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Old 9th November 2007, 10:04 PM   #1
prorms is offline prorms  United States
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Default 12au7 preamp will not work

iam not sure how to hook up pins 4,5 and 9 the heater section
of the tube i tried use a 1.5m resistor to pin 9 cause the voltage supply is 14v so the pin would get 12.15v iam not sure if the tube bad or iam not conecting pin to thier correct spots

ive also include the schamtic below
i also tried hook the preamp to my computer sound card with a audio generator with no luck
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Old 9th November 2007, 11:19 PM   #2
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Don't you require a resistor between the grids and ground ?
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Old 9th November 2007, 11:26 PM   #3
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Ignore pin 9. Your 1.5meg resistor is doing nothing.

Connect 14 volts to pin 4 and 5 through a 10 ohm resistor.
In other words, 14 volt +ve terminal to one end of resistor, the other end of the resistor to pin 4, then connect pin 5 to the -ve terminal of the 14 volt supply.

The 12au7 should draw 150 mA, dropping .15 volts across the resistor, and leaving 12.5 volts across the tube heater.
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Old 10th November 2007, 12:28 AM   #4
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Quote:
The 12au7 should draw 150 mA, dropping .15 volts across the resistor, and leaving 12.5 volts across the tube heater.
That 150mA is obviously a typo.

But Prorms, your design is completely wrong for tubes and you need to see some basic information on the subject, at one of the many educational websites that explain the basics such as Aiken Amps and Fun with Tubes
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Old 10th November 2007, 02:05 AM   #5
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Default Many errors !

Pins 4 and 5 are the heater but you need about 12.6 volts here and the rated current is 0.15 A ( 150 mA ). Pin 9 is the center point of the heaters. If you connect pins 4 and 5 together and apply 6.3 volts between pin 9 and pins 4,5 it will draw 0.3 amps.

Your grids are not grounded ! The input triode must have at least a 1 meg resistor from grid to ground. The capacitor to the grid of the second triode must come before the 1000k resistor shown. That way it gets its resistor from grid to ground.

AND 14 volts is WAY too low for 99% (?) of all tubes ! Only special low voltage types will work at such low voltages.
As far as I know you will need at least a hundred volts at the anode that makes it about 200 volts at the + supply for this tube to work well.
It should work at low voltages like 100 volts supply but certainly might not sound as good as it would at higher voltages. Read that as generating plenty of distortion at low voltages and possibly limited useful output voltage swing.

As Ray_moth suggests , you DO need to read up some more before continuing !
Have fun and be careful.
Cheers.
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Old 10th November 2007, 08:46 PM   #6
prorms is offline prorms  United States
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SO WHAT SHOULD I EXACTLY CHANGE ON THE SCHEMATIC
resistor between the grids and ground ??????
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Old 10th November 2007, 10:01 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by prorms
SO WHAT SHOULD I EXACTLY CHANGE ON THE SCHEMATIC
resistor between the grids and ground ??????

No need to type in all CAPS.. You need to do exactly as indicated in the posts above. Reread them carefully.

Basically you need to remove that 1000K (1M) resistor connected from the plate of the first triode section to ground.

Then you need to add one 1M resistor from the grid of the first triode to ground, and another 1M resistor from the grid of the second triode to ground.

I would also reduce the value of the coupling capacitor between the first stage and the second to about 0.1uF.

220K of plate load resistance is a bit excessive with a 12AU7 frankly. Something like 22K might be more in order.

12V supply voltage is too low, somewhere around 100V would normally be the minimum..

Crackpot idea: With the above value changes something like 60V might actually work. (5 x 9V batteries in series sitting on top of the 14V filament supply. Plate current would be under 2mA so battery life would be several hundred hours. - Alternately buy a small switching supply designed for 12 - 15V operation with 100V output. )

Pico Electronics Switchers:

http://www.picoelectronics.com/dcdclow/pe62_63.htm

If you go this route make sure you protect the switcher from polarity inversions at the input with a series diode, and do not ever short its output!
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Old 10th November 2007, 10:24 PM   #8
bigwill is online now bigwill  United Kingdom
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Speaking of running tubes from low voltages - my all 6SN7 aikido, which normally runs from a 300V supply will still run with a somewhat distorted output at.... 0V? I'm guessing this is something to do with the cathode emissions letting some of the signal through passively without any gain. The amp is completely silent without heaters.
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Old 11th November 2007, 03:57 AM   #9
prorms is offline prorms  United States
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if i change the 220k resistors to 22k my out put singal goes to 75mV pk form 750mV in the sumulator let see if understand
resistor between the grids and ground ??????
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