6SN7 low plate voltage for lower gain?

[Johan Potgieter]

What operating point do I use for the 6SN7?
5 or 10mA,180v anode?

Ryssen,

As a first approximation choosing the operating point such that Va is about half the H.T.+, is a good starting point. At this point not exactly sure at which ht+ voltage you will operate. If you take the given 180V as the supply and not Va itself, then for a 47K anode load, Va should be at about 90V with a cathode bias voltage of 3,6V at an Ia = 2 mA (my graphs are rather small and inaccurate at that point). That is closely approximated with Rk = 1,8K.ohm. a 10µF bypass cap should suffice for lowest frequency.

 

[Ryssen]

I did another calculation yesterday,with 150v anode,about Vg -5v for 3 red leds
with 1.65v each,4,5mA the B+ becomes 362v.

 

[Ryssen]

If I do a C-L-C filter how much uF do I really need for 2 channels?
I thought of 450uF-L-450uF but that might be to much for this single tube....?

 

[Ryssen]

Played a little in PSU designer,clearly it was to much...

 

[Ryssen]

Correct, unless you need to block a DC voltage.

Ivè built this from the schema and it is working I connected my sigalgenerator and my scope and the gain is 2.
But the problem is that with signalgereator connected I get dc of about 120mv on both the input and the output,and if I turn the geneator of it rases to 0,4v.
Does this mean I have to have an inputcap and another cap at output??

 

[kevinkr]

Input and output capacitors beyond the one shown on the plate may not be required provided that the output end of the coupling capacitor is actually referenced to ground by a resistor. Depending on the size of that coupling cap and other components a resistor of 100K - 1Meg may be suitable.

 

[Ryssen]

I had some 1Mohms that I soldered at the output to ground.
Now the dc is about 35mv at output and 220mv at input,I even tried the 1Mohm at input to ground,but no difference.So the dc must come from the input through the feedback resistor,or?

 

[BinaryMike]

It may be useful to know DC voltage polarities. Also check with output shorted to ground.

 

[Ryssen]

They are NEGATIVE,if I short the output nothing happends to the input, still -220mv,and the outputs goes to zero.
If I turn off the signalgenerator the voltage att input goes to -450mv.
Strange..

Edit: I am using Led bias.

 

[Ryssen]

Just as I start it up the voltage on output goes to -2v and then slowly down.
I tried 3 different tubes but they are all the same.

 

[BinaryMike]

Ah, so you're getting a little 'contact potential' bias from the grid. Suggest you shunt the input jack with 100K, assuming your signal source has no problem with that amount of loading.

 

[Ryssen]

Mounted the 100k at the input jacks,when I discovered that the signalgenerator has a knob saying "offset"....
dialed it in for minimum offset so when the tube is warm the offset both in and out are at about 10mv,fluctating sommetimes to 50mv some more when the tube is cold.
It worries me a little what happends when I connect the DAC (ES9023) and the bassamp (Class D)don´t know if it has a blocking cap.
I could try with another source like the computer but then again I don´t know if it has a cap.

 

[GoatGuy]

Not really a small signal tube but ⁶BX7 is pin compatible and sounds fantastic in preamps. Heater current a little high perhaps at 1.5amps. I have a pre that takes 6AS7 for lowest gain, then ⁶BX7, ⁶BL₇, and finally ⁶SN₇ which gives most gain. They all work great even tho none are optimized in this circuit.

I agree with you: the whole "BX7" line (either 6 or 12 bx) is a much under appreciated valve. If you are a "guitar guy", the 12 series is a great pin-for-pin substitute for the AX 7 line. Changes the tone significantly, both (usually) cleaning up the gain linearity and “fattening” the sound. But that isn't "hi fidelity".

In my few-dozen preamps, I found that the BX kind of "decompresses" the incoming signal, increasing dynamics without otherwise coloring the audio stream. Vocals are especially decompressed. Sounds more like "you're there" instead of "listening to a great recording". Neat trick.

GoatGuy

 

[Ryssen]

A little update:I now have input caps,and using the computer as tone generator!
So now the problem is solved on the inputside,so on the output during warm tube/playing 1khz I have -5-20mv,it is during start that there can be -300mv
that must be harmful to the speakers??
I might as well put caps on the output to...?

 

[Ryssen]

Now I have caps at the outputs to shunted with 1Meg resistors.
But even now I get DC (up to 2 volts)at startup until it starts to go down to about 0,5mV.
How can I get that AFTER the cap??

 

[Ryssen]

Now I´m down to 50k on the output ,the the dc gets down in a couple of seconds maybe 4-6 seconds.
Is there any drawback on loading the output this much,or is is it to much?

 

[Ketje]

The output capacitor must load to the cathode voltage it is attached to.
The current to get there commes trough the output resistor.
If the cathode raises slowely than the output doesn't have to jump up at starttime.
You could try like this (just an idea), gain is adjustable between <1x to ~5x
Mona

 

[Ryssen]

Hmm,thanks for the build tips.sorry to say that I allready made the box and theres no place for another tube.
If I dont get this one working (will connect the bassamp tomorrow)I will make a new top for the box and build with youre schema.

But I wonder,ain´t caps supposed to block DC?Then why dont they?

 

[BinaryMike]

Caps don't block DC in the same sense as a reverse-biased diode. They develop an opposing force when charged. The energy used for charging isn't lost, but stored as dielectric stress and returned to the circuit when terminal voltages allow it. Does that help?

 

[Ryssen]

It does help,thanks.
As I don´t have a delay on the output I guess I´ll have to be my own delay by turning on this tube before the bassamp.