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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 09 Dec 2013, 08:46 
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Hi, Nice. I like the measurements on the trannie. As much as I think that NFB can mess things up in this instance it might be of use. (A non-critical listening amp). I believe I would use about 6 db and wrap it around the whole amp from output trannie to catode of the driver. If you tweaked the network for bass and treble boost - say modestly at 3 db you might find the sound more suitable. It would entail removing the LED from the driver cathode and replacing it with a resistor of around 1200-1500R to implement. The NFB would also reduce the distortion a bit. All at the expense of some gain although that sould not be a problem with the tubes you are using. There would appear to be excess for a PC application.

Good listening
Bruce

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PostPosted: 09 Dec 2013, 10:00 
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Location: Denver, Colorado
Thanks Bruce.

Before reading your post, I spend most of yesterday afternoon trial and error with various feedback resistors from 33k down to 2.2 ohms. Tapping from the speaker @ 8 ohms to the tube cathode side of the led and also from the 4 ohm tap to the cathode of the led. The result was from no effect to a sort of choking effect of the signal. The sound becomes very "tinny" and the low end rolls off.

On the scope, the output signal is no where close to a perfect signal, but with a 200mV input sign, I have 424mV ptp with a 4ohm load and 884 mV with a 8 ohm load (i have a dummy load)

Reading your post now, I see that the led and its CCS must be counteracting the feedback somehow. Although the sound is somewhat decent at low to medium volume levels, the output does distort at the higher end.

I will try removing the LED's and go with a resistor and bypass cap combo, and then try feedback again.

Since this little output tranny has the 8 and 4 ohm taps, do you think I should tap off the 8 ohm or the 4 ohm for my feedback?

In the end any improvements will be a plus and any suggestions are greatly appreciated.

Thanks
Rogers


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PostPosted: 09 Dec 2013, 17:39 
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Hi, Yes indeed. The LED will pass the NFB signal applied to it quite easily to ground. The CCS is likely OK. I would use the 8 ohm tap as you are working with a higher level signal. The ideal arrangement if you are only going to use one particular set of speakers is to use the tap they are on. But for a generic use, I would go for the 8 ohm one. You may find it useful to split the cathode resistor on the driver into two parts. One about 10 times the size of the other. The lower one being the smaller one. Attaching the NFB there will give you a voltage divider arrangement and often make the values of the loop easier to implement. Also you can use a partial bypass on the lower resistor to get a bass boost (actually a treble cut) as it will send a portion of the NFB to ground. I usually bypass the upper resistor for more gain at a F3 of about 5 HZ. This allows the full NFB to see the cathode. You can fiddle with that resistor as well. The value of the bypass componeents is not easy to compute and I have found that the easiest way is to try caps there and see what the result is. A series resistor and cap can form a shelf response as well. Another alternative is to split the resistor in the NFB loop into two equal one and put a cap, or cap in series with a resistor to ground from the mid point. This can also be used to tailor the response. I find that using clip leads, variable resistors and a scope that can do Bode plots really handy for this. Most folks will not however have such a critter. You can tune it by ear though with pretty good results.

Good listening
Bruce

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PostPosted: 10 Dec 2013, 13:24 
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Bruce,

Thanks for the guidance, I spent several hours last night with various combinations. I had to stop because I thought my head would explode, lol.

Replaced the driver cathode with 1k and 100uF bypass, I tried a 1000 and 100, in series, with the top bypassed and it did boost the higher end. The only thing is tying to the middle point was causing my tube to oscillate, I went back to one resistor.
The voltage was 1.9 before feedback and now 0.8vdc, I will be changing the value of the cathode resistor to higher values and see what I get. I have plenty of room to play with, as the total gain is approx x20-30 depending on roll off.

Feedback really filled in the bottom end, I tried various combinations, but ended up with 660 ohms right now. I will most likely be changing those. I "really" need a decade box, as I had resistors all over the bench, but very fun as the lower end becomes more punchy as the values change. Gain loss can be seen on the scope, but negligible on the speakers (Old School Radio Shack Minimum 7's in the steel case)

As for the scope, I have a Tektronix TDS3012B (DPO) at my disposal. I downloaded open choice, but still have to get the scope connected to my computer. I searched the net for options to run Bode plots, but most need developer skills to set the system up.

