Who knows about tubes...

I have a 100-watt Marshall JCM 900 (4100 dual reverb) in need of new tubes so I'm looking at my options. It has four EL34's at the moment. I'm thinking of ways to get more lows out of it without having to mod it too much. Someone mentioned replacing the preamp tube with something else, like a 12AU7.

Any advice?

Don't know about those tubes in particular, but I can attest to different tubes giving wider bandwidth.
When I changed out my ECC83s (pre amp tubes) I not only gained better tone, but also better bass response.

Good luck with your hunt!

Hi Qvart; Tomek is right that changing out tubes will change the sound of the amp but it will not change the amp's fundamental frequency response. The amp will sound different; maybe clearer highs, better balance and so on but due only to subtle differences in tube materials, mfrg process, and age. At the end of the day, nothing much will have changed.

This is a table of amplification factors for the 12A_7 family:
12AU7, Amplification Factor = 17
12AY7, Amplification Factor = 40
12AV7, Amplification Factor = 41
12AT7, Amplification Factor = 60
12AX7, Amplification Factor = 100

As you can see, subbing in a 12AU7 will reduce the preamp's total gain and, at the same time, provide more headroom but you may not care for the 12AU7's apparent lack of edge. But if you want to do a little tube-rolling, I'd suggest you start with a 12AT7; it will change the sound of the amp but not frequency response.

If this is your amp Here, it's a complex, hybrid design using ICs, op amps, and transistors along with tubes. I'm sure there are other ways but if you want to change the frequency response of the amp, you have to change either (or both) the frequency response of an active component - like a tube - or mess with the signal characteristics.

The pics below are taken from the linked schematic. In the upper panel, that's V2B, the 'back half' of V2 - 12AX7 ahead of the Master Volume controls and the phase inverter. It has an unbypassed 1.2K cathode resistor that sets that section's bias. Adding a bypass cap; as small as .68uf or as large as 25uf will filter high frequency out of the signal. .68uf is a value commonly found in Marshall designs while 25uf is commonly found in older, vintage gear. Roughly, the .68 will eliminate some hf, the 25uf will eliminate more.



The lower half is the EQ section with several capacitors highlighted. These caps set the frequency range of the bass/mid/treble controls. As you know, messing with tone is a subtractive process; to increase bass, the controls subtract treble and vice versa. If these caps are board-mounted, it will be a PITA to get at them; particularly if it's done trial-and-error; you at your tech, he changes something, you play, you like/dislike, he changes, you play, and so on. If a change of tubes doesn't do it for you, you might want to consider the bypass cap. You can be there when the tech tugs the chassis, locates R18, and tacks a cap in place; you can play it right there and decide; more, less, or it's a big waste of time. Good luck with your amp! John

I don't really think different tubes will get you more low end - a bigger or better output transformer might, and different speakers or cabinet will for sure.

Walter Broes said:

I don't really think different tubes will get you more low end - a bigger or better output transformer might, and different speakers or cabinet will for sure.

Click to expand...

Hi W: other than the condition where a replacment brand/set happens to do a better job with lows (or highs), they won't really do anything and you're right about the speaker; any model with better bass response would do the trick. J

Walter Broes said:

I don't really think different tubes will get you more low end - a bigger or better output transformer might, and different speakers or cabinet will for sure.

Click to expand...

I guess I didn't phrase my comment very well - I don't expect to get "more" low end out of it so much as I'm looking for ideas to cut the shrillness out of the hghs. It's a lead amp (with a Peavy - headbanger!) cabinet so there's only so much to work with without extensive mods. I've read up on some of those but it's a bit much to get into right now, and I don't necessairly want to alter it drastically. Since I need new tubes and don't have any real experience or knowledge beyond the little bit of research I've just now begun I figured I'd ask you all. Good responses! I appreciate it!

There is a guy here who works on Marshalls at the shop I frequent, so I'll be chatting with him about all of this too.

Looking at some info lately I'm thinking about Tung-Sol EL34B's. Any experience with those? They don't look like tubes that would alter the sound a whole lot from what I have now but they've gotten good reviews.

