No gain, unresponsive eq, and squealling on new build

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Major Sparky
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No gain, unresponsive eq, and squealling on new build

Post by Major Sparky » Mon Feb 25, 2013 5:11 pm

I'm having some problems with my newly built JMP 2203. All the voltage readings are right and all the testing went well. I turn it on and plug into the high input and immediately get a squealing sound. The sound is almost always there and is worse at higher volumes.

I tried pluging it into the low input and thankfully get no squealing. Everything seems ok with that input, except I'm getting no gain whatsoever even with the preamp volume turned all the way up. Even when I crank the amp through my attenuator I'm not getting even half as much breakup as I get with my JMP 1987.

Furthermore, the treble, middle, bass, and presence pots don't seem to do anything at all. I know Marshall EQ's can be subtle, but these just don't work. I tested all the pots prior to installation to to make sure they were good.

I checked over to make sure everything's in the right place and it all seems to be. Here's some shots of the amp. Any help? I built it using the metro instructions. The only changes are some wire colors on the board. All pink wires are violet, all black wires are white, and all red wires are orange. The rest of the colors are the same.

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toner
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Re: No gain, unresponsive eq, and squealling on new build

Post by toner » Mon Feb 25, 2013 6:41 pm

Does it squeal in a shielded head case?

Lift your preamp grid wires (pin 2 and 7) up away from other wires and chassis near the sockets (probably not related to the squeal but can cause hum noise).

It's normal for the low input to stay clean, by the way.

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Re: No gain, unresponsive eq, and squealling on new build

Post by toner » Mon Feb 25, 2013 6:52 pm

V3 pin 6 and 7 wires are very close together, if not touching, near the middle of the socket. Clip those bare ends off.

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Re: No gain, unresponsive eq, and squealling on new build

Post by Major Sparky » Mon Feb 25, 2013 10:03 pm

Haven't built the head cab for it yet so I don't know if the shielding would help. I looked at v3 and pins 6 and 7 weren't touching but the wire for 7 wasn't soldered in well and was lose. I clipped the ends but I don't have time to fire up my soldering iron and fix it now.

Any ideas as to what could be causing my eq problems?

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Re: No gain, unresponsive eq, and squealling on new build

Post by OnTheFritz » Mon Feb 25, 2013 10:11 pm

I'd suggest fixing the bad solder joint and go from there.
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toner
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Re: No gain, unresponsive eq, and squealling on new build

Post by toner » Mon Feb 25, 2013 11:53 pm

Major Sparky wrote:...Any ideas as to what could be causing my eq problems?
The tone stack and pot wiring looks okay so I'm not sure.

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Re: No gain, unresponsive eq, and squealling on new build

Post by Major Sparky » Tue Feb 26, 2013 12:35 am

I fixed the wire, but I'm still getting squealing on the high input. I don't have any idea what the problem could be.

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Re: No gain, unresponsive eq, and squealling on new build

Post by toner » Tue Feb 26, 2013 1:16 am

It probably shouldn't squeal even out of a head case since you have shielded cables on V1 but there's only one way to find out unfortunately. A shielded case may kill the squeal.

You may want to move the NFB wires around to see if that helps. The one connected to the impedance switch is very close to the V4 and V5 grids. Does the presence pot affect the squeal?

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Re: No gain, unresponsive eq, and squealling on new build

Post by demonufo » Tue Feb 26, 2013 2:57 am

Yes. You REALLY want to move that NFB wire to the impedance selector. That wire should be kept as far away from other stuff as is possible, and it seems wound around other leads at the moment.

The layout of the chassis itself isn't really helping things. What chassis is that?
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Re: No gain, unresponsive eq, and squealling on new build

Post by Major Sparky » Tue Feb 26, 2013 9:44 am

It's a 100 watt style chassis from tubedepot.

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Re: No gain, unresponsive eq, and squealling on new build

Post by demonufo » Tue Feb 26, 2013 9:50 am

Don't like the stuff covering the power tube sockets, or the location of the impedance selector at all. Still, you have to work with what you've got. Try moving the NFB lead and see what happens. If you can't keep it away from other stuff, you might want to think about resorting to standoffs.
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Re: No gain, unresponsive eq, and squealling on new build

Post by Major Sparky » Tue Feb 26, 2013 10:23 am

What do you mean by using standoffs?

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Re: No gain, unresponsive eq, and squealling on new build

Post by demonufo » Tue Feb 26, 2013 11:39 am

http://en.wikipedia.org/wiki/Standoff_%28separator%29" onclick="window.open(this.href);return false;

Then get some tags that wrap around the wire, and screw to the standoff.
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Re: No gain, unresponsive eq, and squealling on new build

Post by Major Sparky » Tue Feb 26, 2013 11:44 am

I could drill new holes and shift the speaker jacks and impedance holes over. Though I'm not sure how much that would help because they'd still be over the tube sockets no matter where I put them.


Would it be worth rewiring the chassis and better routing the wires? I'm thinking about doing that just to ensure I have as little hum as possible.

Also, does the board look ok?


EDIT: I'd like to state again that I have no problems with squealing at all on the low input. The only problem I have is that the EQ is unresponsive. Could the NFB wire really only be effecting the high input?

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Re: No gain, unresponsive eq, and squealling on new build

Post by demonufo » Tue Feb 26, 2013 2:11 pm

Bear in mind the hi input has a whole other gain stage on top of the lo input (the lo input, v1b - but in your case it appears as if v1a is actually feeding v1b rather than the other way around, which is the norm).
It's not just a small difference, it's a big difference.

However, I don't think the NFB lead is the issue here. You're going to have to check your work thoroughly. The EQ section should be pretty obvious. Not like Fender or Mesa obvious, but all the same, you should be able to notice quite a lot of difference there, especially on the treble. And on the presence. Which I would expect to work regardless of the EQ section. Perhaps you should thoroughly check your grounds AND your wiring around the pots. Are you certain the buss on the pots is making a good connection with ground? Although looking at your pics and board I can't see anything amiss (despite a rotten headache at the moment and a different layout to what I'm used to)

Ideally, the secondaries on the OT need to be twisted together, and the lead dress improving (although again, this isn't your problem). Best way to do that would be to move the jack sockets and the impedance selector one space further away from the fuseholders, but I dare say you already have a rear plate for this, stopping you from doing so. The primaries should also be twisted on the OT. Can't see them, so I'll assume you have already.


Also, just for shits and giggles, remove the NFB wire altogether from the impedance selector, and tell me if it's still squeals?
So I like purple, okay!!!!!!

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