Taming the JCM 900

DC135er · 11-04-2009, 10:46 PM

I'm thinking of replacing the 6L6's in my 4102 with a set of 5881's in an attempt to cut back on the excessive, yet all too sweet, overdrive delivered by this magnificent amp. I currently use the amp on the 50% output setting but there's just too much O/D. Times have changed and I'm doing alot more "coloring" these days as opposed to lead solo work. At any rate, I either need to tame her or get a new amp and I just can't spring for that at this point in time. I have a couple of questions and, of course your thoughts and ideas are more than welcomed.

1. I believe the 5881's have a lower voltage than the 6L6. Am I correct in assuming this will help clean her up?

2. Should I be visiting the preamp stage instead of the power amp stage?

3. Will using the 5881's be detrimental to the amp in any way?

4. Do you think using a 12AT7 in the preamp section would achieve more or would it be more beneficial than changing the power tubes? If so, which tube should I replace?

American Viking · 11-04-2009, 11:02 PM

If you're looking for less distortion I think I can help you out. For one, the power tubes is not where you distortion is coming from as output tube distortion is vastly different than preamp distortion. You won't encounter power tube distortion until you hit insanely high volumes on a 100w amp. You will get much less power tube distortion if you leave your amp set to full power, or full 100w as the tubes won't break up earlier. Biasing 5-10mA colder than normal will yield less grit and have more punch. For less power tube distortion, tubes with higher plate dissipation will yield more headroom, but I don't think changing anything here is what you're after.

For what you're really looking for is taming down the preamp section and yes, a 12AT7 will definitely cut back on the preamp distortion, which is what I'm absolutely positive is what you're looking for.

I personally do not see a big difference between 5881 and 6L6. Some tube manufacturers even put the same tubes out calling them either/or.

One last thing, the 900 you have in particular has a solid state diode clipping circuit for added gain. If you were able to bypass this you will lose all solid state distortion and solely be relying on the tubes for your gain. This can take out a lot, but I would highly recommend trying the 12AT7 first, as if you clip this out, you'll never be able to sell the amp as is and it will lose resale value, that is, if you're considering selling it in the future. For even less distortion there is a 12AU7, but you should try out the AT7 first, as the AU7 is going to probably not make it sound very good, but it is an option and you may be able to find one cheap to toy around with. You can put AT7's in other positions as well, but the phase inverter is where it's going to have the biggest impact on gain. I used to run my 800 like that but have since just stuck with all AX7's.

Adrian R · 11-04-2009, 11:09 PM

Replacing the 6L6s with 5881s will compress the tone more which I believe is precisely what you do not want. To achieve a cleaner tone with more headroom the 6L6 is really the only option for that amp.

Large bottled KT66s would be even better for a cleaner, high headroom tone, but will not fit as Marshall put the tube sockets too close together.

You could start by replacing the 6L6s with a really cold set of JJs 6L6GCs and bias them as cool as possible. And then, yes..the AT7s would be a cleaner alternative to AX7s.

Without buying a different amp..(the 900s series by nature is a higher gain amp) your last option I believe would to install lower output pickups on your guitar. This would clean things up as well.

DC135er · 11-04-2009, 11:42 PM

Quote:

Originally Posted by American Viking

You can put AT7's in other positions as well, but the phase inverter is where it's going to have the biggest impact on gain.

Thanks A/V,
Can you tell me which one is the phase inverter? And you are correct, I would never go in and modify the amp in any way. The amp has a purpose and I don't want to take that away.

American Viking · 11-04-2009, 11:46 PM

If I'm not mistaken you will have less headroom with a KT66 as these will breakup much earlier than a typical 6L6.

The phase inverter should be the preamp tube closest to the power section, or last in line depending on how many tubes are in the amp. For this one I believe it is V3. It won't hurt the amp to put the AT7 in any position. You should experiment in all positions in my opinion. It really changes the overall tone when put in other positions, but like I said, the phase inverter is where it's going to target the gain.

DC135er · 11-05-2009, 12:02 AM

Quote:

Originally Posted by Adrian R

You could start by replacing the 6L6s with a really cold set of JJs 6L6GCs and bias them as cool as possible. And then, yes..the AT7s would be a cleaner alternative to AX7s.

