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 Post subject: Audio half of regen rcvr - advice?
PostPosted: May Tue 17, 2022 7:14 pm 
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3-tube shortwave regen rcvr, works well and headphone volume is adequate. However, I think it should be able to drive a small speaker to reasonably comfortable levels, at least on the stronger signals. Any advice on component selection that might yield more audio? The OPT is from a derelict AA5 of long-forgotten make.

Thanks!

Tom

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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Tue 17, 2022 8:03 pm 
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I don’t see anything fundamentally wrong with your circuit. I’ve used a similar one with my regens and it produces an adequate amount of power to drive a speaker. You might try changing the 6AK6 cathode resistor to around 150 ohms and the plate load resistor of the 12AU7 to around 220k. But I don’t think that’s going to make a whole lot of difference. If the detector is the first half of the 12AU7, I’d try a 12AT7 or a 12AX7 in its place. That’ll definitely give you more audio, but the detector may or may not work well. It’s worth a try though.

Darrell


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Tue 17, 2022 9:12 pm 
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Thanks Darrell. In my case, the detector is a 6C4 and the second half of the 12AU7 is used as a grounded grid RF amp. I'll root through my stash and see if I have any 12AT7/AX7's. If so, I'll play with those this afternoon and report back. Will try your other suggestions, too.


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Tue 17, 2022 11:08 pm 
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A 12DW7 is a dissimilar triode that’s a 12AU7 and a 12AX7. Unfortunately, they’re kind of pricy.

Darrell


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Wed 18, 2022 1:05 am 
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You need more audio voltage gain before the 6AK6. The single 12AU7 triode stage gain is only about 10. Not nearly enough, especially for the low audio output from a regenerative detector.

Years ago I made a regen with a 12AT7 (GG amp and detector). The audio section was two high mu triode stages (12AX7), driving a 6AK6. B+ about 150VDC. Adequate but certainly not overwhelming volume.

Any chance you can add another audio stage? If not, as Darrell suggested, try a 12AT7 triode (in circuit gain around 30) or a 12AX7 triode (about 50).

Do you have any sharp cutoff pentodes like the 6AU6? Voltage gain 100~150.

Rob


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Wed 18, 2022 9:48 am 
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shinkuukan wrote:
You need more audio voltage gain before the 6AK6. The single 12AU7 triode stage gain is only about 10. Not nearly enough, especially for the low audio output from a regenerative detector.

Years ago I made a regen with a 12AT7 (GG amp and detector). The audio section was two high mu triode stages (12AX7), driving a 6AK6. B+ about 150VDC. Adequate but certainly not overwhelming volume.

Any chance you can add another audio stage? If not, as Darrell suggested, try a 12AT7 triode (in circuit gain around 30) or a 12AX7 triode (about 50).

Do you have any sharp cutoff pentodes like the 6AU6? Voltage gain 100~150.

Rob


All that is required is to increase the value of the anode resistor in the 12AU7, to give more gain say to 220k at least. Adjust the cathode resistor so that the anode voltage is about 1/2 the B+, then also increase the grid resistor of the output tube from 510k to around 1Meg. This way you will have at least x 4 the gain you currently have at audio frequencies.


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Wed 18, 2022 1:40 pm 
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Quote:
All that is required is to increase the value of the anode resistor in the 12AU7, to give more gain say to 220k at least. Adjust the cathode resistor so that the anode voltage is about 1/2 the B+, then also increase the grid resistor of the output tube from 510k to around 1Meg. This way you will have at least x 4 the gain you currently have at audio frequencies.


No change in gain. Stays about 11 or 12. See page 3, Brimar 12AU7 data:

https://frank.pocnet.net/sheets/184/1/12AU7.pdf


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Wed 18, 2022 9:59 pm 
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Thanks -- I don't want to get in the middle of a debate here, but before I saw the most recent 2 or 3 posts, I had already made changes based on hbrnut's suggestions: I changed the 6AK6 cathode resistor to 150 ohms, changed its grid resistor to 270K ohms, and tried both a 12AX7 and a 12AT7 (neither brand-new). Empirically, I would say that the 12AT7 makes for a slightly better improvement than a 12AX7, at least in this particular case. The main thing is that the volume out of a speaker has been improved a bit.

Yes, I have a 6AU6 but that would be a major switch if I want to keep the whole thing to 3 tubes. Theoretically, I suppose, something like a 6U8A could replace the 12AU7 (now 12AT7) and use the triode as the GG RF stage and the pentode as a first audio to drive the 6AK6. I have vague plans to build a band-switching regen (I have the parts on hand to give it a try) and will keep in mind the 6U8A notion for it (or some other triode-pentode).

