#1
I am going to be building a jcm 800 and was wondering how difficult it would be to add an extra pre-amp tube. I am assuming I would have to adjust my transformer specs and obviously wire it up a little different.

All in all is it more trouble than its worth, especially if its a first build?
Gibson SG Standard
Ibanez S2170FB
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Marshall 1960A
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#2
yes its more trouble than its worth instead use that tube to make a valvecaster
#3
Its possible to add another tube, and it won't be all that difficult. But I do want to ask what you hope to achieve from this?

If you want a higher gain tone you could always change the bias points of the preamp stages, and add in a bypass capacitor to the second and third preamp stages.
#4
This is UG, of course MOAR GAIN in the objective, lol. To be honest, I do not know much about building an amp although I think putting it together by a schematic should be pretty easy considering how simple the circuit is., and im pretty handy with that sort of thing.

I have read about some mods that would help it out, but I find it hard to believe that adding a few more caps or changing a few values would change it as much as adding another tube (but what do i know).

edit: any other tips would be welcome too, im planning on using component kits from turretboards.com and transformers from Mercury Magnetics.
Gibson SG Standard
Ibanez S2170FB
Peavey JSX
Marshall 1960A
TEXAS A&M
Last edited by sacamano79 at Aug 8, 2010,
#5
The second gain stage has a 10k resistor for the cathode and no bypass cap. This gives you a gain of about 8 for that stage. You may want to leave this stage alone, as it is going to heavily clip one side of the waveform.

If you bypass the cathode resistor of the third stage, 820 ohm, with a 25uF capacitor you bump the gain of that stage up from 40 to 61. Another way to get a higher gain sound would be to try a 330 ohm resistor instead of the 820 ohm resistor there. That pushes the bias point up and will cause the wave form to clip even more on one side.

If you want a fatter sound then you can remove the .001uF capacitor across the volume pot. To get even more low end, I would change the 22nF capacitor in the tone stack, connected to the bass pot, with something like a .047-.1uF

I would also add screen grid resistors to the power tubes in a JCM800. Something like 100ohm 10 watts connected from the screen voltage supply to the screen-grid of the tube.

With these mods you have to be wary of oscillations though. If you add too much gain you end up with a whaling banshee, but if you don't go too crazy you should get something nice.

Like I said, I wouldn't mess with the second gain stage, as that is going to give you a specific sound, but you can always boost the gain on the third stage a bit, or change the operating point to change when clipping will occur.

Its better to add small amounts of gain through out the preamp, as you can shape the sound more like this, than to add a lot in one place. If your still thinking about adding a tube stage then you can do that, but it will likely increase the chances of getting an oscillating mess. Thats one of the reason that Bogners and many other high gain amps are built on a PCB. Its much easier to get oscillations worked out once and then know that you don't have to do that on the next amp as well.
#6
Wow dude I really didn't expect that in depth of an explanation, but thanks a ton for your time. I am comparing what you said to this article:

http://tone-lizard.com/Ultimate_JCM800.htm

A lot of the ideas are the same, so I am going to do some more research before modifying anything. I will eventually experiment with different mods and will probably not add another pre-amp tube after your explaination.

Again, thanks a lot.
Gibson SG Standard
Ibanez S2170FB
Peavey JSX
Marshall 1960A
TEXAS A&M
#7
I'm sure you can get by with 3 gain stages and get a fairly nice sounding amp. The good thing is that if you don't add a huge amount of gain, you can still get a nice clean tone by rolling the volume back.

Another thing you could do is vary the ammount of negative feedback you give the amp. The 100K resistor, R22 I believe, could be replaced with a potentiometer wired as a variable resistor. 250K might be a good choice. I would test the amp without this first though, and see if you like it. If not you can always add this in. Negative feedback reduces gain somewhat, and makes the circuit operate more linearly. This produces a better frequency response though. Vox amps have no negative feedback, which is why they go from clean to crunch without much middle ground.

Its no problem as far as answering questions go. I only do it when I have the time, so when I do answer I want to give a very concise answer. I also find that people are more inspired to figure out more on their own when you start them off with really good information.
#8
One more noob question, if you are going to look at this thread again:

When you say you "bump the gain up from 40 to 61", this is in db correct? If it were V/V that would be a ton.

And how do you know these figures? Looked into 800's a lot or somethin?
Gibson SG Standard
Ibanez S2170FB
Peavey JSX
Marshall 1960A
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#9
It would actually be a lot more gain if it were in dB. Its voltage gain that I gave you. Its really only about a 3 db increase. Which is what a bypass capacitor should do. The value of this capacitor determines what frequencies get this 3dB boost, so you could selectively boost certain frequencies, but when I think higher gain I also think tight low end.

I've done a few mods to JCM800s for friends, and I've got a few programs that assist in quickly finding operating points for standard gain stages. The load line plotter from the valve wizard website helps quite a bit.

Some of the figures come from previous experiences with other amps, while some come from my thoughts at the time of looking at the amp schematic. As far as the screen grid resistors go, those are pretty standard. Some people like to use even higher values, like 1K for screen grid. I think Hiwatts may have used 5k.