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Old 21st August 2003, 04:59 AM   #1
zobsky is offline zobsky  India
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Question help troubleshooting this diy pre-amp

I'm trying to build a simple 6sn7 - 6bq5 se amp using a single 6sn7 dual preamp tube. i built the preamp section but cannot get ANY sound out of it. ...connected to the amp input of a cheap powered computer system or to my headphones. there is no sound whatsoever, not even the hum i would expect from using only capacitative filtering. i have verified the integrity of the solder joints and tried swapping tubes. ...no success. i took out the volume pot and hardwired the source directly to the grid, ...this should give me distorted sound at least, but no success again.

i know the power supply section works, ..delivering 350 V brought down and filtered to 250V B+ for the plate. the cathode is about 10V above ground. I'm sure the high voltage DC gets to the coupling caps just before it is blocked.

with all this in mind, i'm sure i must have done something fundamentally stupid and i appeal to you to help me out or offer hints.
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Old 21st August 2003, 05:02 AM   #2
zobsky is offline zobsky  India
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closeup of socket
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Old 21st August 2003, 05:08 AM   #3
zobsky is offline zobsky  India
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the guts
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Old 21st August 2003, 05:15 AM   #4
DSummer is offline DSummer  United States
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Where are the plate resistors? Double check the circuit...
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Old 21st August 2003, 05:58 AM   #5
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You need a plate load. Circuit won't work without it.

Try 30k for Ra and change Rk to 870 which should get you started.

Some basic tube circuit theory would do you the world of benefit.

Study this.
http://www.tubecad.com/articles_2003..._Amplifier.pdf
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Old 21st August 2003, 08:24 AM   #6
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If you don't have a plate resistor...then what you have built a voltage to current converter.

A change in the voltage of the grid causes more electrons to flow through the plate causing a change in current! Yep current not voltage.

The simplest current to voltage convertor is a resistor...therefore you need the plate resistor. V=R*I...

Thus the larger the resistance (resistor) the bigger the voltage swing!...try something between 15k and 68k..the lower the resistor value...the most high frequency extention..the higher the resistor...the opposite happens..you get less high frequencies.


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Old 21st August 2003, 02:03 PM   #7
zobsky is offline zobsky  India
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i redrew the circuit with my actual component values and included the power supply voltage dropping resistor (of about 33K value to givea B+ of 2504V). Shouldn't this act as the plate resistor, or is the 100uF smoothing capacitor in the power supply doing something it shouldn't (sending the amplified signal to ground by presenting a lower impedance path)?

I haven't added in a grid resistor. when i had the pot connected, i should have acted like one to ground the input. should i put one back in?


yes, that is a good article on the grounded cathode amplifier, ..i have been reading through it.

thanks
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Old 21st August 2003, 02:17 PM   #8
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Hi,

Quote:
Shouldn't this act as the plate resistor, or is the 100uF smoothing capacitor in the power supply doing something it shouldn't (sending the amplified signal to ground by presenting a lower impedance path)?
Yes.
Disconnect the 100F cap and you'll have amplification.

The cap across the 33K shunts any AC present on the anode to ground.


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Old 21st August 2003, 02:35 PM   #9
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And add a grid leak resistor on the grid...
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Old 21st August 2003, 03:10 PM   #10
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Hi,

Something like this:

Cheers,
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