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Old 20th December 2004, 09:06 PM   #31
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SY

I have hum in one channel as I haven't fired the other one up yet until all bugs are gone. It will have same hum I am sure.

Hum stays at the same level and makes no difference with tube swap. I have a box of 12AX7 tubes and have tried 9 with same result.

I would think that with DC on heaters it would be quiet.

I need a tall glass of red wine, maybe the whole bottle. Good thing I am on vacation now.

Any ideas?
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Old 20th December 2004, 09:24 PM   #32
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Yes, I'd recommend a nice 2001 Michel Ogier "La Rosine" Syrah Vin de Pays.

If you feel ambitious, disconnect the feedback loop, short the input of the cathode follower to ground and see if you still have hum.
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Old 20th December 2004, 09:31 PM   #33
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Hi,

Quote:
I would think that with DC on heaters it would be quiet.
Not necessarily...
I'd work with both channels live, the B+ is probably common to both channels and with only half the load it may be stressing a tube.

Is the heater supply common to both channels? Maybe it's earthed at the socket side of the channel you're not using?

Cheers,


P.S. May be best to finish the bottle after the problem's been solved............
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Old 20th December 2004, 09:41 PM   #34
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Frank

The channels are totally separate from one another. I could score the board down the center and seperate them. The heaters aren't hooked up on the empty channel nor is the power supply. The tubes are out also.

I will wait on drinking the wine until the line stage works correctly.

SY

It sounds like a fine wine but here in Iowa there isn't too much other than ripple available in the wine section. I will look however.
Here a wine is great if it has a cork in the bottle.

If you feel ambitious, disconnect the feedback loop, short the input of the cathode follower to ground and see if you still have hum.

Ah, you are thinking that with about 20db of gain my problem may lie there? I will try it.
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Old 20th December 2004, 09:54 PM   #35
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No, simpler than that- I'm trying to see in which stage the hum is originating by working backwards from the output. Thinking a little deeper (i.e., clearing my throat after saying something stupid), I realize that you can't actually take the CF grid directly to ground. Best you can do is take the second stage tube's grid to ground. So, OK, let's start there.
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Old 21st December 2004, 02:00 AM   #36
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Ok, second stage's grid to ground. Still have noise.

I changed the feedback resistor value from 100K to 27K and the noise went down some in level. The output is down from 29V with 3 volts in to about 11volts out. It is more of a buzz now.
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Old 23rd December 2004, 12:36 AM   #37
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Default Stab in the dark

Check the 12AX7 CF output tube cathode voltage. If its above 100V you may need to cut some tracks etc. to float its heater supply (use the raw AC). By "float" I mean really float it with just a cap from one side to 0V or alternatively tie it to say +80 to +100V DC.

Cheers,
Ian
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Old 23rd December 2004, 02:09 AM   #38
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I replaced the 100K feedback resistor. I eliminated the DC on the heaters and tied a 100ohm resistor from each 3.15v leg to gtound. The noise is less now than with the DC on the heaters.


The heaters are wired using 22ga wire and don't have their own traces so there is nothing to cut. I will have to look up the voltages that I have written down for every stage. If my memory is correct there is about 100v cathode voltage.

Are you saying to inject 80-100 VDC into the heater voltage?
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Old 23rd December 2004, 02:49 AM   #39
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Hi,

Joe,

This is what I suspect is causing the problem:

Quote:
360V on that CF's plate seems rather steep too....
and is what I think Gingertube is concerned about as well, namely heater to cathode leakage.

Cheers,
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Old 23rd December 2004, 03:41 AM   #40
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Hi Frank

Well, I too was concerned with that much voltage so I lowered it for trial purpose. It still had no effect on the hum. I can get rid of the hum if I pull out the CF tube but then what good it the linestage.

Voltages across the resistors: With 360V feed.
R103 .908, R104 17.4, R105 198, R107 .760, R108 246, R109 112

To me it would appear that the 112V was too high across R109, and I don't think the cathode voltages are correct in the first stage. Can I rebias both the cathode follower stage and the first stage or am I barking up the wrong tree? Can I re do the first stage and remove the feedback connection and jumper across R104? That would put R102 to ground(470K) and give me a cathode resistor of 1.6K.

Is Ian wanting me to make a voltage divider and throw B+ into the heater circuit of the cathode follower only or the whole heater supply?
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