the incoming signal level to as low as possible, and then cranking the audio up to compensate. Then I run the regeneration right on the edge of oscillation. That usually does it. However, sometimes you just can't separate them. This is, btw, why I like to make the audio amplifier part of the radio as quiet and hum-free as I can. I don't want to rely on the regen detector part for volume.

Try making some SW coils. With a Hartley design (like you have), it's not difficult. Just make the tap point around 1/4 to 1/3 up from the bottom. That usually works fine. Don't fuss too much about getting the exact number of turns or spacing, etc. It really doesn't matter in this application. Check the frequency range with another receiver (I use a cheap Chinese digital radio for that purpose). If it seems there is too much feedback, just take a turn or two off the bottom of the coil. If it seems there is not enough feedback, take a turn or two off the top. I actually don't bother with the winding formulas. I just grab a form, put some turns on it and see where I end up. If I like the range I get, I use clear fingernail polish to hold the wire in place.

Rob


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