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Old 10th July 2014, 11:50 AM   #1
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Default Resistor question

Hi all,

I have a HIFi brand KT-88 tube amp. Its kinda unknown and google doesnt get much info when i search for it.
Its point to point wired, pretty nice to. I replaced all the cheap wiring and installed TJ Jensen Foil coupling caps. I also replacedall resistors with NOS Allen Bradley JAN resistors.
I noticed that on the pre amp tubes (6J8P) there is a 390k resistor to ground on the same pin where the audio signal is connected from the volume knob.
I played around a bit with different values and when i went to 600k the sound really opened up. Does anyone know what kinda value is normal in this spot?
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Old 10th July 2014, 11:58 AM   #2
Mooly is online now Mooly  United Kingdom
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Well I'm guessing you mean a resistor from the grid to ground. That is needed to bias the valve but the value isn't critical and works (at AC or signal conditions) to define the low frequency roll off of the stage as it forms a filter with any input coupling caps. So increasing the resistor means that the low frequency cut off point is lowered... all other things being equal.

Is that what you mean ?
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Old 10th July 2014, 12:00 PM   #3
coresta is online now coresta  France
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resistors is a very very deep problem .Your experience seems to confirm mines ... did you try an higher wattage : twice the half value serial connected ? (but twin wattage)
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Old 10th July 2014, 12:04 PM   #4
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Thanks for both replies. The pin where the audio signal is wired to also has a resistor soldered on it that goes to ground. I figured it determes how much of the signal is allowed to reach the pre amp tube.
I used two 300k resistors in series and the improvement in sound is enormous, hence the question :-)
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Old 10th July 2014, 01:01 PM   #5
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Dephazer View Post
I played around a bit with different values and when i went to 600k the sound really opened up. Does anyone know what kinda value is normal in this spot?

I figured it determes how much of the signal is allowed to reach the pre amp tube.
I used two 300k resistors in series and the improvement in sound is enormous, hence the question :-)
There really is no "normal" value there. It depends on the design goals for input sensitivity and needs of the pre tube.

All you have done is up the resistance in parallel with the Vol. pot which will make it more sensitive. The volume will rise more quickly and come up to your normal listening level on a lower number. But you will also reach the clipping point at a lower volume number, too.

An audio taper pot gets higher resistance more quickly as you turn it so you will also be compressing that value into a narrower band on the volume control.

I'd undo it.

HTH

20

Last edited by 20to20; 10th July 2014 at 01:07 PM.
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Old 10th July 2014, 01:04 PM   #6
coresta is online now coresta  France
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Quote:
Originally Posted by Dephazer View Post
I used two 300k resistors in series and the improvement in sound is enormous, hence the question :-)
I know , thats the problem : if you TWICE the wattage, it'll be TWICE enormous, any explanation from DF96 won't explain anything (i've been trying 5 watts, and even 10watts : endless limit ... )

Last edited by coresta; 10th July 2014 at 01:08 PM.
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Old 10th July 2014, 01:13 PM   #7
SY is offline SY  United States
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Quote:
Originally Posted by 20to20 View Post
you will also be compressing that value into a narrower band on the volume control.
If this gives the user the psychological impression that the sound has "opened up" and that the "improvement" is "enormous," then it's a cheap and easy one compared to the usual audiophile snake oil.
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Old 10th July 2014, 01:15 PM   #8
coresta is online now coresta  France
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nope Sy , did you ever try a 250watter resistor in a 12AX7 cathode ?
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Old 10th July 2014, 01:20 PM   #9
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I'm sure someone has and reported the fantastic (literally) results to all of his friends. His wife in the next room may have even commented on the startling improvement.
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Old 10th July 2014, 01:27 PM   #10
20to20 is offline 20to20  United States
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If this gives the user the psychological impression that the sound has "opened up" and that the "improvement" is "enormous," then it's a cheap and easy one compared to the usual audiophile snake oil.

Yup! I kinda like a 12 band grahic EQ in that spot with dancing LEDs sitting next to my orange goo lava lamp, listening to a DBX tape of Alan Parsons on a player with yellow analog meters and pulsing red peak light.

But that's just me.

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