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Old 13th January 2009, 09:28 AM   #71
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Quote:
Originally #67 posted by korneluk


C2 and Chb are clear. I do not see the C1 cap at the input and the 6SN7 grid leak resistor goes straight to ground.

here is a clearer picture:

Click the image to open in full size.


Hello korneluc

thanks for your question. I am using an input triode. The 'well known factor' for this triode is > 4 therefore I don't need this cap.

Kind regards,
Darius
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Old 13th January 2009, 09:53 AM   #72
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Default Pentode output

@ #70
Thanks Sheldon
@ #69
Hello JLH
Example #1 brakes the 'ultrapath'. #2 is the better solution, you can take the g2 voltage from +B1.

Kind regards,
Darius
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Old 13th January 2009, 12:02 PM   #73
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Folks,

The Cunningham CX-350 data sheet specifies that "the '50 is not recommended for use in resistance-coupled circuits having a grid resistor". However, in the original LW circuit the grid of the 50 is directly connected to the plate of the 224. See schematic posted above.

Does this arrangement violate the spec?

-- josé k.
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Old 13th January 2009, 12:34 PM   #74
kmaier is offline kmaier  United States
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Very true.... but later when the type 50 in ST glass became available, RCA changed this (newer manual)... stating that any conventional type of coupling may be used provided the resistance added to the grid circuit by this device does not exceed 10,000 ohms. That's a pretty low resistance, implying that the 50 can get into a runaway condition.

In any case, you bring up a valid concern. In a traditional self-bias circuit, you could get runaway with a higher grid resistance and things would get ugly... I have a couple 2A3 tubes which do this, hence they may be a bit gassy. As you have a DC coupling from the 224 with a current flow of 1ma (thru the 224), it's unlikely to have the runaway problem as the 1ma of current should be far greater than the grid leakage current of the 50 triode. I'd probably be inclined to increase the driver idle current to be on the safer side which would improve the slew rate for the driver.

Regards, KM
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Old 13th January 2009, 02:23 PM   #75
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Default @ #73 and #74

Please read the radio news articles, thanks.

http://rl12t15.blogspot.com/2007/09/...tin-white.html
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Old 13th January 2009, 03:22 PM   #76
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Default Re: @ #73 and #74

Quote:
Originally posted by oldeurope
Please read the radio news articles, thanks.

http://rl12t15.blogspot.com/2007/09/...tin-white.html
Hallo Darius,

I have part 3 of the Radio News article series (not in your blog) where it shows the actual part values using a 45 tube. Please email me offline and I'll be glad to send them to you.

Grüße

-- josé k.
jose_korneluk AT bellsouth.net
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Old 13th January 2009, 04:02 PM   #77
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Thumbs up @ #76

Thanks José,
I sent you an email.

In my old circuit I had some trouble with drifting bias current for the RL12T15.
The resistor 220KΩ+390KΩ was much too high and little voltage drops made
big changes in bias because of the relatively low cathode resistor 1KΩ.
The Loftin White topology solved this problem and reduced the AC load impedance
for the driver stage. schematic
comes from

Kind regards,
Darius
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Old 13th January 2009, 04:48 PM   #78
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Default Re: @ #76

correction:

Quote:
Originally posted by oldeurope
The Loftin White topology solved this problem and reduced the AC load impedance
for the driver stage.
The Loftin White topology increased the AC load impedance for the driver stage.
Means less load for the driver stage.

Have a look at #69 The triode works in no load operation (current source). Loftin White makes this possible.
See this one bootstrap

comes from
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Old 13th January 2009, 06:11 PM   #79
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Does your 220K resistor present higher dynamic resistance
to the ECC83's plate when so bootstrapped? More constant
current than pure resistance?

If so, was that your main intention for this bootstrapping?
Or you had some other important reasons in mind?
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Old 13th January 2009, 06:28 PM   #80
Sheldon is offline Sheldon  United States
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It takes some work to get all the operating values set for a true LW amp, as they are all interdependent. However, once set, the operating points are very stable and self correcting.

On question Darius, on the clever bootstrap. How is the output impedance calculated? The feedback from the bootstrap should increase the impedance. From this article: http://www.tubecad.com/articles_2002...ted/page5.html

It suggests that the amount of the output fed back to the input is calculated according to the ratio between the resistor from plate to feedback cap, and the rp of the input tube (in this case, about 225K). So for this example, about 50% of the output is fed back to the input, in phase. I assume that the output impedance of the follower would be increased by approximately double?

Sheldon
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