Amps

[kaoslord]

for more bass, 12ax7LPS or EHX in V1 is the way to go.

A 5751 in V1 will also give you a better clean sound

[Soulcrusher_X]

Also, try one of the TungSol reissues. It's like they took the EHX and made it better. Same clarity, gain and grind, but made it fatter and a little cleaner. If you want more headroom out of a 12AX7, that's the one.

The JJ and Chinese 12AX7s have a more gradual breakup, so you can get some dirt if you dig in a bit more at lower gain settings. The TS and EHX don't break up until you slam the strings a little harder.

If you want something that's REALLY fat, try the Mullard RI. It's round and warm like the JJ, but with an extended bottom end.

[Soulcrusher_X]

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kaoslord wrote:

Up ! Anyone knows a simple mod to reduce gain and to bring clarity to the OD channel ???

I think that the gain is not usable past noon,

there is too much low end "ompf" and compression.

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Which amp do you have? The 22, 50 or 100?

[Soulcrusher_X]

Well, one trick for the 50/100H that will help a little bit for clarity AND the notorious FX loop overloading is to put a 750-1k ohm resistor in parallel to R25, or just swap out R25 for something around 860R-1K. Then bypass R21 with a 1-4.7uf cap depending on how much low end you want. Personally, I think anything bigger than 4.7uf adds a bit too much "woof" to the low end. It will bring the loop level down to between 0dbv and -4dbv, AND it will add a bit a clarity. It does make the amp a tad stiffer, though.

[kaoslord]

I have the 100H, still with stock (rebiased) power tubes

the normal channel is OK, I just want a bit more clarity and less "woooffff" in the lead channel

[kaoslord]

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Soulcrusher_X wrote:

Well, one trick for the 50/100H that will help a little bit for clarity AND the notorious FX loop overloading is to put a 750-1k ohm resistor in parallel to R25, or just swap out R25 for something around 860R-1K. Then bypass R21 with a 1-4.7uf cap depending on how much low end you want. Personally, I think anything bigger than 4.7uf adds a bit too much "woof" to the low end. It will bring the loop level down to between 0dbv and -4dbv, AND it will add a bit a clarity. It does make the amp a tad stiffer, though.

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thank you, this is an interesting mod.

somebody also suggested me to snip the C19 cap to lower the gain on V3. any thoughts ?

[Soulcrusher_X]

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kaoslord wrote:

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Soulcrusher_X wrote:

Well, one trick for the 50/100H that will help a little bit for clarity AND the notorious FX loop overloading is to put a 750-1k ohm resistor in parallel to R25, or just swap out R25 for something around 860R-1K. Then bypass R21 with a 1-4.7uf cap depending on how much low end you want. Personally, I think anything bigger than 4.7uf adds a bit too much "woof" to the low end. It will bring the loop level down to between 0dbv and -4dbv, AND it will add a bit a clarity. It does make the amp a tad stiffer, though.

---

thank you, this is an interesting mod.

somebody also suggested me to snip the C19 cap to lower the gain on V3. any thoughts ?

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While this is does reduce the gain by a little bit, it will also loosen up the low end. Before you clip anything, there is one thing you can try. It's part of a mod that I did on one of my SLO clones. Put a 15k resistor in parallel to R32. This should bring the total resistance to around 10.5k. Then put a 1m resistor in parallel to R36. Total resistance should be around 500k. The thrid stage is a "cold clipper", which is designed to compress and distort easily. It also can make the low end a little on the "loose" side. The two added resistors should help that stage from clipping too hard, and keep the low end from getting too messy. Hopefully, this helps a bit. If it's a step in the right direction, then we can try some other things to get you where you want to be.

If you want to do some serious changes, you could always change the lead circuit to the actual SLO lead channel, which will tighten things up considerably and retain most of the core tone.

[käsebrot]

@ Motocooney:

Could you please tell me, what exactly you've done to get the Predator Mod on the 22H?

I also have a 22H, but have no idea, how to transfer the numbers and values from that mod for the 50H to the 22H schematic as they are both different.

That would help a lot!

[LProoster]

Hi guys,

Been a reader of this fine forum for quite some time.

I just wanted to say thanks to the regular contributors on here, like grimespace and edge, im sure there are quite a few others that have helped me.

