Diagnosis of Marshall 8100 Valvestate head problems

Thread Starter


Joined Mar 16, 2015

As I already told you there are parts directly related to the current protection of the TR8 and TR10.
For TR8 the resistors R116, 117, 118 and transistor TR12 and D5 make a current limiter.
For TR10 the resistors R96, 97, 115 and transistor TR11 and D4 make a current limitter.

The calculations for the current are similar to the calculations at TR9.
It will result in 1.92 volts accross R118 , this will give a limit of 1.92 / 0.33 = 5.82 A
For R96 it will result in 2.14 Volts , this will give a limit of 2.14 / 0.33 = 6.48 A.

Hi Bertus, great response to the very amp I'm working on/trying to understand. I understand your voltage divider calculations but when I use a voltmeter to take readings at R117,118 and 116 I get very low readings. .007 vdc. I thought that maybe the transistor wasn't turned on so I put a 1khz signal through amp and also played my guitar through but very little change. The amps works great so perhaps it's my measuring technique. I measured from one side of the resistors to ground and also across but nothing like the 1.92 volts from your calculation. What am I missing? Thanks.


Joined Apr 5, 2008

In normal operation, the voltage will be low.
The transistor will only be activated when an overcurrent situation occurs.