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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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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 12 Jan 2017, 18:43 
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On your mood? Now we're getting technical. What about the weather on the day?

Thanks Matt for the explaination. I was using 100K steppers in my T4 Class D Tripath amps but run out of them. So I used a 250K stepper. The amp hummed loudly. As soon as I went back to the 100K it was fine and this value still works fine with my two stage tube preamp.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 12 Jan 2017, 21:10 
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mwhouston wrote:
I used a 250K stepper. The amp hummed loudly. As soon as I went back to the 100K it was fine and this value still works fine with my two stage tube pream
My guess is that the Tripath has a relatively low input impedance. With the 250kΩ on the front it was essentially unloaded and the input was acting as a noise antenna. I looked for some documentation on the T4 but was unsuccessful. Might be worth a look.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 12 Jan 2017, 21:14 
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Input impedance is 22K but they have a lot of gain.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 17 Jan 2017, 10:11 
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Joined: 26 Dec 2016, 07:10
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Hello, I am a new member and this is the first time I have posted here. I have been going through as much of the posts about the Lacewood amp's progression. I am in the process of building the Lacewood 2.0 amp. I also like the 6V6 tubes and the SE configuration has the smoothest sound in my opinion. I am a little confused with the main choke selections for ver 2.0. I would like to use Edcor throughout the amp or even the Triad for the choke. The choices are what is confusing me.

Suncalc wrote:
I would also replace the Hammond 193J with an Edcor CXC100-7H-150mA. If you want it under the hood you could use Triad C-16X 6H 200mA choke as the main unit.

Suncalc wrote:
RossD wrote:
And I think you mean a Triad C-14X rather than C-16X.
Yup. I meant the C-14X.


193J is a 10H 150mA choke while the suggested replacements are 7H or 6H. So is it the Henries don't matter and try to get the mA to match when finding a suitable replacement. Wouldn't the Edcor CXC125-10H-200mA be a good substitute?
Thanks for a great site with all this information available.


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 17 Jan 2017, 20:43 
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84bronze wrote:
193J is a 10H 150mA choke while the suggested replacements are 7H or 6H. So is it the Henries don't matter and try to get the mA to match when finding a suitable replacement. Wouldn't the Edcor CXC125-10H-200mA be a good substitute?
I apologize for the confusion. It all goes back to the history of the Lacewood amp and reuse of components.

The original Lacewood power supply was built to preclude any chance of power supply ripple coming through to the tubes. As such, it was FAR over designed. Since I retained the choke for the V2.0 That is what got used. But let's walk through the math.

The the original power supply design has a design point ripple factor of 0.022 at the first capacitor. This equates to -33dBv of ripple. I want at least -90dBv of ripple at the tubes these days. Using the 10H-32µf filter stage gives a ripple reduction of -45dBv. This puts ripple at -78dBv. Just 12 dB shy of my target.

Now the lacewood V2.0 uses a split power supply filter with interstage isolation provided by choke filtering on each branch. This provides a lot of filtering overall because the filter factors from the two stages are multiplicative, i.e. dBs add. So looking at the second stage 1H-100µf filter, it gives a ripple reduction of -35dBv. This gives a total ripple reduction at the power tubes of -113dBv. This is a full 23dBv of filter margin.

What all this means, is that with the same second stage filtering, the main LC filter only really needs to supply 22dBv (90-33-35=22) to meet the -90dBv requirement. If the 10H choke is substituted by a 7H choke, the new 7H-32µf filter section provides 42dBv of ripple rejection (for a total of -110dBv) and a 6H choke gives 40.6dBv of ripple reduction (for a total of -108.6dBv). So either option is more than sufficient to meet the -90dBv overall ripple requirement.

For a better understanding of filter sections, you can take a look at part 2 of my power supply article here.

Oh... and to answer your question: Yes, the Edcor CXC125-10H-200mA would make an admirable substitute for the Hammond 193J.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 17 Jan 2017, 23:10 
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That's good reading for designing power supplies! I haven't seen those calculations in a long time. So there's a threshold for where your design on paper needs to be to keep the ripples down for power coupling to the tubes. So if there was too low a choke than there would be voltage ripple distortion. So keeping the value up at 10H would help keep the amp in class A? It would keep the voltage out more stable but also lower the voltage at the same time? So without installing a larger power transformer it's a give and take balancing act? -90DBv is an ideal voltage "quality" (for lack of a better term) for class A?

For the Lacewood 2.0 we need the higher voltage to run SE UL , so the Triad C-14X would work good or something similar from Edcor.


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 18 Jan 2017, 17:07 
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84bronze wrote:
So there's a threshold for where your design on paper needs to be to keep the ripples down for power coupling to the tubes. So if there was too low a choke than there would be voltage ripple distortion.
Correct.

84bronze wrote:
So keeping the value up at 10H would help keep the amp in class A?
No. The power supply ripple is about clean dc power that doesn't induce hum or noise into the signal/power chain. Amplifier class is about active device conduction angle. An amplifier with a conduction angle of 360° is class A. It is class A1 if it does not draw grid current, it is class A2 if it draws grid current at any time during the conduction cycle. Controlling amplifier operating class is much more about the bias point than the supply voltage.

84bronze wrote:
It would keep the voltage out more stable but also lower the voltage at the same time?
I'm not sure what you mean by this. A larger choke increases the ripple reduction, but the output voltage reduction by the choke is more about the current and winding resistance than the inductance.

84bronze wrote:
So without installing a larger power transformer it's a give and take balancing act?
Yes. As with all things in Engineering, it's always about balancing and tradeoffs.

84bronze wrote:
-90DBv is an ideal voltage "quality" (for lack of a better term) for class A?
This is a value suggested by Herbert Reich in chapter 14 of his text "Theory and Application of Electron Tubes" for microphone circuits. I have found that this is a good value to use as a design point for modern power amplifiers. For modern high gain systems (>40dB), I would endeavor to achieve at least 20dBv better filtering. But it not required very often.

I hope this helps.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 18 Jan 2017, 17:42 
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Thanks Suncalc. I'm going with the Triad C-14X .


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 19 Mar 2017, 18:11 
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I found locally 390 ohm 5w resistors, instead of the 392 3w resistor of your circuit.
Is that really matter?
Best regards.


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 19 Mar 2017, 18:32 
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lowhifi wrote:
I found locally 390 ohm 5w resistors, instead of the 392 3w resistor of your circuit.
Is that really matter?
No problem, should be fine. Measure them before you put them in just so you know.

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