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

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?
 
Last edited:
pelachile said:
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?
Click to expand...
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
 
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?
 
pelachile said:
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?
Click to expand...
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.
 
Actually when I say on the older side, I should have said on the really older side. 47th week of 2008.

Good to know about the bias. I found V6 was bad. My tech has a 60 year old vacuum tube tester from the Navy that will let you test both sides of the tube and I had one side that was dead. We replaced it with a known good tube but the squeal problem was still there. From what I understand V7 provides the gain for OD1 and OD2 correct?

I have also read that running the bias a little hotter will smooth out some of the top end and make the gain in the OD channels a little less brittle.

Will a bad micro controller effect the operation of the loop? The LED for the switch on the front panel lights up, the loop works, but the switch doesn't turn it off or on, using the foot switch makes no difference either.

I tried the factory reset but no change. Some how the when coming out of standby after initial startup the entire pre amp is bypassed and it is if I'm just plugged straight into the return of the power amp. However, when I push the mode selector switch the pre amp is engaged and the amp remembers what mode I was in previously and switches to the next mode. So if I shut the amp down on the green mode in the clean channel, power the amp back on in stand by, the pre amp is engaged. When I bring it out of stand by the pre amp is disengaged and the LED goes out. When I push the selector switch the pre amp is engaged and switches to the orange mode.
 
a faulty controller could behave like you say, it handles all the amp switching and the memory. You may also have something as simple as a faulty standby switch that arcs when engaged and interferes with the micro and/or a faulty C95.
if you start the amp with the standby in the playing position does it work well?
 
I never thought of that. I will give that a try and report back. What is the maximum amount of bias you would recommend for the EL34 JJs? Also, I have thought about trying 6AC7s instead of the EL34s. Any thoughts on that?
 
pelachile said:
I never thought of that. I will give that a try and report back. What is the maximum amount of bias you would recommend for the EL34 JJs? Also, I have thought about trying 6AC7s instead of the EL34s. Any thoughts on that?
Click to expand...
you can try 6CA7, KT77, KT66, KT88, 6L6, 5881... not sure the bias range is broad enough for all of them though but, other than that, you are completely safe trying them.
 
Good to know. I want to stick with an EL34 flavor though. I love KT88s and 6L6s for clean tones, but not so much for the more distorted stuff. For distorted tones I prefer the EL34s. The green mode on the clean channel is really, really good though with the EL34s so I am hoping to get a little bloom out of the low end with the 6AC7s but retain the bite in the mids that the EL34s provide.

As far as the C83 mod, does that just tame the amount of gain, or does it tame some of the high end fizziness as well? I'm fine with the gain as it is because it helps to add punch, articulation and girth to palm mutes, I just don't like the some of the fizziness that comes from high frequencies in the signal (I'm guessing here). Kinda sounds like someone is tearing a piece of paper which makes it sound brittle. I am hoping that raising the bias to about 68-70% will help alleviate some of that problem.

I really appreciate you taking the time to respond to me, it's amazing to be able to talk to the guy that designed my favorite amp. Sorry if I am hogging up all your time on here.
 
Hello. Just wanted to ask. Are there any updates about this preamp?
I really liked it and hoped to get one for myself. Is this project already dead?
 
pelachile said:
Good to know. I want to stick with an EL34 flavor though. I love KT88s and 6L6s for clean tones, but not so much for the more distorted stuff. For distorted tones I prefer the EL34s. The green mode on the clean channel is really, really good though with the EL34s so I am hoping to get a little bloom out of the low end with the 6AC7s but retain the bite in the mids that the EL34s provide.

As far as the C83 mod, does that just tame the amount of gain, or does it tame some of the high end fizziness as well? I'm fine with the gain as it is because it helps to add punch, articulation and girth to palm mutes, I just don't like the some of the fizziness that comes from high frequencies in the signal (I'm guessing here). Kinda sounds like someone is tearing a piece of paper which makes it sound brittle. I am hoping that raising the bias to about 68-70% will help alleviate some of that problem.

I really appreciate you taking the time to respond to me, it's amazing to be able to talk to the guy that designed my favorite amp. Sorry if I am hogging up all your time on here.
Click to expand...
removing C83 will reduce the gain and increase the bass.
Also, as you point, maybe some of the fizziness you hear is from the bias too low, may be also a matter of testing different tubes, specially the input tube
 
okewaja said:
Hello. Just wanted to ask. Are there any updates about this preamp?
I really liked it and hoped to get one for myself. Is this project already dead?
Click to expand...

Hi, that preamp evolved towards a low power head. The project is not dead at all and it was supposed to be launched by a famous brand last year but that company changed hands and all the projects got messed up... so I am not tlaking to other companies or perhaps I end launching it under my own brand.
In any case the project is alive, hopefully it can see the light relatively soon!
 
You must log in or register to reply here.
Back
Top