cozmacozmy
Active Member
Hello, been lurking these forums for years now and finally come across a problem that I have no clue what the cause is. I have a 1990 4500 JCM 900 DR with Sovtek EL34wxt (originally an EL34 model, unmolested and extremally clean inside) that I've had for many many years. I have not played the amp in a long time and figured I would buy a plate current probe to measure the bias to just see what it was. I wasn't aware of any problems prior to this testing and I have triple checked to make sure I didn't cause this issue while removing the chassis to do the bias testing... I just don't see how I could of caused this??? I did not play this before pulling the chassis so I am not sure if the time since playing this (years ago) is what failed, and just started to show up...
So... When I hooked up the 2 plate current bias probes under the tubes & hooked to my ammeter (A snap on Vantage pro graphing multi meter) on the ma scale and with my Fluke 88 on the voltage pin #3 to monitor plate voltage. When I went to turn it on, the voltage starts out at 480v and starts dropping when the tubes start flowing. The plate current rises to around -35ma as soon as the tubes warmed up and started flowing. The voltage starts dropping at this same time and as it gets around 400v the current is -80ma and getting worse by the second. At this time the plates start to go red. Adjusting the bias pot has no effect on the readings at all!
From all the reading I have done and pin voltage testing, the only thing I can see is the missing negative control grid voltage. Visually I don't see any burnt components, leaking caps (although all are original) I measure the 22k across the bias pot. I even put in a pair of GT El34 from my buddy and the red plating was even worse with the current hitting -330ma at one time and an extremely red plate. The amp smells extremely hot too while this is all going on! Tried this with and without all the 3 preamp tubes also. I have swapped tubes side to side and to my buddies GT EL34. Even pulled the preamp tubes and nothing seems to change anything.
With no power tubes in place I get
pin 3 482v
pin 4 482v
pin 5 -.007 to -.010 depending on where the bias pot is at. (this is the only difference i see moving the bias pot)
Shouldn't I be seeing around -35v on pin 5
Any ideas at to what I should do to pin point this down?
So... When I hooked up the 2 plate current bias probes under the tubes & hooked to my ammeter (A snap on Vantage pro graphing multi meter) on the ma scale and with my Fluke 88 on the voltage pin #3 to monitor plate voltage. When I went to turn it on, the voltage starts out at 480v and starts dropping when the tubes start flowing. The plate current rises to around -35ma as soon as the tubes warmed up and started flowing. The voltage starts dropping at this same time and as it gets around 400v the current is -80ma and getting worse by the second. At this time the plates start to go red. Adjusting the bias pot has no effect on the readings at all!
From all the reading I have done and pin voltage testing, the only thing I can see is the missing negative control grid voltage. Visually I don't see any burnt components, leaking caps (although all are original) I measure the 22k across the bias pot. I even put in a pair of GT El34 from my buddy and the red plating was even worse with the current hitting -330ma at one time and an extremely red plate. The amp smells extremely hot too while this is all going on! Tried this with and without all the 3 preamp tubes also. I have swapped tubes side to side and to my buddies GT EL34. Even pulled the preamp tubes and nothing seems to change anything.
With no power tubes in place I get
pin 3 482v
pin 4 482v
pin 5 -.007 to -.010 depending on where the bias pot is at. (this is the only difference i see moving the bias pot)
Shouldn't I be seeing around -35v on pin 5
Any ideas at to what I should do to pin point this down?