Thunderchief "power amp" section?

[vanhansen]

As some know, I am planning on doing a JFET version of the Marshall JCM 800 2203.  I already have the preamp schematic together using JFETS using the Thunderchief and 1959 Super Lead schematics as a guide to what the ROG guys did, but I used values from the 2203 schematics I have.

Now, my question is this.  What part of the Thunderchief does the "power amp" stage begin?  I'm guessing it is at the 22n cap coming off the 3rd JFET.  Is it before or after?  I'm thinking it is before.

A few other questions (for clarification):

Does the dual LPF start at the 15n cap after the 4th JFET?

Also, if someone wanted to put the tonestack in, that would go between the 3rd and forth JFET but before the 22n cap, correct?

I'm just trying to get a better grasp on where the stages start/stop on the Thunderchief so I have a better understanding on how to tackle my experiment.  If it's OK with Brian and Gary, I'd like to use the "power" section of the Thunderchief so I can get that Celestion speaker simulation as well.  Granted I may need to tweak some values here and there but it looks to be a great starting point.

[B Tremblay]

Feel free to use the Thunderchief as a starting point for your own emulator.

The third stage of the 'Chief represents the phase splitter of the amp, while the fourth stage based on one of the EL84 stages.

Correct, the dual LP filter is located between the 15n output cap and 100k Master Volume.  

Here's a schematic that includes the full tonestack and the buffer that drives it, but note that it is untested:
http://runoffgroove.com/tc-tone.png

[RDV]

Thunderchief with Marshall Tonestack

RDV

[vanhansen]

Thank you both, Brian and Ricky.  This answers alot of my questions and clears up alot.  I'm getting a better hang of reading the schematics now and a better understanding of the signal flow.  You guys are great.

I'm noticing a lot of similarities between the Super Lead and 2203, assuming that the Thunderchief was taken off the 1959 Super Lead Standard and not the Master Volume, although the 2203 is similar to that one as well.  A few components are different here and there but not much.

The fun part will be making the final JFET sound more like 6550's rather than EL34's.  I'll probably start with just the voicing from the Thunderchief and then go from there.  There is a good chance that a "possible" mod will be for 6550 voicing.  Should be interesting.

[Ardric]

I can confirm that the Thunderchief w/Marshall tonestack as drawn by ROG does work.  I built my first one on perf and stuffed it into an old Nvidia video card box as a proof of concept.  Here's a couple of pics:




I used all J201's from mouser, and boxed film caps 'cause I had a pile.  The layout mostly follows the ROG perf design; I just inserted the follwer and tonestack parts in the middle.  I socketed the final coupling and low-pass caps, as well as the cap in series with the feedback loop.  I wired it for 12VDC with a switch, LED, battery clip and jack.  I had motorboating problems until I put a 100 ohm resistor in series with the pre's V+.  Even without shielding it's not that noisy.

I put in a set of pre-out/amp-in jacks so mono or stereo fx can be inserted.  My old Digitech GSP7 can then help out with eq and reverb.



It's a stereo power amp board for PC speakers that I found a bag of at a computer swap meet.  They use a TEA2025B 16-pin DIP and will output 2x2.4W into the pair of old 8-ohm Sony 6" full-range alnico speakers from ebay.  At bedroom volume levels it's more than enough.  I've since modified another with a Ruby-style buffer and bridged output.  The buffer really helps.  I'm gonna perf up little stereo buffers for the rest of them.

I hot-glued in a set of chopsticks to reinforce the "soundboard".  It doesn't rattle, but it doesn't project much bass either.  It sounds waay better through the closed-back 12" on my Traynor YGM-3.  I need to get it into a real cab.

I've built the TC, EC and Prof Tweed as stomp boxes and am pleased with all of them.  Thanks to ROG and the great discussions I've read in this forum!

[B Tremblay]

WOW!  That's incredible!  Very, very nice work.  Great enclosure and photos, too.  Also, thanks for confirmation of the TC w/ stack schematic.

I've found that buffers are absolutely necessary when running a guitar signal into IC power amps.

Thanks again for sharing your experience with the circuit!

[vanhansen]

That is very nice.  Glad to know the tonestack works well.

Brian, what schematic of the Super Lead did you use in creating the Thunderchief?

[B Tremblay]

Gary had already drawn the FET-based schematic when he sent it to me, but I believe he was using one from Schematic Heaven.  Unfotunately, that site is currently down.

[vanhansen]

Yeah, it is down but I was able to locate the site in the web archives, web.archive.org.  Some of the schematics are still there and I saved the ones I wanted but I couldn't get the 1959 Super Lead preamp schematic.  I did get the reissues.

If anyone has the 1959 Super Lead preamp schematic from Schematic Heaven, please PM me.  Thanks.

[vanhansen]

Nevermind, I found the schematic.  I had several but wanted to look at the one Gary used.  This could be it.  It's the same name as what was on Schematic Heaven and was created on the same day as the poweramp schematic was.  Anyway, if anyone is interested in it, it is here:

http://dumbdog.org/schematics/marshallamps/1959_superlead_preamp.pdf

Once I get all my schematics in order, I'll put some on my site.

[B Tremblay]

When hunting for a tube amp schematic, I always visit Duncan Amps' excellent page first:
http://tdsl.duncanamps.com/schematics.php

One of the best features of that page is that they tell you if a link is "live" or not!

[vanhansen]

I went there a few days ago.  Unfortunately all the ones I was looking for were dead (schematic heaven links).   :roll:  So my search trudged on until now.