I don't think it's worth the trouble to add another voltage amp (say, a 6C4) preceding the 6AK6. The chassis is kinda small, 4 x 6 and I would find it a bit of a squeeze. The performance of the receiver as a regen is pretty decent -- employing a GG RF stage is definitely worthwhile.

I might post a short youtube video of it in operation in the next few days.

Tom


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Fri 20, 2022 5:45 am 
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Tom, if you don't already know this site, I think you might find it fun.

https://www.ampbooks.com/mobile/vacuum-tubes/

Click on, say, 12AU7 (aka 6C4). It shows a Vpp (=B+) of 325, the plate load resistor 22K. Change those values to your original 6C4 values, 160V and 47K. Click on Ip vs Vp.

Now you're a new page displaying a graph of the 12AU7/6C4 load DC and AC load lines. It doesn't look right because the calculator assumes 5.3mA plate current and a grid bias of -8 (an impossible combination for a 160V B+). If you change the plate current to 2mA, and the grid bias to -2V, hit RECALCULATE, you'll get a new plot. The DC load line and the AC load lines intercept at 2mA and 2V. The cathode resistor value is calculated and appears at the top: 1K. The calculated plate voltage is 64V.

You can play around with different values of desired grid bias and plate current (the cathode resistor value will be recalculated), see how they affect the load line plot. You can return to the first 12AU7 and reset the available plate voltage and plate load resistor values.

You'll notice the load lines (with 160V and 47K) are way down near the bottom, where the curves are very 'curvy'. Which means of course distortion. These tubes were meant to be used with much higher voltages. If you increase the B+ to 250, lower the plate load resistor to 22K, you'll see the load lines are up in the straight line section. Much less distortion. When the B+ is not high enough, you have to accept the compromise in performance. But lots of triode amps use less than optimum voltage and they work OK. Eg, the 12AV6 triode in AA5 radios runs on just 100~110V.

Anyway, it's educational and more important, fun to try out different combinations of B+, load resistors, plate current and grid bias.

Rob


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Fri 20, 2022 6:03 pm 
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Rob, I had not seen that website before and so 1) I bookmarked it and 2) played with it a bit. Dynamite! Thanks very much! It should help me cut down on the dumb questions. :oops:

Tom


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Sat 21, 2022 12:53 am 
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A reader asked for a schematic, so here it is. Hope it's complete. I also took a couple of quick snaps of the rcvr itself. The front panel is re-used from an old project and so the meter (upper right) has no function here. Looks kinda cool, though.

Tom

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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Sat 21, 2022 7:53 pm 
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How many turns on the coil ( 9 )

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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: May Sun 22, 2022 4:38 pm 
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battradio@: The coil in the photo originally had about 9 turns. The coil form has an approx. diameter of a little over 1 1/4 inches. I have since re-wound it so it has 8 1/2 turns. It is tapped at 1 1/4 turns from the cold end. With the particular capacitors associated with it, the coverage is approx. 9.3 to 10.3 mHz. I also wound a 40/41 meter coil. It has 12 turns with the tap point also 1 1/4 turns from the cold end.

As with any relatively primitive rcvr like this (and I'm quite sure you know this) factors determining actual coverage are dependent on things like stray capacitances, inductances and lead dress. To determine the needed number of turns I use a simple spreadsheet I wrote for myself a long time ago, but this only gives me a starting point. I have always found that there is a fair bit of difference between calculated frequency coverage and reality.

Tom


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: Jun Wed 01, 2022 5:45 pm 
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The OPT from an AA5 radio is only 2,000 ohms. The tube manual shows a 6AK6 needs a 10,000 ohm output transformer, just like a battery portable radio. With such low plate current, you could use a cheap "70 volt line transformer" at the 0.62 watt tap. There are plenty of taps to choose from for best results https://www.parts-express.com/70V-10W-L ... er-300-040


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: Jun Tue 14, 2022 5:19 pm 
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I'm arriving late to this discussion, but I wanted to mention a couple of ideas.

First, as mentioned, the 6AK6 wants a load of 10k, not the 2k or 2.5k from an AA5 transformer.

Second, make sure your cathode capacitor is large enough. A rule of thumb (for comms audio, not hi-fi audio) is that C = 5000/R, so for R=330, C wants to be at least 15uF.

Finally, try a different tube. The 6AH6 has the same pinout as the 6AK6 but almost 4 times the gm. The biasing would have to be reworked, and perhaps the screen voltage, but you'd get a lot more gain in the stage. Even a 6AU6 would be a noticeable improvement.


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 Post subject: Re: Audio half of regen rcvr - advice?
PostPosted: Jun Tue 14, 2022 6:30 pm 
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Thanks, KK6GM. I appreciate it. More helpful info for the file. :D

Tom


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