Without buying a different amp..(the 900s series by nature is a higher gain amp) your last option I believe would to install lower output pickups on your guitar. This would clean things up as well.

Thanks Adrian. I thought about the pickups but those DiMarzio PAF Pros are the best pickups I've heard and, while I know they're a little hotter than Gibson's H/B's, I love the attack I get with them. One thing I hadn't thought of was what A/V said about running the amp at full power and simply turning it down.

MajorNut1967 · 11-05-2009, 12:10 AM

Quote:

Originally Posted by American Viking

If I'm not mistaken you will have less headroom with a KT66 as these will breakup much earlier than a typical 6L6.

Right Viking, The KT66 & the 6L6 are kissin cousin to an extent, but the KT66 has much bigger sound and will break up earlier then the 6L6. The KT66 will handle more voltage the your average 6L6 (Except for the 6L6-GC) KT66: plate 500v screens 400v (6L6: plate 360 Screen 270v). The KT66 will give you some Marshall growl, but it is real hard to find good ones.

wkcchampion · 11-05-2009, 02:48 AM

stupid but effective:
what about dialing in less gain? ^^ Like 1 or 2 or 3
You may also try to lower the volume on your guitar, try to play with half

RobS · 11-05-2009, 04:50 AM

Have a chat with Bob at Eurotubes. He sells a 'smooth' tube option for the 900's which use balanced preamp tubes. These will smooth out the sound for you.

As for 5881 vs 6L6. Well I recently switched the other way from 5881s to JJ6l6's and I do not think I'll ever go back. I find the 6l6 to be a much better tube allround for this monster wether it be for clean /crunch or heavy sounds.

As mentioned by Adrian R, biasing a little colder, say 30-35ma with the 6l6 will tame it down but it will lose it's punch. But itsounds like your'e after the clean fenderish sound? If so, try the Sovtek 5881 WXT biased around 33ma. But really the JJ's are better.

To give an example, with the sovteks I used to run bass at 10, mid 4 and treb 2. Now with the JJ's I run bass at 6, mid 5 and treb 4.5. With this I feel it has that classic Marshall sound and it is very tight. Backing off the gain on ChA to say 5 and cranking the vol gives a very clean sound with minimal breakup. It's all experimentation.

What sound exactly are you after?

Rob

Jesstaa · 11-05-2009, 05:50 AM

Quote:

Originally Posted by RobS

Have a chat with Bob at Eurotubes. He sells a 'smooth' tube option for the 900's which use balanced preamp tubes. These will smooth out the sound for you.

As for 5881 vs 6L6. Well I recently switched the other way from 5881s to JJ6l6's and I do not think I'll ever go back. I find the 6l6 to be a much better tube allround for this monster wether it be for clean /crunch or heavy sounds.

As mentioned by Adrian R, biasing a little colder, say 30-35ma with the 6l6 will tame it down but it will lose it's punch. But itsounds like your'e after the clean fenderish sound? If so, try the Sovtek 5881 WXT biased around 33ma. But really the JJ's are better.

What sound exactly are you after?

Rob

I might have to try the JJ 6L6s, I was kinda put off JJs after I got a bad quad, but seeing as they're cheap, might as well give them a try again. This thread could be interesting, I have wayy too much gain on my amp, I wanna take it back just one notch, so it's heavy and crunchy, but not muddy and blah

DC135er · 11-05-2009, 09:20 AM

Quote:

Originally Posted by wkcchampion

stupid but effective:
what about dialing in less gain? ^^ Like 1 or 2 or 3
You may also try to lower the volume on your guitar, try to play with half

Channel 1 is o.k. I'm running the gain on 2. I've tried lowering it a tad but it sounds real bad if I go lower. Lowering the volume on the guitar works great but that drive channel is just way too hot.

RobS, I'm not after the Fender sound...I don't think, anyway. I have tried the HotRod Deluxe and, while I liked it, it just wasn't my cup-o-tea. I used to have a 1972 BandMaster at one time. Fantastic amp but it was too clean for my tastes and it didn't have a mid pot nor a gain pot. What I'm really trying to achieve is to be able to use the drive channel and hopefully eliminate the need for the Bad Monkey or the Guv'nor. Don't get me wrong, I will keep these pedals on my board but I really want to use the amp for the O/D and use the effects for color.

