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DC path to ground?

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Posted on November 13, 2014 at 07:58:40
Nickel Core
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Joined: February 11, 2005



Hi,

Please take a look at the schematic of my fixed bias supply.

My question is, what is the DC path to ground from Bias_VT25?

Is it only determined by the resistance on the bottom side of the pot? Or is it this resistance in paralel with the top side of the pot (the bias PSU?). Or is the resistance path on the top indefinitely high because of the diodes?

:-/

NC

 

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RE: DC path to ground?, posted on November 13, 2014 at 08:50:48
Tre'
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Posts: 17294
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 R9 in series with R5 and that's in parallel with a bunch of other resistors providing a path to ground.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: DC path to ground?, posted on November 13, 2014 at 09:01:55
Nickel Core
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 Ok it will be in parallel with the bunch of resistors for the Bias_300B side.

But will it also be in parallel with the DCR of L4, R2, he DCR of L2,L5 and the internal resistance of diodes?

Or does this side not provide a DC path to ground?

NC

 

RE: DC path to ground?, posted on November 13, 2014 at 09:51:34
Tre'
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 "But will it also be in parallel with the DCR of L4, R2, he DCR of L2,L5 and the internal resistance of diodes?

Or does this side not provide a DC path to ground?"

I don't think the diodes will conduct in reverse.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

I'm sure they will not but..., posted on November 13, 2014 at 10:01:35
Nickel Core
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 Looking at the path TO ground, you go with the arrows of the diode (conducting, be it switching).

Looking at the path from ground to grid, you of see the diode blocking.

So, I was (and am) confused about the DC path (from grid) to ground.

If course this is basic EE engineering, but sometimes even the basics confuse me.

Sorry for this.

NC

 

RE: I'm sure they will not but..., posted on November 13, 2014 at 10:40:16
Tre'
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Location: So. Cal.
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 I see what you mean.

The diodes are conducting toward ground for grid current to flow but those diodes are so far removed from the grid, with much lower resistance paths in the mix, that I don't see that it will matter in practice.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

But it will matter.., posted on November 13, 2014 at 11:02:16
Nickel Core
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Posts: 870
Joined: February 11, 2005
 when calculating/simulating the AC response for the Grid Choke / Coupling Cap combination.

It so appears the whole bias supply is 'in' the DC path to ground, not only the DCR of the grid choke (supply bias via the grid choke).

I thought at first only the resistors on the bottom of the pot mattered, but everything else also I see now.

So best to do simulations including the the bias supply. Will try that and see if I have different results. If path via the supply has considerably lower resistance, this will matter me thinks.

NC

 

RE: But it will matter.., posted on November 13, 2014 at 12:00:08
Tre'
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 OK, but if all those values are part of the schematic won't the program automatically take all that into consideration when it runs the sim?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: But it will matter.., posted on November 13, 2014 at 13:17:05
Triode_Kingdom
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 Grid current will only reach the diodes after it exceeds the time constant of the power supply filter. In other words, short excursions into grid current will be absorbed by C3/L4.

 

RE: But it will matter.., posted on November 13, 2014 at 13:28:41
dave slagle
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 NC was asking about DC behavior.

I have to ask under what circumstances (aside from low level signals on an A2 biased tube) the DC behavior comes into play.

dave

 

RE: But it will matter.., posted on November 13, 2014 at 14:21:19
Tre'
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Posts: 17294
Location: So. Cal.
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I thought DC was anything (moving, periodic or whatever) that doesn't reach both sides of the zero voltage line?

The above is all DC. The grid current, even though it's periodic, is DC, no?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Not to hijack the thread, posted on November 13, 2014 at 16:18:37
deafbykhorns
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Posts: 1067
Location: Florida
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 but how did you model a power transformer using inductors in spice?
Im still learning the program and curious.

Add a directive K1 L1 L2? but not sure what the .995 does

 

SPICE Directive, posted on November 13, 2014 at 19:50:16
Triode_Kingdom
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 In LTSPICE click Help, enter keyword "inductor," then click "Mutual Inductance" for the Help file. In a nutshell, the expression "K1 L1 L2 L3 .9" means L1, L2 and L3 are coupled. The last number is the coefficient of coupling.