Hopefully cleaning out the pots and changing the tubes will get it back in good playing order. The pots are noticeably better so on to the tubes...

Hi Q: as you wish but consider a 12AT7 in the first stage - new old stock / non-descripts can be had for short money. If you like the effect of the diminished gain, then treat yourself to something fancy; NOS/used Mullard, Brimar, or Phillips Miniwatt; they really do make a difference. Good luck with your amp! John

capnjuan said:

Hi Q: as you wish but consider a 12AT7 in the first stage - new old stock / non-descripts can be had for short money. If you like the effect of the diminished gain, then treat yourself to something fancy; NOS/used Mullard, Brimar, or Phillips Miniwatt; they really do make a difference. Good luck with your amp! John

Click to expand...

Looks like Ifound the right person!

Thanks for the advice. I wouldn't want to lose too much gain (it goes to 20!) so I might just have to try it out and see. I could go ahead and get the quad plus one to try and end up with one extra of some kind.

As John suggested, a 12AT7 or other tube with less gain might work, but it's hard finding AT7's that aren't microphonic these days, especially for the first preamp stage of a Marshall. A NOS Philips 5751 might be your best bet if you can find one - I'm currently using those to tame the crunch in my tweed Bandmaster replica - works great.

Qvart said:

... I wouldn't want to lose too much gain (it goes to 20!) so I might just have to try it out and see. I could go ahead and get the quad plus one to try and end up with one extra of some kind.

Click to expand...

Hi Q; it's worth a try; low risk but with good up-side potential. Not sure whether giving up some gain will be all that noticeable but you will notice there's more headroom. Good luck! John

Walter Broes said:

.... but it's hard finding AT7's that aren't microphonic these days, especially for the first preamp stage of a Marshall. A NOS Philips 5751 might be your best bet if you can find one ...

Click to expand...

Hi Walter; agree, finding good tubes is risky business. It isn't enough that they test good; they can test good and still be microphonic. On the web, there are two pretty good sources for NOS/used tubes: KCA and Tube World. Both usually higher than run-of-the-mill eBay tube pimps but even some of them are stand up people. The key is testing the tube as soon as it arrives; sellers are reluctant to replace/refund if months go by before hearing from the buyer. J

capnjuan said:

If this is your amp Here, it's a complex, hybrid design...

Click to expand...

I couldn't see your link earlier (I was mobile-LTG'ing) but yes, that's it: model 4100. I have a copy of those schematics but I'm not adept at deciphering them. But this is exactly the kind of new thing that gets my research-geek-fu tingling.

Again, valuable advice. Thanks!

Well, in the end, I don't want to lose too much gain - I tend to crank it up and like the crunch - but if I can cut the shrillness (and that's the best word I can think of to describe it) it's worth a shot. I'll check out the tubes you all suggested and the dealers you recommended.

BTW: I got the Marshall many many years ago when I was in a punk band. Played it through a friend's Fender 4x12 and it was great for that style. Years later and no punk band I'd rather take the edge off of the highs. Honestly, to get the lower sound that I think would be ideal I'd be better off looking for different gear. Turning the mids and highs down and cranking up the bass results in a lot of muddiness. But I do like the Marshall for it's rock power. As for the cab, it kinda fell into my lap: traded a cheap Ibanez guitar (I got for free) for it. It has two Peavy speakers (I have no idea what model) and two Celestion G12K-85's. Definitely geared towards metal.

I can't complain too much. It's good gear and even if I ended up with four more EL34's it's worth having.

This has helped clarify some things for me though so when I talk to the Marshall guy at the shop I'll have a better grasp of the concepts and options (and I'll be picking up my DCE1. Didn't get "the works" but I bet it'll be sweet after the setup!)

Ick. That is not a tube amp, at least on the preamp side. Sure, they tossed two 12AX7s in there, but that is a solid state amp. Changing tubes will make minimal differences. Try what you want, no need to be too conccerned about microphonics since the signal has already been through several stages of solid state amplification before it ever hits a tube.

