Gain Boost on JTM

[Mattfig]

Can anyone tell me how to do this? I thought I knew but have clearly proven I don't by constant mistakes...

I want this to be a HUGE boost too...Or a three way boost? Any ideas?

 

[JKMV12]

short answer: the plate load resistor, bias point, and grid stopper resistor will affect the gain. now you also have to consider the capacitors in conjunction with the resistors b/c they affect the gain at certain frequencies.

long answer: someone more experienced will have to chime in. i don't know what the JTM circuit looks like so i can't be too specific. there are a lot of factors to consider. changing one resistor may give you more gain but it may also change the frequency response. for example: if you increase the load resistor value...you increase the possible output voltage swing (more gain) but you will also have to change the bias point b/c now clipping will occur on one part of the output swing more than the other (not necessarily a bad thing...just depends on how you want to bias the stage). now if you decide to change the bias...that means changing the cathode bias resistor. but if you have a cathode bypass capacitor and/or a plate bypass capacitor, you may have to change those too b/c they work in conjunction with the resistors to affect the frequency response of the stage. Depending on how the stages are designed...it can be a simple mod...or it can be a complicated mod. I am not an expert but from what i do know there are a lot of factors to consider. Someone else who has more experience with the actual circuit can probably help you more.

 

[Graham Pearson]

The PCB for my black faceplate JCM is part # E110901 REV-A. The other modules I have with the same PCB are the Tweed, Top Boost, Plexi, SL+, 1086, and Ultra.

If you check out my module spreadsheet, you can see the differences in resistor and capacitor values for each of these modules. Perhaps this can be a starting point for you boosting the gain of your JTM to that of an SL+ for instance.

Hope this non-scientific approach helps.

 

[SacredGroove]

Can anyone tell me how to do this? I thought I knew but have clearly proven I don't by constant mistakes...

I want this to be a HUGE boost too...Or a three way boost? Any ideas?

Is it a newer or older module? Since the pcb layouts have changed, it might be easier to either post a email a gutshot. :wink:

 

[Tbag]

the most noobiest rookie mod is to SPDT in and out R7 from 100k or whatever it is to nothing (no part)

You can also switch in and out C13 with a .68uf 50v electrolytic capacitor. So make sure the polarity is right or kaboom!

You can also do both at the same time for maximum effect which may sound great depending on your rig. But play with values. You may like 2.2uf 50v also.

The R7 change will add gain and barely change your tone, and with the C13 mod you will get a nice mid girth hump reminiscent of a PLEXI, but also a gain boost, for the good! (earlier JCM800's did not have this and is a typical mod done on the real amp, among other things)

Atleast here you shouldn't have a huge volume jump.


Goodluck dude! (and dudettes!)

 

[RADE01]

I was going to say the 'removal of R7 resistor' mod too. That will greatly increase the gain.

 

[Mattfig]

Thanks guys...And Graham, don't know how I missed that final spreadsheet but I just downloaded it and it's awesome...Thank you very much for doing that!

 

[jitterybit]

Think Bruce would/could sue?
Dozens of companies sell custom modules for the API 500 series. Tons of DIY as well. API even setup the VPR alliance to ensure current draw compliance.
You could patent the tech to an extent I guess...

 

[ak37]

I use a SPDT on a PCB mounted pot. I remove the jumper on the R12 and come out from the out (the farthest hole from the module front) to the center lug on the switch. Then on the front switch post add a jumper wire (no resistor). On the aft lug of the switch add a the resistor. I used a 470ohm. Tie them both together and connect them back to the return on the board. It is sort of an illusion of gain boost because with a resistor in line in the non boosted position, you will turn the gain up higher to get the same result as without a resistor. When you pull out on the switch it removes the resistor out of the path and you get a boost in gain. I also flip the switch over on the housing to have the switch posts on the top. I can send you some pics. I have converted my Tweed into a JCM monster though! I still have the switch installed though so it is still relevant. It has become one of my favorite modules. I am thinking of getting another Tweed though just to have this mod. I have recently aquired a sickly Top Boost that I got fixed that I use for my clean. I just love that Tweed tone.

 

[Kapo_Polenton]

the most noobiest rookie mod is to SPDT in and out R7 from 100k or whatever it is to nothing (no part)

I've been flirting with the idea of installing an in out switch for my R7 on my modded Brahma. I'm going to assume that this is done with an on/off SPDT switch and one lead of the resistor in R7 and the other to one side of the switch (by wire) to the other spot on R7? Is there any cleaner way of doing this or is this the common "boost" trick? Seems like it would be the best option but not sure i feel like drilling into my nice faceplate. An internal trim pot would be cool to dial in the amount desired.