The Creator
New Member
Hi there experts, I had my JVM410h dropped to the ground from 3 feet after a gig, and it was not played until it was checked.
So when doing the check, I would do some moddifications as well.
First I saw that the R26 was blown, this could be a bad tube. The tubes were originale from 2008 so i guess it was time to fail!
I deassembled everything and did NFB mod, Resonance mod, C83 with switch, put in a noisegate, and changed several resistors to metal-film and some diodes to lower the noise. All these tips can be found in the MOD Bible.
When all was completed, I started to do some voltage measurements, without tubes. This I looked up in a post in the forum.
This was to check that all the powertubes had right voltages, and were bias-able.
Result after 30min warmup:
In StBY=off: Pin 5 =adjustable from -29 to-47V, Pin 3 Platevoltage=470V, Pin 2=67V. All these values are OK right?
I put the new set of tubes in, and measured again:
StBy= ON Pin 5 =adjustable from -29 to-47V, Pin 3 Platevoltage=419V, Pin 2=56V. Voltages on pin 3 and 5 dropped, NORMAL ??
Measured voltage across screen resostor to +10V and above !!!
Measured voltage at BIAS connector to +100mV and above !!!
I shut down the amp fast, and took the new tubes out, if they could be the reason.
Tested again with old tubes, and the same result as above. Shut down fast.
Tested agin with only one set of powertubes V3&V4 and preamp tubes, same result.. BIAS is way to high !!!
I have not changed any resistor values in the powersection, only the NFB, this was set to 47k at test.
Can anyone tell me what is going on here???
Can I test the amp with only powertubes in, to see if there is problem with the preamp tubes?
So when doing the check, I would do some moddifications as well.
First I saw that the R26 was blown, this could be a bad tube. The tubes were originale from 2008 so i guess it was time to fail!
I deassembled everything and did NFB mod, Resonance mod, C83 with switch, put in a noisegate, and changed several resistors to metal-film and some diodes to lower the noise. All these tips can be found in the MOD Bible.
When all was completed, I started to do some voltage measurements, without tubes. This I looked up in a post in the forum.
This was to check that all the powertubes had right voltages, and were bias-able.
Result after 30min warmup:
In StBY=off: Pin 5 =adjustable from -29 to-47V, Pin 3 Platevoltage=470V, Pin 2=67V. All these values are OK right?
I put the new set of tubes in, and measured again:
StBy= ON Pin 5 =adjustable from -29 to-47V, Pin 3 Platevoltage=419V, Pin 2=56V. Voltages on pin 3 and 5 dropped, NORMAL ??
Measured voltage across screen resostor to +10V and above !!!
Measured voltage at BIAS connector to +100mV and above !!!
I shut down the amp fast, and took the new tubes out, if they could be the reason.
Tested again with old tubes, and the same result as above. Shut down fast.
Tested agin with only one set of powertubes V3&V4 and preamp tubes, same result.. BIAS is way to high !!!
I have not changed any resistor values in the powersection, only the NFB, this was set to 47k at test.
Can anyone tell me what is going on here???
Can I test the amp with only powertubes in, to see if there is problem with the preamp tubes?