Updates on the Classic 30 mods?

[BobW]

Steve, et al, I applied the tone cap changes (orange drops) about 18 months ago, and liked the changes. It was a great improvement, but was wondering if anything new has been discussed to further improve the tone? Am not completely happy with the lead channel, it's still a little too gritty at the top end. Has anyone tried any *umble type mods? increasing the Rp, adjusting the coupling caps, etc? I may try Ray Ivers suggestion using zeners to clamp the large negative grid swing.

[Ray Ivers]

Bob,

If you're thinking of trying the Zener thing on the preamp tubes (and hopefully reporting back with some intel) that would be outstanding - I only tried it on the power stage, but it should also work on the preamp tubes.

Ray

[BobW]

Ray,

I plan to give it a try hopefully later in the week after the usual fires on Monday and Tuesday mornings. It would be best to look at the nominal signal gain w/scope to determine a ballpark Vz in the preamp section. My bassman *umble is a good test candidate since it's easily accessible.

[Joe N]

Steve, et al, I applied the tone cap changes (orange drops) about 18 months ago, and liked the changes. It was a great improvement, but was wondering if anything new has been discussed to further improve the tone? Am not completely happy with the lead channel, it's still a little too gritty at the top end. Has anyone tried any *umble type mods? increasing the Rp, adjusting the coupling caps, etc? I may try Ray Ivers suggestion using zeners to clamp the large negative grid swing.

you can replace c4 (470pf) with a higher value like .002, .0047, etc... and that will allow more lows through, but I wouldn't go to high or it will be really loose sounding. you can do the same with c4, but can go higher to .022uf if you want. you can clip c1, which is an ice picky JCM800 like trble peaking cap, or replace it with a higher value like .002, which will sculpt your lower mids. lowering the 150k RP's to 100k will help too. thats a start

[BobW]

Joe, I appreciate your comments, but gather you have not performed any particular component value changes in this amp? The reason why I'm asking is because this amp (Classic 30) is used mostly as my alternate amp in smaller clubs. I can try a few changes by trial and error, but am mostly interested in proven changes someone has already tried. This amp is not the easiest to tweak and reassemble w/o chassis buzzes, etc. I have already tried a few gain stage changes, added snubbers to tame the icepick highs, and may just leave it at that. thanks for the suggestions.

[Joe N]

Joe, I appreciate your comments, but gather you have not performed any particular component value changes in this amp? The reason why I'm asking is because this amp (Classic 30) is used mostly as my alternate amp in smaller clubs. I can try a few changes by trial and error, but am mostly interested in proven changes someone has already tried. This amp is not the easiest to tweak and reassemble w/o chassis buzzes, etc. I have already tried a few gain stage changes, added snubbers to tame the icepick highs, and may just leave it at that. thanks for the suggestions.

not particulary the classic models, but have done extensive mods on 5150's which are assembled the same, and a heap load of other high gainers, low gainers, and everything in between etc for years.... So I do have alot of working experience under my belt, and I know the math behind what does what and most complaints of fizzy, ear piercing gain are a result of a few key things, not always, but mostly. sometimes the OT, will greatly improve things, sometimes it wont. The truth of the matter is somebody can tell you that x mod gives you y result, but you really don't know unless you sniff solder. in your case you may just need high bandwidth restriction. snubbers work, but I would use grid stoppers instead if at all possible. not only can you roll off the highend, but you have the added bonus of preventing blocking distortion. Point is Sniff some solder, find out for yourself.
Joe N

[Joe N]

another good tip if you are tweaking a pcb amp, but don't want to take it apart is to cut out the components so that there is still a bit of the lead still sticking out of the solder pads, and then solder the new components to those from the top of the board. that way you can tweak, then take it apart when you finalize the changes. just another idea

[Ray Ivers]

BobW,

I have modded several Classic 30's, but none recently... if you want to greatly reduce raspiness on the overdrive channel, that shouldn't be too difficult to accomplish, but I agree that it would be best to have a concrete idea of at least what frequency response you want before pulling the chassis. If you had a good EQ to throw in the loop for testing purposes, that would wonderful IMO - you can determine the exact HF break point that sounds best to you, then do the math, open 'er up, and start changing out components to the values indicated by your calculations.

