6550 Push Pull noise in power stage

famousmockingbird · 14th April 2011, 11:01 PM

The amp is a JCM800 50w combo. Here is the schematic:

http://www.drtube.com/schematics/marshall/4210.gif

I am getting a static/crackling sound from the amp with the master volume at Zero and no input. Noise is still present when all the preamp tubes are pulled. I replaced the master volume pot, coupling capacitors around the master volume pot, and diodes, and that didn't help. This all started when I lost my bias voltage and my tubes redplated. I purchased new tubes and replaced everything in the bias supply section. I got my bias voltage back and now I have this terrible noise. Screen resistors test fine @ 1k ohms, but I was curious if they could still be the culprits. Also with the amp on standby, when powering down the noise can be heard through the speaker as the caps drain off.

richwalters · 15th April 2011, 06:55 PM

You didn't mention IF you changed the Bias adjust presets ? Screen resistors if carbon types can withstand but modern types will often go high. One of my tricks is to desolder one/or all and measure while the heat from a soldering iron is applied. There will be some drift, but a duff one will go wild.

richy

Frank Berry · 15th April 2011, 07:07 PM

Perhaps one of the 6550 tubes is bad.

14th April 2011, 11:01 PM	#1
famousmockingbird diyAudio Member Join Date: Dec 2010 Location: Amesbury, MA	6550 Push Pull noise in power stage The amp is a JCM800 50w combo. Here is the schematic: http://www.drtube.com/schematics/marshall/4210.gif I am getting a static/crackling sound from the amp with the master volume at Zero and no input. Noise is still present when all the preamp tubes are pulled. I replaced the master volume pot, coupling capacitors around the master volume pot, and diodes, and that didn't help. This all started when I lost my bias voltage and my tubes redplated. I purchased new tubes and replaced everything in the bias supply section. I got my bias voltage back and now I have this terrible noise. Screen resistors test fine @ 1k ohms, but I was curious if they could still be the culprits. Also with the amp on standby, when powering down the noise can be heard through the speaker as the caps drain off.

15th April 2011, 07:07 PM	#3
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	Perhaps one of the 6550 tubes is bad. __________________ Frank

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15th April 2011, 06:55 PM	#2
richwalters diyAudio Member Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru.	You didn't mention IF you changed the Bias adjust presets ? Screen resistors if carbon types can withstand but modern types will often go high. One of my tricks is to desolder one/or all and measure while the heat from a soldering iron is applied. There will be some drift, but a duff one will go wild. richy