Looks from the schematic like there are multiple versions, some with a 6L6 output stage and some with EL34s.Marshall had several years of making faux tube amps as marketing gimmicks...does the presence of a tube or two somewhere in the signal chain of opamps and IC's make it a real tube amp? I say heck no.

count_scrofula said:

Ick. That is not a tube amp, at least on the preamp side....

Looks from the schematic like there are multiple versions, some with a 6L6 output stage and some with EL34s.Marshall had several years of making faux tube amps as marketing gimmicks...does the presence of a tube or two somewhere in the signal chain of opamps and IC's make it a real tube amp? I say heck no.

Click to expand...

Good point. And not completely lost on me when I pulled the housing out the other day. But again, I'm no expert.

This one is the EL34 model. The tubes are noticeable. Doesn't sound like solid state amps I've played. Rock machine to go with the Guild rock machines. That's one helluva combination if you want an AC/DC kind of sound!

count_scrofula said:

Ick. That is not a tube amp, at least on the preamp side. Sure, they tossed two 12AX7s in there, but that is a solid state amp. Changing tubes will make minimal differences. Try what you want, no need to be too conccerned about microphonics since the signal has already been through several stages of solid state amplification before it ever hits a tube.

Click to expand...

Hi Count: Well ... the Marshalls call it "... all valve..." and it has these features:

Separate preamp gain controls: Channel (A) 1-10, Channel (B) 0-20
Tone network with Treble, Middle, Bass, and Presence controls
Individual master volume controls for each channel
Separate reverb controls for each channel
Manual or footswitch operation for channel selection
Effects loop with sensitivity control
Direct and recording compensated line ouputs
Impedance selection switching
Two speaker outputs
High/low power switching (pentode to triode)
Valve failure LEDs
100 watt models (model numbers 4100, 4101, 4102) feature 4 x 5881 output valves, 3 x ECC 83 preamp valves
50 watt models (model numbers 4500, 4502) feature 2 x 5881 output valves, 3 x ECC 83 preamp valves
Note: some models came factory equipped with EL34 power tubes instead of 5881s ... apparently including Q's.

The features; switching, sensitivity, compensated line out ... the rest of them are all done with ICs/Op Amps/transistors but the signal amplification is being done by the three preamp tubes and, if they weren't using ICs for reverb send/return, there'd be 4 tubes in the preamp instead of three.

Call it what you want; tube or transistor and, rather than debate whether subbing in an AX7 makes a difference, maybe we ought to wait to see what Q has to say. CJ

Qvart said:

... I wouldn't want to lose too much gain (it goes to 20!) so I might just have to try it out and see.

Click to expand...

Hi Q; I think your amp probably has gain to burn so I wouldn't get too hung up on this point. Get the tone you want and then concern yourself with with whether the guy in the back row can hear you. Walter Broes is a professional musician and he's pointed out on a number of occasions that speaker efficiency is as important to 'loudness'/perceived volume as output watts.

Also, the 5751 he suggested has an amplification factor of 70 falling between the AT7 and the AX7; eBay search string for 5751s. If you are headed in the right tone direction, but want to see what a little more gain will do, then you might consider a 5751 in the first gain stage. If at the end of all this screwing around, you get the tone you want but think you've given up some volume, you might want to consider Walter's suggestion of more efficient speakers ... they count just as much. John

capnjuan said:

Note: some models came factory equipped with EL34 power tubes instead of 5881s ... apparently including Q's.

Click to expand...

Yep, those are all the features. It would be nice if there were separate tone controls for the two channels, but I only really use the B channel anyway...

capnjuan said:

Hi Q; I think your amp probably has gain to burn so I wouldn't get too hung up on this point.

Click to expand...

...with the gain up around 16 or so. Beyond that is too much so you're probably right about have some to spare.

capnjuan said:

Get the tone you want and then concern yourself with with whether the guy in the back row can hear you. Walter Broes is a professional musician and he's pointed out on a number of occasions that speaker efficiency is as important to 'loudness'/perceived volume as output watts.

Click to expand...

For now I don't have to worry about that - there's no audience!

Depending on the sound I get out of this amp/cab combination after new tubes and whatnot I might consider some different speakers if I do end up in a project down the road. The Fender cab I used before handled the sound better than this Peavey does.