DC135er · 11-05-2009, 10:53 AM

What I've gathered:
1. Put the amp back to full power and turn it down. I feel so stupid I didn't think of this before but that will be option #1.

2. Replace the phase inverter with a !2AT7 is option #2. Try this tube in all positions. A friend of mine has one he said I can have so this will be cost effective as hell.

3. Stay with the 6L6's and bias at 30 - 35mA.

4. Try the Sovtek 6L6WXT's and bias at appx33 mA.

5. I just finished sending an email to Eurotubes so we'll see what Bob says.

RobS, As far as the sound I'm after, I don't know if you know the song "Carry On My Wayward Son" by Kansas, that would be the sound I'm after in the drive channel. I looked it up on YouTube and back in 1976 they were using Marshalls, to my delighted surprise. The guitar player I'm watching was using what looked to be a Gibson Les Paul Special and the other was using a Strat.

LesPaulopolis · 11-05-2009, 01:25 PM

Quote:

Originally Posted by DC135er

As far as the sound I'm after, I don't know if you know the song "Carry On My Wayward Son" by Kansas, that would be the sound I'm after in the drive channel.

It's going to be tough. For that rhythm part you could use Ch. B, but for the staccato solos you're better off cranking Ch. A with medium gain and an OD pedal (IMO). Ch. B won't do that 'cleanish' lead tone thanks to the diodes. If you bring the gain down it sounds neutered...I'm sure you've experienced this.

Have you tried running high-power and pulling 2 tubes? Worth a shot cuz it's free! Make sure to half the head's impedence if you do. I like the tone this way over using half-power (triode) mode and 4 power tubes.

Just some more options for ya.

I've been trying to get a "Journey" or "Boston" lead tone w/the 4100 Ch. B and it just doesn't happen (too much gain). The Kansas lead tone has less gain than those tones.

Starting to think this is the most Mesa-sounding Marshall out there (when using it as-is). Weird, right?

Rock on!

Dewey Finn · 11-05-2009, 01:57 PM

When you say to half the amps impedence do you mean lower the output from say 16 to 8?

LesPaulopolis · 11-05-2009, 02:32 PM

Courtesy of Wilder Amplification (who knows what he's talking about):
"A common trick we do with 100 watters is pull either the two inner or two outer pairs of power valves out and set the impedance selector to the 8 ohm setting on a 16 ohm cab and setting it to 4 ohms for an 8 ohm cab. This effectively doubles the impedance seen on the other side of the OT, satisfying the double impedance requirement for two valve/50 watt operation. But this is the ONLY application in which you would run double the impedance that the amp is set for."

DC135er · 11-06-2009, 01:24 AM

WOW! I just got back from a rehearsal tonight. The first thing I tried was setting the amp back to full power...nah, that wasn't gonna get it. What happens next is too cool.

I figure I'll try the 12AT7 as we'd talked about earlier so I pull the back plate off and I'm facing the speakers. Keep in mind I bought this amp used so I would have something decent for the youth band at our church and I never really got to do a full inspection on this bad boy. So I'm looking and what do I see...4 Sovtek 5881 WGC's. So I figure that's wrong, right? Does anyone know if there were any production model 4102's that came with 5881's?

So here I go after that phase inverter and I figure I'll just check all the pre's first. I pull the first and it's a JJ ECC 803S. Cool. I pull the next one and it's a JJ ECC 803S. Cooler. I pull the third one, the one right next to the transformer and it has no name on it, it has no classification as to whether 12AX7, AT7, nada. All that is written on it is where someone wrote with a black ink marker VG1. Any ideas here as to what this could be or what VG1 means? At any rate, being as this had nothing on it I decided to try it there first, as American Viking suggested trying it in each spot. I think I nailed it the first time. I'm not really sure if it's the phase inverter but DAAAAM what a difference it made. The amp is no longer muddy, it has clarity, chime, grit and definition such as I've never heard before. Crap I need to learn to play all over again as she now allows all my mistakes to be heard.

