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DC coupled design without cathode resistor?

Hello guys. I have built inthe past one dc coupled amplifier and enjoyed its sound very much. But there were weak bass and dynamics. I felt that the cathode resistor is a bad thing.
My schematic of the amp is here:

[IMGDEAD]https://scontent-prg1-1.xx.fbcdn.net/v/t1.15752-9/83289294_229847544681050_4874448720277536768_n.jpg?_nc_cat=110&_nc_oc=AQmHaBD7vyabnVR4q-lTh4pZVpOsIUNZmwO82v9N8Kh5dRyVErHBs_S5LLCmsittg4A&_nc_ht=scontent-prg1-1.xx&oh=678da3c9596e240a3020a534cc7a0278&oe=5EC2EC38[/IMGDEAD]I tried changing the ultrapath into cathode cap and vice versa but no difference in sound.

Do you think that it is possible some DC coupled without that big cathode resistor? Something like this,justvery big negative grid supply instead of the Rk voltage drop? Will it work? What are your opinions?
[IMGDEAD]https://scontent-prg1-1.xx.fbcdn.net/v/t1.15752-9/s2048x2048/85061331_637517260125821_4546793799933755392_n.jpg?_nc_cat=102&_nc_oc=AQkrGf-WsD9kkbIVdU7Zrbr9GQhhwSd0ho_QqBfYdCLATXZMKllObzM9VwO_JAqkCBs&_nc_ht=scontent-prg1-1.xx&oh=97432d21d3bdbeec54bec6868442be4c&oe=5ED50B52[/IMGDEAD]Thank you. Best regards, Michal
 

Tjj226

Member
2012-12-02 2:49 am
That particular implementation wont work.

What you are doing is setting your preamp plate voltage to be negative 20v. You would have 150v across the preamp tube IF you had a coupling capacitor and set the fixed grid bias to be the proper -20v instead of -170v.

Because the preamp tube's plate voltage is negative with respect to the cathode, the tube won't turn on or do much of anything.

Not to mention that even if this did work, you would find that you would have to make your -170v supply VERY quiet because you would be injecting all sorts of noise right into the grid of the output tube. No bueno.
 
Last edited:
That particular implementation wont work.

What you are doing is setting your preamp plate voltage to be negative 20v. You would have 150v across the preamp tube IF you had a coupling capacitor and set the fixed grid bias to be the proper -20v instead of -170v.

Because the preamp tube's plate voltage is negative with respect to the cathode, the tube won't turn on or do much of anything.

Not to mention that even if this did work, you would find that you would have to make your -170v supply VERY quiet because you would be injecting all sorts of noise right into the grid of the output tube. No bueno.
Thank you. I have also seencathode follower directly coupled to output tubes grid, but the negative voltage was injeczed into the cathode resistor rather than grounding the cathode resistor.
I can guess that this supply carries all the cathode follower current but what other things are happening?
Thanks. Best regards, Michal
 
It's pretty hard to see the image of what you have built now, can you post one more like your second image?

Weak bass/dynamics would have me asking you what your OPT is and what the cathode bias resistor value is and how much capacitance you have across it.

Do you have access to any kind of measurements tools? FFT or oscilloscope? Often times it helps to quantify what you're hearing.
 
You could direct couple and avoid a cathode resistor by stacking power supplies like this:

direct-coupled-a2.png