• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube Amp CCS?

Status
Not open for further replies.
Jump to Latest
#22
Could be something to do with my preference for setting bias, which is to ignore the second and minimize as far as possible the remaining harmonics. The bypass seemed to alter the gain a miniscule amount consistent with eliminating the 10-20 ohms cathode impedance of two LEDs. You may recall seeing the one of the attached graphs, the second is the same tube/jig/session with a 1000uF bypass as described above. It's entirely possible the effect is due to a cancellation against the LEDs' non-linear impedance instead of feedback though.
 

Attachments

#23
FWIW, when I looked at THD of the signal across the unbypassed LED in my 6SN7 jig, it was higher than the THD at the plate; this is with a small resistor load, not a CCS. That suggests cancellation to me. I'd like to repeat that with two series LEDs- hopefully this weekend. I have a new soundcard that needs a workout...

I think we've strayed a bit OT. Jojo, I'd look for a different recipient for a CCS transplant.
 
#24
Jojo, I'd look for a different recipient for a CCS transplant.
Click to expand...

ok then... maybe I should really really really go build that HF87 clone we once talked about in another thread. :angel:

one last question though. you said that plate CCS don't like cathode resistors, I remember that in a preamp I opened the voltage amplifier (which looks a lot like my input tube) has a plate ccs and an unbypassed 1.8K resistor in the cathode, then it is direct coupled to a cathode follower with a ccs on it's cathode.

could this be a poor approach in the use of ccs circuits?

EDIT:

If I'm not mistaken, it looks a lot like this...

An externally hosted image should be here but it was not working when we last tested it.
 
#25
The point is that the power supply rejection ratio (PSRR) is pretty much RL/ra, so making RL very large by using a CCS as anode load not only makes the stage very linear because of the horizontal loadline, it also rejects power supply noise. But if you leave the cathode resistor unbypassed, its value is multiplied by mu so that ra becomes ra + mu x Rk, thus destroying your PSRR.
 
#27
The improvement gained by giving a cathode follower a CCS is not that great - not as great as that gained by giving a common cathode stage a CCS anode load. Why not replace that cathode resistor with an LED or two?
 
#31
I think SY has already done a part of my homework...

He posted this circuit abotut a year ago - which is exactly what I was about to do an experiment on. :scratch:

An externally hosted image should be here but it was not working when we last tested it.


😀 😀 😀

I'm confused as to how to get the 8mA current flowing. SY also mentioned the use of a 20K dummy load to get the 8mA before inserting the tube. The dummy load is for the ccs right?
 
#32
With the tube out of the socket, connect the 20k resistor from the collector of the current source to ground. Put a voltmeter across it, then adjust the trimmer for 8mA (that's 160V across the 20k). Then power down, pull out the resistor, put in the tube and you're good to go.
 
#33
thanks SY! this is getting very exciting for me 😎

I can always direct-couple this stage to a cathode follower with a bottom ccs right? I'll try to draw it right away.

You see, i was inspired by your heretical linestage so I tried it on a 5692 - sans jensen input transformer (i don't have one) and I was blown away. I heard the difference and I liked it, and that's all that matters to me. 😎 It's a surprise how a handful of leds and sand can improve what is already a beautiful linestage.
 
#36
The effective load for the cathode follower is the CCS (a very high impedance) in parallel with the actual load (normally a medium impedance). In that sense, yes, the load dominates and the CCS really is just setting the DC operating point of the cathode follower.
 
#39
sy, if i'm interpreting you correctly, z out of a ccs loaded cathode follower will be essentially rp inparallel with the "infinite" ccs load???

as for dc, does the tube view itself as being simply tied to ground or does it "see" the ccs load resistor?
 
#40
No, Zout of a cathode follower is roughly 1/gm. I was talking before about the effective load resistance for the tube, not the output impedance.

For DC, the tube's cathode to grid voltage will take care of itself assuming you haven't asked the CCS to do anything unreasonable.
 
Status
Not open for further replies.
Top Bottom