6922 or 12AX7

[DF96]

I have another question:
- why deliberately put a high gain low distortion valve like the 12AX7 in a low gain high distortion circuit? Is this some sort of guitar FX box?

 

[stocktrader200]

Funk 1980,
The gain is set to what is required rather then the tube max.
the pot wiper resistor is to allow the op amp bias if the wiper is noisy
the inputs are tuner , CD , iphone and phono so high impedance is not nesscessary
12AX7 and 6922 are the tubes I have on hand

 

[famousmockingbird]

Why not put the volume pot in front of the 12AX7.

Why use such a high gain tube?

 

[stocktrader200]

the 6922 with 220uf bypass cap is the better choice?

 

[famousmockingbird]

If you feed the input with 1Vrms from a CD player you are looking at a lot of distortion from your arrangement, something like 5%. Better off placing the volume control at the input, you are operating these valves in their not most linear region. If you want to use these valves I suggest increasing supply voltage and or attenuate the input.

 

[funk1980]

Funk 1980,
The gain is set to what is required rather then the tube max.
the pot wiper resistor is to allow the op amp bias if the wiper is noisy
the inputs are tuner , CD , iphone and phono so high impedance is not nesscessary
12AX7 and 6922 are the tubes I have on hand

Perhaps a bit daft, but my question were kinda rhetorical. I haven't done the math on the 6922, but with the 12ax7, you'll add a lot of distortion which is something you reallywant to avoid with a preamp...

I don't understand your remark about the noisy wiper. In the case the 220k resistor would serve any purpose, the pot needs replacement anyway. All it does now is reduce the impedance even further.

EDIT: And the 5k6 really doesn't make any sense.

 

[stocktrader200]

I made the assumption that, like with a transistor, tube gain is close to Rp / Rc .
I want to drive the pot with 15 to 20 v p-p signal with 0.5% tube distortion
My assumption my be wrong

 

[rayma]

I made the assumption that, like with a transistor, tube gain is close to Rp / Rc .

You can't set the gain simply by adjusting the plate and cathode resistors because this affects the DC bias conditions too much.

 

[famousmockingbird]

I made the assumption that, like with a transistor, tube gain is close to Rp / Rc .

mu = gm*Rp

Gain of a common cathode stage is: Av = (mu*Ra)/(Ra+ra)

I want to drive the pot with 15 to 20 v p-p signal with 0.5% tube distortion

I feel like I might be misinterpreting you, design goal is to have about 20db gain with .5% THD.

 

[DF96]

Gain of a degenerated common cathode stage is approximately
mu x Ra / (ra + (Rk x mu) + Ra)
so it is only approximated by Ra/Rk if Rk is large enough - but large Rk almost certainly means wrong bias in the simple circuit. You would need to tap the grid resistor partway up Rk to get sensible bias.

In most systems no preamp gain is needed. All that happens is that the gain (and noise, and distortion) added by the preamp is attenuated away again by the volume pot. Of course, if you want a valve FX box then choose the wrong valve with the wrong bias and the wrong supply rail voltage so you get the right distortion.

 

[famousmockingbird]

To echo what DF96 is saying gain isn't needed with most systems. Most power amplifiers are driven to full power with just 1Vrms input. Most of todays sources already put out 1-2Vrms.

If you need some gain or want a little harmonic distortion added to the signal then I propose this circuit trying to use the parts you have in your schematic. The volume control is in front of the valve I just used two resistors to model it. Simulation shows 20db gain and .5% THD at 1Vrms out.

 

[Vincent77]

Several very high end and very expensive preamps use the 6922.

That doesn't mean the 6922 is the best choice a DIY-er could make. It is used in the "very high end preamps" only because it's readily available in large quantities from current production...

 

[stocktrader200]

😀Thanks famousmockingbird, I will try that circuit