JCM 900 SLX 5881 conversion to EL34 question

Amp Mad Scientist said:
Fuse is 3.3 ohms...
why does pin 1 and 8 read 14 ohms?
Should read the same as the fuse does...

Socket 1 pin8: 14.3 ohm
Pin 1: 14..3ohm
Socket 2 pin 8: 14.3ohm
pin 1 : 14.3ohm
Click to expand...
Ok im sorry i thought you were asking for pin to ground. Pin 1 to 8 on each socket is 3.2 ohms, fuse is 3.3 ohms.

Im sorry im driving you crazy with this ignorance and i appreciate your time none the less. I hate to keep you held up
 
Clockworkmike said:
Ok im sorry i thought you were asking for pin to ground. Pin 1 to 8 on each socket is 3.2 ohms, fuse is 3.3 ohms.

Im sorry im driving you crazy with this ignorance and i appreciate your time none the less. I hate to keep you held up
Click to expand...
Install the fuse.
Measure pin one to ground.
Measure pin 8 to ground.
Do this for both sockets.
 
Clockworkmike said:
Socket 1 is 15.3ohm to ground on pin 1 and for pin 8 to ground
Socket 2 is 15.1ohm to ground on pin 1 and for pin 8 to ground
Click to expand...
should be 3.3 ohms.
bad wire or bad circuit track.

Pull the rear board out.
Look for cooked circuit tracks or burn marks.
We need to test the wire between pin 1/8 and the rear circuit board.
 
Amp Mad Scientist said:
should be 3.3 ohms.
bad wire or bad circuit track.

Pull the rear board out.
Look for cooked circuit tracks or burn marks.
We need to test the wire between pin 1/8 and the rear circuit board.
Click to expand...
Wire from the fuse to both pin8s seems good on both leads as far physically speaking. Heres shots of the board from beneath. 20210214_183359.jpg20210214_183406.jpg20210214_183414.jpg20210214_183422.jpg
 
syscokid said:
SCREW IT!!!!!!!!! JUST GET A NEW AMP FOR GOD'S SAKE... :rolleyes2::rolleyes2::rolleyes2:
Click to expand...
2 things wrong:
Wrong screen resistors (should be 1000 ohm or 2200 ohm)
5881 resistors are 470 ohm and too small for EL34.
Wrong bias setting (should be -48 VDC on pin 5)
Bad connection at cathodes of output tubes. (should be 3.3 ohms to ground)

Everything else tested OK.
Except the speakers still read funny....and we haven't checked the speaker cables.

Still more to do...
 
Amp Mad Scientist said:
2 things wrong:
Wrong screen resistors (should be 1000 ohm or 2200 ohm)
Wrong bias setting (should be -48 VDC on pin 5)
Bad connection at cathodes of output tubes. (should be 3.3 ohms to ground)

Everything else tested OK.
Except the speakers still read funny....and we haven't checked the speaker cables.

Still more to do...
Click to expand...
So the screen resistors are undersized? Biasing could be fixed with just an adjustment i assume?
 
Amp Mad Scientist said:
All 3 things need to be fixed.
The amp is drawing too much current because of bias volts too little, and screen resistors too small.

Don't put the board back in yet.
You need to pull the screen resistors.
Click to expand...
This is excellent information my friend. I could upsize the screen resistors from 1k 5w to 2200 ohm 5w I guess? The bias could be bumped up if its low ( between 6 and 9 volts more?) The cathode ohms is another issue though since they are higher than what it should be from pin 8 to ground
 
Clockworkmike said:
This is excellent information my friend. I could upsize the screen resistors from 1k 5w to 2200 ohm 5w I guess? The bias could be bumped up if its low ( between 6 and 9 volts more?) The cathode ohms is another issue though since they are higher than what it should be from pin 8 to ground
Click to expand...
Look on the rear board.
There are 2 BIG resistors which should be 470 ohm 5 watt. (470 ohm is for 5881)

These need to be changed to 1000 ohm 5 watt.
Or 2200 ohm 5 watt.
Modern EL 34 needs at least 1000 ohm screen resistor.
EL34L is high current and should be 2200 ohm.
EL34 II is early break up.

Which EL34 do you have?

well.gif
 
Im really sorry that ive dragged everyone around with this BS. I feel as if Ive certainly wasted your time at least. I know i dont know dick about this stuff, but i appreciate all the effort poured into helping me definitely. I guess its like trying to walk a child thru a Trigonometry lesson
 
Amp Mad Scientist said:
Look on the rear board.
There are 2 BIG resistors which should be 470 ohm 5 watt. (470 ohm is for 5881)

These need to be changed to 1000 ohm 5 watt.
Or 2200 ohm 5 watt.
Modern EL 34 needs at least 1000 ohm screen resistor.
EL34L is high current and should be 2200 ohm.
EL34 II is early break up.

Which EL34 do you have?
Click to expand...
Theyre just regular Svetlana EL34 and the screen resistors are 1k 5w. But it certainly wouldnt hurt going to 2200ohm?
 
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