Noobish quention: How to add a pot in a tube pre amp?

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ctechdx

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12ax7_schematics.gif


Hello folks!

I´m studing to build a simple tube preamp like this one in the schematics. I´m doing some software simulation and the preamp works. It increases the signal. BUT I would like to be able to control the amount of increasing gain. How can I add a pot to control the gain?

I did change the first 1M resistor to a 100k potentiometer , wiring the variable pot pin to the tube´s grid (pin 2). And it seems to work all right in the measureaments. But i´m not sure if I don´t this right. Thanks!
 

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volker

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gyraf said:
Warning: Potential Security Risk Ahead

Firefox detected a potential security threat and did not continue to www.shine7.com. If you visit this site, attackers could try to steal information like your passwords, emails, or credit card details.
It just doesn't have SSL. Latest browsers are getting a bit paranoid, mostly for no reason. It's not like you're gonna type in your bank credentials anywhere on that site...



ctechdx, you can connect it like any other volume pot that also works as a grid leak resistor, see attachment.
 

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ctechdx

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volker said:
It just doesn't have SSL. Latest browsers are getting a bit paranoid, mostly for no reason. It's not like you're gonna type in your bank credentials anywhere on that site...



ctechdx, you can connect it like any other volume pot that also works as a grid leak resistor, see attachment.

Thanks Volker!

But the Image you shared looks the same as the original. I dont get how can i add the pot.

I did some experiments and added the pot before the tube input in the tube's grid. And It works. The simulation shows the volume working, but im not sure If im doing this right. Could you please explain me better? I Will try to share what i did here.
 

ruffrecords

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The 100K pot you added to the input should work fine with the wiper connected to the grid of the first tube. You should leave a 1Meg connected from the grid to ground as well because pot wipers are notorious for adding noise if there is the slightest dc flowing in the pot. To be on the save side connect the pot wiper via a 100nF to the grid.

Cheers

ian
 

ctechdx

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ruffrecords said:
The 100K pot you added to the input should work fine with the wiper connected to the grid of the first tube. You should leave a 1Meg connected from the grid to ground as well because pot wipers are notorious for adding noise if there is the slightest dc flowing in the pot. To be on the save side connect the pot wiper via a 100nF to the grid.

Cheers

ian

Thank you so much Ian! I gonna try this on simulador and will share the results.
 

volker

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ctechdx said:
Thanks Volker!

But the Image you shared looks the same as the original. I dont get how can i add the pot.

I did some experiments and added the pot before the tube input in the tube's grid. And It works. The simulation shows the volume working, but im not sure If im doing this right. Could you please explain me better? I Will try to share what i did here.
Look again, I placed the pot in front of the second triode. The question where it is better suited depends on your expected input level. If you know you won't overdrive the input, or actually want to, then placing it in front of the second triode is beneficial in terms of noise.
 

ctechdx

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ruffrecords said:
The 100K pot you added to the input should work fine with the wiper connected to the grid of the first tube. You should leave a 1Meg connected from the grid to ground as well because pot wipers are notorious for adding noise if there is the slightest dc flowing in the pot. To be on the save side connect the pot wiper via a 100nF to the grid.

Cheers

ian

Ian I did try what you suggest, and seems to work fine (Please check the schematics attached if I did it right). With the oscilloscope I can measure the pot working on signal gain. BUT,  seems that I´m having a huge gain boost with this preamp, even without the pot, the gain seems to be around 32 dBs!

I´m not sure if i´m doing right measures here. The input signal is playing 1khz  1vpk. The measure of the input voltage is 707mV and the output is 32volts!

Does this preamp gives all that boost? With the pot on 5% it already adds 6db of boost! What am I doing wrong?

How many dbs of gain should this preamp drive?

Thanks! 
 

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ruffrecords

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As Scott said, the 12AX7 is a high gain tube - it has a mu of 100 so a stage gain of over 40X is not unusual. How much gain do you need from this preamp? Once you decide that you can decide whether it may be better to use a different tube.

