Is this pre-amp useful at all?

[Sir Cuitous]

Has anyone used this circuit from the TubeCAD website?:

http://www.glass-ware.com/tubecad/CC4.gif

How would you interpret the 1m resistor between the triodes? Is it bridging the grid pins? I am interested in any opinions on this.

[Kramusha]

Can you give us more information? I wonder, why there is a voltage divider at the second grid. Because of this, the first grid is positive against the kathode and the triode will be destroyed.
An other thing: The grid of the first triode has no connection to ground --> The first triode will be destroyed.

Stefan

[cbarrow7625]

The first grid does have a ground path. The grid resistor for the first stage is a combination of the 1M resistor & R2. The 1M resistor also creates some negative feedback (or feedforward, as the case may be) to V2.

R1 & R2 just keep the bias level of both tubes at about 1/2 B+ (give or take, it's hard to know the full intent without resistor values).

This circuit is really pretty similar to a typical LTP phase inverter (obviously, no long tail here, just a high value cathode resistor to pass the signal from one cathode to the other.

V2 is a common grid amplifier (signal injected intoo the cathode, appears non-inverted at the output) with some negative feedback coming from the 1M resistor.

Looks ot me like a relatively low gain (maybe even unity gain) non-inverting buffer stage. I hope it sound good because it seems like an awful lot of trouble to go through for what you get out.

Chris

[Sir Cuitous]

Thanks for sharing your thoughts with me. I am a new builder and learning fast. I believe the idea behind this circuit is to feedback the inverted signal for some hum/noise reduction and that it would be relatively low gain. I will see if I can get some resistor values. So the 1M resistor does connect the grids? Is that how you read this schematic? It seems a little odd since there already appears to be a feedback loop across the triodes.

[cbarrow7625]

What exactly do you mean by "there already appears to be a feedback loop across the triodes?" Where?

The signal path is this:

Signal comes into Grid 1A, signal goes out cathode 1A, signal goes in cathode 1B & is amplified appearing at the plate of 1B.

At the same time this is happening:

Signal comes into grid 1A, goes through 1M / R2 voltage divider and into grid 1B where it is amplified appearing at the plate of 1B where it is summed with the signal from the other path.

Since the amplification of each signal is nearly identical but the one going through the voltage divider is lower in level, I would assume that this stage does have some gain.

There is no real "feedback" going on in this stage other than localized feedback from the cathode of V1A. I think the 1M / R2 junction would be more properly referred to as "feedforward". Remember, signal can only be input into a grid, signal cannot be fed from a grid to go elsewhere. If you are thinking that the grid of V1B is somehow feeding back into the grid of V1A, you would be mistaken.

Hope this helps.

Chris

[Sir Cuitous]

Thanks - yes very helpful. I was mistaken. I will need to stare at the drawing and re-read your paragraph a few times.

My only remaining question is exactly how does that 1M resistor connect? The drawing is not clear to me.

[clyde]

Hifi Differntial Input?

This Appears To Be A Differential Amp. Probably Used As An Input Stage On A Hifi Amp.

The Common Cathode Resistor Creates A Constant Current Source. When The Input Signal Is Positve, The Voltage On V2's Plate Goes Positive And V1's Plate Voltage Goes Negative. When The Input Signal Is Negative, V2's Plate Goes Negative And V1's Plate Goes Positive. The Output Is Essentially The Difference Between V2's Plate Voltage And V1's Plate Voltage, And Is In Phase With The Input Since It Is Taken From V2's Plate.

The Humbucking Effect Comes About Because Of Common Mode Rejection. That Is, When Both Plate Are At The Same Voltage, No Signal Is Applied To The Input, With Noise Likely Being The Only Thing Present, It Is Cancled From The Circuit.

I Belive That These Stages Can Have A Moderate Amout Of Gain To Them.

As Far As The 1 Meg Grid To Grid Ressistor, If I Read The Circuit Right, It Does Appear To Be Connected Between The Grids. Maybe For Dc Coupling V1''s Grid To V2's To Ensure That That They Are Biased The Same, Possibly For Stabillity. Might Also Be The Reason For The High Value.

Not Sure Why It Is Connected This Way. What Few Common Cathode I've Researched, Different Means Of Biasing Are Used. Derivng A Dc Voltage Form The Cathode And Apllied To V1 Grid. Maybe Done This Way For Simplicity Of Circuit And Still Maintaining Good Stability?

If Anyone Else Knows, Please Let Us Know.