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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: 05 Aug 2009, 22:27 
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Location: Toronto, Canada
Both VoltSecond and Bruce Heran suggest modifying the B+ by increasing the capacitance after the rectifier bridge:
VoltSecond wrote:
The screen supply (C9, 100u) is much more sensitive to noise than the B+ itself (C8, 220u) so it needs to be stiff (low impedance).
The supply that feeds the driver tubes (C5, 22u) also needs to be low impedance. This will reduce crosstalk between channels and reduce the sound of the electrolytic in the circuit.

To do this VoltSec recommends Nichicon UVR2E331MRD 330 uF 250V cap (Digikey part number 493-1199-ND) in all three positions.

Meanwhile, Bruce increased the capacitance on the B+ circuit by running a 10 uF Solen capacitor in parallel with C8, but partly this was in response to power supply issues arising from wiring his transformers in Ultra-Linear Mode.
Bruce Heran wrote:
"The B+ had one more section of filtering than the output transformers. Switching to UL mode bypassed the filter section."

So my question is this: does it matter than C8, C9, and C5 are different values? I mean, swapping in a 330uF cap for a 220uF one seems like a different thing entirely than using a 330uF cap to replace a 22uF one!

Thoughts, reflections?
-Tay

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In progress: K-12G
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PostPosted: 06 Aug 2009, 22:50 
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Hi, I love questions like this. In modding a K-12 my experience is that three things matter, how much do you want to spend (you can turn a $150 kit into a $400 project), how much improvement do you want and will the stuff fit on the circuit board. So to start, the 330 vs 220 cap variation is not likely to be greatly significant. Both are much better than the originals. I could easily support either value. The second part of the question is something I have found based on a number of amps. It is generally accepted that electrolytic caps do not respond well at high frequencies. There also seems to be a general relationship in that larger value ones are worse than smaller ones. (please don't everyone pile on that - there are exceptions I know). So when ever I use a large filter cap it is always in conjunction with a poly cap of some sort. Polys have good high frequency response and compensate for the electrolytic's deficiency. Now at the same time, I like quiet amps, no hum, no noise and I will do extra things to get rid of it. Thus nearly everything I build has a 4 (sometimes 5) section filter. The last section is often a large value poly cap. Size and cost of these caps are factors. The cost for example a 10uf poly is about 5-10 times that of an electrolytic of the same value. The size is equally large. I have found though, that amps that use this arrangement (4 filter sections the last being a poly cap) seem to me at least to have superior sound over ones that do not. Several of my project amps are dead quiet at idle even on sensitive speakers (mine are 94 db/watt). No hum, no noise. There are lots of variations and factors that can apply here so if you didn't get enough of an answer, post again.

Good listening
Bruce

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PostPosted: 06 Aug 2009, 23:48 
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Interesting, thanks, Bruce.

At this point it's more of a $400 amp than a $200 one, that's for sure! No need to tell my wife that until we are enjoying HER favourite music in our living room by the fire, etc., etc.!

I've done about as much as I can do without building an enclosure. None of the scrap wood around the wood shop I have access to is worthy or correctly shaped, etc., so I picked five feet of very nice red oak from home depot. That will form the sides, front, and back. For the bottom I'm using a metal radiator-screen material also from Home Depot.

My plan is to use sheet brass for the top, with holes with the tubes pointing through, but I'm having a hard time finding a local supplier. I found a place that will ship it to me, though.

At this point I really wish I had an oscilloscope, because once I get this this assembled I can sense already there will be tweaking involved and good metrics would help a lot. My dad has several, alas, he lives far away!

-Tay

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PostPosted: 07 Aug 2009, 09:04 
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Hi, It sounds like a good build. I like wood cases and use them when I can. If nothing more than wood side panels to add a touch of class. I found some screen door ornate mesh that looks good on projects. It is designed for the kick panels on screen doors. The stuff I found is brass colored aluminum and has what looks like a clover leaf pattern. A good scope is invaluable. Many used B&K and others are out on Ebay. It is hard to tell if any particular one is good though. I use three. A DSO from MCM, a Velleman DSO that works through a PC and a dual trace B&K analog. You still need a good signal generator with any of them. No sense measuring something if it already is distorted. The most useful item I have is a HP 336A distortion analyzer. It is certainly not the newest (about 20-25 years old) but great. With a scope you are hard pressed to see anything below about 5% distortion, the analyzer can go down to about 0.01%. Very helpful in tweaking. If you go all that route be prepared to throw lots of $$ at it all. If you don't plan on going off into the design area, I would not. Best bet is a analog dual trace and signal generator. Most bang for the buck.

