Tweaking a 12XXX

[Twistingcrow]

Hi.
I'm writing here as I hope to get some insight on tweaking the infamous 12XXX and I'm sure this will inspire other builders to go ahead and experiment as well. The 12XXX at hand was built by myself about a year ago. I wanted one to get that van Halen Unchained sound I saw on Youtube (great video George!). However, my tastes have changed a bit since then and I'm now leaning more toward a cleaner sound. I thought at first that with a 100 watter I'd always have all the clean headroom I'd ever want but it actually ain't so... The amp gets into heavy crunch territory quite fast and now I would really want a LOUD clean sound. I would also love to increase the MIDS so that the amp cuts more through the mix (there's another guitar player in the band and he cuts more through the mix with his Fender Hot Rod than I do with my 12XXX!).

My B+ is 452V and the amp is biased at 70% with TAD E34LB-STR tubes.

So to you tweakers: have you got any tips on some things to change inside the amp to have more mids and most of all a cleaner sound?

Here is one pic of the amp board:



I have read somewhere that changing the values of these three circled caps (or taking the .68uF cap out altogether!) would change the amp's tone. But to what extent? What will increase/decrease? For instance what will happen if I take the .68uF out? What will happen if I change it and place a 330uF instead? Same thing with the 560pF caps... And what about the 0.0022uF cap? What happens if you change it to 0.022uF?



Thanks a lot to everyone! Glad this forum is back!

PS: Now, don't tell me I should sell it and build a JTM45/100 instead...

[Tone Slinger]

Just go with Super Bass specs and you should have what you described

[Carbia]

For more mids, put a 50k mids pot instead 25k and try a .68 uF presence cap.

For less gain remove the 0.68uF cap across the 820 resistor, use the 16ohm tap for NFB and lower the Nfb resistor to 27k.

In addition, you can raise the preamp voltage to get more headroom lowering the supply's resistors.

[Twistingcrow]

Thanks a lot for your replies guys!

[Strat78]

Yeah but, the 45/100 would just kill for your purposes now. You're 12xxx is a beautiful build don't you just have a hankering to tackle building the king of all Plexies! ValveStorm has got everything you need to kick that Fender hot rod into next year.

[ValveStorm]

Now there's an idea! Strat78 has built both so he would know!
I have had the Marstran trannies (good thing because now there is considerable lead time on those), and the Brian H boards (no longer available) to build one of the JTM 45/100s for years. One of these days I will get around to it!

[Twistingcrow]

I know guys... I'd rather try and tweak it first... But building a JTM45/100 is on my amp list for sure... I haven't seen any clear layout for that one though...

[neikeel]

I know guys... I'd rather try and tweak it first... But building a JTM45/100 is on my amp list for sure... I haven't seen any clear layout for that one though...

Do you need a layout? The shared cathode Marshall is pretty straightforwards and the schematics are pretty clear too.

A good Superlead can do loud and clear too (I got rid of my 2x12 HRD as it could only do cleans).

Suggest 1.5k/25uF on channel 2 with 0.022uF coupling caps, 250pF 56k tone stack and 27k on 16ohm tap.

100pF bright cap

efficient speakers

[emmjaydubya]

I thought at first that with a 100 watter I'd always have all the clean headroom I'd ever want but it actually ain't so... The amp gets into heavy crunch territory quite fast and now I would really want a LOUD clean sound. I would also love to increase the MIDS so that the amp cuts more through the mix (there's another guitar player in the band and he cuts more through the mix with his Fender Hot Rod than I do with my 12XXX!).

Funny, this is the same issue I have had with my 12,000 build. Great sounding amp but my other guitar player cuts through way better. Part of it is that I run it at 90v, in the next week or so I plan on changing the bias resistor and upping the voltage to 100v or higher to reduce the compression and squish, maybe even 120v! And, Carbia is right, the 50k mid pot works, you definitely get a mid bump with it.

First thing I noticed about your build is the carbon comp resistors in certain spots. Those can be a little fuzzier sounding than the metal films you have in other locations, might try swapping those out, won't cost much. Also you might try 6CA7s instead of EL34s. The 6CA7 doesn't have as much grind as the EL34 in my experience.

Most importantly, Strat78 is right! You should build a 45/100! I just finished one and about to fire it up this weekend, so keyed up to play it. I used Amp Archives photos and the Billy Batz layout (although not completely historically correct):

http://forum.metroamp.com/viewtopic.php?f=26&t=11946

I feel like this circuit along with the KT66s suits the sound I like since I love my little JTM45, I think this will just be a lot more of it. King of All Plexies indeed!

[neikeel]

Yes but a 45/100 is not as loud as a JMP. As you know I have had at least one each Marshall 100w in recent times as well as clones ( two types of 45/100, JTM100, 68 and 69 spec JMPS, 71 SB and SL and all types of 50w - alas no 45 anymore) and a well set up JMP 100 through a G12H cab is bloody loud.

If you overdo the mids it will not cut through IMO, a slight scoop (like the Fender ) helps.

Carbia's suggestion of removing or bypassing a dropper (the 10k next to the NFB resistor) will increase headroom a little, experiment a little but a 45/100 will be no louder...............

[Twistingcrow]

Yes but a 45/100 is not as loud as a JMP. As you know I have had at least one each Marshall 100w in recent times as well as clones ( two types of 45/100, JTM100, 68 and 69 spec JMPS, 71 SB and SL and all types of 50w - alas no 45 anymore) and a well set up JMP 100 through a G12H cab is bloody loud.

If you overdo the mids it will not cut through IMO, a slight scoop (like the Fender ) helps.

Carbia's suggestion of removing or bypassing a dropper (the 10k next to the NFB resistor) will increase headroom a little, experiment a little but a 45/100 will be no louder...............

Louder isn't really the issue as both are darn loud anyway and I need a PPIMV either way. Clean headroom is the real issue... The 12XXX crunches even on 4. If the JTM45/100 crunches from 6 or 7 then it will do. I want the cleanest Marshall ever. I even thought of building a Fender Vibroverb, but heck, I prefer Marshall glassy cleans...

But thanks to everyone for your replies! I will experiment with the 12XXX first and try to turn it into the cleanest 12XXX ever.

[emmjaydubya]

I agree, it's not a volume thing, my 12,000 is louder than sin. But my Matchless DC30 combo will be heard over it any day of the week. And that's because it has a very strong midrange cut, open back cab, mismatched speakers and CLARITY. I would even venture to say that my JTM45 can be heard alongside the 12,000. Clarity is the key though, and the 45 gasses out early. Good luck!

[Tone Slinger]

Straight up SB specs and maybe get a nice quad of 6550's

[bill bokey]

If you remove the 5nf bright cap and replace it with a 100pf there should be a lot less crunch on 4. And if you want even less gain remove the 0.68µf bypass cap on V2 cathode.
And as it's already been said, 27k on 16ohm for the NFB will help also.

[Twistingcrow]

Thanks a lot to everyone again! I've changed a few things already and I'll report back soon with a couple of pics and my results.