Fireboxx3":4rblnkfr said:
I used a bias probe on each power amp tube.
But, I don’t think it may be the tubes any more and here’s why:
I’ve only had it since Saturday so bear with me here. I don’t know what model it is. I only know that it’s a 2204 and it was built in the year 1984. It’s been modded before and it has 2 holes in front that have been plugged up. So, the 2 that work are horizontal. It says master model mk2 lead 50 on the back. The sticker that would normally say the model is blank. So I don’t know if it had 4 inputs and someone permanently patched 2 of them together or...
I don’t know everything about Marshall’s. I don’t know what to look for to figure out what model it is or anything else. I only know I love the way they sound. I can take a picture of the insides if that would help.
Photo’s always help. If it is a 2204 it came stock with only 2 input holes. If the pots are soldered directly to the PCB it is the latter model which IMO sounds consistent with the original 2204 lay out and lead dress.
Anyway, put the JJ’s back in and bias to about 60% dissipation which should be about 30-32 mA/mV depending on the probe you are using and see if it resolves, improves or degraded the issue.
Always trouble shoot using a second cord (guitar and speaker) guitar and cab if possible to eliminate those possibilities. Also plug straight in bypassing any effects. Ensure bias is set correctly.
I am not certain of your experience so I must tell that these amps contain lethal voltages both AC & DC. DC can store in the large dual caps for weeks at a time. Never place two hands inside the chassis at the same time and ensure to make certain the amp is off and unplugged when not necessary.
Now, if you replaced the JJ EL34’s and biased to about 30mA/mV and the issue persists I am assuming the amp is board (PCB) exposing the components. Get a non conductive chop stick or pencil in order to start the tap test. It is at this point you will listen and look closely for anomalies.
When ready turn amp on with guitar pluged in. Turn amp down to a quiet tolerable volume. Leave guitar in the stand and pluck the low E string then move over the amp in a memorable pattern to check for shorts caused by cold solder joints, wear, etc... Pay special attention to the bottom side pin connections of the power tubes. Ensure tubes are not loose. That is to say not easily moved around by poking with the probe or finger.
BE SAFE. Take your time. You will need to repeat this process a few times in order to cover the “circuit”.
It is also great practice to spray the pins of all tubes with a quick drying electronic cleaner. Work tubes in and out a couple times. Also hit all jacks use cord in and out, hit pots as well.
Complete a visual inspection again. I can’t count the number of times it was easily identified with a slow detailed visual inspection.
Good luck, David
