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 Post subject: Re: 6SN7 preamp
PostPosted: Wed Nov 30, 2016 5:52 pm 
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Cow

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Location: Catalonia New Europe State
The pot is a 100K log ladder with resistors.


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 Post subject: Re: 6SN7 preamp
PostPosted: Thu Dec 01, 2016 3:46 am 
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Muriel
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Hi,

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


It does not matter what your heater regulator is. Heater MUST BE AC grounded.

felipe wrote:
I don't use feedback, see attached schematic.


That circuit should run a single triode of 5687WB, not two in parallel. Current will be around 10mA per channel.

Place a 3.3MOhm resistor in parallel with the battery. Personally I would also add a 10nF silver Mica or C0G ceramic capacitor.

This circuit has a bit more PSRR than what I referred to earlier at around 12dB, still, noise on the +B will bleed into the audio circuit.

If using a 100k Pot, make sure to use well shielded, but low capacitance cable for connections, I tend to make my own using PE (never PTFE) tubing, Silver Wire & copper braid.

As this circuit still has 22 dB gain possible options are to get something more usable are:

1) Arthur Loesch / First Audio Presence style - place a 10K attenuator on the output, first it drops gain to 20dB and you attenuate tube noise along with all the rest. For 2V standard inputs I tended to use a voltage divider on the input, to lower gain to around 8dB (so 4:1 attenuation - e.g. 33k/11k - in practice 33k/10k) while also having some higher gain inputs and of course a Phono stage. In these preamp's I implemented a neat trick, using the tube heater for input switching - as I only had two sources (LP & DAC) this worked splendidly using one tube per channel...

2) Jim Fosgate / Thorsten Loesch style - place a 680k resistor from output to input (before the battery, not tube grid in your case) and a 100k resistor in series between input and attenuator. Now you have around 8dB Gain using local feedback (single stage) which lowers noise, distortion and output impedance, the way feedback does. The around 14dB feedback work quite well, with a Z out around 600 Ohm in practice.

Either way works, I tend to prefer #2 nowadays, but it is a matter of taste.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Thu Dec 01, 2016 10:02 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,

It does not matter what your heater regulator is. Heater MUST BE AC grounded.

Ciao T


I'm sorry my ignorance, how can I do?

TIA
Felipe


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 Post subject: Re: 6SN7 preamp
PostPosted: Thu Dec 01, 2016 10:33 am 
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Muriel
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Hi,

felipe wrote:
Kuei Yang Wang wrote:
Hi,

It does not matter what your heater regulator is. Heater MUST BE AC grounded.

Ciao T


I'm sorry my ignorance, how can I do?


In the easiest one, a simple wire linking negative of heater to audio ground. As you have DC heaters and the tube cathode directly grounded, this will suffice.In this case it of course DC and AC grounded.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Thu Dec 01, 2016 3:54 pm 
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Cow

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Posts: 191
Location: Catalonia New Europe State
Kuei Yang Wang wrote:
Hi,

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


That's heater elevation? How can calculate to have +20V? If isn't ask to much: could you draw a schematic or link to see how to do?

Thanks for support really appreciate.
Felipe


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 Post subject: Re: 6SN7 preamp
PostPosted: Sun Dec 04, 2016 3:30 am 
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Muriel
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Hi,

felipe wrote:
That's heater elevation? How can calculate to have +20V? If isn't ask to much: could you draw a schematic or link to see how to do?


Sorry, no time. Just one resistor ftom +B to the heater - and another from heater - to ground. For arguments sake, 22k from heater - to ground and 240k/1W from +B to heater -.

Then put a 100uF/35V cap (electrolytic ok - watch polarity) across the 22k resistor.

But for starters just link heater - and ground. In your case that should be all that is needed.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Sun Dec 04, 2016 4:25 pm 
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Cow

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Location: Catalonia New Europe State
Like attached pic?

Where have I to measure 20V:
Across 240K?
Between 240K & 22K to ground?


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


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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Dec 05, 2016 4:32 am 
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Muriel
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felipe wrote:
Like attached pic?

Where have I to measure 20V:
Across 240K?
Between 240K & 22K to ground?


