Results 1 to 4 of 4

Thread: JCM900 Preamp + AX84 Single Ended with channel switching

  1. #1
    New Member
    Join Date
    Feb 2011
    Posts
    2
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0

    Unhappy JCM900 Preamp + AX84 Single Ended with channel switching

    I'm building AX84 Power Amp :http://www.ax84.com/static/corepower..._Schematic.pdf
    clean channel preamp : http://www.ax84.com/static/corepream..._Schematic.pdf
    and JCM900 preamp for lead channel: http://www.heavil.ru/cxema/marshall-...preamp.gif.jpg

    I would like to have channel switching just like JCM900 had but on this power amp. Original schematics use multiple chips etc. but i want to do it old school without them. I'm thinking of using relays or opto-coupl, but i don't know where to put them in the circuit. I was thinking of using "clean channel" as first on input and then with a switch turn on the "lead channel" section. I looked into it in older threads and googled it but didn't find sufficient data on this. If someone would be so kind and help me out

    0 Not allowed! Not allowed!

  2. #2
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,753
    Thumbs Up/Down
    Received: 1,884/23
    Given: 1,463/35
    Rep Power
    27
    Dear Mastersamo: your guitar signal should go into both preamps, which then will run "in parallel", although you'll use either one or the other.
    Since you have another, completely independent clean preamp, you can switch between them using Led_LDR or Relays; FETs or CMOS gates aren't well suited for the high signal levels usually carried by tube preamps.
    Remember to have an extra switch to kill gain on the distorted channel (besides switching it off) so as to avoid noise feedthrough/crosstalk when unused.
    *This* version of JCM900 you link does not have any switching.
    The one which does, is a *very* different beast, and might be considered a giant SS distortion pedal, driving a tube power amp.
    Yes, it does have a tube, but since it does not distort, it doesn't add much to the final sound .

    0 Not allowed! Not allowed!

  3. #3
    New Member
    Join Date
    Feb 2011
    Posts
    2
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by J M Fahey View Post
    Dear Mastersamo: your guitar signal should go into both preamps, which then will run "in parallel", although you'll use either one or the other.
    Since you have another, completely independent clean preamp, you can switch between them using Led_LDR or Relays; FETs or CMOS gates aren't well suited for the high signal levels usually carried by tube preamps.
    Remember to have an extra switch to kill gain on the distorted channel (besides switching it off) so as to avoid noise feedthrough/crosstalk when unused.
    *This* version of JCM900 you link does not have any switching.
    The one which does, is a *very* different beast, and might be considered a giant SS distortion pedal, driving a tube power amp.
    Yes, it does have a tube, but since it does not distort, it doesn't add much to the final sound .
    Hmmm, thanks J M Fahey on your comment. Do you think that it's a waist of time trying to build this JCM900 preamp and should choose another preamp? If so could you name me some pedals/preamps that have that JCM900 sound. But i don't want solid state kinda pedal, so low-or-even-ANY solid state stuff in pedal, i'd like to see it full-tube, even if i don't get that exact Marshall-Crunchy-Lead sound. Should i signal that's going into preamp that's not being used ground or just let "floating in the air"?Also i would like to know if i should go with the KT88 or EL34, since this is gonna be driving a 1x12" i think that kt88 will give me more bottom-end and closer to Marshall-sounding amp even though the use EL34's. Pls help me out

    0 Not allowed! Not allowed!

  4. #4
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,753
    Thumbs Up/Down
    Received: 1,884/23
    Given: 1,463/35
    Rep Power
    27
    Do you think that it's a waist of time trying to build this JCM900 preamp and should choose another preamp?
    No, what I told you is that you should not build the "regular" SS preamp; the one you linked to is the less common, non switching "good" all tube one.
    I don't even understand why they carry the same name.

    Should i signal that's going into preamp that's not being used ground or just let "floating in the air"?
    You need at least a DPDT relay; the ones with 6 contacts, same as "true bypass" switches.
    I've drawn a block schematic to show you:

    You see the relay coil drives two spdt (single pole dual throw) switches, where the center pin touches normally the upper one by spring pressure (nc= normally closed); when the coil passes current it atracts the leaf switch, opening the center_to_top contact and closing the center_to_bottom one.
    We can use this to do a lot of things, just be creative.
    In this case, (as drawn) , the upper switch normally selects the upper (dirty) preamp into the output jack; the lower (clean) preamp is disconnected from it.
    If you activate the relay, both sets of switches change; the upper one selects the clean Pre, the Dirty one gets disconnected.
    Isn't that enough?
    Usually no, remember input jack goes to both preamps all the time.
    Although you have not selected the dirty one, it is still working full blast, doing all kind of nasty things to poor guitar signal
    This very loud and distorted signal can easily get into the clean channel; you'll hear the clean sound mixed with a faint but annoying distortion in the background.
    So we use the second switch to ground signal somewhere in the Dirty preamp, silencing it.
    The "triangles" mean Gain Blocks in each Preamp, may mean one or more Tubes.
    Also i would like to know if i should go with the KT88 or EL34, since this is gonna be driving a 1x12" i think that kt88 will give me more bottom-end and closer to Marshall-sounding amp even though the use EL34's.
    *Personally* I would use 1 pair of EL34 which is the classic sound.
    Having a 50W output lets me reasonably clip it, for best/fattest sound, without killing everybody around.
    A too loud amp forces you to use buzzier preamp distortion all the time.
    Good luck.

    EDIT: *now* I see you will use a SE power amp.
    Forget my 50W suggestion (or not, why not go all the way and have a giggable amp?)
    My preference for EL34 still remains anyway.

    0 Not allowed! Not allowed!
    Last edited by J M Fahey; 02-15-2011 at 01:47 PM. Reason: Slow mind.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Single-Ended Hum
    By Sir Cuitous in forum Tweed Builders
    Replies: 49
    Last Post: 02-10-2010, 05:05 PM
  2. Marshall JCM900/4100 PS and channel switching problem
    By jaz10 in forum Maintenance, Troubleshooting & Repair
    Replies: 18
    Last Post: 01-26-2009, 09:35 PM
  3. Single ended 12a_7 ???
    By walkman in forum Theory & Design
    Replies: 9
    Last Post: 09-02-2008, 10:17 AM
  4. single channel rack preamp project (marshall 2204 mod)
    By grrrunge in forum Build Your Amp
    Replies: 12
    Last Post: 04-04-2008, 02:41 AM
  5. 2 channel single ended
    By H Chandler in forum Theory & Design
    Replies: 5
    Last Post: 07-14-2007, 05:50 AM

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •