Need help with Caswell #39 preamp schematic

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erigm
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Need help with Caswell #39 preamp schematic

Post by erigm » Fri Jan 09, 2009 7:07 pm

Ok, this is the #39 caswell mod I've drawn as outlined by Tim Caswell himself on his website. Now I need all of you help to figure out the missing values.
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Re: Need help with Caswell #39 preamp schematic

Post by xbearxau » Sat Jan 10, 2009 1:03 am

The values in blue I think should be pretty much spot on. Plate resistors at 100k look right. 22n coupling cap is pretty much the norm, as are the 470k/470k voltage dividers. Depending on your prefered sound, you may not even want the additionl bright cap Ive labeled as 500p.

Regarding the cathode resistor and bypass cap, this is where the extra 'kick in the pants' comes from. I think being an extra gain stage ( and still retaining the cathode follower) you may find its biased very cold, almost like a SLO amp. Id start at around 30k and work back from there. Bypass cap can be adjusted to suit once you get that extrs gain stage biased right.

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Re: Need help with Caswell #39 preamp schematic

Post by erigm » Sat Jan 10, 2009 1:30 am

xbearxau,

Are those values verified or conjecture. I agree that the values you inputed are the most likely (good idea putting them in blue). I figured when designing this mod Tim might've thought "I'll just add another stage just like the second stage", thus possibly a 10k cathode resistor value. He also said it was very bright, which would led me to leaving in the gain control treble caps, and thus I would think your 500p bypass on the mixer after the third stage is probably a correct guess as well. I also just guessed on the bypass cap brought in by the toggle on the second stage. It's a common cap and sounds good when bypassing 10k in this position, but this is not a verified value. Thanks for the input! I'm hoping someone who has one of these amps or has seen or worked on one might be able to verify this circuit.
erigm

xbearxau

Re: Need help with Caswell #39 preamp schematic

Post by xbearxau » Sat Jan 10, 2009 2:35 am

Just conjecture, although Id feel reasonably comfortable with the blue values. And not just for the 'lets just copy the previous stage' factor, but electrically they make perfect sense. Or I should say they make perfect sense based on the values YOU have already placed on the schematic.

As for the cathode values, I still think you'll find 10k a touch too low. Just have a feeling she'll squeel like a pig with even that much gain. Are you going to build this yourself? If its done on turrets, obviously it will only take a few minutes to swap out components as needed.

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Re: Need help with Caswell #39 preamp schematic

Post by erigm » Sat Jan 10, 2009 12:34 pm

xbearxau,

I plan on trying it in a Laney Protube AOR50H with an turret board installed that I use as my test mule. It's had about 20 different preamp circuits in there already.

I don't think 10k is too low for two reasons; 1) If you look at a schematic of a Sovtek Mig100h, or any modern 4 stage amp, you'll find much more gain than this, 2) I've had the "stage II" mod in this amp before with pretty much the same circuit but with 1k5/22uF values on the cathode without squealing (have you seen the schematic for this mod?).

The values I placed on the schematic a drawn from text that Tim Caswell put on his site regarding the #39 amp.
erigm

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Re: Need help with Caswell #39 preamp schematic

Post by Kapo_Polenton » Sun Jan 11, 2009 11:37 pm

Interesting Caswell put this on his site? Guess the #39 mod won't be soo secret anymore eh? 8) I have read people who had the mod say it was very bright. I think this is the reason Slash's tone cut soo well on that first album. Probably why it was tweaked for live work.

