First amp build: low output and gated sound

[Max999]

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?

 

[knockbill]

Can you post a schematic...

 

[Max999]

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?

 

[knockbill]

The grounding layout may contribute,,, AC heater leads near signal wires,,, loose connections... Its hard to say without seeing the chassis...

 

[Printer2]

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.

 

[rayma]

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?

 

[indianajo]

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.

 

[Max999]

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?

 

[hex69]

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.

 

[Max999]

Thnx Hex, but I am already using the preamp shields.

 

[klingo]

Hi,

You could try the humdinger pot instead of heater center tap, heater elevation is always a good idea when you have a cathode follower idling at max Vkf/180v.
The Valve Wizard