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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: 27 Apr 2020, 16:29 
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Sebastián wrote:
My question is this: can I use a CRC filter instead of the choke?

What values ​​would the resistors be?
Because the current draw is small on this unit, you can use reasonable sized resistors. I would construct the filter as shown here. This will give about 75dB of ripple reduction for each channel and will provide about 75dB of additional channel separation over and above the PSRR of the SE stages.
Attachment:
filter.jpg
This is just three 1KΩ resistors and three 100µf capacitors. It's pretty simple.

I hope this helps.


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PostPosted: 27 Apr 2020, 19:00 
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https://www.calculator.net/ohms-law-calculator.html
If you use the ohms law formula wheel then you can estimate the pre amp current draw also power disipation and voltage drop accross the resistors and design your own.

I know all of the fomulas off by heart other than 4 of them, which I should learn actually and is what I am doing today.
Instead of schematic calculation the old way try to use the new formulas.

Sure I can rearragne the formulas I know off by heart then I would get the other 4 that I don't know off by heart but its faster and more straight foward to just use a few most basic ohms law to do the same calculation.


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PostPosted: 27 Apr 2020, 19:10 
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Joined: 10 Apr 2020, 23:53
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Location: Uruguay
Suncalc wrote:
Sebastián wrote:
My question is this: can I use a CRC filter instead of the choke?

What values ​​would the resistors be?
Because the current draw is small on this unit, you can use reasonable sized resistors. I would construct the filter as shown here. This will give about 75dB of ripple reduction for each channel and will provide about 75dB of additional channel separation over and above the PSRR of the SE stages.
Attachment:
filter.jpg
This is just three 1KΩ resistors and three 100µf capacitors. It's pretty simple.

I hope this helps.


That's great, Matt! Thank you very much! :thumbsup:

It helps me a lot!

Thanks for making this schematic to help me, I really appreciate it.

Warm regards,

Sebastián


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PostPosted: 27 Apr 2020, 19:16 
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Joined: 10 Apr 2020, 23:53
Posts: 7
Location: Uruguay
ILoveHiFi wrote:
https://www.calculator.net/ohms-law-calculator.html
If you use the ohms law formula wheel then you can estimate the pre amp current draw also power disipation and voltage drop accross the resistors and design your own.

I know all of the fomulas off by heart other than 4 of them, which I should learn actually and is what I am doing today.
Instead of schematic calculation the old way try to use the new formulas.

Sure I can rearragne the formulas I know off by heart then I would get the other 4 that I don't know off by heart but its faster and more straight foward to just use a few most basic ohms law to do the same calculation.


Thanks for your help, ILoveHiFi! This forum is fantastic! :up:


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PostPosted: 28 Apr 2020, 16:22 
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Joined: 02 Oct 2014, 20:36
Posts: 47
Suncalc wrote:
Sebastián wrote:
My question is this: can I use a CRC filter instead of the choke?

What values ​​would the resistors be?
Because the current draw is small on this unit, you can use reasonable sized resistors. I would construct the filter as shown here. This will give about 75dB of ripple reduction for each channel and will provide about 75dB of additional channel separation over and above the PSRR of the SE stages.
Attachment:
filter.jpg
This is just three 1KΩ resistors and three 100µf capacitors. It's pretty simple.

I hope this helps.

hey!!! i'm stole this schematic for the baxandall power suply,and its just perfect!!! nice sound improviment and chanell separation...lots of learning here,thankyou!!!


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PostPosted: 28 Apr 2020, 17:52 
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Joined: 06 Jun 2008, 18:23
Posts: 5267
Location: Australia
I pull still go; CLCRC. Caps are: 100, 100, 200. The last R is set to limit the HT to what is required and can be anything from 100omhs to 1K. Choke mostly Hammond 5H low current. Quite small.

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PostPosted: 20 May 2020, 16:42 
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Joined: 20 May 2020, 15:49
Posts: 8
As far as electronics go, this is my first build of any kind. I am no EE. Not even close. This is a learning experience. The 4SUniversal didn't look too particularity imposing, so I went for it.
Image
Later I was urged to move a few things and optimize the pre for one particular tube, the 12au7. As the learning (hopefully) progresses, I'm beginning to draw load lines and find there is a huge difference between current the 12ax7 (~2ma) vs 12au7 (~10ma) desires. To that end, I've reduced the plate resistor to 22.5k and the cathode resistor to 750ohm with good quality Ohmite 11 and 12 watt wirewound resistors, and come up with this.
Image

When looking at the 4s's tubed power supply, the supply voltage is stated ~270v@1.8ma. Is this correct? Will this power supply work and if so, what modifications will be required?
Am I on the right track?
And yes, I have searched but with this thread and all the off-shoots of this thread, my eyes hurt from all the reading.


