Restoring an old Garnet amp
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erogenousjones17
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I have no idea what's going on in this thread, but I love it just the same. Oscilloscopes, wires everywhere, all manner of esoteric units and terms... Trevor, you're like a mad scientist! :laughing7:
mayfly
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Here's a shot of the 60Hz hum with all the volume controls at full. The bottom trace is the speaker output, and the upper trace is the input to the phase inverter:
Here's another shot one gain stage back. Lower trace is still the speaker output, upper trace is the stage ahead of the phase splitter:
and here it is another stage back. This is essentially at the grid of the last preamp stage:
You can see that the noise is injected pretty early on, and it is essentially 60Hz, with only a slight amount of 120Hz on there. It does not seem like it's power supply ripple, rather it's either the heaters, or it's crap from the air.
I took some time to move grid wires away from the heaters and did some other cleanup. Unfortunately there was no real effect. It may be being injected somewhere around the tone stacks, which is almost impossible to shield with a point to point wired amp.
More work to do here...
Here's another shot one gain stage back. Lower trace is still the speaker output, upper trace is the stage ahead of the phase splitter:
and here it is another stage back. This is essentially at the grid of the last preamp stage:
You can see that the noise is injected pretty early on, and it is essentially 60Hz, with only a slight amount of 120Hz on there. It does not seem like it's power supply ripple, rather it's either the heaters, or it's crap from the air.
I took some time to move grid wires away from the heaters and did some other cleanup. Unfortunately there was no real effect. It may be being injected somewhere around the tone stacks, which is almost impossible to shield with a point to point wired amp.
More work to do here...
fdesalvo
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Usually people twist their heater wires and keep them either perpendicular to or away from the signal wires, but you don't have the latter option with your current lead dress. Do you have them floating above ground using the ol' resistor off of the pilot lamp to ground method? You could still twist them to eliminate hum from these wires or wire up a small DC filament supply on the cheap. Also you may have to use shielded cable for all leads going to preamp grids.
mayfly
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Keeping to how the amp was originally built, they just pushed the heater wires onto the chassis and didn't even bother to twist them. I just did it the same way - more just to see and to re-create the same, er feel. hum might just be part of the feel :icon_biggrin:
I haven't tried the old elevate the heater center tap trick yet, but that might be in the cards. BTW - garnets used a neon bulb at 110VAC for the pilot - so I won't put it on the pilot just yet...
I haven't tried the old elevate the heater center tap trick yet, but that might be in the cards. BTW - garnets used a neon bulb at 110VAC for the pilot - so I won't put it on the pilot just yet...
fdesalvo
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Wow if I'd have suspected you had a neon pilot, removing it would be the first step I recommended to quiet things down lol! That neon bulb could seriously inject some hum mojo lol. Imagine stuffing a neon beer sign into the chassis :laughing3:
Elevating the heaters would be the easiest one to try first. Alls you needs is a pair of 100Ohm resistors IIRC.
Elevating the heaters would be the easiest one to try first. Alls you needs is a pair of 100Ohm resistors IIRC.
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