After many years of playing small ss practice combos i finally acquired a JCA50H late last year. I built a angled front 2x12 homemade cab for it to sit on.

When I got the amp I knew there was going to need to be some mods done. Because the sound was definitely not what I thought it was gonna be. It was pretty weak and sterile until you turned it up to about 5.

Swapped every tube out of the thing, put a JJ 5751 in v1 to lower the gain, and JJ's in the rest. Pulled the stock power tubes, put in Sovtek 6l6WXT+'s and set the bias, it was a little cold at 20ma

Also did the depth mod...holy crap. That knob should be on there from the factory.

Like several others I did not care for the blue on the front so I used my leftover grillcloth from the cab build and some piping and redid the head. Thinking about getting the alternate JCA logo to put on the front, the 333 ain't goin back on there.

Thought I'd post some pics, I'm very happy with the BA rock machine that I have now.

Thanks again for all the help!

[LProoster]

new to this still, think i got the pics right this time

[Soulcrusher_X]

Hey! I like the Grillecloth covering there. Very classy looking.

[LProoster]

thanks! thats what I was going for, it didn't turn out to bad for my first cab build

[Soulcrusher_X]

What kind of speakers in the cab?

[LProoster]

Weber sig12b, the ceramic ones, 50w and $50 a piece,
I think they're prob the weakest link in my chain, will
Upgrade when I can afford to

[Motocooney]

Sure no prob.....

Predator Mod for JCA22h

C2-.047uf
C7-220pf
C5-.0022uf
C6-1n
C8-3.3uf
R5-470k
R10-680k
R9-500k
R18-220k

I lifted a leg of C5 afterwards as I found that the OD channel was brighter than the normal channel

Preamp tubes are ruby high gain V1&V2, tungsol reissue V3, old Marshall 12ax7 C4, Millard reissue V5
JJ el84 power tubes biased to 21ma ish.

[käsebrot]

Hi there,

thank you very much!

For what I know, the C2 cap is between the 1. and 2. gainstage. Normally it's 22nF - and that is really high, 47nF seems a lot too high, will produce mud in base. You will boost very low frequencies, which are out of the speakers range - and also very high frequencies, which will lead to more fizz. I would rather go with 4,7 to 10nF here.

C7 at 220pF is a good idea, I already have done this. The bigger JCAs have a stock 270pF/1M (C7/R10 at 22H) combination. The change of C10 to 680k and/or the C7 at 470pF (as stock at 22H) would be the "crunch mod". C7 only affects the crunch channel as far as I know - but the crunch and overdrive channel responses a little bit to each other. You will hear it when leaving the gain in overdrive channel at a certain postion and turning up and down crunch gain - while playing in the overdrive channel.

C5 should be a mid/treble filter and C6 is the bright cap for overdrive gain. They both only affect the overdrive channel. I have already tried out several values for both, but think, the stock 1nF (C5) and 470pF (C6) fits best. You can also try 470pF at C5 - that would be more Marshall style.

I can't say anything, how the changes of C8, R5, R9 and R18, which you have mentioned, would work. Haven't tried this yet.

But I could recommend to try a 39k slope resistor (R26) instead of the stock 47k. That helps the amp to get a little bit brighter (like Orange and Diezel - Marshall would be 33k) and get a bit more bite.

You also could change the treble cap (C14) to a higher value (stock is 470p), f.e. 500 or 560 to get the upper mids more aggressive.

If the amp sounds too loose, a change of the NFB resistor may help. Stock is 47k at 8 Ohms, try 51k or 56k and you get a more aggressiv and tighter rhythm sound. But therefore, the leads wouldn't have that soldano sweetness.

If you have too much fizz overall, try to change the fizz cap (C20). Stock is 270p. Higher values give you less fizz. I actually use 220p, but I also lowered the C2.

I am not an amp tech - but have talked about this amp a lot with one. So I know a few things. Maybe some of these, I have posted, will help.

Best regards!

[käsebrot]

Should be: "The Change of R10 to 680k" not "C10"

[Motocooney]

The values I listed are the same as edge's predator mod.
The 47nf in c2 does seem high. I used a .0047uf (4.7nf) there.

Also, the change in the nfb resistor you mentioned is kind of covered by the depth mod.

[käsebrot]

Oh ok!

You wrote "C2-.047uf" < that would be 47nF not 4,7nF. ; )