The JCM 900 and modification... Thoughts? - Page 2

LesBalls · 10-29-2009, 11:46 AM

Using a volume pedal in the effects loop is very different from using an attenuator. Putting a volume pedal in the loop is basically like adding another master volume.

Using an attenuator between the power amp and the speakers allows you to drive the power section harder but at lower volumes.

A volume pedal does not allow you to do this. Even if you turn the master on "10" when you bring the volume down with the volume pedal, you are driving the power tubes less.

Now, you may be driving them harder than you would with just your master volume control and no volume pedal.

But, this is different than driving them hard with an attenuator.

*** Also, keep in mind that an effects loop is between the preamp section and the power amp section. An Attenuator goes between the power section and the speaker cabinet.

LesPaulopolis · 10-29-2009, 12:00 PM

I agree 100% with the prev. 2 posts.

Using a volume pedal is NOT the same as an attenuator. However, as TPR stated, and I agree, it sounds "better" at lower volumes. Also, you can avoid messing around with the touchy gain to volume dance. On a 100w amp, I would describe low volume pedal + cranked amp as "less muddy" than just using the amp at the same volume. Will this trick "completely" change your tone? No. However, nuances like this DO make a difference to the discerning ear. Maybe it's placebo, maybe not.

Just my $.02

donkost · 10-29-2009, 01:02 PM

I'm liking this volume pedal in the loop idea more each time I read about it, for several reasons. If it gives a slightly less muddy tone at lower volumes that's great. The other thing I like about the idea is that it's an easily adjustable solo boost without altering the gain structure. Using an EQ or MXR micro amp in the loop also works as a preset volume boost, but controlling the overall volume with your foot on a volume pedal has to be better that trying to adjust a little knob or slider on a stomp box. I'm wondering if a volume pedal in the loop affects other items in the loop such as a delay.

TPR · 10-29-2009, 01:18 PM

Quote:

Originally Posted by donkost

I'm liking this volume pedal in the loop idea more each time I read about it, for several reasons. If it gives a slightly less muddy tone at lower volumes that's great. The other thing I like about the idea is that it's an easily adjustable solo boost without altering the gain structure. Using an EQ or MXR micro amp in the loop also works as a preset volume boost, but controlling the overall volume with your foot on a volume pedal has to be better that trying to adjust a little knob or slider on a stomp box. I'm wondering if a volume pedal in the loop affects other items in the loop such as a delay.

Yup, the boost in the loop will also boost other fx in the loop. I personally like getting a bit more delay in the solo as well as the volume, so I think it works out pretty well.

And I definitely believe the tone sounds less squashed with the volume pedal in the loop compared to just using the master volume, even at the same perceived volume, so I don't think it's a placebo, at least at bedroom level volume. I haven't really tried it at higher volumes, so I guess that's something new for me to check out. Woo-hoo!

LesPaulopolis · 10-29-2009, 03:12 PM

Using a vol. pedal as a solo boost is a tricky thing live. You can purchase ones that have a 'set' lowest volume and highest volume (instead of 'all the way off' to 'all the way up'). Mine does not have this feature, so after rolling up for a solo, it's very hard to find the spot you used to be at for rhythm. A 'boost' pedal like a SD-1 or and EQ is a better solo volume booster at a gig IMO.

TPR · 10-29-2009, 04:15 PM

Quote:

Originally Posted by LesPaulopolis

Using a vol. pedal as a solo boost is a tricky thing live. You can purchase ones that have a 'set' lowest volume and highest volume (instead of 'all the way off' to 'all the way up'). Mine does not have this feature, so after rolling up for a solo, it's very hard to find the spot you used to be at for rhythm. A 'boost' pedal like a SD-1 or and EQ is a better solo volume booster at a gig IMO.

Yup, good catch. I have the Little Alligator which has the minimum volume set, but didn't think about that when I posted. Also, I starting using an EQ pedal instead so I could shape the solo tone. I was kinda stuck on the volume pedal in my post since that's what the OP asked about, but I think the EQ is better for a few reasons.

LesPaulopolis · 10-29-2009, 05:39 PM

Right on, bro.

Although they're not too glamourous, volume pedals rock!!!

V-man · 10-29-2009, 06:14 PM

Unfortunately, I tried the volume pedal twice and the experiment was a miserable failure for me. I cranked the gain and master volume, setting the pedal in the "closed" position. Then as I roll forward it goes from no sound right into a "booming" bass-buzzy, rattling clean, then a hair more and I have that great crunch, but I'm at 75% of the volume of the amp going full bore. Another inch and it's full bore.

