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Old 24th March 2013, 05:29 PM   #1
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Default Parts Box 300b

Its been a while (about 1.5yr) since I have built any amps, but I have had some parts burning a hole in my closet that required me to get off my butt and do something with.
What I built was a fully regulated 300B amp using a 6C45n driver tube CCS loaded into a power drive then into the 300B. I know of the mixed reviews of the 6c45n but I have them and I wanted to see if I could tame it. The OT is temporarily a Edcor XSE 15-8-5K and I am pushing 80mA bias @375V. The 6c45n is 150V at 15mA.

I have never really measured a SE amp so I am curious of others perspective of the results. The amp sounds nice but the measurements are ? I am happy with the noise floor however. at 1Watt(2.82V into 8R) i measured 1.004% THD. Is this pretty normal for a 300b SE? I would think there is alot I could do to make it better. The freq responce is 27(-3dB) to 90k(-1dB).

After working with my mullards, I have no feel for what would be good.
Thanks for your insight
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Old 24th March 2013, 09:02 PM   #2
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It sounds decent because of the distortion's spectral distribution.

Post a schematic, please. Maybe combination bias, without a bypass cap. on the cathode resistor, can be employed. That would linearize the 300B.
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Old 24th March 2013, 11:18 PM   #3
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I will get a schematic up after the next day or so. I have to put my toy away to finish up some school work before spring break. I also want to get a few more measurements as well, particularly of the driver.

Its been a while so I am rusty on the test equipment which is why I posted such a lousy looking spectrum. Is that a normal harmonic distribution and magnitude for a 300B?
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Old 26th March 2013, 06:11 AM   #4
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Quote:
Originally Posted by SGregory View Post
Its been a while so I am rusty on the test equipment which is why I posted such a lousy looking spectrum. Is that a normal harmonic distribution and magnitude for a 300B?
For a VT like the 300B, single-ended, that's a normal harmonic profile. Looks pretty good, what's called a "waterfall" distribution. You could improve on the harmonic distortion performance by including a higher quality (more $$$$) OPT and/or applying gNFB. Some SE designers/users would consider the latter "heresy". You've already gone heretical by including SS in the design, so why not wrap some gNFB around that 300B?

It's also a good thing that you opted for the "power drive" topology. Lots of complaints about less than stellar sonics from 300B's stems from inadequate grid drive. Power triodes like the 300B present the driver with a less friendly load than pents, due to the higher reverse transfer capacitance that gets magnified by Mr. Miller, the presence of grid current even before Vgk actually goes positive, and the much larger input swing. If you don't take that into consideration when designing a driver, you're likely to be disappointed with the end result.
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Old 26th March 2013, 10:15 PM   #5
45 is offline 45  Italy
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Originally Posted by SGregory View Post
Is that a normal harmonic distribution and magnitude for a 300B?
Distribution yes, magnitude no. 1% at 1W and 1KHz is a bit high.
The simplest 300B amp I have seen used a 12AX7 with the two sections in parallel and self-bias. That's something I would never built..!!
However the max Pout was close to 10W and THD surprisingly low: 0.1% at 1W and less than 2% at 8W! Some harmonic cancellation led to such result. The sound was not bad at all. Not my favorite but surely better than one might think...
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Old 28th March 2013, 04:17 AM   #6
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Thanks for the feedback, 45, Eli and Miles. Have a little time to play so worked on my measurement system and came up with the attached. 0.39% THD+Noise is a much more realistic number. IMD appears to be a bit high so need to look at OPT and Ht regulators. Blue is Output at 1 Watt into 8 ohm, the red is the driver signal into the 300B offset bu 14dB
The IMD surprises me. Curious to see if OPT or regulator is responsible Schematic soon.
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File Type: jpg Ver4 Spectrum_THD_1.jpg (323.4 KB, 212 views)
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Old 28th March 2013, 06:29 AM   #7
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The IMD doesn't surprise me at all. It's 120 Hz ripple mixed with your signal frequency (and its harmonics). If you aren't already using well-regulated DC supplies for the B+ and the 300B filament supply, I strongly suggest that you implement those.

In my 300B amp (see my website), I use a 6N6P driver tube. I get about 0.18 % THD at 1 W, 3 % THD at 11 W.

~Tom
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Old 28th March 2013, 11:08 AM   #8
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Thanks Tom. After I posted the original spectrum, I saw the distortion value of your amp and have been using them as a benchmark for this project. I still have some quirks to work out on my measurements as this is a well regulated HT and B+. In fact it should be no different than your amp as it uses the same regs. B+ is a C-L-C-(21 madia)-C.

I am not sure what the best measurements will be yet, I plan to work on optimum driver bias and then refine the output stage.
I'll post a schematic today if I get enough time.
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Old 28th March 2013, 11:38 AM   #9
45 is offline 45  Italy
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Quote:
Originally Posted by SGregory View Post
Thanks for the feedback, 45, Eli and Miles. Have a little time to play so worked on my measurement system and came up with the attached. 0.39% THD+Noise is a much more realistic number. IMD appears to be a bit high so need to look at OPT and Ht regulators. Blue is Output at 1 Watt into 8 ohm, the red is the driver signal into the 300B offset bu 14dB
The IMD surprises me. Curious to see if OPT or regulator is responsible Schematic soon.
I think that most of that IMD is caused by the 300B filament supply as it is nearly absent in the driving signal, in comparison.
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Old 28th March 2013, 11:54 AM   #10
45 is offline 45  Italy
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P.S.
I was assuming that your filament supply is already DC. If not you should do it, in my opinion.
On the other side it is also true that your driver is CCS loaded so, let's say, it is less demanding in terms of HT supply. Anyway IMD overall is not bad at all. The main peaks around 1KHz are at -90 dB! You could see how it looks at higher output. Sometimes people don't see it because they don't have enough resolution.

Last edited by 45; 28th March 2013 at 12:02 PM.
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