help with classic 30 mod please

[12AX7]

Just got my second classic 30, this one a newer model with the boost being footswitchable. However, they didn't change to sound of the boost which as pretty much everyone agrees is extremely middy and loud. I want nothing more then a little volume boost and as little tonal change as possible, none if thats possible. Just a volume boost like maybe the amount you'd get turning the master up a 1/2 number. I can't really wrap my head around it so i'm asking if anyone can tell me which components to change and the values. I have an idea but it's way too hard to get the board out to made a bad choice and have to do t all again. This on has the ew single board insttead of that folded junk in the older ones. But it's still a PITA to remove it. Heres the schematic of the boost. Thanks for any help....

[passaloutre]

The "Boost" is just bypassing the tonestack. The tonestack has a lossy mid scoop, so bypassing it will naturally sound like more midrange.

This is the topology of the boost switch, so there isn't really a way to change that behavior with just changing a few component values. You'd have to change the circuit entirely to use that optocoupler somewhere else to control the volume.

I advise a clean boost or EQ pedal

[12AX7]

The "Boost" is just bypassing the tonestack. The tonestack has a lossy mid scoop, so bypassing it will naturally sound like more midrange.

This is the topology of the boost switch, so there isn't really a way to change that behavior with just changing a few component values. You'd have to change the circuit entirely to use that optocoupler somewhere else to control the volume.

I advise a clean boost or EQ pedal

Thanks, but i can't use a pedal in the front because i don't want more drive. I need purely volume, not OD, and thts wht will happen when using the drive channel.

[sluckey]

Please post your schematic. The C30 schematics I have use relays rather than opto-isolators for switching.

[12AX7]

It' a pdf and when i try and post it it says file is too large

[kagliostro]

Convert the PDF file in JPG then host it at IMGUR and post here the link

Franco

[12AX7]

I will. But i'll post it in a new thread i need to start with a different mod question.

[12AX7]

I figured this out, i found a PDF online editor and deleted all pages but the schematic to drop the file size way down.  So here's the PDF schematic. I see the boost mod mentioned in the blue guitar instructions from Steve Aloha (RIP) and he says to change the cap (c 11) to a 750pf and that will reduce the insane mid boost and volume to a reasonable level. Any thoughts on that?

[12AX7]

The "Boost" is just bypassing the tonestack. The tonestack has a lossy mid scoop, so bypassing it will naturally sound like more midrange.

This is the topology of the boost switch, so there isn't really a way to change that behavior with just changing a few component values. You'd have to change the circuit entirely to use that optocoupler somewhere else to control the volume.

I advise a clean boost or EQ pedal

I found  lot of people who modded the boost by using a smaller cap to eliminate the huge mid boost and changing the resistor to alter the volume. Can't i do that to give the boost less mids and a lesser volume boost? theres have, or were you saying that for some reason you can't because it uses an optocoupler instead of a relay?  When i engage the boost the tone knobs still have an effect.

[sluckey]

Steve Aloha (RIP) says to change the cap (c 11) to a 750pf and that will reduce the insane mid boost and volume to a reasonable level. Any thoughts on that?

Just do it. This boost circuit does not bypass the tone stack. Then tell us your thoughts.

[12AX7]

Steve Aloha (RIP) says to change the cap (c 11) to a 750pf and that will reduce the insane mid boost and volume to a reasonable level. Any thoughts on that?

Just do it. This boost circuit does not bypass the tone stack. Then tell us your thoughts.

I didn't think so, but most here are genius in circuitry compared to me so i wasn't sure. I have it open now and am doing a few mods and i'll do the boost last and check back before i do to see f you have any thoughts on the cap value and resistor. it's so frigging loud in stock form but i think the resistor doesn't need to be changed because i think the insane volume boost is from the cap basically replacing the treble cap value with one several times bigger. So basically dropping the size to about 750pf it should give me a bit of volume without that crazy mid that sounds like plugging a graphic EQ the loop in and boosting 400hz all the way up. No exaggeration, it;s nut's and i have yet to see anyone suggest it's not. Anyways, if u have any suggestions let me know before i get to the boost today, and thanks for the reply.