Maybe there is a better pedal out there (this is an old Morley Wah/vol) but I have a feeling that even a better pedal will yield similar results.

LesPaulopolis · 10-29-2009, 06:26 PM

Ch. B: Gain-16, Vol, 6 pedal about 45% down is how I run it
Ch. A: Gain-7, vol, 5

Too bad you had poor results, V-Man! I'm using an Ernie Ball VP Jr.

V-man · 10-29-2009, 06:38 PM

Yeah, my gain is about where your is, master was a little louder, but I don't see rolling it back a couple of notches to be the solution. I tried it again because I just got a 1992, which has no master volume and would LOVE a solution that didn't involve a few hundred on an attenuator.

TPR · 10-30-2009, 12:29 AM

V-Man, a couple of questions. Has the 4100 been checked out for good tubes, proper bias, etc? Either of those (and maybe other things, like old caps) might explain some of the trouble you're having getting your sound.

Is the 1992 strictly for gigging? I don't know those models, but I assume you don't really start getting the Marshall magic out of it until you have it cranked, so that rules out low to mid volume just for practice. And no loop, right? If that's right, an attenuator is the only thing to help with volume on that. And at gigs, you may find the attenuator gives you a bit more leeway to find the sweet spot at the right volume. I've seen THDs on ebay for $175-225. Also, the Weber ones are less than that. Mine sucked and I had a big hassle sending it back, but a lot of people seem to like them.

V-man · 10-30-2009, 12:55 AM

Quote:

Originally Posted by TPR

V-Man, a couple of questions. Has the 4100 been checked out for good tubes, proper bias, etc? Either of those (and maybe other things, like old caps) might explain some of the trouble you're having getting your sound.

Winged C EL-34s Brand new caps, fully serviced this spring

Is the 1992 strictly for gigging? I don't know those models, but I assume you don't really start getting the Marshall magic out of it until you have it cranked, so that rules out low to mid volume just for practice.

I just got the 1992, but all my amps are primarily used for practice in a residential area, which means the master rarely clears 3 on my 4100. When I do crank the 4100, yes it's something of a different story, and I haven't had much time playing with it naturally overdriven due to the living arrangements. Thus, it's hard to tell how much improvement the volume makes since I am too busy feeling the vibrations hit me in the face and rattle the balls. That opens another can of worms... whether sonic improvements at lower volumes will stick at higher volumes, etc. As for the 1992, I imagine it has 2 flavors, clean & quiet, and savagely LOUD

And no loop, right?

Yes, the 4100 has a loop, but other than rack systems, I have yet to find a stomp box that sounds very good back there. No loop on the 1992.

If that's right, an attenuator is the only thing to help with volume on that. And at gigs, you may find the attenuator gives you a bit more leeway to find the sweet spot at the right volume. I've seen THDs on ebay for $175-225. Also, the Weber ones are less than that. Mine sucked and I had a big hassle sending it back, but a lot of people seem to like them.

I'm happy to hear about some (more) effects loop suggestions, but with the new non-master SuperBass, an attenuator looks like it might be on the short list for me for the benefit of both amps. I will add that I tried the Sonic Maximizer 382 and the Sonic Stomp n my 4100 and the former really impressed me. I took it back to consider whether a 482/882 is in order or if I should look at attenuators.

TPR · 10-30-2009, 07:27 AM

Yup, it was the SL I was wondering about the loop for. I figured it didn't have one. Sadly, I think an attenuator is in your future. If you can get by without it for a bit, you can watch the deals and probably snag one for a good price. Just make sure the ohms match your setup.

Oh, you know about the loop adjustment control on the 4100, right?

Thanks for the info on the sonic max. I was thinking about getting one for my 4500, so now I'll definitely pick one up since it can't hurt to try, right?

donkost · 10-30-2009, 08:59 AM

I'm just looking for an excuse to pick up a volume pedal, and I'd probably go with the Morley as I also have a Morley Bad Horsie II wah. I used to have one of the big box AC-powered Morley Wah/Volume pedals back in the 80's and I wish I would have kept it. Of course I wouldn't have been able to use the wah in front of the amp and its volume pedal capability in the loop at the same time.

hmstrat · 10-30-2009, 03:31 PM

In an interview in "Vintage Guitar" Jim Marshall revealed that doesn't think much of modding his amps...