Cheers

Ian
 

Tubetec

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I see plate volts on the first triode are almost the same as grid volts on the second stage , what would happen if we omit the coupling cap ?  We'd have a DC coupled stage obviously , would there be anything to be gained or lost by doing this ?
Would it increase the load seen by the first triode ,and how would dc coupling effect the operation of the cathode follower ?


 

emrr

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.....and what is happening after your output, a step-down transformer?  Calculate that too.  In many classic circuits an output transformer subtracts the gain of the last tube stage. 
 

ctechdx

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Thanks for all replies! Really helpfull!

My intention on this preamp is more for color than for gain stage. I gonna use this in a professionals line level +4dBU. The signal comes from a DAC to the preamp, and then It get back to an AD converter. Only for coloring, and Record back the tube sound. Then i supose i dont need much gain. These 32dbs are obviosly too much for what i need, and gonna make the pot sensibility too hot. I supose something around 10... 12 dbs is already enough.

What you guys suggest me for this purpose? I have the same schematics for this preamp using a 6922 insted of 12ax7. I supose i can get less gain from a 6922 am I right?

Attached the 6922 schematics*
 

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ruffrecords

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If you want colour rather than gain then I would suggest changing from the current set up where you have a common cathode stage followed by an cathode follower. Instead I would suggest you use an SRPP stage. This has less gain, more colour and a much better output drive capability. I use them all the time in my tube mixer designs. You can also add a 2K4:600 output transformer to give yourself a balanced out and a little more colour.

Cheers

Ian
 

ctechdx

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ruffrecords said:
If you want colour rather than gain then I would suggest changing from the current set up where you have a common cathode stage followed by an cathode follower. Instead I would suggest you use an SRPP stage. This has less gain, more colour and a much better output drive capability. I use them all the time in my tube mixer designs. You can also add a 2K4:600 output transformer to give yourself a balanced out and a little more colour.

Ian, could you suggest me some SRPP schematics? Where can I start?
 

ruffrecords

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scott2000 said:
https://drive.google.com/drive/folders/0B_n67A1hN3qtfkFYQ0xnSWFvbXJISTNrc3p1YTFkUGNpU3gzdWJXa1g5T3JpbXhSN2hXdTg

That is not a bad place to start. It is the public part of my web site where I published the EZTubeMixer design some years ago. If you look at the Eurochannel schematics you will see an 6922 arranged as an SRPP output stage. You can lift this as it is as a starting point. You will probably still have too much gain but you can lose 6dB in a 2K4:600 output transformer and put a pot at the front to lose the rest. I have used this sage in the past in a Tube Tone box I designed for a customer who uses it on Reggae tracks.

Cheers

ian
 

ctechdx

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ruffrecords said:
That is not a bad place to start. It is the public part of my web site where I published the EZTubeMixer design some years ago. If you look at the Eurochannel schematics you will see an 6922 arranged as an SRPP output stage. You can lift this as it is as a starting point. You will probably still have too much gain but you can lose 6dB in a 2K4:600 output transformer and put a pot at the front to lose the rest. I have used this sage in the past in a Tube Tone box I designed for a customer who uses it on Reggae tracks.

Ian thank you so much! I gonna make some tests in a simulator.

The schematic you have suggested me is the Eurocard Sht 3 of 4?

How many volts are the HT1 and HT2?
 

ruffrecords

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ctechdx said:
Ian thank you so much! I gonna make some tests in a simulator.

The schematic you have suggested me is the Eurocard Sht 3 of 4?

How many volts are the HT1 and HT2?

Sht2 shows how HT2 is derived from HT1. Remember, this is the schematic of a PCB that contains two identical amplifiers each of which uses a 6922 as the output stage and half a 12AX7 as the input stage. For your colourful preamp you only need the SRPP output stage (the 6922 bit) so you only need HT1. Its values can be anything in the range 250V to over 300V. it is not particularly critical. Just feed you input from your pot to the 100nF that feeds the grid of the lower half of the 6922.

If you drop the HT to about 200V you can safely use an ECC88 or 6DJ8 instead.

Cheers

ian
 

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