How to improving JCM900 tone? Can it be done?

[satch]

I have a JCM900 4100 Dual Reverb that needs new power tubes.
The amp is equipped with 5881 tubes (the stock ones) and I would like to read about your suggestions for new tubes.
Although this is 100w version, I will probably just replace 2 of the 4 tubes, making it a 50w version through the low mode switch.
I think this amp lacks some bottom end, so a tube that can give me more bottom end would be welcome.
What tube would you recommend? 6L6, EL34 or KT66?


Another thing; I have a 1960B cabinet loaded with Celestion G12T-75. Replacing the speakers to a different brand/model would give me better tone?
Can I put the cabinet switch to stereo mode, use only one input to make it a 2x12 cabinet? I mostly play at home and don't need the 300w.


Sorry for the long post.

[Swarty]

Try 6550/KT88s. They will bias very similarly to a 6L6/5881. The PT is up to the extra filament current as it was designed for EL34s. I've done this and it is a no brainer way to tighten things up.

[satch]

Try 6550/KT88s. They will bias very similarly to a 6L6/5881. The PT is up to the extra filament current as it was designed for EL34s. I've done this and it is a no brainer way to tighten things up.
Does it only need bias? No need to change any electronic components?

[John Phillips]

I would strongly recommend converting it to EL34s. 6L6/5881s are fine tubes, but not in this amp. The voicing was designed around EL34s, and they just don't sound right otherwise (IMO). It's a little more complicated than just adjusting the bias trimmer though - you need to change a resistor in the bias supply, and the screen resistors.

BTW, the 'Low' mode on this amp works by running the tubes as triodes, not by switching off two of them - so if you only fit two tubes, you will then get 25W in Low, not 50W. You will also need to set the impedance selector to half that of the cabinet, whether in Low or High mode.

[satch]

I would strongly recommend converting it to EL34s. 6L6/5881s are fine tubes, but not in this amp. The voicing was designed around EL34s, and they just don't sound right otherwise (IMO). It's a little more complicated than just adjusting the bias trimmer though - you need to change a resistor in the bias supply, and the screen resistors.
Will the bottom end be decreased and the medium incresead if the amp is fitted with EL34?

BTW, the 'Low' mode on this amp works by running the tubes as triodes, not by switching off two of them - so if you only fit two tubes, you will then get 25W in Low, not 50W. You will also need to set the impedance selector to half that of the cabinet, whether in Low or High mode.
So, what's the better way to run it at 50w? Removing 2 tubes and leave on high mode, or using 4 tubes on low mode?
Regarding the impedance, are you saying it should be set to 8 ohm on the amp in low mode? In this case, I have to set the cabinet to stereo mode , which use 2 speakers per input and works on 8 ohm mode.

[John Phillips]

Will the bottom end be decreased and the medium incresead if the amp is fitted with EL34?Yes, but in a way that makes the sound fatter. You lose some of the extreme bottom end, but the increase in thickness in the mids more than makes up for it, and you get a much less boomy and hollow sound, with far more punch and 'Marshall' character.

So, what's the better way to run it at 50w? Removing 2 tubes and leave on high mode, or using 4 tubes on low mode?
Regarding the impedance, are you saying it should be set to 8 ohm on the amp in low mode? In this case, I have to set the cabinet to stereo mode , which use 2 speakers per input and works on 8 ohm mode.
IMO running two tubes is better. Triode mode changes the tone more (makes it softer and muddier), and is harder on the tubes. You can try the difference easily and see which sounds best to you - just pull one of the 500mA HT fuses to turn off either pair of power tubes (the 'fail' light will come on, but that's OK).

If running two tubes, set the amp to half the impedance of the cab. So if you have a 16-ohm cab, set the amp to 8 ohms. Don't switch the impedance on the cab or use half the speakers.

If using triode mode, leave the impedance set normally.

There's also nothing to stop you deliberately running at 25W with two tubes in triode, in which case you still want to reset the impedance to half.

[Balok]

I dont care for the sound of my Jcm 900 at all ...muddy, or if you turn up the treble, harsh..prety awful, and that is the overwhelming consensus here on TGP

now, thats with Celestion 12gt-75s

with Jensen c12ns its the best sounding amp I own..and that's a long and expensive list...a grossly underated amp IMO

[JJman]

I had the same head. I tried the triode mode switch and found that it merely reduced low end (not good.) I switched to EL34's by grounding the socket terminal (#1 or #6?) that needs to be grounded when el34s are used, and increased the screen resistors to 1K. I found that the tone didn't change in any way I could notice. I tried using only 2 tubes and noticed no change since I didn’t play at extremely high volumes anyway. I got the schematic and felt like I was looking at a schematic for a computer. I decided that I needed to buy a Fender. Got a '71 SFDR. Sold the JCM on EBay. I've never missed it.

[Swarty]

As I mentioned, 6550/KT88s will tighten the bottom in these amps. EL34s are a tube that bends/distorts very easily when pushed, which makes them perfect for non-master amps... but to take a high gain, sloppy/slushy preamp and run that into a soft/sloppy output section makes for a real mushy sound. A high gain preamp into a tight/linear power amp is the way to go IMO.

