WTF. What's going on here.

Cassiel · 14th September 2011, 01:55 PM

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.

costis_n · 14th September 2011, 02:13 PM

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.

Cassiel · 02:35 PM

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.

SY · 14th September 2011, 02:51 PM

Cathode-heater leakage? How high do you have the heaters biased?

multisync · 14th September 2011, 02:58 PM

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

Cassiel · 14th September 2011, 03:02 PM

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.

ruffrecords · 14th September 2011, 03:33 PM

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

Merlinb · 14th September 2011, 03:33 PM

Quote:

Originally Posted by Cassiel

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?

Cassiel · 14th September 2011, 06:07 PM

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.

DF96 · 14th September 2011, 06:12 PM

Is the CF oscillating? What type of meter are you using? A DMM should not drag the previous anode down by much.

14th September 2011, 02:51 PM	#4
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Cathode-heater leakage? How high do you have the heaters biased? __________________ If there's a sucker born every minute, where do the rest of them come from?

14th September 2011, 03:33 PM	#7
ruffrecords diyAudio Member Join Date: Jun 2007 Location: Holt, Norfolk	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 __________________ Ian

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14th September 2011, 02:13 PM	#2
costis_n diyAudio Member Join Date: Aug 2008	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.

14th September 2011, 02:58 PM	#5
multisync diyAudio Member Join Date: Jan 2008 Location: Carp	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

14th September 2011, 03:02 PM	#6
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	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.

14th September 2011, 06:07 PM	#9
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	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.

14th September 2011, 06:12 PM	#10
DF96 diyAudio Member Join Date: May 2007	Is the CF oscillating? What type of meter are you using? A DMM should not drag the previous anode down by much.