Ampeg SJ12R schematic

[phsyconoodler]

I got myself an Ampeg SJ-12R 1-12 combo the other day and it sounds good but you know me,I can't just leave it alone!
 I know there is more tone lurking in there!
 Anyone know where I can find a clear schematic? I tried for a couple of hours but can't find anything that I can read well.
  Anyone mod one of these amps before?
They really are not a bad amp at all. The circuit boards look fairly well laid out.
  I popped i a set of Winged C's and it perked it up a bit.Seems a bit stiff and it is plenty loud.I want to modify the boost feature as it sure doesn't 'boost' much except the volume.

[Jack1962]

Email Loud Technologies , they will send you a disclosure form , then you fill it out and they will email you the schematic for any out of production amp.

[phsyconoodler]

I'm going to drop in a pair of jj 6v6's and a 16 ohm warehouse ET65 speaker and see what it sounds like while I wait for the schematic...
  Nobody else has done anything to the Ampeg?

[HotBluePlates]

Try Here.

Click "Get Manual" below the image. You'll get a pdf than can be enlarged, and 100% zoom is quite readable.

[phsyconoodler]

Can't get that on my computer and don't know how to lower the security to let it through,so.......

[sluckey]

try this...

[phsyconoodler]

Awesome! Thanks Sluckey!

Looking at the schematic,this is what I'm going to try.

R4-change to 1.5k(cathode)
R6-change to 100k(slope) I may try 56k first
R7-change to 1.5k(cathode)
R10-change to 25k(boost cathode bypass)
R42--change to 25k(boost bypass cathode)

 What I'm experiencing now is not enough juice in the boost mode which is really the 'normal' of most amps.The boost simply bypasses the normal mode and uses a 100k cathode resistor. which lowers the gain.The 'boost' simply bumps it back to the normal mode.However,the boost is too far apart from the normal at 15db difference,so I think 25k would be more useable.
  My idea is that bumping up the gain slightly and reducing the spread between boost and non-boost will make the amp far more playable. 6V6's are a great option as the B+ is only 400v.

[DummyLoad]

What I'm experiencing now is not enough juice in the boost mode which is really the 'normal' of most amps.

and you won't get much more - the "boost" mode simply switches in .68uF bypass to 1st stage and .2uF of bypass to 2nd stage. try bypassing tone stack or lifting it off ground some; i.e. 100k switched in/out under mid control.

The boost simply bypasses the normal mode and uses a 100k cathode resistor

no, the 100Ks are a bleeder under each bypass caps to help quell switching noise. the caps are always "on" as they have a hi-Z path to ground through the 100K, so they have little effect until the JFET "shorts" the 100k when switched on in boost mode. the cathode resistors are always grounded. 2nd stage always has .022uF of bypass.

--pete

[rzenc]

Hi Keith!

After trying what you said you would, I'd give a shot @ C32, C2 and the R34||C3.
Suggested values to try: Remove C32. Up C2 to 22nF. R34 down to 220K || C3 2.2nF.

Interesting, the amp does not employ NFB to the non-inverting side of the P.I., but injects reverb return to grid pin#7.

Hope this helps.

Best Regads,
R.

[sluckey]

The boost simply bypasses the normal mode and uses a 100k cathode resistor. which lowers the gain. The 'boost' simply bumps it back to the normal mode.

That's not how that boost circuit works. Q3 is a switch. When boost is on Q3 is switched on and shorts R10. That effectively connects the bottom side of cathode bypass cap C1 to ground (just as if you had used a mechanical switch). V1A's gain increases due to the C1 bypass cap. When boost is off, Q3 is switched off, removing the short across R10. Now the bypass cap C1 is connected to ground thru a 100K resistor R10. This effectively removes the bypass cap C1 which in turn, decreases the gain of V1A.

Q4 operates exactly the same. The only difference is V1B always has a small .022 bypass cap (C4) and Q4 switches in an additional .22µF bypass cap (C5) to boost V1B's gain.

