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...
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...
Attachments
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...
EDIT: the 1k resistor in the heater supply should have been a 10-ohm anyway...
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):
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):
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.
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.
Changes
The hum is probably a combo of 60 and 120hz. Breadboarding is probably the culprit of the 60hz, but the supply is for the 120hz.
Since the tube will draw only a mill or so, you could add a 270 ohm, 1/2 watt and 100 uf cap to the plate supply and cathode supply, -12 volts, (after the IC is supplied voltage) with alot more suppression of the 120 hz ripple.
The distortion is caused by low voltage and improper resistances.
If you connect the cathode to the -12 vdc line, a small cap will need to be added between the grid and input jack. I would try increasing the plate current a little, if possible, by adjusting the plate and cathode resistances for the clearest sound. It may work or may not.
Hope this helps.
The hum is probably a combo of 60 and 120hz. Breadboarding is probably the culprit of the 60hz, but the supply is for the 120hz.
Since the tube will draw only a mill or so, you could add a 270 ohm, 1/2 watt and 100 uf cap to the plate supply and cathode supply, -12 volts, (after the IC is supplied voltage) with alot more suppression of the 120 hz ripple.
The distortion is caused by low voltage and improper resistances.
If you connect the cathode to the -12 vdc line, a small cap will need to be added between the grid and input jack. I would try increasing the plate current a little, if possible, by adjusting the plate and cathode resistances for the clearest sound. It may work or may not.
Hope this helps.
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