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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 Jan 2014, 05:34 
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Joined: 26 Jan 2014, 12:48
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Great. I'll head straight to designing the new wiring.
Thank you very much for the data.


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PostPosted: 28 Jan 2014, 04:21 
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Joined: 08 Feb 2011, 21:57
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I hope this question isn't too naive, but I had a question about the B+ supply to the pre-amp tube. On your build do you have two leads coming from the 470k resistor to supply the 250volts? Thanks for any advice,
just putting all the parts together for the build.... :wizard:


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PostPosted: 28 Jan 2014, 20:26 
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bugeyes wrote:
I hope this question isn't too naive, but I had a question about the B+ supply to the pre-amp tube. On your build do you have two leads coming from the 470k resistor to supply the 250volts?
Not a naive question at all. The short answer is yes. But the full answer is a little more complicated.

If you look at Figure 3 on the project page (the power supply schematic) you'll see that there are two different types of ground symbols used. Those that look like a parallelogram are chassis grounds. These are tied to all the chassis metal in the enclosure. Those that look like triangles are signal grounds. These are tied together but are independent of the chassis grounds. Then the two different types of grounds are tied together via the ground lift switch. The text in the project article explains why this is a good idea.

Below is a copy of Photograph 3 from the project page (underside) with some labeling added for clarity.
Attachment:
Screen shot 2014-01-28 at 5.12.05 PM.jpg
This diagram shows which types of grounds are where and how the ground switch ties them together. You will notice that (getting back to your original question) that there are two wires running through the center penetration of the divider plate. The black lead is the high voltage rail coming out of the primary filter inductor, the green line is the signal (and power) ground. These connect to points on the terminal strip under the final filter capacitor. So yes, there are two leads coming off the bleeder resistor. The following picture shows how these leads connect to the terminal strip.
Attachment:
terminal strip.jpg

This should make the grounding scheme that I used more clear. Does all of this make sense? Let us know if you have other questions.


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PostPosted: 28 Jan 2014, 23:44 
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Matt,
thanks for the quick reply and the awesome pictures. Your post helped clear the grounding scheme
that I wasn't 100% on, though the actual high voltage that needs to be applied to both halves of the tube was the thing I was confused about. I wasn't aware you had a terminal strip mounted inside the chassis.
Do you tap the high voltage from the terminal strip and use two leads to apply B+ to the tube?
thanks again for any advise you could give... :wizard:


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PostPosted: 29 Jan 2014, 00:56 
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bugeyes wrote:
Do you tap the high voltage from the terminal strip and use two leads to apply B+ to the tube?

Actually the point on the terminal strip where the black inductor lead is attached also has the positive side of the final filter capacitor and the bleeder resistor attached to it as well. This then servers as the high voltage B+ supply for the preamp.

As you can see in the attached image, the plate load resistors for the two channels are connected directly to this tab as well. You can also see that the coupling capacitors connect to the other side of the plate load resistors at the plate pins on the tube socket.
Attachment:
Screen shot 2014-01-28 at 9.48.25 PM.jpg

Does this answer your question?


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PostPosted: 29 Jan 2014, 05:06 
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Matt,
thanks for the awesome pictures. Yes, now I understand how to route the B+. Now to just to
put all the parts together, I am very looking forward to hearing it.
thanks again.. :thumbsup:


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PostPosted: 29 Jan 2014, 05:42 
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Nice component-2-component wiring.

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PostPosted: 15 Mar 2014, 12:24 
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Hi, I am looking for a Mic Preamp. Does the 4S delivers enough gain to get Line level (+4dBu) from mic level (-40dBu aprox)?


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PostPosted: 16 Mar 2014, 15:52 
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arturo.sallas wrote:
Hi, I am looking for a Mic Preamp. Does the 4S delivers enough gain to get Line level (+4dBu) from mic level (-40dBu aprox)?

I would say not but I'm just guessing.

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PostPosted: 16 Mar 2014, 17:27 
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arturo.sallas wrote:
Hi, I am looking for a Mic Preamp. Does the 4S delivers enough gain to get Line level (+4dBu) from mic level (-40dBu aprox)?
If you look at the gain table you’ll see that if you use the bypassed cathode configuration and a 12AX7 tube, you should get around 34.5dBv of gain. This will take a -40dBu level to approximately -5.5dBu. So not quite. But, whether it will be useful or not is really a function the system you are driving. If it’s going into a consumer amp, this may be just fine. If you’re driving the input of a commercial mixer board it depends on how much gain you can get in the channel off the board.

Also replied here http://www.cascadetubes.com/the-universal-preamp/

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