[nandrewjackson]

Here is what i would try if it were mine, and if the physical connections would allow.


First, break the connection of pin 3 of LDR and top of treble pot.


Second, add a 2nd 220K in series with the other one.


Third, attach pin 3 of LDR to where the 2, 220's meet.


That will make R to ground either 220 K, or 440 K depending on state of LDR, thereby giving volume boost.

[nandrewjackson]

And, after looking at the schematic,  I'd clip out C24 1.5N, at least temporarily to hear a difference.  That's dumping lots of high end to ground.

[12AX7]

Thanks guys. I decided to try the mod steve from blue guitar suggests as i mentioned earlier mainly because it's much simpler so i figured i'd try that first. Instead of 750pf i used a .001  since its close enough. If anything that should have gave more boost than his 820pf suggestion if it even is notable at all. But now the boost has zero effect at all. he talked about trying differnt values so obviously it worked for him, so i'm stumped. The only 2 things i can think of is picking a different value closer to stock and trying a few till it's just right. But i have to do major disassembly to get to those parts so i need to be more sure. Second, he says he changed the resistor to 2.2 meg which i didn't do because i misread it. So i guess i need to go back in and see if that works. Or maybe just a different cap value because with the .001uf theres no change and with the stock .047uf it;s huge. So maybe about 1/3 of that value might be right.
Heres steve's say on the mod....
For the boost cap (C11), I recommend using a 750pf cap. With this value, the boost switch works similar
to a Shift switch on an older Mesa Boogie amp: it kicks the volume up a notch and boosts the midrange
response. For more boost and tonal shift, you can use a .001uf or .0015uf cap instead. Using smaller values
such as these, the 220k resistor (R12) to ground could be eliminated or replaced with a very high value (I
used a 2.2M resistor). With the stock boost cap, I found the gain boost and tonal shift to be too drastic to be
usable for vintage blues tones. With the values I recommend, the boost switch offers an alternative voicing
which retains much of the tone and character of the unboosted mode.
If you choose to replace the boost cap with a lower value as recommended here, you will sacrifice much
of the gain of the stock boost circuit. To recapture some of the gain lost I suggest that you replace the 100K
plate load resistor for V3A with a 120K or 150K resistor as outlined later in this article.

[12AX7]

Here is what i would try if it were mine, and if the physical connections would allow.


First, break the connection of pin 3 of LDR and top of treble pot.


Second, add a 2nd 220K in series with the other one.


Third, attach pin 3 of LDR to where the 2, 220's meet.


That will make R to ground either 220 K, or 440 K depending on state of LDR, thereby giving volume boost.

I may try this. But one question....shouldn't C11 be lifted? Actually one more.....what kind of volume boost you you think adding that second 220k will give? I don't want a big jump, just something like turning the master up say 1/2 a number. I suppose i could put a 500k trimmer there instead and adjust to taste.

[12AX7]

I made a pic of the mod you, nandrewjackson posted to make it clearer to me, as my brain is such that it would be easier to fly than walking and chewing gum . What i'm wondering is now wouldn't C11 cause some lows to be lost when in boost mode? Should it be replaced with a full range cap, whatever value that would be

[nandrewjackson]

You could try different cap values to see.

[12AX7]

You could try different cap values to see.

I have since i posted this. Tried a few and it turns out whatever i use the result is a unpleasent mid sound. I've gotten it to where the boost volume is perfect but the mids make it unusable. What i'm thinking now looking at this is whether the boost cap is affecting the amp's tone even when off. I wonder....if i do the mod u suggested can jump the boost cap ? Then we just have the full signal thru the 220k or 440k total. Because it seems to me with that cap there it's creating a imbalance of frequencies to ground thru the 220k, no?