 

More..., posted on November 13, 2014 at 20:32:26
Triode_Kingdom
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Posts: 10042
Location: Central Texas
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 Also, the default unit of measurement for LTSPICE inductors is the Henry. This is true whether the inductor is a choke or a transformer winding coupled to another winding. When the inductor is a transformer winding, the ratio between coupled windings is expressed by assigning them specific values of inductance. The assigned inductance ratio acts similarly to ratios of impedance. In other words, a transformer with an 8K primary Z and 800 ohm secondary Z can be simulated with a 10H primary and 1H secondary. This ratio will relate to the turns ratio (voltage ratio) in the same way as any transformer.

 

RE: But it will matter.., posted on November 14, 2014 at 05:38:52
dave slagle
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 I guess my test would be how the inductance of a choke would respond to that waveform. If it is pure DC then only the DCR of the choke matters, if it is pure AC then it is the DCR + the inductance that matters.

In the case of a grid choke, I want an example of a situation where the transient signal causes a waveform that only responds to the DCR.

dave

 

RE: But it will matter.., posted on November 14, 2014 at 07:30:36
Tre'
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Posts: 17294
Location: So. Cal.
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 I guess if it is moving and/or periodic then it's going to "see" the reactance of the choke.

If it's caused by musical peaks driving the tube to grid current, from say a song at 120BPM with the kick drum and bass guitar pounding on the down beat, then the period of the occurrence of the grid current might be 30Hz.

A grid choke with 7000Hy has a reactance, at 30Hz, of 1.3meg ohms and the bias of the tube will be thrown way off and for a long time.

WRT bias stability a person would be better off with a grid resistor.

BTW Dave, this is a setup just for you. Now run with it. :-)


Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

What happens to the inductance in the presents of DC?, posted on November 14, 2014 at 12:27:14
Tre'
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Posts: 17294
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 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Wow, that was easier than I thought, posted on November 15, 2014 at 05:50:44
deafbykhorns
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 Same should be true doing a sim for OPT's?
Would you enter a series resistance value as well for the inductor?
Thanks

 

Yes to both (nt), posted on November 15, 2014 at 10:49:12
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
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 nada aqui

 

RE: But it will matter.., posted on November 16, 2014 at 07:39:16
dave slagle
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 Been trying to discuss it for 5 years... even tried on DIY audio and nobody seemed to care.

dave

 

I care, posted on November 16, 2014 at 09:05:54
Tre'
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 and I would like to understand.

It's been said and I've always understood that a grid choke has two advantages over a grid resistor.

1. Higher impedance presented to the driver stage helping to keep the load line for that stage horizontal, lowering the harmonic distortion and increasing the gain.

2. Presenting a low DC resistance to the output tube's grid, lowering the bias offset voltage when the tube is driven to grid current and reducing the time it takes for the bias to re-stabilize.

If the grid current is periodic and thus will "see" the reactance of the grid choke and not it's DCR, that changes everything and I would like to understand the ramifications.

I would think a lot of people here would like to understand the ramifications.

Thanks Dave.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: I care, posted on November 16, 2014 at 09:18:01
dave slagle
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Posts: 5430
Location: NYC
Joined: April 27, 2001
 It is your point 2 that I would like to discuss.... maybe it required its own thread but nobody seems to want to discuss it.

this sums up my stance for the past 5 years or so....



 

Not that it adds directly to your discussion but..., posted on November 16, 2014 at 18:59:04
Steve O
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 ...the waveform you posted is "pure" DC with an AC waveform superimposed on it. IOW, there is a DC component and an AC component.

 

RE: Not that it adds directly to your discussion but..., posted on November 16, 2014 at 19:54:27
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 Yes, and I think Dave's point is the AC component will see the reactance of the inductance and the bias of the output tube will be thrown off and not recover quickly.

So one of the two things that most people think of as being a real plus of having a grid choke just went out the window. :-(

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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