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Old 13th December 2010, 11:44 PM   #21
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I've been doing alot of reading, oscillation can start in the 12ax7 and work itself up to the 12au7. Go figure. so I guess I would have to put grid stoppers on both sets. In the 12ax7 Pins 2 and 7, I already have 47k Caddock TF020 Ultra precision Film resistor soldered into the pins. I wonder if I solder the 1k Carbon comp would it be a problem? This would be where I would put the grid Stoppers for the 12ax7. Maybe someone can steer me in the right direction? Pins 7 is a little Confusing. I know I should place it after the Grid to ground resistor. So does this mean I put it from pin 7 to the first connection and cut the pcb trace there or add it down after the 2M resistor?

Red, I would cut the pcb trace
Blue = Where is would connect the 1k Carbon Comp.


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Last edited by migkiller1971; 13th December 2010 at 11:58 PM.
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Old 14th December 2010, 01:33 AM   #22
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Hi Migkiller ,

the first 1k resistor looks good to me , mean i can understand , because the
resistor is in series to the 47k one .
But the second resistor on the second half of the tube is not in series , its
paralell , to my limited understandig about tubes , thats not the same as in
the first example .

Juergen
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Old 14th December 2010, 01:51 AM   #23
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On the second (pin 7) I think I have two options:

1. Cut the PCB trace to Pin 7 and add the 1k resistor from pin 7 to that connection. (Right)

2. Run the 1k Resistor from pin 7 down passed the 2M Grid to Ground Resistor. (wrong)


I guess the 2nd option would be Parallel and the first Series. Let me read some more..

Last edited by migkiller1971; 14th December 2010 at 02:01 AM.
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Old 14th December 2010, 12:56 PM   #24
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Hi Migkiller ,

from the orginal ( Schematic ) point of view , the first option is right ,
than the resistor is in series .
On the _picture_ the stroke down to Ground is not right , because than
its paralell , from my little understanding about tubes .

Juergen
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Old 14th December 2010, 06:52 PM   #25
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I don't think I can use the 1k grid stoppers in the 12ax7. I think I have to do MATH.... The 12au7 (output) I can use 1K resistors. I can bridge pins 2 and 7 and the cut the pcb trace to the grid.
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Old 14th December 2010, 07:34 PM   #26
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Hi Migkiller ,

have you measured the old resistors ?
Maybe they have more resistance than the newer ones ?

Juergen
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Old 14th December 2010, 08:17 PM   #27
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Quote:
Originally Posted by audiotux View Post
Hi Migkiller ,

have you measured the old resistors ?
Maybe they have more resistance than the newer ones ?

Juergen
Hi Juergen,

I can't measure the old resistor because this preamp came with no grid stoppers which is kind of odd for tube amps.. So I'm actually adding resistors (grid stoppers). I've been running this preamp non stop for two days now and I get no whistling. I have added bypass caps, cleaned up some flux and used Cardas tri EUTECTIC to redo some of the soldering. It sounds amazing. Very Clear and tight especially the bass. I can hear the toms, cymbals and snare especially on Copelands(The Police) drums. WOW..I can hear three people playing instruments instead of mush. it still would not hurt to add grid stoppers and I just have to figure out the resistor size I need for the 12ax7...
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Old 14th December 2010, 09:52 PM   #28
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Hi Migkiller ,

wy chance a running system ?
Are the tubes okay , or do they oscillating ?
Soundwise its good ( to your own words ) , i like how you
describe it ! Hope you enjoy your musik more then ever .

Jürgen
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Old 15th December 2010, 03:11 PM   #29
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Quote:
Originally Posted by audiotux View Post
Hi Migkiller ,

wy chance a running system ?
Are the tubes okay , or do they oscillating ?
Soundwise its good ( to your own words ) , i like how you
describe it ! Hope you enjoy your musik more then ever .

Jürgen

I ran it for 8 hours yesterday and no whistling on the 12au7 Telefunken smooth plate. The tubes are fine. Right now I have NOS GE JG-5814WA (1957) in. I don't think I'll hear any oscillation with these tubes in there. I'm still going to put 1k Carbon Comp on the 12au7 and I guess its trial and error on the 12ax7. I'll start off with 1k and work down and up. Too high a values will probably cut off some High freq.

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Old 15th December 2010, 07:29 PM   #30
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Hi Migkiller ,

very good that all is okay with the tubes + the Yaqin !
Maybe the original resistors have to much tolerance or are
not non inductive , like the carbon thingies .

Juergen
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