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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: 30 Nov 2020, 21:01 
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Joined: 13 Jan 2018, 21:33
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Location: australia
Thanks Lexx21. I’ll look into your circuit. The 6dj8 appear to be available locally to in the JJ, Russian and Chinese varieties depending on stock. Sounds like you have been happy with the performance?

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PostPosted: 30 Nov 2020, 21:43 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, either of the circuits will perform decently. My biggest issue is that the output impedance is pretty high. Depending on what your preamp has for input impedance it can matter. I like to stick to the old 1 to 10 ratio for matching gear and thus keep the output impedances low when feeding a preamp. This is particularly true if you use a passive one like I do that has a Z in of 20K. Of the two circuits I would prefer the one with 12AX7s as there are many choices for the tubes and even though tubes in the same family (12AX7, ECC83, 7025 etc) all spec the same they do not all sound the same and thus you can to dsome extent tailor the sound to you likes. The 6DJ8 family is not as extensive and is more limiting in choices.

Good listening
Bruce

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PostPosted: 30 Nov 2020, 23:39 
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Thanks Bruce. I admit I don’t have the proper understanding about impedance matching but I was hoping it would work ok with the wide band version of the 6sn7/300b set I have. I do have a few stereo receivers class d and class ab I still use from time to time, so it might be prudent to design it to match these as well at least in case I have a failure in the tube amp.

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PostPosted: 20 Dec 2020, 21:32 
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Bruce, I see you also designed the linked phono preamp. download/file.php?id=3269&mode=view
We’re you happy with the outcome? What are the positives and negatives with respect to moving magnet cartridges compared to the Groovewatt?

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PostPosted: 22 Dec 2020, 04:47 
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Previous enquiry can be kindly ignored Bruce. I see you have fielded similar queries and your response to another attached covers the issues you have struggled with and resolved. viewtopic.php?f=9&t=3507&hilit=Phono&start=30#p28590

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PostPosted: 23 Dec 2020, 06:27 
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Has anyone had experience in the use of the linked RIAA component value calculator? No doubt it would be handy to know how to compute the values from first principles but seems a dark art. http://www.mh-audio.nl/Calculators/CalculateRIAA.html

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PostPosted: 23 Dec 2020, 08:47 
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Hi, I use a different one...but they all tend to give the similar results. There are several similar programs. The final value for R1 in them is critical. It is related to the effective output impedance of the stage feeding it. Thus it is sensitive to what tube and what configuration is used in that stage. For example I use a SRPP stage most often with tubes that spec as 12AX7. This gives (just an example) of an output impedance of around 23K. This becomes part of the R1 value that gets calculated. An error in this value will seriously alter the response of the equalization. A feature that I don't like about the one on the web is that it doesn't take into account the value of the grid to ground resistor of the next stage. Often it is assumed to be around 1meg and is not all that significant. I much prefer 100K for lower noise and less sensitivity to hum. That is significant. BTW I use a commercial program to calculate these values.

Good listening
Bruce

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PostPosted: 24 Dec 2020, 00:30 
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Thanks Bruce. I need to study up on input and output impedance.

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PostPosted: 24 Dec 2020, 05:15 
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The attached is an interesting read, also funny, and quite possibly crude for some. Read in conjunction with his phono preamp write up. Has an interesting excel spreadsheet which appears to take into account, plate resistance, impedances and ratios, grid resistors etc. Is there a way to validate the results to a reasonable degree? It certainly gives different results to the calculator I mentioned in previous post.

https://wtfamps.com/2017/03/22/diy-tube ... tor-sheet/

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PostPosted: 24 Dec 2020, 06:44 
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Joined: 01 Jun 2013, 09:05
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Hi,

Years ago Croft made some interesting preamps but there are better alternatives now.
I thought you might find it interesting. (I have no connection to the website).
http://soundmovements.co.uk/diyaudio/Croft_preamps.html

The RTP3 is supposed to be "one of the best" but I have never built it. (Allen Wright)

Regards
M. Gregg

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