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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: 24 Dec 2013, 17:54 
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Hi, Projects like this always seem to be fun to tweak. One thing I see is that the NFB loop is pretty much ineffective as the large capacitor on the driver cathode will shunt nearly eveything to ground.

Good listening
Bruce

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PostPosted: 25 Dec 2013, 01:32 
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Thanks Bruce,

So what would be best, to divide the cathode resistors into a divider network 1k (Ck1 on top) and 100(Ck2 to ground) with the NFB in the middle, or reduce the cap value, or reduce the cap and bypass Ck1, bypass the NFB? I read your previous suggestions, but with this config, which would be most effective.

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PostPosted: 25 Dec 2013, 12:28 
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OK, I think I have it sorted.

Spent some time trying to figure out why my tube would oscillate when I connected feedback between the divider. Since the upper resistor was x10 that of the lower, I went ahead and increased the NFB resistor by the same order. So I have upper resistor of 1k (bypassed with 100uF), lower resistor 100 ohms then increased the feedback to 10K. Tested with a 500 hz tone and the feedback drops the signal approx 17%. The amp seems to "tighten" up in sound, I will now play with the cathode resistor values as my bias shifted a tad to the right with 3.2v @ top of Ck, 3mA of current with the 6N1P and 5.3v 5mA with the 6CG7. Also a lower Ck cap in an attempt for a bit lower end, but not enough defeat the feedback

Is there a book with these configurations? I searched online and found bits and pieces but nothing collective. Something with math would be great.

Merry Christmas everyone!


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PostPosted: 26 Dec 2013, 13:49 
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Hi, You can probably see benefits with a little more NFB. It seems to me that up to about 3 db is not harmful and will usually improve the sound. Try dropping the 10K to about 5K and see what happens. There are lots of texts on NFB but I find it easier to make an educated guess most of the time then tweak with the scope and distortion analyzer. With one exception (the Alpha SE U/L amp) my designs usually are in the 2.0-3.0 db range. Just enough to insure stability. The frequency response and distortion levels are already low enough. All will run fine without it .....but there are some speaker systems with wild impedance curves that might cause some undesirable interactions. They would manifest the problem typically as high frequency (over 15K HZ) ringing. I would suspect that 99% of the listeners would not notice it.

Good listening
Bruce

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PostPosted: 26 Dec 2013, 22:38 
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Thanks,

Will try, I changed the CK to 560 with a 100uF bypass and lower to 68. Will play with the feedback and see what comes up. I also rolled a 6DT8 (mU 60, 6.2mA) and GB6DJ8 (mU33, 6.1mA), both did pretty good with the existing 22k Ra and tail. The hi mU 6DT8 was very punchy, but of course will saturate pretty quickly, and the 6DJ8 was very "full bodied" Im liking it more than the 6N1P.

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PostPosted: 29 Dec 2013, 22:36 
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More tweaks, I added a "sub out" divider as found in Bruce's amp (hope you don't mind). This help enormously to fill in the lower end. My Hammond 269JX was running a little warm, so I changed the pentode cathode resistor to lower the current draw to under 60mA. Changed the NFB to a pot, wired as a variable resistor.

Can now turn the volume up, up, up :thumbsup:


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PostPosted: 30 Dec 2013, 13:07 
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This circuit won't provide much feedback at all. Most of the feedback signal will be shunted to ground via C4 (47µf). A 47µf capacitor only has about 169Ω of impedance at 20Hz, 34Ω at 100Hz, and by 500Hz is down to a minuscule 6.8Ω. So this feedback scheme will only provide minimal levels of feedback and only at the lowest frequencies.

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PostPosted: 30 Dec 2013, 19:14 
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What would you suggest for a replacement for C4, or should I just remove it? I do get some gain from C3 and the feedback does reduce the overall signal out of the amp (on the scope). Just that no matter how much feedback, the total resistance is never more than 68 because of the cathode resistor r3. It sounds pretty good right now, but I am always open to make it sound better. I added the "sub out" because the line OPT that I am using do not have that much bandwidth, although I can see approx 20hz-10khz on the scope during a sweep test.

My goal is to get as much usable gain, without clipping. Considering I am using Tv sweep pentodes and line transformers, the intent was to make an economic desk amp that sounds decent as well.

Tube rolling has been fun as each one highlights different sound signatures. 6CG7(light and airy with crisp highs), 6N1P (better for mid range), 6DT8(punchy), and 6DJ8(overall warm across the spectrum, but not as defined at top as the 6CG7.


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