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PostPosted: 21 May 2020, 22:26 
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Being a complete nube at building anything electronics related, the 4s seemed simple enough for even me. Let the fun and tube rolling begin, and it did. Then someone told me this was a 'one size fits all' pre that mated with some tubes quite well while others were a bit outside their optimum operating range. I was encouraged to optimize it for one tube. I like the 12au7.
Trying to learn some very basic stuff in order to better understand the who and why of these shiny glass bottles that were in everything when I was a kid.
Load lines; to me, the 12ax7 and 12au7 datasheet looks way different, ~5ma vs ~20. Learning to calculate plate and cathode resistors makes me think a 12au7 would operate better with more ma. A 22.5k plate resistor and 750ohm = ~12ma. The volume control was moved to the input and a much larger 2.2uf dc blocking cap replaced the .47uf orange drop. 2.2k grid stop, 1m grid leak, and a 22uf cathode bypass.
Am I moving in the right direction to optimize the 4s for the 12au7 or am I totally lost?


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PostPosted: 25 May 2020, 13:01 
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Kornbread wrote:
Then someone told me this was a 'one size fits all' pre that mated with some tubes quite well while others were a bit outside their optimum operating range.
Actually this is not true. If you look at the load lines posted here, you'll see that each bias point is well placed in the operational envelope and none of the operational characteristics are off nominal for a simple common cathode triode amplifier stage.

Personally I don't like descriptions like "better" or "optimum" because first, they are subjective and second, they presuppose that the tube itself has some perfect operating point, which they do not. The tubes are just one part of a circuit designed to meet a set of requirements. The entire circuit either meets those requirements or it does not.

Kornbread wrote:
Am I moving in the right direction to optimize the 4s for the 12au7 or am I totally lost?
This is a very hard question to answer because you haven't stated what is your intent in changing the circuit. Comparing characteristic curves of different tubes will lead you to draw specious conclusions. I would recommend against it. So for the further discussion, let's limit the discussion to any single triode plate characteristic.

When designing a single amplifier stage there are really three things about which the designer is generally concerned. There are amplification (v/v), distortion (harmonic and intermodulation), and frequency response. When designing a vacuum tube preamp (like the 4S) the discussion is all about how the preamp colors the sound. As such, the main topic of discussion should be distortion. How much do you want? For what harmonics are you designing? And how do you want the harmonics to manifest themselves with respect to drive level? Answer these three questions and they will guide you in your design.

First, it is important to understand triode distortion and how it changes with design parameters and drive level.

At any given bias point, triode distortion is generally almost all 2nd harmonic and is nearly proportional to drive level. This means that the "color" added by the preamp is proportional to drive level. If you place the volume control in front of the stage, you will be limiting the "color" because you are limiting the drive level of the triode. If you place the volume control at the output, then the drive level remains largely constant and so does the color. This point seems to be lost on many people. If you want color, then design for it. If you don't, then use a very low distortion FET stage you could drive with a battery, not a vacuum tube.

It is also important to understand how distortion changes with load. As the load resistance decreases (and so the plate current increases) so does overall distortion. In general, high plate loads (and low plate currents) mean low distortion and low plate loads (and high plate currents) mean higher distortion. So if one wants high distortion (or color) one should choose lower plate loads. If one wants very low distortion the one should choose the highest plate loads they can in the context of the expected drive levels and larger circuit.

You stated that you chose a much lower plate load (22.5KΩ) and higher bias level (Rk=750Ω), thus indicating that you desire greater distortion or color. But you also said that you moved the volume control to the input of the stage, thus limiting drive level and distortion. These are conflicting design decisions. If you are questing for minimum distortion, then the plate load should be as high as permitted by the loading circuit. If you are questing to color your sound, you should move the volume to the output, and chose a bias point to provide the level of color you desire.

So the real question is "What are you trying to achieve?"

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PostPosted: 25 May 2020, 21:39 
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Joined: 20 May 2020, 15:49
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First off, thanks for the reply. It is appreciated. And apologies for the double post, it took several days to get mod approval and I suspected the first post was lost. Anyhow, I'm trying to get at least a basic grasp of things that have always piqued my interest but I was never brave enough to reach out for. This time of social distancing and unemployment has given the time to do just that.
What am I trying to achieve?
I like to design and build my own speakers. To that end, this was going to be the first year attending several meets to evaluate my designs. So this pre has to be fairly accurate. Looking to add a 'little' tube sweetness, not dark chocolate. It does not have to pick fly poop out of the pepper but it must be somewhat resolving. Frequency response does not have to be ruler flat, but close (already ran into a problem with that and the 4S). Both micro and macro dynamics. If the whack of a drum is supposed to slam me in the chest, it better.
From reading your post comparing the Foreplay to the 4S, you basically assess the differences in sound as one being more accurate and the other more pleasing. My preference is somewhere in the middle.
Spending a lot of late nights reading and trying to learn. There is a difference of ~15 ma plate current between the ax7 and au7 and I was attempting to move the au7 it into what I though was a more linear place in the operating curve. Load line below (is it correct) because somewhere I read it would be more dynamic and lower distortion. But it seems that more current across the plate increases distortion? Once again I've got to make a Ue' and do more reading.


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