I was running the amp with the bass at 11:00, the mid at 2:00, the treble at 2:00, and the presence at about 3:00. After the tube swap I had to raise the bass, and lower everything else substantially. I also had to raise the gain on both channels up to right at 2:00. I CAN NOW USE THE O/D CHANNEL.

Some things that were said tonight. Keyboard player stated "wow,that sounds like U2". I had the neck pickup on and the splitter pulled on. We were getting ready to start and one of the singers hears me slam an E5 and she says "Woah, that sounds really nice". I had the bridge pickup on without the splittter. Neither one knew anything about the tube swap as I had arrived real early so I could have enough time to try this and get it right.

If you want to tame that JCM 900 try the preamp tube swap with a 12AT7. It's cheap and it really helped my amp. Thanks to everyone for all your input/ideas.

RobS · 11-06-2009, 04:24 AM

Cool..a happy post for sure.

By the way I meant to say Sovtek WGC as you already have and not WXT.

But anyway your'e happy and that's all that matters....

11-04-2009, 10:46 PM	#1 (permalink)
DC135er Member Join Date: Aug 2009 Location: S. Texas Posts: 43	Taming the JCM 900 I'm thinking of replacing the 6L6's in my 4102 with a set of 5881's in an attempt to cut back on the excessive, yet all too sweet, overdrive delivered by this magnificent amp. I currently use the amp on the 50% output setting but there's just too much O/D. Times have changed and I'm doing alot more "coloring" these days as opposed to lead solo work. At any rate, I either need to tame her or get a new amp and I just can't spring for that at this point in time. I have a couple of questions and, of course your thoughts and ideas are more than welcomed. 1. I believe the 5881's have a lower voltage than the 6L6. Am I correct in assuming this will help clean her up? 2. Should I be visiting the preamp stage instead of the power amp stage? 3. Will using the 5881's be detrimental to the amp in any way? 4. Do you think using a 12AT7 in the preamp section would achieve more or would it be more beneficial than changing the power tubes? If so, which tube should I replace? __________________ One man's ear is totally different from another man's. Some are tone deaf, others have perfect pitch, and yes, some just don't give a s**t.

11-04-2009, 11:02 PM	#2 (permalink)
American Viking Senior Member Join Date: Jan 2009 Location: RICHMOND, VIRGINIA Posts: 1,510	Re: Taming the JCM 900 If you're looking for less distortion I think I can help you out. For one, the power tubes is not where you distortion is coming from as output tube distortion is vastly different than preamp distortion. You won't encounter power tube distortion until you hit insanely high volumes on a 100w amp. You will get much less power tube distortion if you leave your amp set to full power, or full 100w as the tubes won't break up earlier. Biasing 5-10mA colder than normal will yield less grit and have more punch. For less power tube distortion, tubes with higher plate dissipation will yield more headroom, but I don't think changing anything here is what you're after. For what you're really looking for is taming down the preamp section and yes, a 12AT7 will definitely cut back on the preamp distortion, which is what I'm absolutely positive is what you're looking for. I personally do not see a big difference between 5881 and 6L6. Some tube manufacturers even put the same tubes out calling them either/or. One last thing, the 900 you have in particular has a solid state diode clipping circuit for added gain. If you were able to bypass this you will lose all solid state distortion and solely be relying on the tubes for your gain. This can take out a lot, but I would highly recommend trying the 12AT7 first, as if you clip this out, you'll never be able to sell the amp as is and it will lose resale value, that is, if you're considering selling it in the future. For even less distortion there is a 12AU7, but you should try out the AT7 first, as the AU7 is going to probably not make it sound very good, but it is an option and you may be able to find one cheap to toy around with. You can put AT7's in other positions as well, but the phase inverter is where it's going to have the biggest impact on gain. I used to run my 800 like that but have since just stuck with all AX7's. __________________ -I'd rather be a good liver than have one...

