Santiago Alvarez (electronics engineer, JVM, YJM, AFD...)

[pelachile]

I was wondering if removing C83 is the best way to keep the OD1 & 2 Orange & Red from squealing? I have already put a 5751 in V6 which didn't do much. As it is, I am unable to get the gain on either channel past 10 o'clock. I am not really interested in doing a lot of mods because I love the amp pretty much the way it is, I just gotta get rid of the squeal.

@santiall was setting the bias a little on the cold side a part of the design or was that more of a "let's prevent tube issues" type of thing.

One more thing, if I am playing in a mode, let's say green on CH 1 with FX Loop on and then I shut the amp off, when I power the amp back up and switch it out of stand by the indicator LED for the channel mode and the FX Loop go off. How is the memory for the recalling the last state the amp was in stored? Is there a battery or a super cap or something?

 

[santiall]

I was wondering if removing C83 is the best way to keep the OD1 & 2 Orange & Red from squealing? I have already put a 5751 in V6 which didn't do much. As it is, I am unable to get the gain on either channel past 10 o'clock. I am not really interested in doing a lot of mods because I love the amp pretty much the way it is, I just gotta get rid of the squeal.

@santiall was setting the bias a little on the cold side a part of the design or was that more of a "let's prevent tube issues" type of thing.

One more thing, if I am playing in a mode, let's say green on CH 1 with FX Loop on and then I shut the amp off, when I power the amp back up and switch it out of stand by the indicator LED for the channel mode and the FX Loop go off. How is the memory for the recalling the last state the amp was in stored? Is there a battery or a super cap or something?

hi, hmm, mods are something personal, I'd rather try to reduce R128. Removing C83 makes the am a bit dull fo rme, but as said, it may work perfectly for you. Also don't forget that the squeal also has a lot to do with the guitar, how far you are from the amp, etc. I have some guitars that hace issues, many others don't.

The bias a bit cold was more for safety, they were biasing the JCM2000 way too hot to make them sound less harsh and there were issues with that so we decided to be a bit on the colder side. You should be OK around 30-40mA per tube.

The memory is intergrated into the microcontroller that takes care of all the switching, midi, etc. It is an EEPROM, no battery but I haven't heard issues yet with that/ IS your amp very old and has been used a lot, I mean A LOT... you may try a factory reset and see if that helps, otherwise I guess Marshall should be able to replace that part, it is mounted in a socket although I have no idea how they are handling service nowadays

 

[pelachile]

I didn't think about doing a reset. I just had all the mode switching relays replaced so that would make sense I guess. That was a weird situation. My tech discovered small holes drilled into the relays from some reason.

Yeah, my amp is on the older side of things but I think it is new enough to not have the bias drift issues, at least I haven't experienced that yet.

You're right about the guitar thing. The squeal is worse on my older Epiphone Les Paul Anniversary than on my newer Charvel Pro Mod which obviously has more modern electronics and better shielding. The problem is I love my Epiphone with the JVM but I just can't get the gain up enough to give it any girth. Basically 9 o'clock is as high as I can go but I really have to be mindful of how close I am to it and also to be sure to mute strings that aren't being used because even when playing open strings will cause that feedback squeal and just ruin the sound of whatever chord I'm playing. That turns jamming into more of chore and it's just not as fun.

I just want to get this amp right because besides the effects loop, I think this is the most versatile best sounding amp I have ever played. My favorite is the 100W Laney AOR Pro Tube Lead but that is pretty much I one trick pony.

Biasing to 30-40mA is in the 65 to 70% range?

 

[santiall]

I didn't think about doing a reset. I just had all the mode switching relays replaced so that would make sense I guess. That was a weird situation. My tech discovered small holes drilled into the relays from some reason.

Yeah, my amp is on the older side of things but I think it is new enough to not have the bias drift issues, at least I haven't experienced that yet.

You're right about the guitar thing. The squeal is worse on my older Epiphone Les Paul Anniversary than on my newer Charvel Pro Mod which obviously has more modern electronics and better shielding. The problem is I love my Epiphone with the JVM but I just can't get the gain up enough to give it any girth. Basically 9 o'clock is as high as I can go but I really have to be mindful of how close I am to it and also to be sure to mute strings that aren't being used because even when playing open strings will cause that feedback squeal and just ruin the sound of whatever chord I'm playing. That turns jamming into more of chore and it's just not as fun.

I just want to get this amp right because besides the effects loop, I think this is the most versatile best sounding amp I have ever played. My favorite is the 100W Laney AOR Pro Tube Lead but that is pretty much I one trick pony.

Biasing to 30-40mA is in the 65 to 70% range?

Very weird the relay situation... Also, if you have had that much work done on the amp it's possible that the microcontroller has been messed up. Anyway, worst case, the micro is very easy to replace with a new one.

The JVM amps never had any "bias drift" problems, that's another internet myth and also people calling things by the wrong name. Not long ago another user approached me with "the bias drift" problem and it was a faulty tube that was pulling the bias down, the bias circuit was perfectly OK but hey, he was told that his bias drifted...

I asked about the age of the amp as I was curious how old it could be. Some amps are approaching 20 years old...

As for the bias current, yes, that's around 70%, although that's another "internet standard", perhaps a bit on the cold side but that tends to work nicely.