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Topic: Sho-Bud Single Channel Amp Noise |
Jon Irsik
From:
Wichita, KS USA
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Posted 7 Oct 2021 7:17 am |
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I picked up a non-working Sho-Bud single channel amp a couple years a go. I took it to a local tech along with a schematic for the amp and he got it up and running. However, there seems to be a good bit of hiss coming from the amp at idle and last night I noticed when I shut the amp down it took it a very long time to complete power down, probably 30 seconds or so after it was switched off.
Is this normal? Anyone have one of these that might verify what I'm experiencing? |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 13 Oct 2021 2:28 pm
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Do you mean the amp was still working after you shut it off? Or, was it just that the pilot light stayed on for awhile? If it's just the light, and it bothers you, a resistor can be added to drain down the power supply caps. Or, you can change to a different pilot light.
The hiss is usually just a bad (leaky) transistor, or sometimes a bad resistor. I've never worked on one of those amps, but a 'scope will usually track down the main source of the noise. Keep in mind that some old circuit designs have a small inherent amount of hiss in them - nature of the beast when you're dealing with older gear. |
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Jon Irsik
From:
Wichita, KS USA
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Posted 14 Oct 2021 6:22 am
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The light as well as the amp sound continues for a long period of time. As for the hiss, it's gotten worse since the amp was repaired and I've been using it much more in the past month or so. It's always had a bit of noise but now I'm not sure I'd use it on a gig and try to mic it due to the hiss.
Sounds like it's time for a trip to a tech.
Thanks! |
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Don Downes
From:
New Hampshire, USA
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Posted 26 Oct 2021 6:29 pm
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Just an anecdote. I looked up the amp quickly, and saw that it was solid-state as opposed to tube.
I have two amps: a JC-120 (solid-state) and a Fender Blues Deluxe (tube).
The JC-120 makes the same horrible hiss as the one I used in 1978. Google it, and you'll see that it is inherent to the JC-120. I found, at the time I was gigging, that the hiss gets lost (think S/N ratio).
My Fender is stone quite. If I hear any noise in the signal path, I know it's not the amp.
As to the turn-off lag, the power supply capacitors take a while to discharge since they need to be substantial to accommodate loads. Given that the digital electronics of the amp are either Vcc=12V, or Vcc=5V, they will be active until the power supply voltage (Vcc) is less than 12 or 5.
I agree that a bleed resistor will help discharge the caps quicker (as suggested), but putting a resistor on the cap to ground will have an effect as to how long the caps take to charge (Ic/(dV/dT)) because you have now added a parallel resistor to the power supply circuit
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"The secret to a long life is to keep breathing." |
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