Q: A few years ago, when the EL34 power tube became scarce, how did that major changeover to using the 5881 affect the Marshall amp's signature tone?

A: We had to do that, but we weren't happy doing it. The amp's sound was a little cleaner with the 5881s than with the EL34s. That was better for the clean channel, but no so good for the distortion channel. When musicians were able to find the EL34s, they were putting them in themselves, but many forgot that they had to change the bias of the amps as well. So many people were destroying their Marshalls by not changing the bias and also by trying to hotrod them, because if we could have gotten more out of the amplifier we would've done it.

So the hotrodding thing was not very clever. It either ruined transformers or meant replacing tubes.

Here's the rest of the interview:

Vintage Guitar® magazine : Features :

V-man · 10-30-2009, 05:15 PM

Quote:

Originally Posted by TPR

Oh, you know about the loop adjustment control on the 4100, right?

Thanks for the info on the sonic max. I was thinking about getting one for my 4500, so now I'll definitely pick one up since it can't hurt to try, right?

I do indeed. I first tried adjusting sensitivity when running MXR-108 through the loop. It was asstastic all the way around. I never tried a delay pedal, but between the Sonic Stomp box, the 108 pedal, a Lexicon reverb rack and a BBE 392 rack, I have concluded that racks sound great in the loop and pedals do not.

Quote:

Originally Posted by donkost

I'm just looking for an excuse to pick up a volume pedal, and I'd probably go with the Morley as I also have a Morley Bad Horsie II wah. I used to have one of the big box AC-powered Morley Wah/Volume pedals back in the 80's and I wish I would have kept it. Of course I wouldn't have been able to use the wah in front of the amp and its volume pedal capability in the loop at the same time.

You may want to (re)read what I wrote with respect to LP's volume trick. I used an early-90s Morley on my 4100 and it failed to moderate, going from silent to buzzy/muddy clean, to just about as loud as the amp on its own. If a Morley works on your amp like it works on mine, it would only be good to take a "slight" edge off the dbs for a gig setting, not turn an overdriven halfstack into something managable in a residential area.

LesPaulopolis · 10-30-2009, 06:42 PM

Quote:

Originally Posted by V-man

Yeah, my gain is about where your is, master was a little louder, but I don't see rolling it back a couple of notches to be the solution. I tried it again because I just got a 1992, which has no master volume and would LOVE a solution that didn't involve a few hundred on an attenuator.

Must be the diff. between the pedals that are giving us such different results. The VP Jr. has a 'taper' setting where the majority of the vol. comes on in the last 10% of depressing the pedal. Maybe that's why I can get more 'control' and it doesn't go from zero to window shattering/bass overload like you experienced??? Then again, I've got a small electronics lab in my loop with half the stuff before the vol. pedal and half after it and a solo boost out front so YMMV.

If anyone else has the opportunity to test this out let us know your results.

Oh, and just for fun last night I pulled everything out of the loop and went straight-in the front at a similar volume (no boosts).

Results? Well it was just different. The gain was grainy-er...more Mesa-ish. Less of a smooth tone and more pre-amp fuzz sounding. Then I went for the "most smooth" I could (think Journey/Boston solos) and y'know, it was ok and all. Kinda harsh/grainy sustain or not enough sustain and round/smooth...made me turn down all the treble, crank the bass, essentially re-inventing the wheel I already built before throwing a bunch of pedals in the loop/in front.

Had some success turning my guitar controls down to 2 and 4 without much treble in the tone controls and cranking the pre-gain on both channels. This is a good way to avoid the harsh treble problems.

Hope you find what you're looking for, friends.

ekkybedmond · 11-02-2009, 05:47 AM

Quote:

Originally Posted by V-man

I'm happy to hear about some (more) effects loop suggestions, but with the new non-master SuperBass, an attenuator looks like it might be on the short list for me for the benefit of both amps. I will add that I tried the Sonic Maximizer 382 and the Sonic Stomp n my 4100 and the former really impressed me. I took it back to consider whether a 482/882 is in order or if I should look at attenuators.

Hi there, JCM 900/50W combo user here:

1) Using the Sonic Stomp in front of amp surely helps the get more body/bass to the sound.

2) Instead of getting a volume pedal in loop in order to get the volume down, why not use JJ 6V6's in power section. This gets the power tubes into break up much earlier and exploits their potential better.