Measure across 22k and the absolute value is not critical. Treat it as appx 20V. As you have 300V HT change the 240k to 270K, I calculated based on 250V +B.

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Tue Dec 06, 2016 3:18 pm 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
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Location: Catalonia New Europe State
I have 250V, the schematic posted it's only for reference how to connect the elevated heater.

Thamk you
Felipe


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 Post subject: Re: 6SN7 preamp
PostPosted: Sat Dec 17, 2016 9:42 pm 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
Posts: 191
Location: Catalonia New Europe State
Kuei Yang Wang wrote:
Hi,

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


It does not matter what your heater regulator is. Heater MUST BE AC grounded.

felipe wrote:
I don't use feedback, see attached schematic.


That circuit should run a single triode of 5687WB, not two in parallel. Current will be around 10mA per channel.

Place a 3.3MOhm resistor in parallel with the battery. Personally I would also add a 10nF silver Mica or C0G ceramic capacitor.

This circuit has a bit more PSRR than what I referred to earlier at around 12dB, still, noise on the +B will bleed into the audio circuit.

If using a 100k Pot, make sure to use well shielded, but low capacitance cable for connections, I tend to make my own using PE (never PTFE) tubing, Silver Wire & copper braid.

As this circuit still has 22 dB gain possible options are to get something more usable are:

1) Arthur Loesch / First Audio Presence style - place a 10K attenuator on the output, first it drops gain to 20dB and you attenuate tube noise along with all the rest. For 2V standard inputs I tended to use a voltage divider on the input, to lower gain to around 8dB (so 4:1 attenuation - e.g. 33k/11k - in practice 33k/10k) while also having some higher gain inputs and of course a Phono stage. In these preamp's I implemented a neat trick, using the tube heater for input switching - as I only had two sources (LP & DAC) this worked splendidly using one tube per channel...

2) Jim Fosgate / Thorsten Loesch style - place a 680k resistor from output to input (before the battery, not tube grid in your case) and a 100k resistor in series between input and attenuator. Now you have around 8dB Gain using local feedback (single stage) which lowers noise, distortion and output impedance, the way feedback does. The around 14dB feedback work quite well, with a Z out around 600 Ohm in practice.

Either way works, I tend to prefer #2 nowadays, but it is a matter of taste.

Ciao T


Could be 15nF in place of 10nF?

TIA
Felipe


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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Dec 19, 2016 4:20 pm 
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Muriel
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felipe wrote:
Could be 15nF in place of 10nF?


Sure value is not critical, small physical size and good quality matter, so polystyrene, silver mica or more exotic...

Ciao T

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 Post subject: Re: 6SN7 preamp
PostPosted: Mon Dec 19, 2016 5:26 pm 
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Cow

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Location: Catalonia New Europe State
15nF is teflon film & aluminium foil Russian FT-1


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 Post subject: Re: 6SN7 preamp
PostPosted: Fri Jul 14, 2017 2:36 pm 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
Posts: 191
Location: Catalonia New Europe State
Ciao T,

Do you know the input impedance?

TIA
Felipe


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 Post subject: Re: 6SN7 preamp
PostPosted: Sat Jul 15, 2017 9:52 am 
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Muriel
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Joined: Thu Jul 26, 2007 5:45 pm
Posts: 1111
Hi,

felipe wrote:
Do you know the input impedance?


Yes.

But I suspect you would also like a number, right...

Without feedback, you are looking at the 100K pot only.

If using local looped feedback, the input impedance is dependent on the volume setting, with the 100k/680k values I suggested, a 5687WB Tube and a 100k Pot/Attenuator the worst case (volume at max) will be around 60kOhm, lower volume settings get closer to 100k.

Ciao T

PS, Ciao is a universal italian greeting, also often used in germany as "bye", T is my initial.

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"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." Richard Feynman


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 Post subject: Re: 6SN7 preamp
PostPosted: Sat Jul 15, 2017 2:20 pm 
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Cow

Joined: Thu Dec 11, 2008 10:13 am
Posts: 191
Location: Catalonia New Europe State
Ciao = Hello & goodbye in Italian (yes same word!!!)

I'm not using feedback and not 100K pot (I'm using Intact Audio AVC).

TIA
Felipe


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