1987 plexi weird treble noise

sdn25

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I have a 68' 1987 replica build, there is a sort of crashing sound that I get when the mid pot is not maxed out, treble and presence are both at about 9.5, only bright channel used at about 8.5 on the volume with the bright cap disengaged. The sound im describing is something similar to that of using a comb filter on the treble frequencies. Using an EQ I was able to narrow this sound to within 3.5kHZ and 4kHZ. If my mid pot is maxed out the sound only occurs when the treble volume is cranked. I will list the parts in the amp that I think maybe relevant to possibly sort this out:

50k mid pot
0.022 coupler for normal channel
0.0022 coupler for bright channel
33k slope resistor
47k NFB off the 8ohm tap
0.68uF cathode bypass cap

Please let me know if you need any other information or component values to sort this out, Thanks !
 

Pete Farrington

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Maybe it’s oscillating.
What happens to the output valve cathode or anode current with no signal as controls are moved to those settings?
 

sdn25

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Maybe it’s oscillating.
What happens to the output valve cathode or anode current with no signal as controls are moved to those settings?
Same as idle, the noise only happens when a signal is passed, as in it blends into the guitar signal. Some more descriptives: swishing, sounds like the frequency of the noise is in the 4khz range when initially strummed, but settles to about 3.5khz during the note decay.
 

Pete Farrington

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Got any spare high voltage 100 to 500pF caps?
Blocking distortion does as its named, grid conduction causes bias shift causes the signal to get blocked off, and it splutters out momentarily.
 

sdn25

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Got any spare high voltage 100 to 500pF caps?
Blocking distortion does as its named, grid conduction causes bias shift causes the signal to get blocked off, and it splutters out momentarily.
I have a 500pf ceramic lying around, would that work ?
 

Pete Farrington

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Try connecting it between pins 6 and 8 of V2, or between 1 and 3 of V2.
That should all but eliminate the potential for oscillation, though 500pF may roll off a noticeable degree of extreme high frequencies.
Don’t let that that put you off, rather than being a permanent mod, we’re trying to find evidence for the oscillation hypothesis.
 

sdn25

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here are some audio clips you guys can hear what im hearing.
Keep in mind I have a 50k mid pot

mid pot cranked -

mid pot on 7 at first but I move it to about 6 after 2 seconds -
 
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Pete Farrington

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It sounds fine to me, I can’t hear an obvious issue :shrug:
The reverb may not be helping to pick stuff out.
How about a clip where you change the settings to bring the issue in and out?
 

sdn25

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It sounds fine to me, I can’t hear an obvious issue :shrug:
The reverb may not be helping to pick stuff out.
How about a clip where you change the settings to bring the issue in and out?
Here you go. I moved the mid pot from max to about 4, and after the pause at the end I maxed it out again to show the contrast.

 

Pete Farrington

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Ok, the tone seems to get a bit hollow, I think you’re getting a bit of bias shift, due to overdriving the output valve grids so hard. You may be able to measure the pin5 VDC shifting more negative as it gets overdriven ever harder?
 

sdn25

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Ok, the tone seems to get a bit hollow, I think you’re getting a bit of bias shift, due to overdriving the output valve grids so hard. You may be able to measure the pin5 VDC shifting more negative as it gets overdriven ever harder?
I will let you know what happens at pin 5. Also, my grid stopper resistors are 1/4 watt is that fine ? Or does it have to be 1/2 or 1 watt ?
 

sdn25

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You may be able to measure the pin5 VDC shifting more negative as it gets overdriven ever harder?
Just tested it, at idle its -39VDC but at the same settings, when I play it jumps upto about -55VDC. Is this normal? This was when I was measuring pin 5 of v4
 

Pete Farrington

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Is the grid voltage measurably different with the mid at 4 than at max?

To mitigate bias shift, I suggest to increase the 2 x 8k2 HT droppers to 2 x 10k.

Also maybe increase the 5k6 grid stoppers to 10k.

There’s other stuff to try, but they’ll tend to reduce gain a little.
 

Chris-in-LA

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All seems normal. Turning up the mids adds fizz and is always bright. Turn down the treble and mids and the amp gets much darker. You are turning up the EQ past the sweet spot.
 

sdn25

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All seems normal. Turning up the mids adds fizz and is always bright. Turn down the treble and mids and the amp gets much darker. You are turning up the EQ past the sweet spot.
No my issue is that turning the mids DOWN makes it really bright and the treble frequency has this unpleasant distortion. The amp only sounds good when the mids are maxed out, and this is with a 50k mid pot so wouldn't that let more mids through ? when I turn the mids to about 25k on the pot (stock marshall value) the sound that I am describing is very prevalent, it is not present on other Marshalls with 25k mid pots.
 

sdn25

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Is the grid voltage measurably different with the mid at 4 than at max?

To mitigate bias shift, I suggest to increase the 2 x 8k2 HT droppers to 2 x 10k.

Also maybe increase the 5k6 grid stoppers to 10k.

There’s other stuff to try, but they’ll tend to reduce gain a little.
With the mids maxed, I get the same bias shift readings. Is the 50k mid pot changing the way the tonestack responds as opposed to just enhancing the mids? I was trying to make an EVH spec amp and in his recordings where all the controls are maxed out, I don't hear the unpleasant distortion. Oh and I forgot to mention, when I turn all the controls to 10 that unpleasant distortion is present again. The recordings I sent before were with the treble at about 7. With all the controls maxed out, lowering treble knob mitigated some of that unpleasant distortion. Does this help narrow down anything ? should in change the mid pot back to 25k and see how it responds ?

For the grid stoppers, what is the recommended power rating for the resistor?
 

SmokeyDopey

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Ok, I see what you mean by "crashing" sound. I'm hearing like a "splatty" distortion.
At the beginning of the clip it sounds pretty normal. 7 seconds in I start to hear that splattiness, and 14 seconds in it becomes a lot more obvious.

My bandmates and I were testing ways to boost a bassman for a recording, and we plugged in an Ampex preamp and when we gave it some juice it would sound splatty like that.
 

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