Just my 2 cts

danmahoney · Yesterday, 03:09 AM

Hi Guys,
I`m actually waiting for the parts right now to mod my 1990 JCM 900
(4101 combo) to run on 6L6s (Tung Sol 6L6 STR`s).
I know most people go in the other direction but I really love the clean sound I get out of the 900 series with the right 6L6s in there.
I`ve got the spec here that covers what Marshall recommends in order to do the mod.
I`m planning on only changing R31-34 from 2K2 to 470 ohms, and R28 from 220k to 47k. Marshall recommends a lot more.
My question is will I potentially be damaging my amp by doing the minimum I`m planning?
I could change the other resistor values while I`ve got the board out but I don`t have the right cap values to start changing those, and I`d prefer not to start messing around with the leads on the power tranny (unless it really should be done in order to keep the amp running safely).
Here`s a link with detail on the mod from Marshall:

http://www.cowboymafia.net/frankie/5...conversion.pdf

Any suggestions and advice would be much appreciated. Thanks.
Dan

danmahoney · Yesterday, 04:25 AM

Quote:

Originally Posted by danmahoney

Hi Guys,
I`m actually waiting for the parts right now to mod my 1990 JCM 900
(4101 combo) to run on 6L6s (Tung Sol 6L6 STR`s).
I know most people go in the other direction but I really love the clean sound I get out of the 900 series with the right 6L6s in there.
I`ve got the spec here that covers what Marshall recommends in order to do the mod.

...forgot to mention that I already did try to get a couple of different quads of 6L6`s in the amp to bias within spec but they were all running too hot (25 watts and more)...
I would have loved to not have to make any changes at all and I`m definitely into 'less is more', as long as the amp is healthy...

10-29-2009, 11:46 AM	#31 (permalink)
LesBalls Member Join Date: Dec 2008 Location: USA Posts: 63	Re: The JCM 900 and modification... Thoughts? Using a volume pedal in the effects loop is very different from using an attenuator. Putting a volume pedal in the loop is basically like adding another master volume. Using an attenuator between the power amp and the speakers allows you to drive the power section harder but at lower volumes. A volume pedal does not allow you to do this. Even if you turn the master on "10" when you bring the volume down with the volume pedal, you are driving the power tubes less. Now, you may be driving them harder than you would with just your master volume control and no volume pedal. But, this is different than driving them hard with an attenuator. *** Also, keep in mind that an effects loop is between the preamp section and the power amp section. An Attenuator goes between the power section and the speaker cabinet.

10-29-2009, 12:00 PM	#32 (permalink)
LesPaulopolis Senior Member Join Date: Jan 2009 Location: Minneapolis Posts: 250	Re: The JCM 900 and modification... Thoughts? I agree 100% with the prev. 2 posts. Using a volume pedal is NOT the same as an attenuator. However, as TPR stated, and I agree, it sounds "better" at lower volumes. Also, you can avoid messing around with the touchy gain to volume dance. On a 100w amp, I would describe low volume pedal + cranked amp as "less muddy" than just using the amp at the same volume. Will this trick "completely" change your tone? No. However, nuances like this DO make a difference to the discerning ear. Maybe it's placebo, maybe not. Just my $.02 __________________ '68 RI LP Custom JCM900 + EQ & Boost

10-29-2009, 01:02 PM	#33 (permalink)
donkost Member Join Date: Apr 2009 Posts: 64	Re: The JCM 900 and modification... Thoughts? I'm liking this volume pedal in the loop idea more each time I read about it, for several reasons. If it gives a slightly less muddy tone at lower volumes that's great. The other thing I like about the idea is that it's an easily adjustable solo boost without altering the gain structure. Using an EQ or MXR micro amp in the loop also works as a preset volume boost, but controlling the overall volume with your foot on a volume pedal has to be better that trying to adjust a little knob or slider on a stomp box. I'm wondering if a volume pedal in the loop affects other items in the loop such as a delay. __________________ 2009 Marshall JCM2000 DSL100 w/MC412A cab 1988 Mesa/Boogie Mark III 60W combo w/EV12L 1988 Gibson Les Paul Custom 1998 Peavey Wolfgang Special 2008 Gretsch G5120 2008 Ibanez RG2550Z 2009 Epiphone Zakk Wylde L.P. (bullseye) Jonesing for a Strat ever since reading "Blackmore tone" thread Pedaltrain PT-Pro w/numerous stomp boxes

