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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: 17 Nov 2010, 23:10 
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Hi Everyone, It is true the output coupling cap is important. The K40Y9s are hard to beat for cost vs performance and I recommend them. However, IMHO there are better. Right now I'm using Jantzen Silver Z caps. They have an entirely different sound (both measure on the bench the same). A fuller more spacious sound, perhaps some more detail as well. Auricaps (a favorite of mine) sound a lot like the K40s. I have not been able to make up my mind which I like better. What I suggest to all you folk that like to tinker with projects is to use the Euro style screw terminal blocks in the amp to connect the capacitors. Then you can easily swap in and out any you choose to try. That is what I do. You may well be surprised how good and how bad some some caps sound. :thumbsup:

Good listening
Bruce

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PostPosted: 18 Nov 2010, 06:46 
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The K40Y9's I've found are 200V. Is this an issue?


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PostPosted: 19 Nov 2010, 11:57 
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Hi, for this application they are fine as the applied DC is in the 90-120 range depending on how symmetrical the gain of the tube sections is. Ideally it ought to be 1/2 the appied B+, generally it is off a few %.

Good listening
Bruce

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PostPosted: 19 Nov 2010, 12:29 
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Thanks both for your clarification -and- your generous contribution to this community.


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PostPosted: 20 Nov 2010, 13:45 
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Bruce,
Thanks for this thread and your designs and tips. I'm currently working on a headphone amp with 6n1p's, and in looking at the design (Bruce Bender's Morgan Jones derivant), it uses a cap to "float" the heater ground, instead of biasing the heater the way your design does. Doing the (simple) math, if I've got a ~350 DC B+, then I need to bias the heater to between 80-90 volts to not exceed the 100Vh-c. How do you run the numbers to set the components for the bias voltage, since it's a little more than a voltage divider going on here? here's the power supply I'm trying to work with (from http://gilmore2.chem.northwestern.edu/projects/showproj.php?file=bender_prj.htm ). Thanks!

Josh


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PostPosted: 23 Nov 2010, 17:31 
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Ahhh, I misread your PS schematic--it is a voltage divider. How do you calculate the tube supply current ranges across your PS filtering resistors (posted here for clarity) so that you can figure out the voltage range at the top of the divider, and then the resulting bias voltage range that will result on the heater circuit? Is it better to use larger value resistors if you have them to minimize this current, or does that just lengthen the time to bias the heater at startup? While I'm being pesky, what is the function of the cap (C1) that floats the Heater + rail to the B+ ground rail, and would a similar cap from the heater - rail to B+ ground do the same thing?
I'm not questioning your design, I'm just trying to understand it so I can modify what I'm working on in a similar fashion. Hope you (or someone with more circuit smarts than I have) can school me a bit here. Thanks,

Josh


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PostPosted: 29 Nov 2010, 16:30 
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Hi Good questions, For the various resistors in the PS filters, I base them on the load. I determine what voltage I want on the tubes and since I have already calculated the current flow in them It is just a matter of subtracting the final B+ from the available source and calculating the total resistance necessary to do it. Then... sine tubes and number crunching are not usually exact. I measure the voltages and adjust the resistors as needed to get the end value desired.

The capacitor in the voltage divider is for noise reduction. I discovered it while sorting out the final build. It reduced the overall noise by about 3 dbv. Apparently as clean as the B+ was, there was some noise in it that carried over into the heater circuit and finally into the cathode of one of the triodes (don't know which one, but I suspect the lower section).

Hope this helps, I have no problem with folks improving on the things I post. We both learn that way.

Good listening
Bruce

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PostPosted: 16 Dec 2010, 11:12 
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Bruce (or anyone else who might know),
I noticed in your design and construction tips that you say to isolate the input/outputs jacks from the chassis. I read in several other places (http://www.diyaudio.com/forums/diyaudio-com-articles/163575-audio-component-grounding-interconnection.html?garpg=5#content_startl , et al.) that the shield of an unbalanced connector should always be grounded to the chassis near the entrance point to prevent hum. What am I missing here? Is your recommendation to connect the shield of the input to the audio signal ground directly? Thanks,

Josh


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PostPosted: 16 Dec 2010, 14:22 
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Hi Josh, Grounding is not as simple as it would seem. The key factor is that there should be just one central ground point. Generally it should not be the chassis. Books have been written on this issue, but I'll try to give some quick thoughts on it. What you want to do with grounding in audio gear is to avoid ground loops, maintain shielding and have complete signal paths. The easiest way I have found is to use the chassis (if metal) as a shield and not have it directly connected to the signal ground. I use a combination of a fixed resistor in the 120-150 ohm range and a type X2 capacitor in the 0.1 to 0.22uf range to connect the chassis at a single point to the main signal ground. (the single point aspect of all this is critical) The signal ground can be a "star" system or a "buss" both will work. Then in order to have a complete circuit for the signal both incoming and out going, I do not connect the shield wires on the input jacks to the ground where they pass through the chassis. I do of course connect them to the jacks (which are isolated from the chassis). This makes that part of the circuit an extension of the cable connected to it from the source (or to the following equipment). The end of the shield that goes to the circuitry is connected to the star or buss. Thus all parts of the preamp share a common ground point. (and importantly there are no alternate paths) The B+ negative is also connected to the same point. If you do something different you can easily create multiple paths through the grounds. This will allow current to flow (power or signal) via different paths and create what is known as ground loops. If you run a spice program you will see that the multiple paths will result in hum and noise pick up. In high gain circuits the effects of ground loops can completely swamp the actual signals.

A second aspect to grounding is safety. In the US the current standards require either isolation of all metal parts on the outside of equipment connected to the AC mains or having it at earth ground potential. This is easy to accomplish if you use the parallel resistor / cap arrangement and a three wire power cord.

Good listening
Bruce

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PostPosted: 16 Dec 2010, 17:24 
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Bruce,
OK, thanks! So my plan is to connect the (chassis isolated) RCA shields to audio ground reference, and then attach the audio grounds to the B+ ground at one point, and then couple that via the X2 cap and 120R to the chassis and mains safety ground. As long as each of the signal grounds and the B+ ground (B- ?) come together only at one point, and only connect through the RC to the chassis ground, I should avoid loop currents, correct?

Josh


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