11-04-2009, 11:09 PM	#3 (permalink)
Adrian R Senior Member Join Date: Jan 2009 Location: Far North Chicago Burbs' Posts: 170	Re: Taming the JCM 900 Replacing the 6L6s with 5881s will compress the tone more which I believe is precisely what you do not want. To achieve a cleaner tone with more headroom the 6L6 is really the only option for that amp. Large bottled KT66s would be even better for a cleaner, high headroom tone, but will not fit as Marshall put the tube sockets too close together. You could start by replacing the 6L6s with a really cold set of JJs 6L6GCs and bias them as cool as possible. And then, yes..the AT7s would be a cleaner alternative to AX7s. Without buying a different amp..(the 900s series by nature is a higher gain amp) your last option I believe would to install lower output pickups on your guitar. This would clean things up as well. __________________ Adrian R. Amplification: Marshall JCM800 2205 Marshall JCM900 MKIII 2100 Marshall JCM900 Dual Reverb 4500 Marshall MG15 Micro Stack Marshall 1960A 4x12 Cab. w/75w Celestions Sonic 2x12 Cab. MXR Fx. Guitars: Gibson LP Standard Gibson Explorer Gibson Flying V Guild DV 6 Acoustic "Only two tones apply: loud and LOUDER"

11-04-2009, 11:46 PM	#5 (permalink)
American Viking Senior Member Join Date: Jan 2009 Location: RICHMOND, VIRGINIA Posts: 1,510	Re: Taming the JCM 900 If I'm not mistaken you will have less headroom with a KT66 as these will breakup much earlier than a typical 6L6. The phase inverter should be the preamp tube closest to the power section, or last in line depending on how many tubes are in the amp. For this one I believe it is V3. It won't hurt the amp to put the AT7 in any position. You should experiment in all positions in my opinion. It really changes the overall tone when put in other positions, but like I said, the phase inverter is where it's going to target the gain. __________________ -I'd rather be a good liver than have one...

11-05-2009, 02:48 AM	#8 (permalink)
wkcchampion Senior Member Join Date: Oct 2009 Posts: 144	Re: Taming the JCM 900 stupid but effective: what about dialing in less gain? ^^ Like 1 or 2 or 3 You may also try to lower the volume on your guitar, try to play with half

11-05-2009, 04:50 AM	#9 (permalink)
RobS Senior Member Join Date: Apr 2009 Location: Melbourne - Australia Posts: 144	Re: Taming the JCM 900 Have a chat with Bob at Eurotubes. He sells a 'smooth' tube option for the 900's which use balanced preamp tubes. These will smooth out the sound for you. As for 5881 vs 6L6. Well I recently switched the other way from 5881s to JJ6l6's and I do not think I'll ever go back. I find the 6l6 to be a much better tube allround for this monster wether it be for clean /crunch or heavy sounds. As mentioned by Adrian R, biasing a little colder, say 30-35ma with the 6l6 will tame it down but it will lose it's punch. But itsounds like your'e after the clean fenderish sound? If so, try the Sovtek 5881 WXT biased around 33ma. But really the JJ's are better. To give an example, with the sovteks I used to run bass at 10, mid 4 and treb 2. Now with the JJ's I run bass at 6, mid 5 and treb 4.5. With this I feel it has that classic Marshall sound and it is very tight. Backing off the gain on ChA to say 5 and cranking the vol gives a very clean sound with minimal breakup. It's all experimentation. What sound exactly are you after? Rob __________________ JCM900 4100 DR + 1960 A + B + Powerbrake Ibanez PS10CL Paul Stanley Custom Gibson LP Traditional Epiphone LP Custom No pedals......... Last edited by RobS; 11-05-2009 at 06:31 AM.

11-05-2009, 10:53 AM	#12 (permalink)
DC135er Member Join Date: Aug 2009 Location: S. Texas Posts: 43	Re: Taming the JCM 900 What I've gathered: 1. Put the amp back to full power and turn it down. I feel so stupid I didn't think of this before but that will be option #1. 2. Replace the phase inverter with a !2AT7 is option #2. Try this tube in all positions. A friend of mine has one he said I can have so this will be cost effective as hell. 3. Stay with the 6L6's and bias at 30 - 35mA. 4. Try the Sovtek 6L6WXT's and bias at appx33 mA. 5. I just finished sending an email to Eurotubes so we'll see what Bob says. RobS, As far as the sound I'm after, I don't know if you know the song "Carry On My Wayward Son" by Kansas, that would be the sound I'm after in the drive channel. I looked it up on YouTube and back in 1976 they were using Marshalls, to my delighted surprise. The guitar player I'm watching was using what looked to be a Gibson Les Paul Special and the other was using a Strat. __________________ One man's ear is totally different from another man's. Some are tone deaf, others have perfect pitch, and yes, some just don't give a s**t.