10-29-2009, 03:12 PM	#35 (permalink)
LesPaulopolis Senior Member Join Date: Jan 2009 Location: Minneapolis Posts: 250	Re: The JCM 900 and modification... Thoughts? Using a vol. pedal as a solo boost is a tricky thing live. You can purchase ones that have a 'set' lowest volume and highest volume (instead of 'all the way off' to 'all the way up'). Mine does not have this feature, so after rolling up for a solo, it's very hard to find the spot you used to be at for rhythm. A 'boost' pedal like a SD-1 or and EQ is a better solo volume booster at a gig IMO. __________________ '68 RI LP Custom JCM900 + EQ & Boost

10-29-2009, 05:39 PM	#37 (permalink)
LesPaulopolis Senior Member Join Date: Jan 2009 Location: Minneapolis Posts: 250	Re: The JCM 900 and modification... Thoughts? Right on, bro. Although they're not too glamourous, volume pedals rock!!! __________________ '68 RI LP Custom JCM900 + EQ & Boost

10-29-2009, 06:14 PM	#38 (permalink)
V-man Senior Member Join Date: Jan 2009 Posts: 189	Re: The JCM 900 and modification... Thoughts? Unfortunately, I tried the volume pedal twice and the experiment was a miserable failure for me. I cranked the gain and master volume, setting the pedal in the "closed" position. Then as I roll forward it goes from no sound right into a "booming" bass-buzzy, rattling clean, then a hair more and I have that great crunch, but I'm at 75% of the volume of the amp going full bore. Another inch and it's full bore. Maybe there is a better pedal out there (this is an old Morley Wah/vol) but I have a feeling that even a better pedal will yield similar results. __________________ Gibson: '88' Explorer 90, Flying Vs - '02' V-Factor, '06' RI, '07' RI Jackson: '92' Rhoads Tribute, '92' Rhoads PCS, '02' KV2 '92' ESP/Hill King V '04' Epiphone Goth V '81' Marshall 1992 '92' Marshall 4100 '97' Crate Blue Voodoo BV-60 Combo

10-29-2009, 06:26 PM	#39 (permalink)
LesPaulopolis Senior Member Join Date: Jan 2009 Location: Minneapolis Posts: 250	Re: The JCM 900 and modification... Thoughts? Ch. B: Gain-16, Vol, 6 pedal about 45% down is how I run it Ch. A: Gain-7, vol, 5 Too bad you had poor results, V-Man! I'm using an Ernie Ball VP Jr. __________________ '68 RI LP Custom JCM900 + EQ & Boost

10-29-2009, 06:38 PM	#40 (permalink)
V-man Senior Member Join Date: Jan 2009 Posts: 189	Re: The JCM 900 and modification... Thoughts? Yeah, my gain is about where your is, master was a little louder, but I don't see rolling it back a couple of notches to be the solution. I tried it again because I just got a 1992, which has no master volume and would LOVE a solution that didn't involve a few hundred on an attenuator. __________________ Gibson: '88' Explorer 90, Flying Vs - '02' V-Factor, '06' RI, '07' RI Jackson: '92' Rhoads Tribute, '92' Rhoads PCS, '02' KV2 '92' ESP/Hill King V '04' Epiphone Goth V '81' Marshall 1992 '92' Marshall 4100 '97' Crate Blue Voodoo BV-60 Combo

10-30-2009, 12:29 AM	#41 (permalink)
TPR Senior Member Join Date: Sep 2009 Posts: 109	Re: The JCM 900 and modification... Thoughts? V-Man, a couple of questions. Has the 4100 been checked out for good tubes, proper bias, etc? Either of those (and maybe other things, like old caps) might explain some of the trouble you're having getting your sound. Is the 1992 strictly for gigging? I don't know those models, but I assume you don't really start getting the Marshall magic out of it until you have it cranked, so that rules out low to mid volume just for practice. And no loop, right? If that's right, an attenuator is the only thing to help with volume on that. And at gigs, you may find the attenuator gives you a bit more leeway to find the sweet spot at the right volume. I've seen THDs on ebay for $175-225. Also, the Weber ones are less than that. Mine sucked and I had a big hassle sending it back, but a lot of people seem to like them. __________________ The awesome tone coming out of my amp almost makes up for my inability to play. '91 JCM 900 Head w/ EL34s '68 Bassman Head- Blackfaced Hand Built 2x12 Classic 30 Gibson LP Studio w/ PAFs Epi LP Standard w/ G 57s The usual assortment of pedals and stuff

