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Old 14th September 2011, 02:55 PM   #1
Cassiel is offline Cassiel  Libya
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Default WTF. What's going on here.

Hi, I have built a phono preamp and it has HUGE problems. The noise is killing me. OK, the final stage is a 12ax7 cathode follower; it's DC coupled and the cathode has a 120K resistor. Voltage at the grid is 100V and at the cathode 115V. Both channels show this strange reading The only clue I have is that there is 2.5 volts between grid and cathode with the preamp switched off and no voltage on the filter caps. When I pull out the power cord the voltage goes slowly back to zero when I plug it back in it slowly goes to 2.5 volts again. Can you help me? What's going on here? Here's the schematic. Those are not my readings - wish they were.
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Old 14th September 2011, 03:13 PM   #2
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TF that is going on there is that you have a Cathode follower biased at 115-100=15V. Seems the grounded cathode stage should be biased a little hotter by decreasing the cathode resistor, perhaps.
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Old 14th September 2011, 03:24 PM   #3
Cassiel is offline Cassiel  Libya
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Quote:
is that you have a Cathode follower biased at 115-100=15V.
That I know, the reason why I don't. There's a leak somewhere.

Edit - PT is leaking out 3 volts AC. It still doesn't explain my biasing ploblem.

Last edited by Cassiel; 14th September 2011 at 03:35 PM.
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Old 14th September 2011, 03:51 PM   #4
SY is offline SY  United States
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Cathode-heater leakage? How high do you have the heaters biased?
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Old 14th September 2011, 03:58 PM   #5
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it's been a while since I played with tubes, but remember your 2 stages are dc coupled. The plate voltage on the 1st stage is the grid voltage on the second. The plate voltage etc on the first stage will effect the second stage operating point. I don't think the tubes operating point will lower your noise level a whole lot. Is your noise hum or hiss? If hum you can fix it, if hiss maybe not. You may wish to change your design a bit and put the high frequency cut between the first and second stage. That way it will help to cut the high frequency noise. my 2 cents, feel free to ignore it
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Old 14th September 2011, 04:02 PM   #6
Cassiel is offline Cassiel  Libya
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I thought about that, SY. I don't how to test it. My heaters are 55 volts above ground.

Regarding the hum - first I want to fix the biasing. There's something wrong at the PT primary connections - I'm checking the ceramic caps right know. That could explain the hum issue.
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Old 14th September 2011, 04:33 PM   #7
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The problem is probably with your meter loading the points you are measuring giving completely false readings. Try measuring the dc across the 300R in the CF anode circuit. If the stage is working and pulling about 1mA idle current you should see about 300mV across that resistor.

Cheers

Ian
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Old 14th September 2011, 04:33 PM   #8
Merlinb is online now Merlinb  United Kingdom
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Quote:
Originally Posted by Cassiel View Post
Voltage at the grid is 100V and at the cathode 115V. Both channels show this strange reading
That sounds pretty normal; a typical volt meter will pull the voltages down and give you a wacky sense of bias. You said you got -2.5V grid-cathode (correct?) which isn't unusual.

EDIT: Ian beat me to it...

Quote:
The only clue I have is that there is 2.5 volts between grid and cathode with the preamp switched off
Did you mean to type that?
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Old 14th September 2011, 07:07 PM   #9
Cassiel is offline Cassiel  Libya
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First off, thanks to those who have tried to help me.

ruffrecords - I just did that. About 1 mA (.98). So the meter reads the cathode voltage fine (120K/125V) but pulls the voltage down when I measure it at the grid?

Merlinb - "Did you mean to type that?" Yes, I did. See, I have a leaky switch (just found out) and when in off position the power trafo primary still sees around 3 volts so with the preamp switched off I still have a B+ of around 3.7V. No filament voltage though. On these conditions I read -2.5 volts between grid and cathode.
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Old 14th September 2011, 07:12 PM   #10
DF96 is offline DF96  England
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Is the CF oscillating? What type of meter are you using? A DMM should not drag the previous anode down by much.
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