My Preamp

Sch3mat1c · 6th March 2003, 10:59 PM

- This is what I'm putting into that 6x6" cast aluminum chassis.

Comments?

(Hmm, I should probably change the 1k and 220k resistors on the 7025.. I haven't graphed the operating point yet..)

Tim

bournville · 6th March 2003, 11:16 PM

Tim,

What is the purpose of the 3.3meg resistor between the anode and grid of the 5814? With the tone control network do you need the .22 mfd cap between the network and the ground? On a matter of personal taste, I'd find the caps in the network not giving enough variation - I'd prefer a larger cap in the bottom part of the bass pot set up and the same for the treble pot.

Best wishes with the construction.

Gunders · 6th March 2003, 11:29 PM

I think the 3,3Meg resistor is used for biasing the 5814. The cathode resistor is large valued(22k), I don't know the quiescent current through this tube, but I think the current will give a voltage drop over the cathode resistor which is greater than the needed biasvoltage.
So the 3,3Meg and 1Meg resistors is used as a voltage divider to raise the grid voltage for proper biasing.

fdegrove · 6th March 2003, 11:31 PM

Hi,

Quote:

I think the 3,3Meg resistor is used for biasing the 5814, the cathode resistor is large valued, 22k.

Me too...but why the tone controls and high gain?

Cheers,

bournville · 6th March 2003, 11:37 PM

Yes,

The 3.3meg will affect the bias, but I was wondering if it was necessary in a cathode follower stage - I've never used a resistor in that position when I've used a cathode follower, and I cannot recall seeing a circuit using one. It may well be necessary (I haven't worked out the bias position of the cathode follower), I'm just curious about it!

Tube_Dude · 6th March 2003, 11:44 PM

Yes usualy the cathode follower is dc conected to the previous stage...but in this case it'not...then the need of the biasing.

A designer choice!

Jorge

Sch3mat1c · 6th March 2003, 11:50 PM

Lesse..

12AX7 - not really needed.. could probably drop down to an 'AU or 'AT7 instead..
I figure around 2mA bias minimum in the CF (yes the 3.3M biases it at ~40Vk, and also loads down the tone circuit a little). 12AU7 really starts working at 5mA, so why not make a CF that works well too?
I could just as well use self-bias in the cathode, but this will fix it at a much more stable point.

The .22uF fixes the ground return for the tone controls. I could just as well short it, and place a cap in series with the output (going to the 5814 grid; come to think of it, it could be a smaller cap and still work, hmm!).

I'll see what Duncanamp's TSC says about the larger caps...

Edit:
Didn't see your post, Tube Dude..
I sure could omit the first coupling cap, and only use one to fix the tone circuit's AC ground (as shown). But it's probably best this way anyway, after all if a strong DC signal is applied to the input, it would shift the operating point of the entire setup. Only two stages, not much to worry about, but meh.

Tim

Gunders · 6th March 2003, 11:56 PM

He could choose to apply the cathode resistor to a negative voltage instead of ground, then the voltage divider would be unnecessary.

Quote:

Originally posted by Tube_Dude
Yes usualy the cathode follower is dc conected to the previous stage...but in this case it'not...then the need of the biasing.

I think it's a bit tricky to dc connect the cathode stage when it's a tone control in front of it.

As I have said before, I dont know the quiescent current through this tube but I can try to make some assumptions.

The voltage divider will raise the grid voltage to about 46V, if say that the biasing voltage should be about 5-7V the current through the cathode resistor will be about 2,3-2,5 mA and this will also be the quiescent current(am I correct?).

I think this maybe a bit low for this tube, I'm not very sure.. I leave it to some of the experts to decide, I'm just a newbie in this game.

Sch3mat1c · 7th March 2003, 12:02 AM

LOL!.. A negative voltage supply would add five, maybe ten components in exchange for a few more volts of performance, and just to get rid of one resistor to +V.

