MusicMan 210-RP-100 :: ugly clipping

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MusicMan 210-RP-100 :: ugly clipping

Postby Leif B on Thu Feb 16, 2017 10:01 am

I'm not great with transistors, and could use a little direction on this.

The amp uses a common grid (?) power amp, and on the 6L6 cathodes the spec calls for JE1692 NPN transistors. These have been replaced at some stage with BD243C transistors.

The problem I'm hearing is a nasty clipping sound at "hi" power...
input: lo
gain: around 5
master vol: around 4

I feel pretty sure it's in the power amp. It doesn't happen at "lo" power.

I'm leaning toward the 6L6 cathodes. These are pretty new JJs.
I've noticed the DC measurements drifting on one cathode at the socket – swap the valves over, and it remains specific to that socket.
The cathode path: cathode –-> transistor --> 3R9 resistor --> earth/ground.
(I've just replaced the 3R9s)

I've set the bias by measuring 25mv across the 3R9s, as seen in another forum. No joy, still a nasty clipping.

I'm not 100% fluent in the lingo – is this some kind of crossover distortion where I'm seeing one valve in cutoff?

Anyway, here's what I'm hearing, and seeing on the scope: (not the world's greatest scope, but hey)
https://www.instagram.com/p/BQkXaoRAMfN ... r_workshop
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Re: MusicMan 210-RP-100 :: ugly clipping

Postby JJman on Sun Feb 19, 2017 2:01 pm

I can't play the video but a trace on each tube would be needed to diagnose one side vs the other. Could be an issue with something from the driving IC for that side, thru the components connected to the path that goes on to, and including, the cathode transistor. I would scope each point on the good side vs the bad side for a culprit.

Scoping at the 3R9s would confirm if one side is cutting off much differently. Each should cut off assuming it is class AB like almost all PP amps.
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Re: MusicMan 210-RP-100 :: ugly clipping

Postby Vortexion on Sun Feb 19, 2017 3:08 pm

First thing I should say is that I'm clueless with solid state circuitry, but...

I have been able to watch your video, and that scope trace doesn't look like an overload/clipping situation to me. It's more like a burst of parastic oscillation being triggered whenever you hit a note.

For the benefit of JJman and anyone else who can't get the video to work, here's a screen grab of the trace:

Image
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Re: MusicMan 210-RP-100 :: ugly clipping

Postby JJman on Tue Feb 21, 2017 11:55 pm

It does look like your not using a sine wave on the input as mentioned. I don't see any clipping. Just the transient spike.

You can generate a signal with your phone and an app if needed. Just keep the strength low to prevent overdriving the preamp.
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Re: MusicMan 210-RP-100 :: ugly clipping

Postby Vortexion on Wed Feb 22, 2017 6:21 am

JJman, a bit of clarification for you: in the video, the OP is playing a guitar string, and that HF burst isn't a transient: it sits there and rides across the screen from left to right as the note dies. My screengrab caught it near the beginning of its journey. Hope that helps.
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Re: MusicMan 210-RP-100 :: ugly clipping

Postby Leif B on Wed Feb 22, 2017 7:53 pm

JJman wrote:Could be an issue with something from the driving IC for that side, thru the components connected to the path that goes on to, and including, the cathode transistor.



You win. It was indeed the cathode transistor(s). Replaced both, problem solved.

Thanks to everyone for the input.
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