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Grid battery bias on phono....

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fred76

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Joined 2005
#1
grid batt bias on phono....

Hi folks,

Maybe by now most of you are aware of the Artemis Labs phonostage where the 6N1P input tube uses fixed grid batt bias:

http://www.artemislabs.com/docs/PL1_Manual_1.pdf


My phono preamp has 1.5V on the input tube's cathode, so it is possible for me to convert the cathode bias to grid batt bias (coin cell) and in effect delete the 330uF cathode byass cap...

The question is if it is also 'okay' to do this mod on the second stage too since it has the same 1.5V on the cathode with 330uF cathode bypass cap value for the cathode R? Or, is it a bad idea to use fixed grid bias just right after the Passive Eq between 2 stages?😕 Plus, would it mess up the Riaa Eq?😕

Thanks for any responses.

br,
fred
 
#4
Or, is it a bad idea to use fixed grid bias just right after the Passive Eq between 2 stages
Click to expand...


If you apply the bias through the same grid resistor it will have no effect upon the riaa accuracy. Artemis would have probably done it if they didn't use the cathode for NFB. A very cool circuit btw.



but I am afraid a battery, in a tube amp, will explode, sooner or later
Click to expand...

You may be mistaking tube amps with pipe bombs 🙂 If the battery is in the grid circuit it will never explode. And it will have less sonic signature than in the cathode.
 
#7
If you put the 1.5V cell at the input grid of your phono stage tube...Then you will have a DC offset on the coil...You would need a input coupling/blocking cap....or as in the case of Artemis, they used input transformer...
I still don't like the idea that Artemic has the battery in SERIES with the input signal current..... I use batteries as well but don't have them in SERIES with any signal current....

Chris
 
#9
If you apply the bias through the same grid resistor it will have no effect upon the riaa accuracy. Artemis would have probably done it if they didn't use the cathode for NFB. A very cool circuit btw.
Click to expand...

Hi analog_sa,

Thanks for confirming that. My particluar phonostage is zero feedback all throughout, 3rd stage is a mu-follower type circuit (cathode output) - the lower triode acting as ccs w/ unbypassed cathode... Artemis' design seems to havea built-in linestage already (LC loaded 3rd stage with mild NFB), and is more suited to passive volume control units or low mu preamps. BTW, I believe the Artemis circuit is designed by John Atwood (One Electron). And yes, a batt on the cathode is much more colored sounding IME, almost like having a lytic cap placed there. AC modulation is said to be the culprit...

I'm going to use coin cells. Saves space since the actual phono chassis is small. Plus I have access to good coin cell holders with copper contacts and pins.

If you put the 1.5V cell at the input grid of your phono stage tube...Then you will have a DC offset on the coil...You would need a input coupling/blocking cap....or as in the case of Artemis, they used input transformer...
Click to expand...

Hi Chris,

Artemis' 'lower' phono model doesn't have any built in step-up/input transformer. You can use a cap across the input for capacitance loading (3M's Textool thingie), maybe that addresses the issue of DC offset?? Schematic here:

http://www.artemislabs.com/docs/PH1_Manual_1.pdf

fred
 
#10
EC8010 said:


Agreed, different technology, but the point remains; with only a little care batteries in valve kit can be kept cool and explosions avoided.
Click to expand...


Yes, I agree with you.
Anyway I wrote that it maybe a my own (wrong) opinion.
I have no "feeling" with 1,5 bateries...I forgot them in remote controls and in every kind of devices. I'm sure to forget them also in a amp, and you know, this kind of batteries have short life...
I don't like to open(after months or years) an amp(or a remote control or a little radio) and see a "dirty white liquid" all around... 🙄
 
#11
TheGatesOfFate said:
I don't like to open(after months or years) an amp(or a remote control or a little radio) and see a "dirty white liquid" all around... 🙄
Click to expand...

Ah, yes, it's important to use the right type of cell. Alkaline cells have quite a good shelf life (about 5 years), but lithium are even better at 10. You really wouldn't want a Leclanche (zinc carbon) cell in there...
 
#12
cerrem:
If you put the 1.5V cell at the input grid of your phono stage tube...Then you will have a DC offset on the coil...You would need a input coupling/blocking cap....or as in the case of Artemis, they used input transformer...
Click to expand...

Nope. The 47K (typical) resistor from input to ground with no DC current flowing through it ensures that the input jack "hot" is held at 0VDC. The other end of the battery has about -1.5V DC and this is modulated by the AC signal at the other end of the battery. You can think of a battery as a blocking cap that supplies its own voltage to block 🙂

See my blog for a slightly different version of John Atwood's Artemis Labs circuit. Mine is meant to drive only a 100K volume pot so it doesn't have the 3rd tube. An Aikido takes it from there. I used 6SL7 instead of 12AX7 because I hate 12AX7's.

I am planning to add a second half-6SL7 in parallel with the existing one, because the 2 6SL7's will match the 1 6N1P for heater current. I have them fed by a 12.6VDC supply and right now I'm just wasting the 300 MA excess current through a resistor, I might as well halve the output impedance instead.

-j
 
#13
Hi J_Epstein,

Thanks for chiming in... Thorsten made a quick and dirty elcheapo phonostage design using 12AX7 and a 6DJ8 for the 2nd stage, but you could substitute 6SL7 for the AX7 without any change in values. I think some folks here have already built this.

No batteries here, but the second stage uses grid-leak bias:

http://www.diyaudio.com/forums/showthread.php?postid=335624#post335624

T's most recent Euridice version uses fixed grid batt bias right w/ a S&B TVC. Something similar can be done with a potentiometer or stepped atten in use, but the bias components would end up in parallel directly at the tubes grid (or gridstopper): cap || gridR || batt.

...something like what I did to my se amp's input using a 5687 or E182CC in cascode:

http://pic4.picturetrail.com/VOL706/2663514/5325320/107508504.jpg
 
#14
I've built a couple different phonos with 6SL7 as the input tube and I don't think it's the tube for the job. Each of them sounded a little soft & mushy compared to other tubes like (especially) 6C45P. I like the 6N1P I just built, I must say. Seems to be working well as athe input tube.

-j
 
#17
I've built a couple different phonos with 6SL7 as the input tube and I don't think it's the tube for the job. Each of them sounded a little soft & mushy
Click to expand...

Thanks for sharing your impression using the 6LS7 as input. Might also try it out in a bread board partnered with a 6922, its 'high defiinition' sound may just offset the 6SL7's soft character.

fred
 
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