10-30-2009, 07:27 AM	#43 (permalink)
TPR Senior Member Join Date: Sep 2009 Posts: 109	Re: The JCM 900 and modification... Thoughts? Yup, it was the SL I was wondering about the loop for. I figured it didn't have one. Sadly, I think an attenuator is in your future. If you can get by without it for a bit, you can watch the deals and probably snag one for a good price. Just make sure the ohms match your setup. Oh, you know about the loop adjustment control on the 4100, right? Thanks for the info on the sonic max. I was thinking about getting one for my 4500, so now I'll definitely pick one up since it can't hurt to try, right? __________________ The awesome tone coming out of my amp almost makes up for my inability to play. '91 JCM 900 Head w/ EL34s '68 Bassman Head- Blackfaced Hand Built 2x12 Classic 30 Gibson LP Studio w/ PAFs Epi LP Standard w/ G 57s The usual assortment of pedals and stuff

10-30-2009, 08:59 AM	#44 (permalink)
donkost Member Join Date: Apr 2009 Posts: 64	Re: The JCM 900 and modification... Thoughts? I'm just looking for an excuse to pick up a volume pedal, and I'd probably go with the Morley as I also have a Morley Bad Horsie II wah. I used to have one of the big box AC-powered Morley Wah/Volume pedals back in the 80's and I wish I would have kept it. Of course I wouldn't have been able to use the wah in front of the amp and its volume pedal capability in the loop at the same time. __________________ 2009 Marshall JCM2000 DSL100 w/MC412A cab 1988 Mesa/Boogie Mark III 60W combo w/EV12L 1988 Gibson Les Paul Custom 1998 Peavey Wolfgang Special 2008 Gretsch G5120 2008 Ibanez RG2550Z 2009 Epiphone Zakk Wylde L.P. (bullseye) Jonesing for a Strat ever since reading "Blackmore tone" thread Pedaltrain PT-Pro w/numerous stomp boxes

10-30-2009, 03:31 PM	#45 (permalink)
hmstrat Member Join Date: Dec 2007 Location: Texas Posts: 65	Re: The JCM 900 and modification... Thoughts? In an interview in "Vintage Guitar" Jim Marshall revealed that doesn't think much of modding his amps... Q: A few years ago, when the EL34 power tube became scarce, how did that major changeover to using the 5881 affect the Marshall amp's signature tone? A: We had to do that, but we weren't happy doing it. The amp's sound was a little cleaner with the 5881s than with the EL34s. That was better for the clean channel, but no so good for the distortion channel. When musicians were able to find the EL34s, they were putting them in themselves, but many forgot that they had to change the bias of the amps as well. So many people were destroying their Marshalls by not changing the bias and also by trying to hotrod them, because if we could have gotten more out of the amplifier we would've done it. So the hotrodding thing was not very clever. It either ruined transformers or meant replacing tubes. Here's the rest of the interview: Vintage Guitar® magazine : Features : __________________ JCM 900 MKIII (2500) ... JTM 622 ... DSL 401 ... DSL 100 LP Standard, Strat, Tele, Tele, Erlewine Automatic, HM Strat

Yesterday, 03:09 AM	#49 (permalink)
danmahoney Junior Member Join Date: Nov 2009 Location: Montreal, Canada Posts: 6	Re: The JCM 900 and modification... Thoughts? Hi Guys, I`m actually waiting for the parts right now to mod my 1990 JCM 900 (4101 combo) to run on 6L6s (Tung Sol 6L6 STR`s). I know most people go in the other direction but I really love the clean sound I get out of the 900 series with the right 6L6s in there. I`ve got the spec here that covers what Marshall recommends in order to do the mod. I`m planning on only changing R31-34 from 2K2 to 470 ohms, and R28 from 220k to 47k. Marshall recommends a lot more. My question is will I potentially be damaging my amp by doing the minimum I`m planning? I could change the other resistor values while I`ve got the board out but I don`t have the right cap values to start changing those, and I`d prefer not to start messing around with the leads on the power tranny (unless it really should be done in order to keep the amp running safely). Here`s a link with detail on the mod from Marshall: http://www.cowboymafia.net/frankie/5...conversion.pdf Any suggestions and advice would be much appreciated. Thanks. Dan __________________ JCM 900 (4502) (Tung Sol 6L6 STRs) (1999) JCM 900 (4101) (1990) JTM 30 (Vintage Tung Sol 5881s) (1995) JTM series (1x12") ext cab (1995) 1912 ext cab (1997) Last edited by danmahoney; Yesterday at 10:10 AM.

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