I can't complain about my gear though - got the cab for nothing, got a good deal on the amp by trading some unwanted gear, and I sold the DeArmond so I have a little extra cash for this and the setup on my DCE1. And I geek-out learning about stuff like this. You all have given me a lot of good information to work with and things to consider.

Thanks again!
Geoff.

Ha! One of my fave arguments: what is a "real" tube amp? All tube preamp? All tube poweramp? Both? what about a tube preamp and tube poweramp, but a solidstate send/return for the reverb (I had that on a bedrock and a laney). Personally, I think if there is a solidstate gain stage it's not true tube.

It may well be a decent amp...afterall, some folks love musicman amps, tube output combined with a full solid state preamp. How it sounds is the important thing, of course, and changing tubes on your marshall might make a difference, but certainly not to the extent changing the front end tube in a true all-tube amp would. The JCM 900 is supposed to be a honking loud distorted shred amp. It even uses solid state diodes to clip overdriven signals (!!!), which results in even harsher sounds. To reduce the front end gain, you could use lower gain op amps if they are plugged into sockets.

The JCM 900, like the good old musicmans, does have a tube ouput stage, but there are significant differences between output tube distortion and solid state distortion. They are both important to sound, but since we're not discussing output sections, it's a bit irrelevant here. Unless you're in one heck of a big room at extreme sound levels, you're not getting power amp tube distortion out of most amps.

As far as how marshall describes the amp, let me stick a big tooting raspberry sound in as my comment. They also call the valvestate and avt series amps "tube amps", even though the only only tube to be found was a single 12AX7 which allegedly "warms up" the sound.

Newb to the forum here, but I'll throw in my .02.

I've been playing through tube amps for a long time, and endlessly tweaking for tone. There a lots of factors at play, and it's hard to say what will give you the change you are looking for. I've done some changes to my amps that some people might find to have a subtle effect, but to my ears they've made a huge difference.

The biggest thing you can do to change the tonal characteristics is to change the speakers. I think you're using a four speaker bottom? Even changing two of the four can make a huge difference.

On tubes...
Preamp tubes, in my experience, have most effect on the richness of the midrange and on how the preamp "drive" will sound. Crispy vs. creamy, fast break-up on pick attack vs. more headroom. The "gainiest" tubes will tend to have the shrillest high end. There is a fair amount of difference even within the tube type, 12AX7s of different brands and vintages can sound very different one from another, then going down in type through the list previously posted will give even more flavors.

Power tubes... Most of my experience is with 6L6s, but I think my observations will still apply. I have found very large differences in tone with different types. The well-known RCA black plates have a beefy bottom end that nothing else comes close to. Some new production 6L6s I've tried have had tight bottom end and smooth highs, some have been all high end edge with little beef. If you search around you can probably find reviews of EL34 types that give their tonal characteristics.

Last, and perhaps most important to what you are looking for is BIAS. The bias setting of the power tubes will have a HUGE effect on the tone. Bias them cold, and the amp will be clean and bright with more headroom. Bias them hot and the amp will be thick and creamy with more compression and a rolled-off high end. I think this is what you are looking for. Your amp may just need to be biased a little hotter. The downside is reduced tube life. Your tech has to be careful not to go too far with the bias, because he can send the tube into runaway which will burn it up very quickly (minutes) and possibly take out a transformer with it.

As others have said, try out a bunch of preamp tubes. It's a fun, easy way to fine-tune your amp. Unfortunately it can become a bit of an obsession, and you'll start visiting old TV shops and garage sales and buying all kinds of old stuff just because it has some 12AX7s in it that you haven't tried yet!

Hi Jon and welcome to LTG. I agree particularly with your point about the relationship between gain and shrillness and I think, as you do, that it's most easily addressed at the preamp stage. Speakers make a difference but if it's a problem with the amp's fundamental tone, the cost to attempt a fix via preamp tubes makes more sense than thowing money at speakers. The speaker's function ought to be to make the amp's voice heard ... not necessarily fix what's wrong with it. Besides ... the cost of failure with 12A_7s is a lot less than the cost of failure with several unwanted speakers laying around ... :wink: Again, welcome! John