[sluckey]

I see an easy way to get a slight boost without affecting the tone. Well, it looks easy on the schematic. I'm sure it may be more involved inside the amp.

R29 and R18 form a voltage divider at the grid of V2 pin 7. Simply switch a resistor in parallel with R29 to increase the signal level going into the grid. Experiment with different resistor values to fine tune the amount of boost you want.

[12AX7]

I see an easy way to get a slight boost without affecting the tone. Well, it looks easy on the schematic. I'm sure it may be more involved inside the amp.

R29 and R18 form a voltage divider at the grid of V2 pin 7. Simply switch a resistor in parallel with R29 to increase the signal level going into the grid. Experiment with different resistor values to fine tune the amount of boost you want.

No, not a permanent boost. I'm talking about the boost switch in the tone stack thats foot switchable. It has a cap in it and when you kick it on there a huge boost in volume and mids thats to the point of unusable to my ear and from everyone who talks about it. What PV was thinking is anyone's guess.

[passaloutre]

I think he's saying you could use the existing optocoupler to switch a resistor in parallel. Rather than using the optocoupler to switch a capacitor in the tonestack.

This is what I meant in Reply #1: "change the circuit entirely to use that optocoupler somewhere else to control the volume"

[12AX7]

I think he's saying you could use the existing optocoupler to switch a resistor in parallel. Rather than using the optocoupler to switch a capacitor in the tonestack.

This is what I meant in Reply #1: "change the circuit entirely to use that optocoupler somewhere else to control the volume"

Ahh, ok. h didn't mention that and i'm no circuitry guru by any stretch. So now i have that idea and nandrewjacksons idea. But either way heres what i just found....i eliminated the boost cap altogether to see if it affects the amp's tone even with the boost off like i suggested/asked above, and i'm actually a bit shocked by what i found. The entire character of the amp changed. The harsh mids and brightness i have seen so many complaints about is gone ! The tone controls now work  differently, IE: better, more linear. Even the gain knob works better. And at louder volumes there no desperite need to re adjust the tones for getting harsher at louder volumes. I question myself as to whether this could be confirmation bias because if it did what i think i'm hearing then how the H has no one come up with this before with the amp being almost 30 years old?! Crazy. I have to come back to it later and listen again and see if i'm still hearing what i think i am. On a scary side note, after this last test i wasn't even near band level so i dunno why, but i fear i have done ear damage because i have a constant high pitched tone happening in my right ear.

[nandrewjackson]

Yes, you could jumper c11 so it doesn't have an effect on EQ.


C10 has already decoupled the high DC voltage

[12AX7]

2 more questions.....between nandrewjackson and slucky's different ideas, which would be the better solution? I know nandrewjackson's would be less trouble and less invasive because I wouldn' thave to cut traces at the optocoupler. Second question is will both of those possible cause popping when switched, or does the soft on/off of the opto eliminate that ?

[12AX7]

Yes, you could jumper c11 so it doesn't have an effect on EQ.

I didn't jumper it, i just lifted it. The result as i said is great. I don't see a reason for that ground point so why jumper it?

[12AX7]

I see an easy way to get a slight boost without affecting the tone. Well, it looks easy on the schematic. I'm sure it may be more involved inside the amp.

R29 and R18 form a voltage divider at the grid of V2 pin 7. Simply switch a resistor in parallel with R29 to increase the signal level going into the grid. Experiment with different resistor values to fine tune the amount of boost you want.

Ok, i'm gonna try this Slucky. Assuming of course there are no logistic issues getting the opto's contacts there. It'll have to be a long run for two signal wires so a couple questions....
1)-being connected to the grid how would u run wires to the resistor from the opto and the far side of the chassis....2 conductor shielded wire? Or do you think i'm worrying about nothing? 2)- what value resistor do u think is in the ballpark to get a small volume boost? I ask because i might use a trimmer with your value prediction in the middle of it's range to eliminate a million resolders and to be able to dial it in just right, and its in a hard to get to place that would make it a bitch to get to once let alone several times

[sluckey]

Before you go to the trouble of making the mod permanent, just use a couple gator clip leads to temporarily connect the mod resistor across R29. This way you can quickly try different value resistors. It could be that you don't care for this mod. Leave your boost switch off while testing.

