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Old 7th September 2003, 10:12 PM   #1
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Exclamation my first tube amp is running (sort of)- help kill the hum

Hello all-

In the mean time of waiting for Ashok's results from his new tube/SS hybrid amp, I decided to break out my 6GM8's and have a go at getting a circuit up and running with one of them. In the past, I have never been very successful. My attempts never gave good enough output for my headphones. This time, I have sound, and it is pretty nice, but could be better... There is a small amount of distortion in the mids/highs, but it isn't bad. Bass is smooth and warm. My main complaint is the amount of hum I have picked up. It is a midbass hum, maybe 100Hz or so. The entire amp is "breadboarded" so, that may be my problem. The hum completely dies when I disconnect the tube from the opamp input, so I know the problem definitely lies with the tube. Attached is the exact circuit I am using. If you see anything I could do to reduce my hum issue, let me know. Thanks all.

Note on diagram: I did this in a hurry. A lot of the symbols are not normal symbols- ie the opamp and diode bridge... I know they are not the correct ones to use, but they filled the space.

EDIT: The 9-0-9VAC tranny is from an old CD-player. The 9VDC tranny is a RadioShack wallwart. Resistors are 1/4W generic metal film, film caps are polyester, generic electrolytics. If any of that mattered...
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Old 7th September 2003, 10:31 PM   #2
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Hi,

Use a full wave, well regulated DC supply for the tube's heaters and your hum problem should be gone.

Cheers,
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Old 7th September 2003, 10:41 PM   #3
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I had a feeling the hum problem probably came from the heater supply, as it usually does... I don't have another tranny right now that will put out the right voltage into enough current... so I got the 9V wallwart out of the closet. I will be buying another tranny soon and will try it that way... Thanks

EDIT: the 1k resistor in the heater supply should have been a 10-ohm anyway...
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Old 7th September 2003, 11:26 PM   #4
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Default Re: my first tube amp is running (sort of)- help kill the hum

Quote:
Originally posted by needtubes
My main complaint is the amount of hum I have picked up. It is a midbass hum, maybe 100Hz or so. The entire amp is "breadboarded" so, that may be my problem. The hum completely dies when I disconnect the tube from the opamp input, so I know the problem definitely lies with the tube. Attached is the exact circuit I am using. If you see anything I could do to reduce my hum issue, let me know.

Hi needtubes

The hum comes from the not well smothed supply...you must put a RC decoupling in the upper side of the 10K resistor...

As it is , the hum from the supply pass through the 10 k resistor and is amplified by the op amp...

Cheers
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Old 7th September 2003, 11:38 PM   #5
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Hi,

Quote:
As it is , the hum from the supply pass through the 10 k resistor and is amplified by the op amp...
So?
Wouldn't it be best to tackle it at the source iso of carrying it around the PS?

Cheers,
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Old 10th September 2003, 03:32 PM   #6
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Default tube hum etc

Hey there, Just a passing note: Alot of modern made tubes in recent times (to save costs) have differing internal heater contructions. Take the RCA 7199 for example.a brilliant allround audio performer but I found hum on this tube can be a pest to eliminate. Substitute tubes made in recent times use straight heaters, the EF86's and early 7199's used a spiral heater spec designed to bring hum & noise to low spec. Now I'm using Sovteks and other makes
and I'm now finding (somewhat oddly) that putting minus 10 volts on the heater centre tap completely cures my problem. Lift the hum balance wiper off ground and connect to low noise DC source and experiment)
Don't take it for granted, I've also tried positive volts......which by laws of physics should be better, but not always...It pays to do a lash-up and see.

keep it up
rich):
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Old 10th September 2003, 04:47 PM   #7
SY is offline SY  United States
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Dumb question: is your heater supply really configured as drawn? You're running AC to an electrolytic cap?
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Old 10th September 2003, 08:43 PM   #8
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SY- The heater supply is a 9VDC wallwart transformer- not AC.
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Old 10th September 2003, 08:53 PM   #9
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Ah, OK, the transformer symbol confused me. I'd certainly double check the ripple at the filament and look at how the filament ground is being run with respect to the signal grounds.
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Old 10th September 2003, 11:13 PM   #10
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Default distortion mids and highs

This circuit looks similar to Benn's, posted sometime earlier.

With 12 volts and a 10k plate resistor, distortion will be present. Voltage needs to be higher, say 35 or 40 volts.

I take it the 1k in the filament supply is a typo. Probably 1 ohm?

With only one filter in the main supply, even though it's 2200uf, hum will probably be a problem.

Hope this helps.
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