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Designing my first tube preamp
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Old 6th December 2019, 12:36 AM   #11
rayma is offline rayma  United States
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Join Date: Apr 2011
Quote:
Originally Posted by Verbstank View Post
So change the .1uf cap to before the 12au7 tube rather than right before the output? Or are you saying change the placement of the tone circuit, possibly put it instead between the two 12ax7 stages? Or am I wildly off-course?
Remove the 1.3k and 50uF. Connect the bottom of the 100k to ground.
Connect the 0.1uF to the top of the 100k.

You also will need a coupling capacitor between the volume control and grid.
Also a resistor biasing network for the grid to get it at a DC potential that is
a little less than you want for the output cathode. Otherwise the tube will be cut off.

Why do you want a tone circuit for the bass output?

Last edited by rayma; 6th December 2019 at 12:42 AM.
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Old 6th December 2019, 12:51 AM   #12
Verbstank is offline Verbstank
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Quote:
Originally Posted by rayma View Post
Remove the 1.3k and 50uF. Connect the bottom of the 100k to ground.
Connect the 0.1uF to the top of the 100k.

You also will need a coupling capacitor between the volume control and grid.
Also a resistor biasing network for the grid to get it at a DC potential that is
a little less than you want for the output cathode. Otherwise the tube will be cut off.

Why do you want a tone circuit for the bass output?

Ok I understand most of this. Updated schematic attached. I guess I need to read up on biasing a cathode follower, because as you are saying I will eliminate the cathode bias (which makes sense in my brain since I'm taking the output from the cathode now) and now be biasing the grid instead.


But I'm lost on your question of why I want a tone circuit? I want a tone circuit for use as a tone circuit... so I can attenuate frequencies with pots... I must be missing something here, sorry I am new to all of this.
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File Type: png Amp Design.png (59.1 KB, 79 views)
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Old 6th December 2019, 01:00 AM   #13
rayma is offline rayma  United States
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Now at the grid you need a resistor to the power supply, and another resistor to ground.
The values will be determined by the current you want in the tube, but the top resistor could be 1M to start.
Perhaps around 1M for the other resistor also, bu this will vary with your current. What is the power supply voltage?

Last edited by rayma; 6th December 2019 at 01:12 AM.
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Old 6th December 2019, 01:09 AM   #14
Verbstank is offline Verbstank
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Quote:
Originally Posted by rayma View Post
Now at the grid you need a resistor to the power supply, and another resistor to ground.
The values will be determined by the current you want in the tube, but the top resistor could be 1M to start.
Perhaps around 1M for the other resistor also, bu this will vary with your current. What is the power supply voltage?

Attached is addition with resistors.



Power supply voltage is 250vac, again I'm new to all of this, but I've read that that will rectify to 1.4*250=350vdc, then I had some questions about stepping that down but figured if I can get it to 300vdc that should be good for all tubes involved?



I think I only want ~3 or 4ma current to the grid? Just like the other 12ax7 tubes? Or does a cathode follower require a different current?


Thanks for your time rayma
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Old 6th December 2019, 01:11 AM   #15
Verbstank is offline Verbstank
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Oh sorry, I took that to be plate current....
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Old 6th December 2019, 01:13 AM   #16
rayma is offline rayma  United States
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Instead, you may want to keep the 12AX7, and "follow" it by the 12AU7 cathode follower.
This will give more gain and reduce the other parts needed. The 12AU7 grid can then be directly
connected to the 12AX7 plate.

Last edited by rayma; 6th December 2019 at 01:25 AM.
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Old 6th December 2019, 01:18 AM   #17
Verbstank is offline Verbstank
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Quote:
Originally Posted by rayma View Post
Instead, you may want to keep the 12AX7, and "follow" it by the 12AU7 cathode follower.
This will give more gain and reduce the other parts needed. The 12AU7 grid can then be directly
connected to the 12AX7 plate.

