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Topics - spud

#1
Preamps and Effects / Plexizer or is it Plexi-izer
August 13, 2012, 12:08:39 PM
I built this about a year ago and true to form, I stuck it in a box when it didn't work.  I say true to form because if something doesn't work and it's not obvious to me, I shelve it and come back later.   My thinking is that I'm too close to it to see the problem.  Like I said, this one has been on the shelf for about a year...  ::)     Anyway, I was cleaning up and found it so I figured I'd give it another go.  Long story short: after fixing a few boneheaded issues, it seems to be working but is a little noisy so that needs to be addressed - should put in an enclosure and use shielded wire.  I got the plan (schematic really) from DIYSTOMPBOXES from a poster there named Mojotron.  Here's the link to that thread where the original schematic is available.  If you can't get to it there, I can post it or I can email it to you.  Here's the link to the schematic (in the thread at DIYSTOMPBOXES):

http://www.diystompboxes.com/smfforum/index.php?topic=50207.0

Anyway, here's the layout as I built it and a really poor pic (sorry it was done with my phone).  I'll post a better one shortly (have to borrow my daughters new Cannon which just takes some awesome hi-res pics, btw). 

http://s252.photobucket.com/albums/hh7/spudjds/Public/Builds/PEDALS/Plexi-izer/?action=view&current=Plexi-izer_15hx24w_D_08-07-12.jpg

http://s252.photobucket.com/albums/hh7/spudjds/Public/Builds/PEDALS/Plexi-izer/?action=view&current=0805022022a.jpg

I think the sound is a pretty good Marshall emulation.  A cool thing about it is when you turn down the gain close to min and get the tone-stack adjusted right, you can get a pretty close approximation of the AC/DC grind - you know, that not so high gain but very mid rangy, punchy, break-up tone.  Anyway, I think it should make a great pedal or amp pre-amp - which I was intending it for anyway, attach chip amp, away you go.  Right now I'm testing it into a set of old powered computer speakers, so it's sort of not ideal.  I need to try it into a SS guitar amp and a tube one to see how it behaves in those situations as I know that using the computer speakers is pretty artificial as far as eq goes. 

It is kind of large as I did it in perf and it could probably be shrunk if you did it as a PCB but I'm an etching virgin. :-[  One of these days I'll start etching too.   

I'm thinking of ditching the presence but it's a trimpot not a reg pot so it's not that big.  From my testing it doesn't seem to have that big of an effect and the DIYSB thread confirms that, with the values I used.  I might try the others they recommended but I'm not convinced it's of any use at all - seems about the same wherever I set it.

I'll put together a clip of it to give you an idea of how it sounds.  Aside from my playing ability not approaching Mojotron's, his clips (later in the thread above) do represent the distortion character I'm hearing from the pedal.  I guess with this and the Dr Boogie, you could cover the whole gamut.  I have yet to build the Dr Boogie but will have to now to see what's up with that.   Seems like a variation of this sort of thing (JFET tube emulation), anyway. 

Jim
#2
Hi guys,

I just played at a party last night with some guys that I've been jamming with - we're starting to put together a pretty good band.  This was our 6th or so session playing together and the singer has only been playing with us for the last 3 practices so this "gig" (party) was only his 4th time with us.  We played about an hour (we only felt comfortable playing about 12 songs of 25 we've been working on) and I think it went really well - the people kept saying how good we sounded, esp the singer - he is killer.  Well, I'm not happy with my sound.  I have a 1993 Marshall Valvestate 8080 Combo (80w) with the original 8 ohm Eminence OEM speaker in it.  The other guitarist is playing through a Peavy Valveking 100/120 W half stack.  Obviously, his sound is much fuller/deeper, with way more punch.  My amp sounds thin - I think it sounds like a toy.  He says it's the speaker and I need to get a 4 x 12 or at least a 2 x 12 sealed back.  I think that's part of it but I'm also suspecting the SS power amp in my Marshall just won't cut it.  In the room we practice in I can get a reasonable sound out of it but we were playing outdoors last night (backyard party) and my amp was just not putting out even though it's a 80w amp (well that's what it's rated for). 

Obviously, the simple solution would be to buy a half stack like his and that would be it - well, money's tight and I don't have the $700.00 or more it would take to do that (I want a good amp, not something cheap).  So some ideas I've got on how to "fix" my set up for minimal investment of cash. 

1) I have a separate speaker cab - a Jet City 1 x 12 that I got for super cheap at a local music store.  They were getting rid of all their Jet City stuff and the price was so low I couldn't resist.  It has a 16 ohm Jet City Speaker in it made by Eminence (not sure of the "model" as it just says Jet City on it) and is a totally sealed back cab.  So my question is: can I run this safely with my Marshall amp?  My amp has a hard wired cable going to the speaker and no "speaker output" jack.  I could install a jack really easily but it would just be coming off the same leads just going to a jack.  I guess my question on this is can I run the 16 ohm Jet City speaker cab without hurting my amp?  Also, related to that question:  can I parallel the Jet City cab with my amp speaker safely - this would be about 12 ohm load (parallel)?