11-05-2009, 01:57 PM	#14 (permalink)
Dewey Finn Junior Member Join Date: Oct 2009 Posts: 10	Re: Taming the JCM 900 When you say to half the amps impedence do you mean lower the output from say 16 to 8?

11-05-2009, 02:32 PM	#15 (permalink)
LesPaulopolis Senior Member Join Date: Jan 2009 Location: Minneapolis Posts: 250	Re: Taming the JCM 900 Courtesy of Wilder Amplification (who knows what he's talking about): "A common trick we do with 100 watters is pull either the two inner or two outer pairs of power valves out and set the impedance selector to the 8 ohm setting on a 16 ohm cab and setting it to 4 ohms for an 8 ohm cab. This effectively doubles the impedance seen on the other side of the OT, satisfying the double impedance requirement for two valve/50 watt operation. But this is the ONLY application in which you would run double the impedance that the amp is set for." __________________ '68 RI LP Custom JCM900 + EQ & Boost

11-06-2009, 01:24 AM	#16 (permalink)
DC135er Member Join Date: Aug 2009 Location: S. Texas Posts: 43	Re: Taming the JCM 900 WOW! I just got back from a rehearsal tonight. The first thing I tried was setting the amp back to full power...nah, that wasn't gonna get it. What happens next is too cool. I figure I'll try the 12AT7 as we'd talked about earlier so I pull the back plate off and I'm facing the speakers. Keep in mind I bought this amp used so I would have something decent for the youth band at our church and I never really got to do a full inspection on this bad boy. So I'm looking and what do I see...4 Sovtek 5881 WGC's. So I figure that's wrong, right? Does anyone know if there were any production model 4102's that came with 5881's? So here I go after that phase inverter and I figure I'll just check all the pre's first. I pull the first and it's a JJ ECC 803S. Cool. I pull the next one and it's a JJ ECC 803S. Cooler. I pull the third one, the one right next to the transformer and it has no name on it, it has no classification as to whether 12AX7, AT7, nada. All that is written on it is where someone wrote with a black ink marker VG1. Any ideas here as to what this could be or what VG1 means? At any rate, being as this had nothing on it I decided to try it there first, as American Viking suggested trying it in each spot. I think I nailed it the first time. I'm not really sure if it's the phase inverter but DAAAAM what a difference it made. The amp is no longer muddy, it has clarity, chime, grit and definition such as I've never heard before. Crap I need to learn to play all over again as she now allows all my mistakes to be heard. I was running the amp with the bass at 11:00, the mid at 2:00, the treble at 2:00, and the presence at about 3:00. After the tube swap I had to raise the bass, and lower everything else substantially. I also had to raise the gain on both channels up to right at 2:00. I CAN NOW USE THE O/D CHANNEL. Some things that were said tonight. Keyboard player stated "wow,that sounds like U2". I had the neck pickup on and the splitter pulled on. We were getting ready to start and one of the singers hears me slam an E5 and she says "Woah, that sounds really nice". I had the bridge pickup on without the splittter. Neither one knew anything about the tube swap as I had arrived real early so I could have enough time to try this and get it right. If you want to tame that JCM 900 try the preamp tube swap with a 12AT7. It's cheap and it really helped my amp. Thanks to everyone for all your input/ideas. __________________ One man's ear is totally different from another man's. Some are tone deaf, others have perfect pitch, and yes, some just don't give a s**t.

11-06-2009, 04:24 AM	#17 (permalink)
RobS Senior Member Join Date: Apr 2009 Location: Melbourne - Australia Posts: 144	Re: Taming the JCM 900 Cool..a happy post for sure. By the way I meant to say Sovtek WGC as you already have and not WXT. But anyway your'e happy and that's all that matters.... __________________ JCM900 4100 DR + 1960 A + B + Powerbrake Ibanez PS10CL Paul Stanley Custom Gibson LP Traditional Epiphone LP Custom No pedals.........

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