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Old 31st August 2012, 12:19 PM   #1
6J7 is offline 6J7
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Default 6J7 preamp

Hi,

relatively new to tubes DIY, I built my first line preamplifier (capacitor output from plate) with 6J7.

It works well, then tried to connect transformers (LL1681) 13:1 to lower output impedance, I don't notice any improvement.

Actually the preamplifer is connected to a small trends audio class D amp (50K input), later it will be my 807 PP.

My question is:

Is there any interest to add a CF after the first stage ?

By the way, I have hum (50 Hz) problems with 6J7.. is this tube normally quiet when AC heated ?

Thanks

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Old 31st August 2012, 12:35 PM   #2
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Try shielding the top cap (signal grid) wire.
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Old 31st August 2012, 01:02 PM   #3
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Already using shielded wire, shield + hot soldered together at one side only between the 300K and 1K.
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Old 31st August 2012, 03:42 PM   #4
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Is the pin 1 of this 6J7s also grounded? It is the metal envelope grounding pin. Is there any unbypased cathode high value resistor?
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Old 31st August 2012, 04:22 PM   #5
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Pin 1 is connected to pin 8 (cathode), maybe better to connect it to ground ?

300 ohms cathode resistor bypassed with a 220uF.

Here is the schematic.

Hum increased when turning up volume pot.. (quiet source and amp, I checked)

Thanks
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Old 31st August 2012, 05:11 PM   #6
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Schematic correction, g3 externally connected to plate..
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Old 31st August 2012, 05:16 PM   #7
DF96 is offline DF96  England
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If hum gets worse as volume turned up then the hum is either coming in at the input, or it is capacitively coupled and responding to the changing ground resistance. With the input shorted, does the hum disappear when the volume reaches maximum?
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Old 31st August 2012, 06:08 PM   #8
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Quote:
Originally Posted by DF96 View Post
With the input shorted, does the hum disappear when the volume reaches maximum?
With input shorted, the hum is always at the same level.

With source (V-DAC) connected, the hum increase when turning up the volume..



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Old 31st August 2012, 06:23 PM   #9
disco is offline disco  Netherlands
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Move your psu ground from the 100uF cap to the C3 minusses (left and right tied together). Drop the 100 ohm to chassis. Any better?
Next, move the heater grounds there too. The way you had connected grounds made the heater potential run through your signal ground.
It's best to keep groundruns low ohmic by choosing thick solid copper wire or thinner braided copper wire or thinnest silver wire. Be aware for the witch hunt when using silver as it's only done for AF by audiophools (people depending on their ears only).
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Last edited by disco; 31st August 2012 at 06:36 PM.
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Old 31st August 2012, 06:33 PM   #10
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Quote:
Already using shielded wire, shield + hot soldered together at one side only between the 300K and 1K.
Ground the grid lead shield to the chassis. Do not connect shield to hot anywhere.

If your pot has a metal back, ground that to chassis also--or ground the shield to the metal back then ground both to chassis.

I have had better luck with chassis grounds for this sort of thing than grounding to the signal ground. Is that chassis powder coated inside also? Make sure you scrape off some of the coating to get to bare metal.

It sounds to me as though this is an input wiring issue. A pic of actual wiring would be more helpful than the schematic.

Last edited by Joe Roberts; 31st August 2012 at 06:35 PM.
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