JCM 900 mod - 6l6/5881's for EL34's

[Whitey-on-the-moon]

Hi, I have the JCM 900 dual reverb 100 watt combo (4102) which I bought to use instead of a JCM 800 2203 cos the thing was too dam loud.

I especially like the clean channel on the JCM 900 which can be set to sparkling. It's like a cross between a Marshall and a Fender. However the 900 has a lot less beef and power compared to the 800.

Basically I want to fatten the sound more on the 900 and although I read on one forum that there is a loss of clarity on the clean channel I'm wondering if the 616/5881's for EL34's mod will give me what I looking for?

Can anyone out there who's done this mod advise me if it will help and just how much it affects the clean channel?

[django]

as far as i know 6l6's have a better clean than el34s and bottom end, maybe you could try changing some of the coupling caps for more bottom end or increasing the size of the feedback caps

[daz]

just a few thoughts on my 900's and 800's for you to consider. Not trying to make any particular point, but these observations may help you. i owned 3 800's and 2 900 dual reverbs. One of the 900's was thin sounding to a fault. Couldn't find a thing wrong with it. The other one was totally opposite...much fuller and fatter sounding in a band mix than any of my 800's. Both 900's used EL34 and i used the same exact tubes in both. The 800's sounded awesome by themselves, but in a band mix the good 900 ate them all up as far as sheer tone and thinkness/fullness. the 80's got thin and weak in the mix, especially after a set of warm up.

IMO if your 900 doesn't sound thick and full in a band mix you either have some tubes in there that are very detrimental to that amp's tone, or like one of mine you got a bad one. I would suggest truing EL34's, tho with the OT being for 6L6's you may have to use a dufferent OT tap. I'm sure someone else who knows can jump in. Tho if it were me i'd likely get a hammond JCM800 style OT. Have one in my home built JCM style amp and it's a better sounding amp than any of my marshalls were. that way at least you'll know the primary will be correct for EL34's. then you'll just have to probably change the bias set resistor so the pot has a usable range for that tube.

[rockon1]

Generally speaking 6L6's can swap into an EL34 based amp with just the proper bias adjustment. A 6L6 draws less heater current so it wont overtax the tranny. A 6V6 even less. I use 6L6's and JJ only6V6's in my EL34 based Rivera Chubster. I even got lucky in that the bias pot has enough range to accomodate the swap without modification. Going the other way (6L6 to EL34) is usually more tricky . Bob

[Lee W]

I have a the 100 watt head version, and it is super thick. I am running 6L6's. Two tricks, no mods needed: boost the first channel. Use an EQ pedal, or perhaps a nice booster...but you'll have all the open low end of the clean channel, with a lot more grit. The 2nd key, is to use a nice EQ unit (preferably rack with a lot of control) in the loop for your distortion channel. This will really give you a lot more bass that you probably feel is missing when using this more compressed channel.

[Whitey-on-the-moon]

Thanks everyone who's replied. I don't think theres any faults with the amp to worry about. The first thing I did when I bought it was to have it serviced and the valves checked, all seemed to be ok. I think I should consider exploring the effects route before I resort to modifications so as a clean boost I'm probably going to go with the MXR Micro Amp.

As for the gain channel, I really like this too, aslong as I've got the gain around 10 o' clock it provides a really nice crunch. Still looking for a nice distortion pedal to achieve a lead sound I like.

One other question for anyone who knows - The JCM 900 seems to recieve alot of criticism for it's diode clipping. As I say, I personally prefere the sound of the gain channel when set to low. The question(s) is - are the diodes in use no matter what level the gain is set at? Or do they only kick in at a certain threshold?

If I'm using the gain set at 10 o' clock is my distortion at that point purely valve distortion or is the diode clipping circuit in use?

Thanks

Dave

[django]

the diodes will always be adding to the clipping so theres not much you can do to avoid it, if you like the tone then don't worry about it it's still valve clipping if you crank the amp a bit! The pedal route sounds like a good idea, a good clean boost like the mxr preamp like you said would be a good idea.

[Rafphiano]

JCM 900 Dual Reverb

Hi, I have the JCM 900 dual reverb 100 watt combo (4102) which I bought to use instead of a JCM 800 2203 cos the thing was too dam loud.

I especially like the clean channel on the JCM 900 which can be set to sparkling. It's like a cross between a Marshall and a Fender. However the 900 has a lot less beef and power compared to the 800.

Basically I want to fatten the sound more on the 900 and although I read on one forum that there is a loss of clarity on the clean channel I'm wondering if the 616/5881's for EL34's mod will give me what I looking for?

Can anyone out there who's done this mod advise me if it will help and just how much it affects the clean channel?

I bought a JCM 900 Dual Reverb and it had Solvtek 6l6's in it.
No matter what setting, distortion, overdrive, ect.. I used it still sounded like
a Fender. So I bought some EL 34's Grove Tubes Medium Gain and 12ax7 Grove Tubes
installed them with no modification. Sent the amp to a repair shop for biasing.
Now the JCM900 sounds like a MARSHALL. The clean channel is still clean and the Lead channel rips,sings,hard crunch , ect. The change may have not been the correct way, but it Sounds Great No problems.

[jrfrond]

I was working on a JCM900 a few days ago. After I repaired the odd issue (the High/Low mode switch arced across the PCB on the two terminals connected to the OT primary, so I added spike diodes to kill HV transients), I went to rebias the amp and found that the EL34's were running really cold, as noted by the excessive crossover notch on the scope. For some reason, this apparently unmodifed bias supply only swung as low as -44 volts, and was -52 max. In my book, that tells me that the bias supply was set up for 6L6's at some point. I put in a pair of new Tung-Sol 5881's. The amp actually sounded MUCH better than with EL34's, and I am guessing that the tubes will last longer as well. This was for a rehearsal studio amp that gets plain abused, played mostly on 11! :O

[g1]

For a while JCM900's were fitted with Sovtek 5881's. I believe it was due to poor EL34 supply at the time causing warranty issues. There are some circuit changes (bias and screen circuits) but the transformers etc. were the same. There are schematics available for both versions if you search.
As John mentioned, you could have a version set up for different tubes than are in it.
Also seem to recall being unable to get rid of crossover notch in versions with 2k2 screen resistors.