FWIW, reducing the value of R13, increasing C16, and adding a small-value cap across R3 are what I would be looking at, just at a glance; these mods would reduce overdrive-channel 'fizz' without affecting the other channel. If C16 and the new cap across R3 are sized to work together, the reduction in grit can be quite dramatic, while leaving the desirable treble frequencies mostly intact.

Does this amp really have a 470pF cap (C7) to ground after the VR2 gain control? If you need all your pickups to sound like neck pickups, this is definitely the way to go...

Ray

[Enzo]

And don't overlook something simple like putting a 5751 in V2 in place of the 12AX7.

[BobW]

Ray, Much appreciated for the suggestions thanks will give them a try. I had previously lowered R13 to 8.2k, which gives about a 1.3kHz rolloff. Are you talking about R3 or VR3? I had tried a snubber (.0012uf cer and a 250k pot) at V2-6/R3 node to ground to dial out the fizz, but it took too much off the top end as well. I could also try a small value cap across VR3 to act as a LPF.

Thanks Enzo, I don't have any 5751s at the moment, but may try a 12AY7 when I get back from TDY. thanks

JoeN, I thanked you for your input and also happen to have a LOT of amp repair/design experience. 15 yrs. as a military tech + 16 yrs as an electrical engineer, with most of my spare time over the years as a repair tech for two local music stores qualifies as a 'lot'. The initial post wasn't meant to be a pissing contest. I was simply asking was for proven suggestions to help the tone. As I said, this is a secondary amp that I don't want to spend a lot of time on. I have other priority amp work.

"Point is Sniff some solder, find out for yourself."
This comment was unecessary and not appreciated.

Something to consider, next time when someone asks for suggestions on a proven tweak or mod it won't always mean that person doesn't know what end of the soldering iron to hold.
Enough said. peace

[Ray Ivers]

BobW,

I guess I really should have said "jumper R13"; IMO a shelving response is not what's needed here, but rather a full-on roll-off (hey, that's kind of a neat expression) increasing up the frequency spectrum.

I meant putting a very small-value cap - maybe 220pF or so, much smaller than your 1200pF - across R3, in conjunction with another cap across VR3 (C16) achieving the identical rolloff characteristics as the across-R3 cap - that's what I meant about "doing the math", the values would have to be worked out in advance to achieve the desired 12dB/octave rolloff. IME this is nearly impossible to do by ear; you may end up with something you like the sound of, but it's unlikely to be a true 12dB/octave filter, which I feel is the best starting place for tweaking (rather than starting from a complete unknown and hoping for the best, especially on a difficult-to-tweak design you hope to get right the first time). IME one cap won't do the job - using a big-enough value to reduce the fizz also removes treble, as you found out.

Ray

[Joe N]

Ray, Much appreciated for the suggestions thanks will give them a try. I had previously lowered R13 to 8.2k, which gives about a 1.3kHz rolloff. Are you talking about R3 or VR3? I had tried a snubber (.0012uf cer and a 250k pot) at V2-6/R3 node to ground to dial out the fizz, but it took too much off the top end as well. I could also try a small value cap across VR3 to act as a LPF.

Thanks Enzo, I don't have any 5751s at the moment, but may try a 12AY7 when I get back from TDY. thanks

JoeN, I thanked you for your input and also happen to have a LOT of amp repair/design experience. 15 yrs. as a military tech + 16 yrs as an electrical engineer, with most of my spare time over the years as a repair tech for two local music stores qualifies as a 'lot'. The initial post wasn't meant to be a pissing contest. I was simply asking was for proven suggestions to help the tone. As I said, this is a secondary amp that I don't want to spend a lot of time on. I have other priority amp work.

"Point is Sniff some solder, find out for yourself."
This comment was unecessary and not appreciated.

