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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Nov 28, 2016 12:40 am 
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Goat

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Jocko Homo wrote:
Yes, they will operate at 20 V, but why would you?
Sir, do you mean Vds=20V is too much? How would it be a problem? Noise? I know the capacitances go down with increased Vds and that is good. What Vds have you on that measurement preamp you mentioned sir?
(Forgive us from the digital generation that didn´t go deep on semiconductor physics!)
Thanks,
Alex


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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Nov 28, 2016 4:18 am 
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Muriel
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Hi,

felipe wrote:
I have power transformer 0-8.5VAC, I like regulated heaters, could i connect regulated 6.3VDC to pin 5 and 0VDC/GND to pin 4? What have I to do with pin 8, to ground or left unconnected?


I tend to run tube heaters a trifle low, 5.9V DC or so and I always leave the whole heater circuit floating in terms of wiring.

In order to reverse bias the heater/cathode parasitic diode in indirectly heated triodes use a resistive divider from the +B - shooting for > +20V above the cathode voltage (including any AC component), but remaining within the tubes limit ratings for heater/cathode voltage. Then a sensible value elcap to ground to make sure the heater is grounded for AC.

I suspect someone obsessive might use some kind of Amplifier circuit to bootstrap the heater to the cathode with some DC offset.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Nov 28, 2016 4:19 am 
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Muriel
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Hi,

felipe wrote:
What's power consumption expressed in mA consumption for one valve for stereo to design my psu with PSUDII?


Should be around 10mA per tub section, give or take.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Nov 28, 2016 4:20 am 
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Muriel
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Hi,

felipe wrote:
Too much noise, I guess because isn't boxed so no mains earth connected to chassis and also not grounded HV & LV PSU.


Few issues, this circuit needs super clean +B, correct grounding and shielding as there is a lot of fairly high impedance stuff going on.

Ciao T

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 Post subject: Re: Question for Jocko
PostPosted: Mon Nov 28, 2016 4:25 am 
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Muriel
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Hi,

alexandre wrote:
Jocko sir, what Vds would you operate a K170, assuming Id of 8mA?


I am not Jocko, but I would look at the curves of Gate leakage vs. Drain voltage. Upshot, keep it low homie...

If you need to handle high voltages, cascode. And remember, cascodes can be folded.

alexandre wrote:
Application is common gate I/V for TDA1543, balanced.


Cannot see any reason why. Using a BJT is likely better. Or a single pair of FET's in Sziklai. Or just a resistor.

alexandre wrote:
What if I go crazy and parallel 4x K170 per phase like this?


Why? Especially, why add Op-Amp's?

alexandre wrote:
Is Vds=20V ok? Dissipation is 180mW each fet.
Attachment:
jfet.png


I'd say that is pushing it at several corners.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Nov 28, 2016 9:44 am 
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Goat

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Ok, I just checked the datasheet and Pd is 400mW. Better to operate the jfets more conservatively indeed.

Gate leakage increases markedly with Vds, thanks for pointing that. I didn´t remember it would increase by orders of magnitude.

BJTs is what I have right now in a similar I/V. What about higher order harmonics? Let me measure my BJT I/V again. (There´s an interesting talk at the end of the OPA604 datasheet.)

Why opamps? OK, I get it, I like all-discrete electronics.

Thanks,
Alex


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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Nov 28, 2016 4:03 pm 
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Benjamin
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I hate to point out the obvious, but...................

This has nothing to do with a 6SN7. Which is what the thread is about. I can only suggest that you start an appropriate thread.

(In the old days.............well, never mind.)


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 Post subject: Re: Question for Jocko
PostPosted: Mon Nov 28, 2016 4:09 pm 
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Benjamin
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Kuei Yang Wang wrote:

I am not Jocko, but I would look at the curves of Gate leakage vs. Drain voltage. Upshot, keep it low homie...

If you need to handle high voltages, cascode. And remember, cascodes can be folded.


Yep.

Quote:
Cannot see any reason why. Using a BJT is likely better. Or a single pair of FET's in Sziklai. Or just a resistor.


I'll let you 'splain it to him.

Quote:
I'd say that is pushing it at several corners.

Ciao T


One way of putting it.


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 Post subject: Re: 6SN7 preamp
PostPosted: Tue Nov 29, 2016 1:09 am 
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Goat

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Will do.
Jocko Homo wrote:
I hate to point out the obvious, but...................

This has nothing to do with a 6SN7. Which is what the thread is about. I can only suggest that you start an appropriate thread.

(In the old days.............well, never mind.)


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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Nov 30, 2016 9:35 am 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
Kuei Yang Wang wrote:
Hi,

felipe wrote:
Too much noise, I guess because isn't boxed so no mains earth connected to chassis and also not grounded HV & LV PSU.


Few issues, this circuit needs super clean +B, correct grounding and shielding as there is a lot of fairly high impedance stuff going on.

Ciao T


Hi T

Attached PSU used, is enough clean?

[img][img]http://thumbs.subefotos.com/5895b4e9a21b29a90c52aeb4b9ec4345o.jpg[/img][img]

A presto F


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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Nov 30, 2016 9:36 am 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
Kuei Yang Wang wrote:
Hi,

felipe wrote:
Too much noise, I guess because isn't boxed so no mains earth connected to chassis and also not grounded HV & LV PSU.


Few issues, this circuit needs super clean +B, correct grounding and shielding as there is a lot of fairly high impedance stuff going on.

Ciao T


Hi T

Attached PSU used, is enough clean?

[url][url=http://subefotos.com/ver/?5895b4e9a21b29a90c52aeb4b9ec4345o.png]Image[/url][url]

A presto F


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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Nov 30, 2016 11:14 am 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
Real world "oscillate", measurement done with heater regulator floating (not grounded to chassis).

Image


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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Nov 30, 2016 11:17 am 
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Cow

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Location: Catalonia New Europe State
Scope set AC 0.1 V/cm 2ms


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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Nov 30, 2016 12:23 pm 
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Muriel
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Hi,

felipe wrote:
Real world "oscillate", measurement done with heater regulator floating (not grounded to chassis).

Image


Heater floating means two things:

1) Any common mode AC on the heater will couple via parasitic capacitances from the tube heater into the cathode. As there is bound to be some serious CM voltage there, not (AC)-grounding the heater is likely not a good idea.

2) as the heater cathode diode is not reverse biased any Common Mode voltage will be demodulated (rectified) into the cathode.

Noise on the +B, well it means this:

3) The PSRR in the specific circuit is around 6dB plus any looped feedback factor. So think whatever is on the +B goes to the output with some attenuation, but not a heck of a lot. It's a tube, not an Op-Amp.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Nov 30, 2016 5:51 pm 
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Cow

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I don't know if I understand you very well, the heater regulator is DC LM317.

I don't use feedback, see attached schematic.


You do not have the required permissions to view the files attached to this post.


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