[12AX7]

It's under one of those those PV pots with long legs so clipping would be hard to impossible w/o the clips shorting against the pot legs. I suppose if i can thread some wire under each leg....i'll see what i can do. I'll try halfing the value with a 150k same as R29 and go from there because i haven't got a clue how small a value i'll need it to be to give a small boost. I finally nailed the amp's tone so if this works i will be in heaven because i need a volume boost and if this doesn't work i'm gonna have to use the loop and a device and PSU and more wires etc etc, just a PITA. So this would be a god send. 
EDIT: Oh man, it;s even harder to get to than i imagined ! Doing the mod will be easy under the board but testing the board must be in place. Maybe i can find termination points from R29 that are reachable.

[12AX7]

Got it. Found easy to access destinations from each side of the resistor on the schematic, one of them obviously the grid of V2 P7 and too a trimmer and attached one wire to it then touched the remaining wire and adjusted it till i got to right amount of boost and it came to almost exactly 220k. All that remains now is wiring a trimmer to the opto which i'll do UNDER the board so the wires will be away from the board and against the chassis which i think will be less likely to be noisy. I just hope it's not a logistic hell getting the traces to the opto switch side cut easily. Never know with this thing. I'll post the final result. Thnks slucky.

[sluckey]

Post a hi-rez pic of the optocoupler. I may have some ideas about connecting to it.

[12AX7]

I can't get a decent pic at the moment because it's in a dark area under the pots and even with the flash the camera i think is weighing the settings by the lighter areas.  But i'm just now starting to pull the board to do the mod so once the board is out i'll try again in a few and post it.

[12AX7]

Post a hi-rez pic of the optocoupler. I may have some ideas about connecting to it.

Ok, here we go. Don't mind the cap with one lead flying, thats the boost cap i lifted and will be remove before i zip it back up.

[12AX7]

I've already figured out how to do the connections so no need unless you have some reason i'm unaware of i'm just gonna clean up some of the things i've done then if you haven't replied i'll just go ahead with it. I don't wanna pull the opto so one side of the switch needs the trace to be cut ad the other i just have to lift R4, C11 already gone. So there are no connections to the switch on that side so i can just use either pad of the cap or resistor i removed. Post your idea anyways, it'll be a while b4 i get to the boost mod.

[sluckey]

Are the two leads on top of the optocoupler the LDR part or the LED part. If they are the LDR (switch) then just unsolder from the board and stand them up or make a loop. Make your connections to the two loops.

[12AX7]

Yes, the top leads are the switch, and i thought of that but didn't wanna do that because i wanted to put the leads on the underside of the board to keep them away from the board. the only difference is i'll have to cut a trace but not  big deal. My biggest problem right now is how to mount a trimmer under the board so if the volume is too much or little in real world use i can tweak it. a lot easier.

[12AX7]

Well sir, the deed is done and it works fine. Not crazy about the opto's slow off because it's hard to tell if you hit the switch initially, but thats a minor niggle. I used a standoff got the trimmer and obviously it's not optimal since i gotta get back in it and readjust because it;s a bit loud. But once it's set no more need to mess with it. Amp sound fantastic with that boost cap removed. One of the best if not THE best fender meets marshal sounds that reeks of vintage tone an feel. seriously....i always knew these C30s were capable of much more but i'm shocked that was obtainable with a single cap removal ! And the boost is the cherry on top....it saved me a lot of time ad trouble. Oh, and i did use the leads from the opto. It turned out it was a better option to go on top of the board for easier trimmer access and other things. I just hope it doesn't squeal with the wire runs at band volumes.   Thanks all, especially slucky for the help. 

