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PostPosted: 23 Apr 2017, 16:44 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
I've finished a new amplifier.
Attachment:
First_Install.jpg

This is a nominally 6.1W per channel SET using the big 6336 pass tube. It uses one 12AU7 in a cascaded configuration to drive each channel and a GZ34 rectifier with three chokes in the power supply to generate truly awesome channel separation. This is the amp that I've been prototyping in parts for about a year.

Here is the basic amp schematic:
Attachment:
Schematic_signal.jpg

And here is the power supply schematic:
Attachment:
Schematic_PS.jpg
One note, this is directly out of my design notebook. But I did not use the 18kΩ/60H filter element shown. Instead I simply used a 20kΩ dropping resistor because I determined that the additional choking was not required. It made no difference. The resultant amp is still dead silent, and the channel separation is incredible.

It may not seem like it, but this is a big powerful amplifier. In a small room this will make your skull vibrate; it did mine. It also has superb bass response. Better than any of the other amps I've built. Here is the frequency response taken at one watt output.
Attachment:
frequency_response_plot.png

I was afraid that with this kind of bass response that the amp would be a little sluggish. But this is not the case. It remains VERY fast, handling the rapid changes in "Frederick The Great : Sonata in B-Flat - Allegro" played by Michala Petri without missing a beat. I did have some concerns about the big tube oscillating so I tamed the top end a little using a 10kΩ grid stopper on the power tube. This means that it’s about 3.9dB down at 20kHz, but since only about 5% of the population can hear 20KHz, I figured that it doesn’t matter. The tube seems nice and stable, so if someone wanted to use a 5kΩ there, it would restore a few dBs at 20kHz and placate the "20-20k" crowd. Distortion is also very good. THD is about 2.3% at 1W and 4.7% at 6.1W virtually all second harmonic. In overdrive condition distortion jumps to about 7.3% at 8W but it has a distortion characteristic much more like a transistor amplifier.

One final note, this amp generates a lot of heat. The 6336 dissipates almost 75W at idle. In short, this chassis is MUCH too small. At some point in the future this amp will be rebuilt on a much larger chassis. But until then, I'm enjoying it in my office at home. But for the most part I have to keep The Beast somewhat restrained. If I really let him loose, I get complaints.

Thoughts?


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Last edited by Suncalc on 23 Apr 2017, 18:45, edited 1 time in total.

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PostPosted: 23 Apr 2017, 18:23 
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Suncalc wrote:
One final note, this amp generates a lot of heat.
I'd like to clarify this statement. The 6336 tube runs hot; too hot to safely touch. Maximum bulb temperature is 250°C (394°F). This also means that radiant heat load on other components can be high. So if you build this amp, keep this in mind. Caging the power tube might be a very good idea.

Here are some steady state temperatures I just measures on the amp surface.
Attachment:
Measured Temperatures.png

This is not an amp to run if there are curious little kids or pets around.


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PostPosted: 25 Apr 2017, 11:45 
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Hi, Nice amp. My only thought is that the 12AU7s are running a bit on the lean side. I prefer about 4-6 ma as it seems to make them more linear there. It might reduce the distortion level a bit as well. Another possibility is to reduce the grid stopper to about 1K and put a 2-3 db NFB loop around the second stage and output tube to incl the trannie. Make it frequency selective so that it only works above about 25-30KHZ. This would give the stability you wanted without harming the frequency response.

Good listening
Bruce

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PostPosted: 25 Apr 2017, 21:58 
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Location: Winnipeg, CANADA
Real great looking build Matt.

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PostPosted: 28 Apr 2017, 06:50 
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Joined: 30 Oct 2015, 07:44
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Oh my, that is a very nice looking amplifier. Thank you for sharing.


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PostPosted: 25 Jul 2017, 05:54 
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You are professional and excellent.


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PostPosted: 31 Jul 2017, 08:17 
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I must have been sleeping I missed this one completely. Once more really nice woodwork mate. Never heard of the tube before but looks the part. I can sort of imagine a KT150 in the socket.

I'm a bit amazed at the distortion level for 1W I would have thought it would have been just under that 1% mark. I know we struggle to hear distortion under 3% depending on the harmonic but I'm a little amazed it is that high. Matt you are probably going to tell my my Black Lace 6V6, based on your schematic, has been producing that much distortion and I have raved in the past about how good sounding the amp is.

Once more a really nice looking amp and a killer PS to support it. Good work.

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PostPosted: 31 Jul 2017, 10:03 
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When I start with my GU72 SE-amp, I will encounter much the same problems, a fr***ing hot tube PLUS a top anode connetor (700V). I will solve the problems by hard soldering a brass cage than will keep fingers away. Guess the brass eventually also will get very hot, but least not carry lethal voltages.

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PostPosted: 22 Aug 2017, 17:40 
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That's a really handsome amplifier. I like the added touch of the control knob on the front. Simply beautiful, beautifully simple.


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PostPosted: 21 Nov 2017, 11:37 
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Suncalc wrote:
One final note, this amp generates a lot of heat. The 6336 dissipates almost 75W at idle. In short, this chassis is MUCH too small. At some point in the future this amp will be rebuilt on a much larger chassis.
This was my original concern, but after reviewing the measured temperatures and after my recent post on transformer heating and temperature rise here, I've decided that I was just being paranoid.

While "warm", these temperatures are really not out of line with typical Class A transformer specifications. I'm going to stop worrying and just enjoy my big all triode amplifier.
Attachment:
First_Install.jpg


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