Schematics for Acoustic 450 bass head needed

[chris_young]

Hi folks,

i'm a musician and repair tech from germany. I've got an old Acoustic 450 head here which has a defective distortion circuit. Nothing big i guess: the signal still passes through the circuit and the overdrive is sounding proper but there's lots of noise underlying the signal with randomly changing levels. The amp's clean mode works fine.

I haven't opened the amp yet because i would first like to get my hands on a schematic diagram. Can anyone here help me out on that?

Thanks!

Chris

[Enzo]

Before I go digging, did you try the search function on this forum? I know we have discussed the amp several times, and I think we may have posted the schematics. If not, we have them.

[chris_young]

Hi Enzo,

thanks for your reply. I did several searches before i started this thread and found a few results but i couldn't find the schematics. There have been some discussions about technical issues with this amp (with you offering the schematics to another member at some point) but no link. I just found the 470 schem, which eventually could be of help to me, too.

I would be glad if you'd help me out. My address is: chrijung at gmail dot com

Best,
Chris

[Enzo]

Here they are, sorry about the two-part scan.

[chris_young]

Enzo, thank you very much!! I'll post here when the work is done or if i should encounter bigger problems.

Best wishes,
Chris

[chris_young]

Hi everybody,

i just want to let you know that the noise problem of my 450 was caused by a defective P-channel-JFET (Q111) which to me looks like a kind of "highs of dist circuit input to GND"-switch whenever the distortion is deactivated. You can find it on page 4 of Enzo's Scan.

I guess, the idea of the FET is to prevent the distortion from bleeding into the clean sound while the dist circuit is bypassed. Interestingly, when disconnecting the FET and letting only Q104 and Q105 do the switching work, the clean signal is still as clean as it can be to my ears, no matter how i set the gain of the distortion. I replaced the FET nevertheless.

If anybody has additional info about what the FET does, don't hesitate to tell me. Would be glad to learn something new. What i'd still like to know: Doesn't the whole Q111 thing act like a high cut to the clean sound in conjunction with the gain pot? Is this good circuit design?

Best,
Chris

[Enzo]

At 1 whole microfarad, it should shunt more than just the highs to ground when ON.

yes, by shunting the signal to ground before it goes into the dirt channel it prevents any unwanted crosstalk from arising. The other switching elements may be doing an OK job of that for you, but that won't always be the case.

[chris_young]

Hi Enzo,

you're right, it's surely meant to be more than a "highs to GND"-switch. But due to the high amplification of the distortion stage, the 1uf shunt can't prevent that a good amount of signal is still coming off the dirt channel. I tested this by activating the dirt channel and bridging D and S of the FET. The signal looses some dirt of course and sounds muffled but with the gain maxed the output is still high enough for crosstalk to appear if the rest of the switching elements wouldn't do proper work.

Imagine a big muff with the gain moving from 10 to maybe 4 and the tone all the way down....

Thanks again for your help!

Chris

[Enzo]

Ah, that "if the rest of it doesn't work" part is really the heart of the matter. The various parts all work together to get to the goal, no one element is sufficient on its own. No I in team, etc etc.

[chris_young]

:-)

Thx again! I will definitely post again here when looking for schematics or help!

Best,
Chris

[bruceg]

I see you have the schematics for the Acoustic 450. I am new to this forum, I thought once I was logged in I could just click on the PDF's but nothing seems to happen. Can they be emailed?. Thanks, Bruce.

[Enzo]

You can do exactly that, except a server crash erased all our stored files. We have been reposting them as people ask for them. Someone ought to have this one, I can't upload from this computer right now, I will when I get to one that will do it. Meanwhile someone else will probably get to it first.

[Enzo]

Sorry for the delay

[Jazz P Bass]

The files are blank.

[Enzo]

Well, crap, I renamed them, and the system took them in. usualy I can;t even get that far.