swapping preamp tubes on jcm800

[khan]

So, my 800 has an extra preamp tube (got it that way) and gets dirty really fast. I've been toying around with the idea of trying different preamp tubes to see if I can stay cleaner a little bit longer, even though I'm in the High Gain input. Is this the right way to go about this. I'm not expecting to be totally clean but, unless I just barely bring up the preamp volume to where I just start to get signal, it seems to get really dirty really fast. And maybe this is the inherent reaction of this amp with the 12ax7 tubes. Nonetheless, If I were to use a 12at7 or a 5751, would I notice a difference in the amount of gain earlier on in the preamp volume control. Or, am I totally daft at my approach.

I do know about the preamp tubes thread here but, after reading through it a bit, I hadn't really gotten a reply to this question. If I should just keep reading through it, I will but, I was hoping for a short-cut to an answer.

Help! Thanks!!!

 

[SG~GUY]

I am assuming "dirty" means gain/distortion??? ((

(cuz at first I was thinking,...good god!!!,...poor dudes got OCD!! why the hell does he care if he gets a lil dirt on his tubes!! ha!! )))

what do you have for a preamp tube in V1 right now?

every marshall I have ever had a JJECC83S did wonders in V1..,..but yes,...a 5751 in V1 will tame the beast,...marty will be able to point ya in the right direction

 

[khan]

I am assuming "dirty" means gain/distortion??? ((

(cuz at first I was thinking,...good god!!!,...poor dudes got OCD!! why the hell does he care if he gets a lil dirt on his tubes!! ha!! )))

what do you have for a preamp tube in V1 right now?

every marshall I have ever had a JJECC83S did wonders in V1..,..but yes,...a 5751 in V1 will tame the beast,...marty will be able to point ya in the right direction

Lol! I figured my derelict use of terminology would get me in trouble. Yes, by "dirt" I meant gain. Thanks for gently letting me off on that
They're all 12AX7's currently. And I'm sure that extra preamp tube adds to that.
Thanks for the info!

 

[damienbeale]

For what it's worth, I don't think it will make enough of an appreciable difference if you have an extra stage. You'll probably find when you get down to a level you like, the tone has thinned out considerably. It's time to either reverse the mod, or, tone down the gain created in the first three stages a little from each stage. Tech time.

 

[khan]

damienbeale, Are you saying go through the preamp stages and swap out lower "gain" tubes in each of the stages until I get where I want to be? I was thinking of asking my amp guy if it was possible to make the added preamp tube switchable...ie, place a switch in the stage path that would bypass the extra preamp tube. I don't know if it's possible to do but, it may be an option to make that extra gain stage an on-call option.

 

[damienbeale]

damienbeale, Are you saying go through the preamp stages and swap out lower "gain" tubes in each of the stages until I get where I want to be? I was thinking of asking my amp guy if it was possible to make the added preamp tube switchable...ie, place a switch in the stage path that would bypass the extra preamp tube. I don't know if it's possible to do but, it may be an option to make that extra gain stage an on-call option.

No, I'm saying the tubes won't make enough of a difference at all. You're likely going to need to change the circuit.

 

[Purgasound]

Buy a couple 12AT7's. Start with the extra tube and then try just V1. If either of those positions don't clean it up try an AT7 in both spots. See if that at least gets you in the right direction. If not, then the amp needs to be gained down or personally I would add a switch that would remove the extra gain stage from the circuit.

 

[khan]

Thanks, American Viking! I will give the tubes a shot first. I'm ordering them now, in fact. I'll report back on the results.

 

[Bieling3]

. . . personally I would add a switch that would remove the extra gain stage from the circuit.

FOOTSWITCH

 

[rockgod212]

I would take it to a tech and see about modding the 4th stage cathode or the voltage divider going to the stage to tame it. tweak it to your liking.

case in point my homebrew slash #36 has 4 stages, but the voltage dividers are tweaked to keep the gain down. hell I hardly ever use the 4th stage, as I tend to keep it at a stock 800 type gain with various cathode mods to get more gain.

 

[Bloodrock]

My Langner jcm800 has 5 gain stages and shitloads of gain. It was useless for anything but metal until I installed a 12at7 in v1 and a 5751 in v2. Keep in mind v1 is very sensitive especially on gain modded amp. I had to go with nos tubes because all the current production I tried squealed and fedback horribly.

 

[khan]

Thanks, Bloodrock (as in the band from the 60's/70's?) I'm trying the tubes first, since it will be the quickest and maybe cheaper solution, if it works. But the idea of being able to modulate or switch it on/off sounds really attractive as well. Maybe down the road if the tube switch works for now.
Cheers!

 

[khan]

Okay. Here's some new news. I have a box of tubes I pulled from an old tube television and tube radio. Found a 12AU7, a 12AT7 and 6L6 in there among a bunch of other tubes. I put in the 12AU7 in V1, it's a sixties if not older Phillips Tung-Sol. Wow! what a remarkable difference that made. It gave me plenty of room to play with the preamp volume. Whereas before, as soon as I got the volume loud enough to to hear, it was already breaking up. Now, I've got room to play cleaner and a lot more control with the guitar volume as well. Everything can get a lot louder before really starting to break up. Also gives my pedals room to breathe and be a lot more dynamic.
So, I can second those statements that said a tube swap would make a difference. It has and I'm really happy. I may still play around and swap in the 12AT7 to see how that sounds but, at this point, I'm really happy with the results. And, it didn't cost me a thing as I already had the tubes to test with.
Thanks everybody for your input and advice. I really appreciate the knowledge you guys freely share with this forum. A really great resource for all of us.