I used the FFT option on the scope and can see the peak slide as I run sweeps with the ToneGen app. I am not sure of a way to capture the peaks to give a somewhat of a curve. It would be nice to just measure gain RMS and just hand plot or enter in excell to get my curve.

For now I just run sweeps and can see that the gain significantly increases as I approach 1k and higher.

The exciting thing is the changes have cleaned up my signal and decreased distortion, I still have clipping with increased gain, but thats the fun of milking every last watt from this little guy.

Thanks

Rogers


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PostPosted: 10 Dec 2013, 13:48 
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Hi, I suspect you need a grid stopper resistor in the output stage, maybe the input one as well. It depends on what the frequency of oscillation is. If you are getting oscillation it might also be that the phase is incorrect. Since you indicate that the NFB reduces the gain you are probably OK. If it increased the gain then it would 180 degrees off.

Try using a pot for the NFB resistor and adjust it by ear. Then sub in a fixed value close to it.

Good listening
Bruce

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PostPosted: 10 Dec 2013, 15:44 
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Thanks, guess, I will mark this thread as a further work in progress.

Good new though. I was getting a bit frustrated with the distortion and unusable amount of gain, so I re-wired driver filaments down to 6.3 and rolled in some 6DJ8 type tubes. Finally tried a 6CG7 and wow... like a rolling down the highway with an imbalanced wheel and it finally righted itself. Looking at the tube characteristics. , very close to the 12AU7, but less transconductance. Night and day.

I am going to work further with this tube as a driver.


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PostPosted: 10 Dec 2013, 19:15 
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Also fixed the phase shift and can now play with the feedback. Big difference much tighter sound.


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PostPosted: 11 Dec 2013, 18:21 
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Hi Guys and maybe Gals,
I just joined the forum and I am mixture of Ham Operator, Audiophile, and AM DX'er. I have a AM regenerative radio that is a home brew and it has a 6SN7 dual triode tube that is putting out its audio in a baby impeadance matchining transformer. Can I build this amp and drive an 8 ohm speaker with it? Right now I have to use amplified multimedia speaker.
Thanks Dan


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PostPosted: 12 Dec 2013, 03:29 
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Hi Dan,

The amp should work to amplify the audio signal. My design was inspired by another builder, and he used a ECL86 tube, this way you will have your driver stage and power stage in one tube. Here is the link to his page.

http://www.jogis-roehrenbude.de/Leserbr ... _Small.htm

Should not be too bad considering you should already have a high enough B+ and as long as you have enough current in your filament transformer, it will be a matter of coupling the amplifier in. The good thing about my amp is the parts are rather cheap, the Speco transformers are only $6 on eBay. The tubes can be found for a few dollars as well, as they were in old TV repair kits.

I just made a tiny AM regen with a 12AU7 tube and used an opamp for a driver, it powers a small hobby speaker the build is here viewtopic.php?f=16&t=4621

only thing is it will need 12v,

Post a link to the build schematic, maybe one of the members or I can help.

I will reply in this thread: viewtopic.php?f=16&t=4621

Not that this thread gets too much traffic, but considering the source is a Regen Radio, we can move to the Radio thread section.

Thanks


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PostPosted: 24 Dec 2013, 16:29 
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OK, on thing about having the amp on the desk is the temptation to do more tweaks.

I made a few more and most likely last tweaks to the system. My goal was to optimize the amp for various tubes and maximize usable and undistorted output.

I changed the driver heaters to work from 6.3v, the Hammond 269JX has enough to accommodate the additional current.

I lowered the driver plate resistor to better match the 6CG7 and 6N1P tubes internal plate resistance. (22K)

The 6CG7 does well if you are running straight from a CPU sound card or a simple potentiometer attenuator. The 6N1P has a little more punch and is great to make up some of the lost db when using passive preamp networks. Both are warm, with the 6N1P being a bit "punchier" with a slight edge.

This was a fun project and thanks for all who contributed.


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