So are you saying swap back to a 12ax7 where I have the 12au7 now, then add another stage that will be the 12au7 cathode follower directly after the 12ax7 stage? I am limited by the current on the heater wind to use only 3 tubes, and I would like to make this a dual channel pre, so 6 triodes total and 3 for each channel. If that falls through oh well, I can devote all 3 tubes to just one channel, but that would be a last resort.



I keep running into the same problem, my lack of understanding in the power section. How do I go about finding the load resistor for the cathode follower? I chose 100k for the 12ax7 loads and the 10k resistor between the filter caps all from suggestions, and as much as I read trying to understand why these values I still don't have a firm grasp on it.

Last edited by Verbstank; 6th December 2019 at 01:25 AM.
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Old 6th December 2019, 01:21 AM   #18
rayma is offline rayma  United States
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Originally Posted by Verbstank View Post
Hmmm so are you saying swap back to a 12ax7 where I have the 12au7 now, and add another stage that will be the 12au7 cathode follower directly after the 12ax7 stage that gains the tone control? I am limited by the current on the heater wind to use only 3 tubes, and I would like to make this a dual channel pre, so 6 triodes total and 3 for each channel.
Ok, just remember with the cathode follower there is no gain (output voltage about same as input).
Then try the last circuit with the resistors biasing the 12AU7 grid.

The 10k in the power supply will drop some voltage, depending on how much current the tubes take.
The voltage drop would be 10k x total plate current in all tubes added together (in amps).

Given your power supply voltage, then all the resistor values have to be determined by designing.
It's likely the 12AU7 cathode resistor will be less than 100k. That would be more for a 12AX7 follower.

Last edited by rayma; 6th December 2019 at 01:26 AM.
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Old 6th December 2019, 01:30 AM   #19
Verbstank is offline Verbstank
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Quote:
Originally Posted by rayma View Post
Ok, just remember with the cathode follower there is no gain (output voltage about same as input).
Then try the last circuit with the resistors biasing the 12AU7 grid.

The 10k in the power supply will drop some voltage, depending on how much current the tubes take.
The voltage drop would be 10k x total plate current in all tubes added together (in amps).

Given your power supply voltage, then all the resistor values have to be determined by designing.
It's likely the 12AU7 cathode resistor will be less than 100k. That would be more for a 12AX7 follower.

Thanks so much for all the responses and time. So thinking that if they were 3 12ax7 tubes, taking maybe 4ma of current per plate, I would say that the 10k resistor drops the voltage by 10,000 * 0.012 = 120volts? The 12au7 looks like it takes much more current, near 20ma, so then if 2 12ax7 and 1 12au7, 10000*.028=280v? seems like such a high number? But I guess that means I change the resistor value to something lower?


Am I doing this reasoning correctly?


edit: I didn't realize the cathode follower provided no gain, thanks for that
2nd edit: Doh I used max plate current for those numbers vs. normal operation current it seems, but that would still be the same thinking behind it?

Last edited by Verbstank; 6th December 2019 at 01:39 AM.
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Old 6th December 2019, 01:40 AM   #20
rayma is offline rayma  United States
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Quote:
Originally Posted by Verbstank View Post
Thanks so much for all the responses and time. So thinking that if they were 3 12ax7 tubes, taking maybe 4ma of current per plate, I would say that the 10k resistor drops the voltage by 10,000 * 0.012 = 120volts? The 12au7 looks like it takes much more current, near 20ma, so then if 2 12ax7 and 1 12au7, 10000*.028=280v? seems like such a high number? But I guess that means I change the resistor value to something lower?
Usually a 12AX7 will be configured have about 1mA plate current.
The 12AU7 is often used with 5mA-10mA

Here's a more advanced follower circuit, similar to what you need. They also mention the resistor grid bias circuit,
but this one is probably better for your purposes. It takes about 8mA plate current.

So roughly 20mA total current, 10k x 20mA = 200V drop, so reduce the 10k power supply resistor
down to 2.5k, to have a drop of 2.5k x 20mA = 50V, about right.

The Valve Wizard -Cathode Follower
.
Attached Files
File Type: pdf CFollower_1.pdf (34.9 KB, 12 views)

Last edited by rayma; 6th December 2019 at 01:46 AM.
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