Sorry, but it is funny

Yes, around 2-3mA is what I'm going for, more or less.. it should be okay, since 22k is a good load for it, though pretty close to the Rp at this current level. Won't matter much if you ask me, since it won't be going over 10V anyway.

TSC shows that the larger bottom caps gives more even boost vs. cut, thanks for that suggestion. (Using 2.2nF for lower bass cap, 470pF for lower tone cap.)

Tim

Tube_Dude · 7th March 2003, 12:06 AM

Hi Tim!

Quote:

Didn't see your post, Tube Dude..

I was only agreing with the post of Bunville...see it ans you will understand!

We are talking about folowers in general...

Regards

Jorge

Ps.But can i ask as Frank...why the tone controls?

6th March 2003, 11:44 PM	#6
Tube_Dude diyAudio Member Join Date: Mar 2002 Location: Aveiro-Portugal	The Bias! Yes usualy the cathode follower is dc conected to the previous stage...but in this case it'not...then the need of the biasing. A designer choice! Jorge

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6th March 2003, 11:16 PM	#2
bournville diyAudio Member Join Date: Feb 2003 Location: Birmingham, UK	Tim, What is the purpose of the 3.3meg resistor between the anode and grid of the 5814? With the tone control network do you need the .22 mfd cap between the network and the ground? On a matter of personal taste, I'd find the caps in the network not giving enough variation - I'd prefer a larger cap in the bottom part of the bass pot set up and the same for the treble pot. Best wishes with the construction.

6th March 2003, 11:29 PM	#3
Gunders diyAudio Member Join Date: Nov 2002 Location: Norway	I think the 3,3Meg resistor is used for biasing the 5814. The cathode resistor is large valued(22k), I don't know the quiescent current through this tube, but I think the current will give a voltage drop over the cathode resistor which is greater than the needed biasvoltage. So the 3,3Meg and 1Meg resistors is used as a voltage divider to raise the grid voltage for proper biasing.

6th March 2003, 11:37 PM	#5
bournville diyAudio Member Join Date: Feb 2003 Location: Birmingham, UK	Yes, The 3.3meg will affect the bias, but I was wondering if it was necessary in a cathode follower stage - I've never used a resistor in that position when I've used a cathode follower, and I cannot recall seeing a circuit using one. It may well be necessary (I haven't worked out the bias position of the cathode follower), I'm just curious about it!

6th March 2003, 11:50 PM	#7
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	Lesse.. 12AX7 - not really needed.. could probably drop down to an 'AU or 'AT7 instead.. I figure around 2mA bias minimum in the CF (yes the 3.3M biases it at ~40Vk, and also loads down the tone circuit a little). 12AU7 really starts working at 5mA, so why not make a CF that works well too? I could just as well use self-bias in the cathode, but this will fix it at a much more stable point. The .22uF fixes the ground return for the tone controls. I could just as well short it, and place a cap in series with the output (going to the 5814 grid; come to think of it, it could be a smaller cap and still work, hmm!). I'll see what Duncanamp's TSC says about the larger caps... Edit: Didn't see your post, Tube Dude.. I sure could omit the first coupling cap, and only use one to fix the tone circuit's AC ground (as shown). But it's probably best this way anyway, after all if a strong DC signal is applied to the input, it would shift the operating point of the entire setup. Only two stages, not much to worry about, but meh. Tim

7th March 2003, 12:02 AM	#9
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	LOL!.. A negative voltage supply would add five, maybe ten components in exchange for a few more volts of performance, and just to get rid of one resistor to +V. Sorry, but it is funny Yes, around 2-3mA is what I'm going for, more or less.. it should be okay, since 22k is a good load for it, though pretty close to the Rp at this current level. Won't matter much if you ask me, since it won't be going over 10V anyway. TSC shows that the larger bottom caps gives more even boost vs. cut, thanks for that suggestion. (Using 2.2nF for lower bass cap, 470pF for lower tone cap.) Tim

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