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Re: Need help with Caswell #39 preamp schematic

Post by erigm » Mon Jan 12, 2009 12:05 am

Kapo_Polenton,

Just to clarify, Caswell did not post this schematic on his site. He posted a block diagram with some text <http://caswellamps.studioelectronics.co ... agram.html>, and with what he inferred I drew up this schematic. I am fairly confident in the values that are posted. I'm trying to find out the question marked values, and maybe some of my switching might be wrong. I was hoping that someone who has or has seen one of his amps might help out and give some hints. Any input is welcome though, even just some good ole guessing.
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Re: Need help with Caswell #39 preamp schematic

Post by jerrydyer » Mon Jan 12, 2009 1:33 pm

since it says 39 + 39 + 39 i would try duplicating the other two and work from there. should be really hot and probably over into too much clipping but its a great starting point.
vids.
http://www.youtube.com/user/jerrydyer?feature=mhw4" onclick="window.open(this.href);return false;
http://www.dui-specialist.com" onclick="window.open(this.href);return false;

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Re: Need help with Caswell #39 preamp schematic

Post by erigm » Mon Jan 12, 2009 6:36 pm

jerrydyer wrote:since it says 39 + 39 + 39 i would try duplicating the other two and work from there. should be really hot and probably over into too much clipping but its a great starting point.
I'm not sure what you mean Jerry. The "#39" mod is with the toggles on stage two and three off. "#39+" is with one of them on, and "#39GL" is the George Lynch version with both toggles on.
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Re: Need help with Caswell #39 preamp schematic

Post by jerrydyer » Mon Jan 12, 2009 7:25 pm

I was just using a process of elimination or duplication. Besides, I dont think thats a legit 39.. it had 330ufs and 3.3k at least that schem has had the 39 label more than the one you posted..who knows really. just go for it and make your own.
vids.
http://www.youtube.com/user/jerrydyer?feature=mhw4" onclick="window.open(this.href);return false;
http://www.dui-specialist.com" onclick="window.open(this.href);return false;

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Re: Need help with Caswell #39 preamp schematic

Post by erigm » Mon Jan 12, 2009 8:33 pm

jerrydyer wrote:I was just using a process of elimination or duplication. Besides, I dont think thats a legit 39.. it had 330ufs and 3.3k at least that schem has had the 39 label more than the one you posted..who knows really. just go for it and make your own.
This is what I've been trying to say for a long time ... the "chip" mod (the one also posted as the "SIR mod") is NOT the #39 mod. Has anyone read the text here <http://caswellamps.studioelectronics.co ... agram.html>?!?! It's obvious the values (or at least generally) for stages 1, 2, and 4. Stage 3 is the one most in question. Read the text, look at the block diagram, and then you'll see what I'm talking about.

I repeat ... the "chip" mod is NOT the #39 mod.
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Re: Need help with Caswell #39 preamp schematic

Post by marci84 » Sun Jan 18, 2009 4:25 am

we know that the sir 39 has also a clean channel, and from what i red on internet the sir36 (the one used by slash on Appetite) had only the distortion channel, so maybe the chip mod is a simil sir36 mod..i have it on at this moment in my amp and it sounds fucking gaining! but the appetite sound is almost there! just swapping some tubes around to find the best one for that sound (at the moment the tung sol 6550 are too much compressed)

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Re: Need help with Caswell #39 preamp schematic

Post by erigm » Sun Jan 18, 2009 12:54 pm

I don't think the original #39 amp had a clean channel. I believe one half of the added tube (the one previously for the tremolo) was not used after he modded it. The schematic I'm trying to complete is his more current "#39 mod", which has the clean channel, 2203 settings, #39 settings, and fx loop. Again, its all right there on his block diagram and explanation.
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Re: Need help with Caswell #39 preamp schematic

Post by Icarus » Mon Mar 02, 2009 11:33 pm

Hi
Sorry to Hijack
But im wondering if these or other Caswell mods can be made to be a three sound & footswitchable
Meaning A cleaner tone
Crunch rhythm & high gain lead tone ?
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Marshall 1982 JCM 800 2203
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Marshall 1960TV
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Re: Need help with Caswell #39 preamp schematic

Post by erigm » Tue Mar 03, 2009 12:15 am

Icarus wrote:Hi
Sorry to Hijack
But im wondering if these or other Caswell mods can be made to be a three sound & footswitchable
Meaning A cleaner tone
Crunch rhythm & high gain lead tone ?
It is my understanding that this is exactly what the current caswell mod does. I think it comes with a three button footswitch to switch between the three sounds.
erigm

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