[LesPaulMan]

Swarty has been preaching the conversion of EL34's to 6550/KT88's in master volume Marshall's for quite a while. I totally agree. The lack of bottom end in the newer Marshall amps (JCM 900's and the JCM 2000's - DSL/TSL come to mind) really kill the overall sound of the amps. These amps typically have under-sized output transformers, and a resistor instead of a choke. They don't help in the sound deptartment, but certainly help Marshall's bottom line. :(

I have tried JJ KT-88's in an old JCM 900 SL-X. The sound instantly improved. Fuller with better articulation, and a nice solid bottom end.
Also tried 6550's in a DSL 50, and found similar results. (Though it was a nightmare changing a resistor to get the bias right. Can you say ribbon cable).

[bluesaxe]

As I mentioned, 6550/KT88s will tighten the bottom in these amps. EL34s are a tube that bends/distorts very easily when pushed, which makes them perfect for non-master amps... but to take a high gain, sloppy/slushy preamp and run that into a soft/sloppy output section makes for a real mushy sound. A high gain preamp into a tight/linear power amp is the way to go IMO.

What bias current are you setting the KT88's to?

[satch]

Swarty has been preaching the conversion of EL34's to 6550/KT88's in master volume Marshall's for quite a while. I totally agree. The lack of bottom end in the newer Marshall amps (JCM 900's and the JCM 2000's - DSL/TSL come to mind) really kill the overall sound of the amps. These amps typically have under-sized output transformers, and a resistor instead of a choke. They don't help in the sound deptartment, but certainly help Marshall's bottom line. :(

I have tried JJ KT-88's in an old JCM 900 SL-X. The sound instantly improved. Fuller with better articulation, and a nice solid bottom end.
Also tried 6550's in a DSL 50, and found similar results. (Though it was a nightmare changing a resistor to get the bias right. Can you say ribbon cable).
What do you mean with under-sized transformers and a resistor instead of a choke?

Looks like the KT88 are a good option, but if there's need for components replacement, I have to find someone in Lisbon to do the job.

Do you have some link with the changes that have to be made to convert it to KT88?


I was thinking about other thing I could do to the cabinet. Mix two of the actual speakers with other two differente speakers, like, for instance, two V30's. Is this a good idea?

Is there any problem if I set the cabinet to 8 ohm (when only two speakers are used) and set also the amp to 8 ohm?

[Swarty]

With modern KT88/6550s, 40mA is generally good/safe starting point (NOS can take a lot more)...

On the high gain modern Marshalls (900s, TSL, DSL, etc) they went to a much smaller OT than the JCM800 and earlier amps.

A giant resistor replaces the choke. I don't see this as a huge negative, just an indicator of the bean counters influence (resistors are much cheaper).

KT88s only need a bias adjustment, which may or may not require a resistor swap to get into the range... If replacing 6L6/5881, the bias requirement should be very similar and I would not expect a resistor swap to be necessary... EL34s would not be as close and would be more likely to require a resistor swap. However, if you can source some GOOD EI KT90s (I am aware of some bad batches of these), they will bias very similar to EL34s and should not necessitate a resistor swap.

Swapping speakers is not going to fix the amp issues... but it's always fun/enlightening to try... But when using different speakers in the same cab, which one do you mic when playing out?

[John Phillips]

A giant resistor replaces the choke. I don't see this as a huge negative, just an indicator of the bean counters influence (resistors are much cheaper).
It's not a 'giant' resistor, it's a rather small one - given its circuit location - 100-ohm 4W... and badly mounted. One of the more common faults on these amps is the resistor overheating and desoldering itself from the board.

I agree, using a resistor isn't necessarily a bad idea, but if you're going to, you need to spec and mount it properly...

[Swarty]

Just reread my post and changed JJ KT90s to EI KT90s, don't believe there is such a tube as a JJ KT90.

[Rich M]

KT90 were made by EI. I've heard the new ones suck.

If you want your amp to have some real balls,
do these mods to it:
http://web.aoct.org/marshall/jcm900-gain-mod.txt
I had a 4100 that was a thin sounding POS and this amp made it far better. I would do that before you change any tubes as this will only cost you ~$10 in parts. If you don't know how to work on amps, get a qualified tech to do it as there are lethal voltages in the amp, even when unplugged and it is pretty easy to ruin traces on those cheap marshall PCBs.

[yZe]

Alot of cats have the Post Phase Inverter Master Volume goin (PPIMV)

Sposda get it in the sweet spot at lower volumes

[satch]

KT90 were made by EI. I've heard the new ones suck.

If you want your amp to have some real balls,
do these mods to it:
http://web.aoct.org/marshall/jcm900-gain-mod.txt
I had a 4100 that was a thin sounding POS and this amp made it far better. I would do that before you change any tubes as this will only cost you ~$10 in parts. If you don't know how to work on amps, get a qualified tech to do it as there are lethal voltages in the amp, even when unplugged and it is pretty easy to ruin traces on those cheap marshall PCBs.
I can ask someone to perform these mods, but I really have to change the tubes. This amp is from '94 and still has the stock power tubes, except one that was replaced during the warranty period.
I know I should have replaced the power tubes some time ago, but since 2000 I only play with the amp at home.
Now, that I'm getting a new band, I have to try to get the tone right.