Amplifier squeel at zero volume

AmpBuilder225 · 11th July 2009, 04:11 PM

Hello, I have designed amd built a guitar amp from scratch and am testing the power amp section of it at the moment. The phase splitter works great and I have inserted 2 of the 4 power valves (since they are two pairs in parallel) to avoid unecessarily damaging good valves.

The output it wired into a THD hotplate attenuater which has a load setting which allows the amp to operate without a speaker, and it then has 4 or 5 levels of attenuation. All works fine until no attenuation is used or very little is used.

When very little attenuation is used, a very loud squeel is heard from the speakers when the volume control is set to zero but at no other time. When no attenuation is used, the squeel is worse and present at all volume levels.

I'm somewhat baffled by this and was wondering if anybody had any ideas. The speaker cab is about 20 or so feet away in another room so feedback is not being induced by having the amp to close to it.

Thanks for any help!

Mooly · 11th July 2009, 04:59 PM

Valve amp or not

oscillation is caused when certain conditions are met. I would check first the grounding, in particular that the volume control and input ground cannot be influenced by any currents flowing in the output stage, speaker or otherwise.
Is it a squeal ? Not motorboating or anything.

analog_sa · 11th July 2009, 05:22 PM

Lack of grid stoppers? Difficult to say without a circuit.

AmpBuilder225 · 11th July 2009, 05:44 PM

The volume control is situated about 1cm from the main star ground point which is also right next to the output filter caps and I have used 1k5 grid stoppers on all 4 EL34's. I've attempted to attach the circuit diagram but struggled to get it small enough so quality is awful. As far as I'm aware the erroneous sound is a squeal as in high pitched. motorboating is more of a chugging sound if i'm not mistaken?

analog_sa · 11th July 2009, 06:03 PM

Installing grid stoppers on the phase splitter won't take much effort.

11th July 2009, 04:11 PM	#1
AmpBuilder225 diyAudio Member Join Date: Feb 2009	Amplifier squeel at zero volume Hello, I have designed amd built a guitar amp from scratch and am testing the power amp section of it at the moment. The phase splitter works great and I have inserted 2 of the 4 power valves (since they are two pairs in parallel) to avoid unecessarily damaging good valves. The output it wired into a THD hotplate attenuater which has a load setting which allows the amp to operate without a speaker, and it then has 4 or 5 levels of attenuation. All works fine until no attenuation is used or very little is used. When very little attenuation is used, a very loud squeel is heard from the speakers when the volume control is set to zero but at no other time. When no attenuation is used, the squeel is worse and present at all volume levels. I'm somewhat baffled by this and was wondering if anybody had any ideas. The speaker cab is about 20 or so feet away in another room so feedback is not being induced by having the amp to close to it. Thanks for any help!

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11th July 2009, 04:59 PM	#2
Mooly diyAudio Member Join Date: Sep 2007	Valve amp or not oscillation is caused when certain conditions are met. I would check first the grounding, in particular that the volume control and input ground cannot be influenced by any currents flowing in the output stage, speaker or otherwise. Is it a squeal ? Not motorboating or anything.

11th July 2009, 05:22 PM	#3
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	Lack of grid stoppers? Difficult to say without a circuit.

11th July 2009, 06:03 PM	#5
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	Installing grid stoppers on the phase splitter won't take much effort.

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