Regulated b+ preamp

[mattiambro]

Hi, I'm working on a preamp/linestage    based on ecc84/6n14p. It is a very simple design as It has a gain stage and a cathode follower. It provides also some ripple noise (around -62db full scale), so I try a regulated power supply, with a lm317. Nothing changed,but if I take my b+ from adjust pinout It Is less noisy, what am I missing?

[tubeswell]

How much filtering have you got for the source voltage? (I.e. ‘before’ the chip)?

[mattiambro]

There is 100uF right before the IN pin of317. Rectifier also has capacitors since It Is a voltage multiplier. I have 100uF aftet the chip too.

[HotBluePlates]

... preamp/linestage    based on ecc84/6n14p. ...

Are you using a 1960s ECC84 from a European manufacturer?  OR are you using a 6π14π (which people sometimes write as "6n14n")??

The latter tube is a Russian-version of a EL84, not an ECC84.  Not a preamp tube, different pinout, etc.

Hi, I'm working on a preamp/linestage ... It provides also some ripple noise (around -62db full scale), so I try a regulated power supply, with a lm317. Nothing changed ...

Preamps are typically very low current, which suggests that if you had hum you have a construction or basic-design flaw.  Most preamps do not need regulation to be very hum-free.

An exception is that some tubes have heater-to-cathode leakage, and so couple hum into the audio.  If you have any unbypassed cathode resistors, try using a large cathode bypass cap (100-220µF).

[mattiambro]

Tube are double triode type, so no 84like. But thanks for askin. The cathode resistor of gains stage It Is bypassed, the load of the cathode follower not.

[HotBluePlates]

... The cathode resistor of gains stage It Is bypassed, the load of the cathode follower not.

Is it just those 2 stages?  One common-cathode gain stage, and one cathode-follower stage?  Do you have a schematic you could share?

I'm asking because any reasonable preamp should be drawing not more than 2-3mA, so any power supply should be loafing. 

It could be that the tube has heater-to-cathode leakage hum, and the cathode-follower side is where the hum would be unavoidable.  If this is the case, DC heaters are one option, and pre-testing/selecting tubes (or using a different tube type) will be the other option.

[mattiambro]

Here schematic and a build.

[mattiambro]

And

[tubeswell]

Your schematic is drawn incorrectly.

The LM317 needs a cap to ground after it. (See following diagram) https://theorycircuit.com/wp-content/uploads/2022/04/30v-variable-dc-power-supply-circuit.png

[mattiambro]

Yup Sorry I wrote It wrong but It has negative lead to ground

[HotBluePlates]

Here schematic and a build.

Do I see correctly that you're trying to use a winding both to power heaters, and also to be part of the volts applied to the voltage-quadrupler?

If yes, then you probably have hum from that winding being at an uncertain (though probably too-high) voltage-to-ground (and also unbalanced with respect to ground).

I would also raise the 10kΩ(?) plate load resistor, as a quadrupler probably isn't great at supplying a large current.  And that high current isn't needed for the amplifying chore here.

[mattiambro]

Yes, It Is indeed, I tried balancing the heaters ( via humdinger or virtual ct)but does not change the hum. And yes It Is above ground but nothing more than 30V at max. Also yes at the plate there Is less Amps than It should but honestly If I raise the load I get more gain and I don't need It.
Anyway thanks for the help so far. Any ideas?
PS. The goal here for me is to out everything in a teko b4 enclosure and arrange a 2 Channel box as you can see from the photo.
Cheers

[PRR]

LM317 is a 40V part. Yes, it is "possible" to float it in higher-voltage systems; but it often dies at turn-on surge.

Millions of clean quiet tube amps worked without regulators.

[HotBluePlates]

... If I raise the load I get more gain and I don't need It. ... Any ideas? ...

Just noticed the 3.9kΩ cathode follower load.  That's crazy-low; more extreme current-draw.

The power supply approach seems ill-advised.  It works great for small-current, not high-current.  But the preamp circuit is high-current everywhere.  That's a recipe for hum and unstable B+...  Funny enough, regulators work best when the basic power supply is pretty stable to start with.

I understand you get more gain with a larger plate load resistor for the 1st stage, but I'd suggest you split the high resistance into 2 parts, take the output from somewhere in the middle.  And definitely raise that cathode follower load resistor (that stage is a slight-loss no matter what).

[mattiambro]

Millions of clean quiet tube amps worked without regulators.

Yeah I know, in fact I was using a schematic like the one you suggested  before trying with the 317.

... If I raise the load I get more gain and I don't need It. ... Any ideas? ...

Just noticed the 3.9kΩ cathode follower load.  That's crazy-low; more extreme current-draw.

The power supply approach seems ill-advised.  It works great for small-current, not high-current.  But the preamp circuit is high-current everywhere.  That's a recipe for hum and unstable B+...

Ok I get this, but even if I use a fullbridge instead of voltage multiplier the issue does not change at all.  I built one with 45k load First and second stage but the sauce Is the same. I guess I'll try some test ( I ready something about cathode leakage in a Tubng-Sol manual) , and maybe running the heater from another source. Thanks you always, have a nice day.

[mattiambro]

No noise with external heater source.

[mattiambro]

I guess I Will use and external 9v plug for heater regulated with a lm317 and a CRC for B+

[PRR]

> Just noticed the 3.9kΩ cathode follower load.

But also zero bias on grid. Might be less than 1mA, depending.

[mattiambro]

How would you bias It?

[PRR]

> How would you bias It?