Something to consider, next time when someone asks for suggestions on a proven tweak or mod it won't always mean that person doesn't know what end of the soldering iron to hold.
Enough said. peace

The sniff solder comment wasn't meant to be undermining, and I don't know how it could have been mistaken for such. maybe it wasn't worded clearly, but my point was that you can figure out on paper what freq. you want to kill, and where you want to kill it, but you wont know if you like the change until you actually do it. sniff some solder to me means, the only way to really know is to do it. I cant remember how many times I have asked Trace Allen for specific modding help, and while sometimes he gave specifics, sniff some solder is the best advice anyone has ever given me. I didn't assume anything about you, I was actually trying to be very helpful. I'm sorry if you mistook my comment for something that was mean spirited or degrading, It surely wasn't meant to be

[daz]

This is a favorite subject of mine. I've had 4 of them and i'll tell you what i've done and the results. I feel like a broken record because i've mentioned this at various forums a million times, but the way my C30 sounds i just want to let anyone who might be thinking about it know. I did a number of steves mods, or i should say mods posted at his site. Not sure which are and aren't his. But in any case, there are 3 mods that i found work and work great. the rest i changed back to stock because they either didn't sound good or weren't my taste. I like a vintage sound without any low end flab and a master with a bit of gain. Not a lot, but enough to play any classic rock and blues.

The first mod i'll mention because i felt it was a huge improvment was to cathode bias the amp. It's actually a very simple mod. there's a gif schematic on steves site that shows the changes, but enzo recommends one difference that i found to be better......a 100 ohm cathode resistor instead of the recommended 60 ohm. This mod really opens the amps tone up and makes it transparent and much more pure and organic like a simple tube amp. the next was the C4 swap as mentioned. The mods recommend a 680 or 820 PF cap, and i found the 620 worked much better. It helps the lead channel a lot and drops the gain down a bit. the processed nasally sound is gone. The last was the input where i simplified it to a classic fender or marshall input. theres loads of garbage there that i think caused the amp to not take pedals very well.

The result is nite and day. every complaint i've ever heard about this amps tone is gone, and i'm not exaggerating. the tone is full and real/organic and so much better than stock it's not funny. The only thing that keeps it from being squarley in the high end boutique relm is the harmonic content. It's actually very good, but not at the level i've heard in some of the boutique stuff. But i did compare it to a number of boutiqe amps one day at the local high end shop and i honestly preferred the C30. It wasn't as good in some areas, but the overall tone is now so much better that it's competitive with many generally desirable amps that cost much more. I also put in a presence and a pot in place of the boost to allow it to be variable. (very cool mod !)

Anyways, these mods are the ones that survived my taste test. to give you an idea of the tone i like, i love marshalls from the 800's on down and my preference for tone and the way to get it is the typical SRV "volume control as tone/gain control" ........amp in OD mode and the volume to clean up. In this context it's just beautiful.

[Enzo]

If you look at the input of the stock C30, the series cap C8 is a huge .1, that should be fairly transparent to any freqs we might care about, but it blocks DC. I sure would not want to guarantee anyone that they will not have pedals with some DC offset at the output.

The shunt cap C10 is a small 39pf. Its purpose is to prevent RF from entering the amp - the old my amp picks up radio stations condition. 39pf oight not the roll off highs you care about much, but if so, then replace it with a 22 or an 18. I like the protection. I can test this stuff too, since there is an AM transmitter a half mile away. This is a high RF environment here.

[daz]

If you look at the input of the stock C30, the series cap C8 is a huge .1, that should be fairly transparent to any freqs we might care about, but it blocks DC. I sure would not want to guarantee anyone that they will not have pedals with some DC offset at the output.

The shunt cap C10 is a small 39pf. Its purpose is to prevent RF from entering the amp - the old my amp picks up radio stations condition. 39pf oight not the roll off highs you care about much, but if so, then replace it with a 22 or an 18. I like the protection. I can test this stuff too, since there is an AM transmitter a half mile away. This is a high RF environment here.

Hmmm......i'm pretty sure i removed that, tho it's been a long while. But if i recall i just have a 1 meg ground reference and a series resistor. But in any case i've never had any problems with pedals or radio interference. In fact, as stock it didn't take pedals well at all but does now. Not sure why because i trust what you say Enzo, but i'll leave it for now. If i ever have a problem with radio interference tho at least i'll know the answer.