Amp Tech Thread / Ask a tech Q

[dunwichamps]

That isn't a SC anymore...

oh no but im kool with it. I have another 120 to refurb if I so wish.

[Mr. Foxen]

Think I may have bought a broken Sound City L120 earlier. Will no doubt need help from you guys to get it sorted. Guess it's time I learnt how to fix amps as well as the pedals and guitars.

Guess I start with Oli's trick of stripping and cleaning everything and taking it from there.

Replace spewy electrolytic caps at very least, all of them ideally. After that is where I run out of ideas.

[moose23]

Yeah of course. Fairly sure it needs all new caps and a few tubes.

[Hemisaurus]

i went overzealous in my strip down of the SC120R, im doing a full rebuild into a Hiwatt/Matamp GT120 mashup with El84 driven verb

Yeah I missed the tech question here?

'nother thread please.

[Mr. Foxen]

Been sorting an amp I bought as 'serviced and working' on Ebay. Check what was in there:



Anyway, after recapping it, replacing all the dodge chained up resistors with the right ones and cleaning tons of exploded cap goo out of it, it gives me a fair squealing noise and bunches of distortion, any clues where to start poking at stuff? Valves are all new, since I have just had a delivery.

[Hemisaurus]

No comment

[Jor el]

Can any of you people apply your super powers knowledge toward my home theater surround sub woofer?
It sounds muddy, then kicks in like a ebow-moogerfooger combo.

[Hemisaurus]

If it sounds muddy, you may be feeding it too much high. Or your sub out is distorting. Or your driver is
busted. Or your amp is busted.

If it sounds like it's dragging, and sounds good at loud levels, most likely your driver.

[Jor el]

I'm gonna try and F with it this weekend.

It easily pops its P's...and ultimately, belts oot solid, loud, tone that can only be stopped by yanking the power.

[Hemisaurus]

There are probably DVD's available with test-tones and useful setup stuff.

[Jake]

Has anyone ever seen a conversion of a "standard" sized 2x15 bass cab (vertical) into a 4x12 guitar cab (staggered speakers) before? I can't seem to find any online examples of someone doing this...? Just curious as to if there's any pitfalls to avoid or whether it's even a worthwhile undertaking. I'd like it to end up something like this Sceptre 4x12.

[Metal and Beer]

Heavy Tim from Witches Tit took an old 2x15 and made a 4x12, I believe all he did was change the baffle ? (obviously the dimension was big enough to accomodate..)

/cloudy on more deets

[Pissy]

I've wondered about cab philosophy when it comes to trying things like this.  is the ideal enclosure volume listed for a specific speaker additive?  As in if a speaker has 1.5cu ft listed as the ideal volume, would two of them with 3 cu ft work or would it be more ideal to separate them?

[Mr. Foxen]

I've wondered about cab philosophy when it comes to trying things like this.  is the ideal enclosure volume listed for a specific speaker additive?  As in if a speaker has 1.5cu ft listed as the ideal volume, would two of them with 3 cu ft work or would it be more ideal to separate them?

Its per speaker, and dividing isn't really necessary unless its designed to blow some speakers and carry on like the SVT cab. You do get a bit more bass sensitivity from adding more speakers than higher though, so you can get away with a little less volume per speaker as you add more and have same low end, but the expectation is more bass from a bigger cab anyway. The separator would act as a brace to the newly bigger panel that might have some flex, so there is engineering considerations, best making it a brace though and not taking up as much air space.

[liquidsmoke]

Ampeg SS150 solid state head, volume dips way down after playing it for awhile. Works fine if the preamp is bypassed. My Laney "GH"(no idea why they called it that) solid state head does the same thing.

Cold solder joint issues probably?

Both of these amps are usable to me as power amps through the rear with pedals but since I'm now actually liking the sound of a pedal through the Ampeg's preamp(can dial in super heavy and bassy tones) and able to get quite a bit more volume that way I want to try to get this thing fixed.

[Hemisaurus]

It's unlikely you'd get a fault like that on both amps, what is different when you run with the preamp vs. on bypass, are you adding in a pedal, differentbcable, different pedal output? Does the volume go down and stay down, or become intermittent, how much of adrop are we talking?

[liquidsmoke]

The Laney will cut in and out after awhile, it happens through pedals and going from my guitar directly to it. It only happens on one of the channels and it's the "clean" one unfortunately. The distortion channel sounds pretty shitty so I don't use it.

I'll try Ampeg direct later today to make sure.

I feel like this is kind of what I get for buying older SS amps but I really like the Ampeg and want to keep using it through it's preamp.

[Hemisaurus]

SLM (Ampeg / Crate) does have some build quality issues, taking it apart, redoing the solder on all the pots, jacks and any large components, especially those mounted to the chassis or a heatsink is a good idea, as is cleaning the pots, and checking your cables.

Dunno about the quality of modern day Laney, but the same treatment won't hurt.

[liquidsmoke]

I think the Ampeg is from before they had anything to do with Crate but yeah after doing some Google searching it seems this problem isn't uncommon. The Laney is 20-25 years old I believe, very rare in the states. Got it on Craigslist with a 4x12 for pretty cheap.

[liquidsmoke]

Okay so I got things figured out with the Ampeg. Channel B is now a lot more quiet than it used to be. It's still loud but not loud enough for practice. Something must have went wrong inside. Used to be MEGA loud. Whatever happened happened at practice earlier in the week.

[Hemisaurus]

Well if you can solder the above is a good starting point for clearing up age related problems, after that you start looking for bad components.

If you can't solder, the tech you take it to will probably start by doing that.

SLM were responsible for the VH series Ampeg's, which had a lot in common with the Crate GX's.

[liquidsmoke]

Looks like I'll be working on it with a friend with help from someone who knows more or I'll be taking it in. Hopefully it's fixable. The B channel is great by itself for heavy rock sounds and I've realized that with pedals it's a monster for mondo gainy sounds.

[Hemisaurus]

Well, like I said, just look at the board, see where the stresses can be, which is where pots, jacks or switches are soldered to a PCB and also bolted to, or project through the chassis. Likewise, any large components, big capacitors, power resistors, anything with a bit of mass that can cause a solder joint to loosen overtime. Then look at things that are bolted to heatsinks, like power transistors, also any board to board connectors.

All these things undergo stresses over time, which can cause a solder joint to go bad.

Take your soldering iron, with a little bit of solder on the tip for heat conduction, and apply it to the joint, wait 'til the solder on the joint melts and flows making a good joint, wipe your iron tip, apply another little bit of solder, move on to the next joint.

Between doing this, and the fact you will be unplugging and replugging any connecting cables, you will cure a whole lot of problems. The strip it and rebuild it method is a great way to cure simple intermittent problems.

[liquidsmoke]

Cool, thanks for the guidelines. I hope to get started on it soon after I get back into town from a trip next week.

Does the type of solder I use matter? I only have what my dad had lying around for decades. It looks fine but I don't know what type it is.

Also wondering how much risk of getting shocked there might be.

[Hemisaurus]

As long as it's rosin-cored solder for electrical work, not stuff for plumbing, you should be OK. If you can find a little bottle of rosin flux as well, a drop of that on each joint as you heat it helps clean it up and let's the solder flow more easily. Don't worry if you can't find it, I haven't used it in about ten years, but if you see it, grab it.



Not too big a shock hazard in solid state, the supply rails are usually 70V or less, but some caps can hold a charge still (only at tens of volts, not hundreds like in a tube amp), you can check them with a meter, or just short them out carefully. I did know a tech who used to search for low voltage live parts with his wet finger, but I'm not going to recommend that