Guys,
I have a 1978 Marshall 2204 that I have been fixing up and when I got it had a number of minor problems all of which I fixed but I have found the following problem with V4 - power tube which has me foxed.
When the amp is switched on and idle, all the tubes come up as normal and everything looks stable. All voltages look correct etc.
After switching stand by on and with both the master and pre-amp volumes on zero I am getting the following on both V4 & V5
pin 3 = 374
pin 4 = 374
pin 5 = -32
(please note this amp does not have screen resistors ... yet... and I have recently fitted 5.6k grid resistors and 1 ohm bias resistors as it did not have any)
So i start running a 1Khz test signal through the amp. If I turn the volume up to around 2/3 with the pre-amp on 2-4 etc the tube in V4 starts to red plate etc. So I have to reduce the volume asap or switch to standby to stop either the tube failing or the HT fuse (500mA) blowing.
I have tested all components on the board and I have tested the OT (just in case) for shorts etc and apart from a dodgy feedback resistor, plate load resistor (which I have replaced) and installing some cathode 1ohm resistors on the power tubes everything is normal.
I have tried to bring the volume up slowly and and measured the pins on V4 & V5 etc and I have noticed that the current is not staying stable on the tubes when a signal is applied (presumably normal) but V4 is drawing a lot more current and I have also noticed that I am reading AC on V5 on pin 3 but not on V4 pin 3? NB:this only happens when a sufficient signal starts to go through the amp.
There must be something fundamental going wrong somewhere but I (currently) lack the experience to find the source of the problem.
Please note, when I pull V1 tubes and let the amp idle with master up to 10 everything is ok, it is only when a high enough signal goes through that things start to deteriorate?
Any advise / pointers greatly appreciated...
Regards
Sean-
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