[12AX7]

One thing i noticed is the reverb is now a lot less wet. The reverb return is right there at the grid where the boost mod is, but it's not when i use the boost that the very is weaker, it's like that even with the boost off which is odd. Any ideas? I thought maybe i used the wrong side of R5 but then it wouldn't affect it when not in boost mode. I'll have to make sure of tht tody but at this hour i can't turn the amp on.
EDIT: i forgot i didn't use R5 to physically connect, i wet right to the grid pin so tats not it. I double checked and one wire from the trimmer goes to the V2 grid pin 7 and the other to the R30 at the junction of R30 and R29 and i just checked to be sure it's not on the wrong side of R30. So this is odd. No big deal because theres enough verb on 10 but i just thought that might affect the amp tone having to keep it on 10 so i thought to ask. While on the subject, anyone know of a really great reverb pan that'll work in the C30? The stock is ok but not as good as i'd like.

[sluckey]

one wire from the trimmer goes to the V2 grid pin 7 and the other to the R30 at the junction of R30 and R29

That's not wired correctly. Make sure it's wired as per the attached pic.

[12AX7]

Ok, i see now. It IS wired as your pic, i just didn't mention the junction of R30 and R29 are going to the switch. i have a trimmer on a standoff above V2 nd the wiper is soldered to pin 7. One side of the trimmer goes to the opto. The junction or R29 and 30 go to the other leg of the switch. I was describing where the trimmer is connected,  the switch notwithstanding.

[12AX7]

I also noticed  hum like a PSU type hum when the boost is OFF which seems odd. If i mess with the wires it gets louder/softer and a screwdriver touching them gets much worse so the wires are acting like an antenna. But why when it's off rather than on? But when off it's goes away 100%.

[sluckey]

Do this test... Measure the resistance across R29 with boost switch ON. Now turn the amp power OFF and disconnect one wire from the trimer. Finally measure the resistance between the two leads of the optocoupler. Report your resistance readings and I'll explain why I asked for this test.

[12AX7]

Do this test... Measure the resistance across R29 with boost switch ON. Now turn the amp power OFF and disconnect one wire from the trimer. Finally measure the resistance between the two leads of the optocoupler. Report your resistance readings and I'll explain why I asked for this test.

One lead removed from the opto and the leads from opto are completely open with amp off,  resistance with amp on across R29 is 52k. Heres another thing i noticed that my be a clue....with the boost off the noise is there even with both channel volumes at zero.

[sluckey]

OK, I was thinking that maybe the optocoupler LDR OFF resistance might not be an open circuit, but that's not the case. As for the noise, replace the mod wires with shielded cable and ground the shield on one end only. Maybe that will help.

[12AX7]

I'll give it a try now. But shielded 2 conductor or two shielded 1 conductor?

[sluckey]

Shielded 2 conductor (STP) might look better. Either should work equally well.

[12AX7]

That did the trick, thanks. Tho i have to secure the cable near the opto because he leads are so thin and weak they'll break in no time. Hot glue dosn't work because the heat melts it. And no place to tie it down. ARG !

[sluckey]

Maybe use some silicone bathtub caulk (RTV).

[12AX7]

I didn't wanna do this figuring it would cause the noise to happen again, but i tie wrapped it to a pot leg figuring worse case i clip it off and go back to the drawing board. But it worked w/o any noise. Thanks again Slucky. 

[tubeswell]

Good luck messing around on those C30 boards!


I had an old C30 years ago that I did Steve Ahola's mods to back when Steve was still alive, and compared notes with him at the time. The best 'tone tweaks' results I found was changing the tone stack caps alongside the boost switch cap change.


IIRC, I put a huge cap in the bass cap - 470nF (instead of the 47nF in Steve's mod). This really opened up the bass frequencies quite a lot. However, because of all the other filtering crap in the pre-amp, using 470nF doesn't have an OTT impact on the bass - just a pleasant much-more-rounded bass tone - night and day. But because of those finicky PCBs, I gave up on C30s years ago. Won't touch them nowadays.