| K200B-1 left channel makes a loud whooshing sound [message #23227] |
Thu, 02 July 2015 18:27  |
gui_tarzan
Messages: 24
Registered: May 2015
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Junior Member |
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The right channel is pretty quiet but the left channel (as you're looking at the front) is a lot more than just a hiss. It's a full-blown whoosh. It gets loud as you turn it up so I'm thinking it's in the pre-amp section. I don't have much trouble with tube amps but solid-state amps are still pushing me out of my comfort zone so any help would be great.
--Jim
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| Re: K200B-1 left channel makes a loud whooshing sound [message #23228 is a reply to message #23227] |
Thu, 02 July 2015 20:32  |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Yes, the volume control on these amps does two things. One it changes the gain of the stage and two it grounds out the signal from the output of the stage. If the noise is controlled by the volume control, then the first transistors are suspect.
Look up the schematic in the technical section and you'll see what I mean.
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| Re: K200B-1 left channel makes a loud whooshing sound [message #23245 is a reply to message #23227] |
Fri, 03 July 2015 13:20 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Here's a real basic explanation.
Okay, I'm looking at PC105. The input transistor is Q100. The signal comes in from the input jack and goes through R118 and C100 to get to the base of Q100. The base is sort of the same as the input grid of a tube triode.
The signal comes out from the collector and goes to the base of Q101. The output comes from the collector of Q101 and goes on to the input of Q102 and then to the tone controls. The last two transistors bring back the lost signal level from the tone controls.
The volume control when turned CCW grounds the signal output from the input stage through cap C104. When the volume control is turned CW, it grounds the emitter circuit of Q100 which increases the gain of that stage.
Basic transistors are pretty much the same as tube triodes.
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| Re: K200B-1 left channel makes a loud whooshing sound [message #23252 is a reply to message #23227] |
Fri, 03 July 2015 22:59 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Not really a tone stack, that's more like the typical Fender circuit. This is more like a cut and boost Baxendall circuit.
I already explained the volume control. It turns up and down the gain of the first stage and also turns up and down the output signal. When the gain is turned down the output volume is also turned down. When the gain is increased the output volume is also increased.
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| Re: K200B-1 left channel makes a loud whooshing sound [message #23281 is a reply to message #23268] |
Mon, 06 July 2015 22:46 |
gui_tarzan
Messages: 24
Registered: May 2015
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Junior Member |
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So I did some searching here for the input transistors and they cross (apparently) to a fairly common transistor which I forget the number of at the moment but I have some. Are they really that standard? What I mean is, if the specs are in the ballpark, is close really close enough in this case?
--Jim
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| Re: K200B-1 left channel makes a loud whooshing sound [message #23283 is a reply to message #23227] |
Tue, 07 July 2015 02:41 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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There are two types of transistors in the input stage, one is an NPN and the other is a PNP. Almost any transistor will work in these circuits, but low noise ones are best.
Okay, let's look at the first transistor stage and how the volume pot works to change the gain of the first stage. The gain of the first transistor stage in the preamp is partially controlled by the value of the resistor from the emitter to ground. In rough terms, the lower the value the higher the gain. Because the volume pot is connected to the emitter resistor of the first transistor, the resistance that appears on the volume pot is part of the resistance of the emitter circuit to ground. When the wiper of the volume pot is turned CCW the full resistance value of the volume pot is added to the resistance of the emitter circuit, which reduces the gain of the stage. As the pot is turned CW the wiper reduces the resistance in the emitter circuit until the resistance of the pot is zero in the emitter circuit increasing the gain.
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| Re: K200B-1 left channel makes a loud whooshing sound [message #23309 is a reply to message #23227] |
Wed, 08 July 2015 02:39 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Both of those caps are there to block the negative dc voltage from ground. All audio circuits tube or transistor have two voltage components dc power supply and ac signal.
Both of those caps will allow the ac signal to pass to the volume control, but block the dc (negative) voltage from the ground circuit.
The ac signal output from the 1st stage comes from the emitter of Q101. The output of the circuit is sent through R106 and then through C104 to the volume control. When the volume control is turned CW the signal coming from C104 is raised up from ground by the value of the volume control. When the volume control is turned CCW the signal from C104 is sent directly to ground through the wiper of the pot.
R106 and R107 basically form a voltage divider for the ac signal coming from the emitter of Q101. When R107 is bypassed by the bright cap C105 the ratio of high frequencies to low frequencies is altered.
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