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Old 29th October 2004, 07:45 AM   #1
garbage is offline garbage  Singapore
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Question current flow, which direction?

there's something that's nagging me regarding current flow in the below schematic.

Click the image to open in full size.

i am of the opinion that total current drawn will flow thru R2, before splitting off to the left where the choke is and thru R1.

ie, if 60mA is total current drawn by 300b and 10mA is drawn by driver,
R2 will have 60mA flowing through and R1 will have 50mA flowing thru, with 10mA flowing to the choke.

or it it the other way around where R1 will have 60mA flowing thru and R2 will have 50mA, with the 10mA flowing to the choke?
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Old 29th October 2004, 08:11 AM   #2
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Default Some comments

1st I don't believe that the circuit is practical but lets look at it and answer your questions anyway.

The entire bias for the 300B must be developed across R1.

The driver tube current must also flow through the 300B.

R1 will have the total current through it and that current will split
- some (50mA using your values) to 0v via R2
- some (10mA using your values) to 0V via the Driver tube and R3

The driver tube will have only the voltage developed across R2 at its anode to operate from.

In its favour I believe that you would get some form of bootstrap action to assist in signal swing to the300B grid as a result of the choke connection to the R1/R2 node (low impedance signal source) and that the driver tube would therefore be running in an almost constant current mode as a result of the bootstrap action.

Maybe it just more clever than it appears at first glance and will work after all.

Any other takers to correct my or garbages miss-conceptions.

Cheers,
Ian
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Old 29th October 2004, 09:49 AM   #3
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Hi,

No bootstrapping as the AC current is shorted ("oil") directly across the cathode of the 300b.......

neat circuit !

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Old 29th October 2004, 10:05 AM   #4
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Default DRD Circuit

I believe the circuit garbage posted was the DRD circuit offered by Welborne Labs? http://www.welbornelabs.com/drd.htm

Anyway, it's probably easier to think of the current flow in terms of conventional current (i.e. from + to -) here. The current flows from the B+, through the OPT and 300B and R1. Here the current splits, passing through the choke to the driver tube (I think it's a 6N1P...) and the 300B grid while the remainder is passed to ground through R2.

Thus the bias for the 300B is developed by the combined voltage drop across R1 and the DC resistance of the choke (which would not be negligible since the choke is probably >30H)

I'm rather new at this, so don't take my word as gospel...

The circuit (except for the "Ultrapath" connection - i.e. the oil cap) is similar to Jeremy Epstein's "Free Lunch" direct coupled amplifier. http://home.earthlink.net/~jeremyepstein/freelunch.html He has documented this quite well and explains with example how to change the operating point
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Old 29th October 2004, 11:04 AM   #5
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Sorry, I didn't read your post carefully

If 10mA is drawn by the driver and 60mA by the 300B, 10mA flows through choke, 50mA through R2 and 60mA through R1
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Old 29th October 2004, 11:11 AM   #6
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"50mA through R2 and 60mA through R1"

I don't think that is correct. 60mA flows through R2 and 50mA flows through R1. R2 is providing the ground path for both tubes.
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Old 29th October 2004, 11:22 AM   #7
garbage is offline garbage  Singapore
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Default Re: DRD Circuit

Quote:
Originally posted by Guido Tent
Hi,

No bootstrapping as the AC current is shorted ("oil") directly across the cathode of the 300b.......

neat circuit !

regards

Quote:
Originally posted by audiousername
I believe the circuit garbage posted was the DRD circuit offered by Welborne Labs? http://www.welbornelabs.com/drd.htm
yes, this is the Vaccum Tube Valley DRD circuit that is used by wellborne labs.

i actually had a drd calculator to calculate the resistor chain at this thread.

the reason why i'm asking about the current directions is because someone asked in AA, about how to size the resistor. so i pointed him to that calculator.

but seems like we have different opinions as to how the current should flow.

i thought that R2 will have 60mA flowing through and R1 will have 50mA flowing thru, with 10mA flowing to the choke.

the direction of current flow will determine resistor sizes in the DRD resistor chain.

if i made a mistake, i would like to correct the drd calculator so that in future, people wanting to build this circuit can benefit from the calculator.

as of now, there are a opposing camps of thoughts. would someone kindly confirm the current flow?

thanks.

cheers,
garbage
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Old 29th October 2004, 11:24 AM   #8
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Default current

In DC terms

If 60mA is drawn by the 300B the maximum current anywhere in the circuit is 60mA because the 300B is the only connection to B+

R1 is connected to its cathode so the full 60mA flows through it

10mA is drawn into the driver tube through the choke, so the remaining 50mA flows to earth through R2.

Is the above right? or have I lost my brain.... did a 3 hour English exam this morning... eep
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Old 29th October 2004, 11:27 AM   #9
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Sorry .....

You are correct and I am wrong. It's very early in the morning.
I haven't had my morning cup of coffee yet.
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Old 29th October 2004, 11:30 AM   #10
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Everyone makes mistakes. Just hoping my commentary on Hamlet was accurate too lol
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