Tube buffer!!!! (image accessable...... for real this time)

[emj12]

http://i1164.photobucket.com/albums/q569/edvinj/scan0001-1.jpg

Above is a schematic of a tube buffer I am designing. This is in a way suppose to replicate

an active pickup circuit. The input voltage will be 250V. The 1M pot is a gain control, and the 10k a PSRR pot.

Any suggested modifications to better fulfill the purpose of an active pickup circuit are welcomed.

[swinginguitar]

sadly, we the public can't see the contents of your C: drive....

[defaced]

Still can't see the image.  You'll need to upload the image to an image hosting site.  Links to your gmail won't work.  

[ORK]

It`s there now indeed:

[Mike Burgundy]

What exactly are you trying to do with it?
Duplicate an active PU circuit with triodes, or replicate its functionality?
For functionality, what is that exactly?
Active PU circuits do:
-high input Z
-low output Z (combine these two = buffer)
-*maybe* some tone shaping.
I may very well be missing something, but I'm not quite seeing why there's a need for *two* dual triodes - so what's the reason behind that? I'm just curious.

[amptramp]

There sould be a resistance to ground from the input grid.  This would be separate from the pulldown resistor.

You have what looks like a tone stack in the upper triode grid of a cascode stage.  These controls will have no effect since they do not see any signal.

The output stage needs a resistor from grid to ground.  Grid resistors are needed to keep the grid from charging to a negative voltage by virtue of intercepting electrons coming from the cathode.  This happens until the grid voltage cuts off the tube current.

There is no need for a plate resistor in the output follower stage.  It reduces the dynamic range and allows the input capacitance to be increased by Miller effect.

The voltage at the upper grid of the cascode section is one third of Vcc, so the plate voltage of the lower tube will be only a few volts above that.  Usually, cascode stages are run near the same voltage on upper and lower triodes by having equal resistance from upper grid to ground and upper grid to the supply.

You do not need a resistor in parallel with the volume control.

[emj12]

Mike B.

Indeed the purpose is to obtain high input Z and low output Z, which is attained from the common cathode stage and the cathode follower.

The middle stage ( a cascode amp) is to obtain gain and high input sensitivity.

There are no tone circuits or adjust, as I would like this buffer to be bandpass.

The circiut aside from the upper cascode tube is a PSRR, which is basically a powersupply noise rejection circuit with a 10k pot for adjustment.

[emj12]

[Mike Burgundy]

Okay thanks - so full-range, maybe slightly tweaked to less-than-full-range, no problem there. Hi-in, Lo-out Z. How much gain do you need?
I'm just asking, tube circuits IMHO often work best when they're kept simple as possible.
That said, keep a keen eye on bias requirements for the tubes!
The first and last triodes as shown won't bias properly. I don't quite get the noise rejection circuit (at first glance I thought it was an SRPP stage - no cigar but funny how the names are alike), I'd like someone to explain that to me.

Mike B.

Indeed the purpose is to obtain high input Z and low output Z, which is attained from the common cathode stage and the cathode follower.

The middle stage ( a cascode amp) is to obtain gain and high input sensitivity.

There are no tone circuits or adjust, as I would like this buffer to be bandpass.

The circiut aside from the upper cascode tube is a PSRR, which is basically a powersupply noise rejection circuit with a 10k pot for adjustment.

[alfafalfa]

What tube are you using and what is the plate voltage ?

[amptramp]

emj12 is correct, the circuit shown is a PSRR (power supply rejection ratio) improver directly from this source:

http://www.tubecad.com/march99/page3.html

with the article starting on page 2 (the previous page).

An excellent treatise on SRPP is here:

http://www.tubecad.com/articles_2002/SRPP_Deconstructed/SRPP_Deconstructed.pdf

The tubecad site is probably one of the best on the web for basic understanding of tube circuits.

[emj12]

Supply voltage is 230v and the tube will be 12ax7s

[Mike Burgundy]

Hang on, I missed the second schematic. First triode should bias (though I haven't checked to see if it biases optimally), but I still don't see the second one biasing - the grid is floating at anode level. Amptramp: thanks, got some reading to do. I do hope Aiken gets his site back up too, that was a nice resource as well.