ValveKing "JCM" mode using Volume Boost switch

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RickBlacker
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by RickBlacker » Tue Aug 28, 2012 11:51 pm

Got any before and after sound samples?

chrisliua
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by chrisliua » Wed Aug 29, 2012 9:33 am

Since the switch still allows you to have both the original gain structure and the "JCM mode" one, the're no real downside in giving this a try... if you don't like it, just keep the switch set to original mode, and then if you ever need to drop the gain mode, you have that option.

Combining this with the clean boost mod gives you 6 different modes... Not bad for a budget amp!

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Re: ValveKing "JCM" mode using Volume Boost switch

Post by RickBlacker » Wed Aug 29, 2012 2:53 pm

Nice, I may have to try this on my VK100 head. I've done most all other mods, the original JCM mod was the only one I never tried.

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skynyrd
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by skynyrd » Sun Sep 09, 2012 5:19 pm

I did this and the diode mod on my vk112 and I noticed from the pic`s of your board mine differes some and I think i might have 1 of the 2nd gen VK`s. I went ahead and did the mod and when I kick in the vol boost the volume drops BADLY,,I even swapped up the resistor that was removed and put in a 750K and it still had a big variance in volume. BUT the plus side is with the lead boost off it nails the Kossoff sounds from free with the gain dimed, kick in the lead boost and its right between a hot rodded Plexi and a JCM. I am very happy with it as is,,its very open and crunchy almost zero compression in the OD. I may do some more looking around see what could be different between 1st gen boards and 2nd gen and see if I can complete the mod to its full potential. Thanks for posting this it was a great help, I picked up the VK112 strictly as a practice amp so i`m not lugging my main amp back and forth, I wouldn`t hesitate to gig with the VK now.

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65 SG
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by 65 SG » Sun Sep 09, 2012 8:38 pm

I am glad to hear a good review of the mod, albeit with some compromise. BTW there is no schematic difference between the older and newer chassis, other than a shielded cable (on the later models), AFAIK. Its normal in "JCM Mode" for the volume to drop off quite a lot - one entire "VK mode" gain stage is bypassed and there's really no easy way to get that signal level back.

That's why I added the "Phase 2" mod, to drop the volume of the "VK Mode" also to equalize the two volume levels somewhat which, as you have noted, has another very nice benefit (it sounds better in "VK Mode" too). It is a compromise but, after awhile, one gets accustomed to the lack of volume. Think of it as the "angel's share" of the volume - worth the sacrifice in exchange for...sonic salvation :wink:

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skynyrd
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by skynyrd » Mon Sep 10, 2012 4:16 am

I love the VK sound now I could change R116 I think and bump up the OD a little but will leave it as is for now. I noticed while doing the mod comparing my board to yours pictured I`m missing JP 104 on mine and figured it must have been the different board mentioned on the other site. But it is a great mod none the less many thanks.

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65 SG
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by 65 SG » Tue Sep 11, 2012 7:11 am

Instead of changing the value of R116, just bypass it with a small electrolytic, that will do a lot more to bump the lead gain range in VK mode (but won't affect it when in JCM mode). I'd also go after R105 and/or R108 - decreasing R105 / increasing R108 would get more crunch into both lead modes and voice the amp more for classic rock and blues. Hmmm...

Changing R102 to a 150k or 180k will raise the gain of the lead channel (both modes) a tad and get some of the "lost" volume back, although that will tend to add back "fizz" and noise. I'm going to try this myself soon and see how it sounds. The tweaking never ends!

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Re: ValveKing "JCM" mode using Volume Boost switch

Post by chrisliua » Tue Sep 11, 2012 1:05 pm

Ideas on value changes for R105 and R108

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skynyrd
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by skynyrd » Tue Sep 11, 2012 4:24 pm

Chris I`m thinking about taking 105 down from 220K to 110K and bumping 108 up from 220K to 440K and see what happens.
Six 5 I see 1 booboo I did,,I didn`t put the 750K resistor across the 2 legs of the switch,,I may try a 1 meg since its the closest I have.

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65 SG
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by 65 SG » Tue Sep 11, 2012 5:24 pm

Those suggested values sound like a pretty good starting point. Note that bumping R108 up in value will also tend to brighten the lead channel, slightly.

A 1M across the switch (Phase 2) will get you some volume equalization but won't do as much to tame the VK distortion. Depending on what you're after, that might be just fine.

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skynyrd
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by skynyrd » Wed Sep 12, 2012 4:07 am

Six 5 I think for what ever reason my VK side of the mod already got tamed down, there must be some sort of difference in our 2 boards. I compared the pic of your board to mine and where you soldered the res/wire to JP61 you have an extra jumper between it and the little yellow cap and mine has and extra cap where yours has a jumper, everything else matches up. The vk side now with the lead boost off is total plexi, then kick the lead boost in and its like a mild JCM. When I hit the JCM switch there is a major drop in volume and I have to crank the post to about 8 to get any kind of volume, and the tone is very thin and bright. I removed the A/B pot so the volume is at full speed, did the D101/D102 mod, and the VK sound is sweet,may try the crunch mod after I pick up some various resistors to find the right match, I will just continue to work with the VK side of the equasion as its teets now.

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65 SG
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by 65 SG » Wed Sep 12, 2012 8:42 am

Guess I better take a closer look at the two schematics!. Arghh...but - Thanks

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skynyrd
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by skynyrd » Wed Sep 12, 2012 1:52 pm

Well I did the R105 and R108 mod,,MOST EXCELLENT ! ! ! I dropped R105 from 220K to 100K and bumped up R108 from 220K to 320K,,I had thought about doubling R108 but figured to drop R105 more than half and just work on medium increases in 108 and the first try was enough for me, not too much crunch but a really good crunch that isn`t too bright or harsh. I did the R100 mod so in case I kick in the lead boost it would`nt treble itself to death and it came out great,,many thanks to Six5 for all the mod info and help,,this amp now has a tone all its own thats pretty impressive.

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Re: ValveKing "JCM" mode using Volume Boost switch

Post by CharlieP » Wed Sep 12, 2012 6:42 pm

Success is great to hear!
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Re: ValveKing "JCM" mode using Volume Boost switch

Post by sebwhite » Tue Nov 20, 2012 7:16 am

so i did this mod, and just as other replies have stated - my board in my vk100 head looks to be the same as far as component layout but most of the numbers are different on the silkscreened board, and the jcm mode for my vk was successful as far as less gain, however, for whatever reason between fast picking on lower notes and soloing it kinda cuts out like a stompbox with a battery that is just starting to die.

any thoughts of what i messed up?

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