[phsyconoodler]

Ah,yes,I see now.  Hmmm...I think I'll just drop the cathode resistor down to 1.5k and work on the slope.
The more I play the amp I find I like it in both modes.It just needs a tad more initial gain and shift the tone controls a bit.
  I think the 6V6's and a speaker will put it where I like it.

[phsyconoodler]

OK,UPDATE: changed the tubes and speaker.The 6V6's don't sound very good in the normal mode,chimey and pretty ok in the boost mode.Not liking them overall.
  The speaker is nice. WGS ET65.
So in went the 6L6's again. They are staying this time and so is the speaker.Very,very nice tone!
 I haven't modded anything in the amp yet and frankly it's fine as is in both modes.I really don't see any need for the footswitchable modes however,and would just as soon use a toggle for that.
   The thing I've learned here is the amp is pretty damn good stock.I wonder why they weren't more popular?
My three knob tonebender clone is marvelous through this amp!A tubescreamer is glorious for blues!

 So why can't people build stuff like this anymore for the masses? Simple,compact,powerful,toneful combo.
So far it's power tubes and a speaker.And my strat with texas special loves this combination.I'll try it live on the weekend and see.
 

[HotBluePlates]

So why can't people build stuff like this anymore for the masses? Simple,compact,powerful,toneful combo.

Cause St Louis Music (now part of LOUD Technologies) was too busy building Crate amps to sell for cheap.

And at the time those were made, the big thing in amps (other than the newish boutiques like Kendrick, Budda and Victoria) were either Fender tweed reissues/copies or amps with a lot more gain. Ampegs generally didn't have the mass appeal that Fender did, and didn't garner the same high prices in the vintage or reissue market.

I think that last bit has to do more with the nature of Ampeg's market-share in the 60's, and that relatively few guitar players had experience with, or lusted after, them.

[phsyconoodler]

Loud has dropped the ball horribly.After getting rid of American production and shipping all production over to Vietnam and China,the quality has dropped considerably.Chinese Crates are awful!
  You have a hard time getting parts from Loud,or at least in the last couple of years anyway.
Big business is just that: big business. F&^$ the little people like maybe Americans and Canadians who can actually make good products.
 Profits rule these days especially at Fender and Marshall.
    I am disappointed in the way things are heading.Blackstar is another bad example. SMT technology has no place in a guitar amplifier.None whatsover.
  Going green doesn't mean ROHS.It means less stuff in landfills.
I used to be a Tech at a major music store and the parent company sent us a no-fix list and it just basically forced obsolescence on 10 year old products.They were perfectly able to fix the older stuff as we were in the shop,but they want to sell more new gear and older parts were less available,so they sent us a bulletin about what was ok and not ok to send in.Then they put a paragraph underneath that said "curiously enough,most older tube amplifiers can be repaired and parts are readily available,so they don't apply"

[DummyLoad]

most of the $#!t coming from china should be torpedoed in the pacific to keep it out of our landfills; it should have been diverted to chinese landfills when it left the factory instead. think of how much they'd save on shipping...

--pete

[phsyconoodler]

So I tried to convince myself that the speaker was better but I popped the stock one back in and it sounds just as good.
Gonna save the ET65 for another amp.....
  Changed the slope resistor to 100k and it made the amp too trebly.Back to 33k.
changed the treble cap from 470pf to 220pf,too trebly,back to 470k
-going to change the V1 bypass cap to 22uf or even 47uf.Should do the trick for more gain and less treble.
  I have to change the whole tonestack to take advantage of a more Fender-like slope resistor.Don't feel like it cause it sounds good as is.Just can't use treble higher than 3 with my strat with texas specials.Fine with humbuckers but I don't use humbuckers most of the time.
 Put a Mullard in V1 and it made it chimier.
 Actually kind of fun trying stuff,but it's a pain to pull the circuit boards out so I'm not going to anymore.
Leaving well enough alone and enjoying the amp as is.