2) As I mentioned about paralleling the Jet City cab and the internal speaker, if it's not safe can I install an 8 ohm power resistor (50w or more) wired in series with my internal speaker (or the cable) to raise the impedance to 16 ohms and with that in parallel with the jet city it would be ~8 ohm load for the amp.

NOTE:  One thing I've heard is that it's safer to go up in speaker impedance (like to 16 or even 12) than to go down (to 4 or 2) with a SS power amp (or any amp really) - is this true?  Does this matter?

3)  Is it ok to "seal" up my combo cab as it is now an open back (almost the entire rear is open)?   The other guitar player said it would make my amp overheat but there's nothing at the bottom of the chassis that is dissipating heat - no vents, no heat sink fins, just a bank chassis.  If needed I could put a small fan somewhere to help reduce the heat but at this time, the amp has no heat issues and is always pretty cool running so I'm not sure if this necessary.  Related to this, if I can seal it up, I'm thinking of leaving a "port" to see if I can "tune" it for best sound/bass response or should I just seal it completely.

4) Another thing is I've considered replacing the internal speaker with a high efficiency model like the Emi Wizard or even a Vintage 30 as I'm not sure of the OEM speaker's efficiency rating - probably sucks.  I've also heard good things about the Private Jack.  So that might be a low cost option - probably not more than $100.- dollars or so.  I've also considered replacing the speaker in the Jet City cab with a Wizard or Private Jack or something along those lines.  I'm open to any opinions on speakers that might be good for this (keep in mind that this amp is Marshall's attempt to create a JCM like sound in a lower priced SS/Tube hybrid amp so that's the sound "signature" that this amp is intended for).  Obviously, I'd do the internal one first but I was concerned that if I replaced it with an 8 ohm one, I'd have the same issues like I discussed in #1 (12 ohm total parallel impedance load).  If I replaced it with a 16 ohm, I'd have 8 ohm's in parallel but if I wanted to just run the internal speaker alone, I'd be back to a 16 ohm load.  So back to the question #1....is 16 ohm ok?

Well, sorry for the long email.  Really appreciate any insights from you guys on what would be the best option/solution or even if you have other ideas.  I realize that it would ever get to being the equal of the half stack but I'm just looking to get the sound NOT TO SUCK - as I'm feeling it does now.  I guess I really need more bass/low end, more punch/tightness, and I need to move more air. 

And yes, I fully realize that a longer term solution is to build a 50w or so Tube head and get a decent 2 x 12 or 4 x 12 setup.  I'm working toward that but I'm currently in the middle of a Tweed Deluxe build that is now in it's third year!  Still haven't drilled my chassis yet so I'm still not committed to my layout and keep changing it (the wonders of VISIO - I call it virtual amp building).  Anyway, after I get this issue resolved with my current setup and if I can get it to sound Ok (and I make progress on that Tweed Deluxe), I'll get to work on that 50 watter - probably a Trainwreck variant as I just love that tone it has. 

Thanks in advance for all the input/advice -

Jim  (aka Spud)

Prologue: 

BTW if you're interested here's my Tweed Deluxe stuff as it stands now:

http://s252.photobucket.com/albums/hh7/spudjds/Public/Builds/AMPS/5E3/

Also, here's all my DIY and Guitar stuff on Photobucket:

http://s252.photobucket.com/albums/hh7/spudjds/Public/

I welcome any comments. 

Thanks again.
#3
The Newcomer's Forum / Orange Crush 15R
January 12, 2011, 04:11:36 PM
Picked up a broken Orange Crush 15R off ebay for cheap.  I asked Orange about schematics and got no reply.  Also on their un-official amp site the guys said that the SS ones made in China are like red-haired step children, they don't really support them - I guess they are viewed as throw away as they are so cheap.  Anyway, haven't been able find any schematic from any source - anyone have any ideas? 

From looking at the guts, it's a TDA030 chip power amp with 3 TL072 chip pre-amp, looks like a some kind of Sym Diode clipping circuit and reverb - so I guess the TL072 are for 3 pre-amp circuits, clean, dist and reverb send/recieve - does that sound reasonable.  It does make a very faint scratchy sound but that's it.  Also, from the look of the board something may have been spilled on/in it.  The outside shows no evidence of that but it was probably cleaned up to sell it.  Owner said it used to work but hadn't been used it in a while and when he pulled it out of his closet, it was like this. 

I suspect it might be a TL072 but need to get some and replace them - I'll add sockets while I'm at it as everything is soldered to the board.  The board itself is pretty spacious and not crowded at all, it's all through hole components and not SMT - so it's human accessible.  The cab is pretty attractive - orange and pretty "beefy".  If I can get it up and running I might sell it but if not I'll gut it and use the cab for my own creation  :tu: 

Well, if anyone has any ideas, send em my way.  Maybe I should try to trace the board out and make my own schematic - might be a useful exercise to learn something...DOH!    ;)

Later,

Jim