Do guitar amps tend to self-oscillate? - diyAudio
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Old 24th June 2012, 05:15 PM   #1
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Default Do guitar amps tend to self-oscillate?

I've got a solid state bass amp that, when you boost the treble, will self oscillate if the gain is high. The severity depends on what impedance is connected to the Hi Z input, but would this be common or acceptable? I'd think a manufacturer would be very reluctant to put out a product where some control combination could cause self-oscillation. OTOH, with the amount of boost available, it might be hard to prevent. I know a lot more about hi-fi amps than instrument amps, so enlighten me!
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Old 25th June 2012, 01:33 AM   #2
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Ultimately you are dealing with a manufactured product. Things happen in the manufacturing process far from the eyes of the people that designed it. Parts get substituted where price is the most important specification. Checkout procedures are written so people without technical training can do functional checkout of finished products. The people manufacturing amps may not understand what it is or how it is used. This happends frequently when manufacturing is moved offshore and the parent company receives amplifiers boxed for shipping on pallets. Sometimes they are shipped straignt to distributors. I'm surprised more don't burst into flames when you plug them in.
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Old 25th June 2012, 08:55 AM   #3
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If it oscillates then it's faulty - take it back abd have it replaced.
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Old 25th June 2012, 02:34 PM   #4
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Quote:
Originally Posted by Conrad Hoffman View Post
I've got a solid state bass amp that, when you boost the treble, will self oscillate if the gain is high.
I have an old 80s solid state Marshall (type 51-something) guitar amp that also peeeeps when you turn the gain fully open.
Later versions of this amp had a small ceramic cap (few pF) somewhere in the preamp to cancel this. So yes, it sometimes happens.

YMMV
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Old 25th June 2012, 02:38 PM   #5
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not so sure it is defective. i would love an explanation, but with the mountains of gain with some of these amps, it is inevitable.

have you ruled out feedback from your bass?
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Old 25th June 2012, 03:03 PM   #6
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Is this fault new? Perhaps inside it has degraded caps in the PS.
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Old 25th June 2012, 06:01 PM   #7
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I had a Sunn Coliseum Slave that did that.
It did to the owner and only once for me.
So, I cleaned it and Sprayed the controls out and reseated all of the connectors to the board.
As far as I know it has never done it again.
I don't know exactly what the cause was but it never came back.

I also had a Peavey Renown 400 the destroyed a speaker because of ultrasonic oscillations.
The cause of that was a bad Molex connector to the board.
So, I cut the connector off and soldered the wires directly to the board and it never did it again.


FWIW

jer
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Old 25th June 2012, 06:39 PM   #8
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Ah ha, so it does happen. This is an older amp and it oscillates with a sig gen on the test bench, so acoustic feedback is highly unlikely. I've got a message in to the manufacturer to see what's normal. The amount of gain is tremendous when you boost the treble, and there are three ways to do that- treble control, "bright" button and a 5-band equalizer. I'm not above adding some non-factory shielding if it would help. Thanks!
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Old 25th June 2012, 06:54 PM   #9
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It Is hard to tell without seeing it or a schematic, But my guess would be maybe a cracked trace or bad solder joint somewhere.
If it as a Graphic eq or any eg for that matter if the negative feed back path is open this could cause to much gain and cause it to oscillate.

I had described a little about this here,

little help here

little help here

Check the control to see if maybe the element has worn out or had opened on one end.
As this could cause it to be separated from the negative feedback path making it only have a positive amount of gain.


jer

P.S. In some eq circuits there there is a positive feedback return path and a negative feedback return path unlike the link above.

Last edited by geraldfryjr; 25th June 2012 at 07:04 PM.
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Old 28th June 2012, 06:02 PM   #10
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Quote:
Originally Posted by Conrad Hoffman View Post
I've got a solid state bass amp that, when you boost the treble, will self oscillate if the gain is high. The severity depends on what impedance is connected to the Hi Z input, but would this be common or acceptable? I'd think a manufacturer would be very reluctant to put out a product where some control combination could cause self-oscillation. OTOH, with the amount of boost available, it might be hard to prevent. I know a lot more about hi-fi amps than instrument amps, so enlighten me!
It is normal and quite common to have squealing oscillations in a tube guitar amplifier. These types of oscillations most commonly originate in the first preamp stage.
Sometimes this is cured by selecting the proper grade tube for V1A, selected for low microphonics.
Installing a 7 pf, 1000V cap, between plate and grid (between pin 2 and 1 OR pin 7 and 6 on a 12AX7) is a very widely used snub circuit.
This design was used by both Fender and Marshall to prevent squealing oscillations.
That's what you were looking for.
The cap should be a high grade silver mica, soldered directly to the pins of the socket. Production amps use a ceramic disk capacitor.
This is ONE of the techniques used by manufacturers to stop the ringing.
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