Good listening
Bruce

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PostPosted: 07 Aug 2009, 09:13 
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My dad has an old (1970's?) Tektronix with an audio analysis module plugged in. I'm not sure how good that signal generator is, though.

I hope to build more projects like this, I tend to do a big project of some sort every year or two. In the past these have mostly been computer related, but analog circuits sure are a lot of fun. If this amp turns out as good as I hope I might try a pair of monoblocks in the future...

-Tay

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Complete: Water-Cooled Quad-Core PC with 4GB RAM, 2X 750GB Hard Drives in Raid-0, 2X Radeon 3870 video cards in Cross-Fire


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PostPosted: 20 Aug 2009, 19:18 
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People of great knowledge,

good evening.

In a few posts about K-12G I have read about removing the DC-blocking caps (between the audio input and the volume pots) and replacing them with wires to increase the bass response.

Here is a stupid question: If these caps are supposed to block DC, what is going to happen to DC once the caps no longer block it?

In other words, if those caps aren't necessary, why did the designers put them there ... and if they are necessary, what are the risks and trade-offs of not having them?

Thanks in advance .... :confused:


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PostPosted: 22 Aug 2009, 20:09 
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Hi, I don't claim to be of great knowledge, but have a fair amount of experience with amp design and still have a modified K-12 that does daily duty in my office.

The input caps are there to do as you noted, block DC from the source. Nearly always the grid of the input tube (true in the K-12s) is at or very close to ground potential. If the source has a dc component or offset and it is applied to the input tube's grid it will alter the bias. A tube's bias is the voltage between the grid and cathode. It is usually negative and it establishes the operational point for the tube. Think of it as a way to control and in most cases preset the amount of current that flows through the tube. A source signal applied to the grid will essentially alter the voltage between the grid and cathode and thus alter the flow of current. This in turn is how most tube circuits amplify. So (sorry to be so long winded, but I don't know how much background you may have) any dc applied to an amplifier that does not have the capacitors will cause problems. That is the bad news part. The good news is that nearly all well designed equipment (signal sources) has little or no dc in the output. All of amps I design do not use input caps. But I have checked all of my sources to verify that there is no dc present at their outputs. Now jump back to the K-12. Generally speaking, the lack of bass is not due to the input cap. A little quick math and if I recall the parts values correctly with the stock caps in place the -3db point will occur at about 7 HZ (other components will have a greater influence). My experience has been that using better quality caps of the same size (my K-12 has .22 uf Auricaps in place of the stock .22ufs that came with it but ) has a much more dramatic effect on the sound than removing the input one (mine is still there in that amp). A second improvement is to use better output transformers. All this being said, there are limits to what a K-12 can do. They are excellent buys at the price, but IMHO it is unreasonable to expect them to equal bigger and more costly equipment. The best way I know of getting better bass out of a K-12 based system is to get better speakers or perhaps use a powered subwoofer. I hope this helps.

Good listening
Bruce

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PostPosted: 27 Aug 2009, 19:17 
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Not strictly on topic here, but wood cases on K-12Gs were mentioned, and here's my rather amateur attempt at it.

Image


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PostPosted: 28 Aug 2009, 17:19 
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I am enjoying it very much. It's been built and functional for quite a while, but it is just finally in it's enclosure.

Interestingly enough, the transformer setup that I have right now is quieter (hum-wise) than the best I was able to come up with, when they were all in-plane!


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PostPosted: 28 Aug 2009, 17:27 
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Hi, You have found one of what I call one of the fundamental truths of audio. The placement of the power trannie with respect to outputs (and indeed lots of other parts) is quite important. Designs with lovely symetrical transformer placements can easily have residual hum levels much worse that layouts that seem haphazard. Nice build. BTW my modded K-12 runs most days in my office and since it was one of my first tube projects it also has fond memories.

Good listening
Bruce

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