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First amp build: low output and gated sound
First amp build: low output and gated sound
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Old 21st March 2019, 03:59 PM   #1
Max999 is offline Max999
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Default First amp build: low output and gated sound

I tried my first amp build, a Marshall 2203, and it has very low output and a gated fuzz sound.


The tubes are replaced and should work. The output tubes feel hot to me, and before there is silence, the amp screaches softly. I suspect oscillation.
Or I forgot some connections offcourse.

Does anybody has an idea where to start checking with these symptoms?
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Old 21st March 2019, 05:39 PM   #2
knockbill is offline knockbill  United States
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Can you post a schematic...
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Old 21st March 2019, 05:48 PM   #3
Max999 is offline Max999
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Sorry, schematic is attached now.

I have full output and lost the gated sound now: bad ground in the p.i.

Now the amp sounds good but hums even with a grounded input.
Hum is most likely a bad grounding scheme? What could be other causes?
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File Type: png 2203 schematic.png (254.8 KB, 56 views)
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Old 21st March 2019, 06:02 PM   #4
knockbill is offline knockbill  United States
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The grounding layout may contribute,,, AC heater leads near signal wires,,, loose connections... Its hard to say without seeing the chassis...
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Old 22nd March 2019, 12:03 AM   #5
Printer2 is offline Printer2  Canada
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Pull the input tube, turn it on how is the hum. If still there turn off and pull the second tube, turn on and check, if still there repeat with PI tube and turn on.
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Old 22nd March 2019, 12:34 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by Max999 View Post
Now the amp sounds good but hums even with a grounded input.
Hum is most likely a bad grounding scheme? What could be other causes?
There are pots after the first stage, and also after the tone.
Does the hum reduce when either or both pots are turned to minimum?
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Old 22nd March 2019, 01:58 PM   #7
indianajo is offline indianajo  United States
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Perhaps the power transformer is too close to the gain stages. Does it have a ferrous wrapper? If not you you may have to build a steel wall around it.
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Old 22nd March 2019, 02:38 PM   #8
Max999 is offline Max999
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Moving the input wire of V1 away from the heater wire helped a ton but there is still hum Knockbill.

Printer2, pulling V1 kills most of the hum.
Rayma, turning the preamp pot down solves most of the hum.
Indianajo the PT is on the other side of the chassis.

Hmm this sounds like V1A is the culprit, but what to do with it... Time to elevate the heaters?
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Old 22nd March 2019, 02:57 PM   #9
hex69 is offline hex69  Canada
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Quote:
Originally Posted by Max999 View Post
Moving the input wire of V1 away from the heater wire helped a ton but there is still hum Knockbill.

Printer2, pulling V1 kills most of the hum.
Rayma, turning the preamp pot down solves most of the hum.
Indianajo the PT is on the other side of the chassis.

Hmm this sounds like V1A is the culprit, but what to do with it... Time to elevate the heaters?
I experienced something similar once, maybe it could help.. was playing with my hand between the output transformer and V1 then I noticed some variations in the hum, so I think it would be a good idea to shield the preamp tubes using those aluminium spring loaded cap.. hum goes away.
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Old 22nd March 2019, 05:50 PM   #10
Max999 is offline Max999
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Thnx Hex, but I am already using the preamp shields.
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