Amp Tech Thread / Ask a tech Q

[Danny G]

No this is the mid 70s SVT.

The SVT-II promptly and completely crapped out when I put it back together. Which leads me to suspect I just made the problem much worse... :\

[Lumpy]

Have you tried putting a shim in the neck? It's super easy, and you can adjust the neck either way, depending on where you place the shim.

Plus, you only need a business card or something of about the same thickness.

There's a shim already - a piece of sandpaper folded up in there (covers about a half-inch area, towards the butt end of the pocket, and it's doubled up).

The stock bridge that came with it was heavily shimmed, and the neck was bowed (and it still had fret buzz). I'll play this one a bit as-is (it's not great but playable now, with a replacement Mustang bridge), and I'll get this to a tech eventually. It needs a pro setup. Too many variables for me to do a good job.

[Pissy]

http://www.warpedneck.com/

I called this place once and considered using them for my basket case G&L neck.  I think that it's like $75 plus shipping, and you can do it with only sending the neck to them. 

Of course, they want you to send you're whole guitar to them and have them set the whole thing up which costs more, but you don't have to.  I still may do it.

[mortlock]

serious question..aside from demonic possession, I need an answer to my issue.

so here goes. I am experiencing an unusual static-y hum from my bass only when volume knob is turned to the 'on' position. if I touch the strings or any metal on the bass [bridge, volume/tone knob, tuners] it pretty much goes away. I haven't changed anything in my set up and this is a new problem.

im thinking its a ground issue but it gets weird.. bare with me.

I run a stock fender jazz bass with the only mod being both pickups wired together with one tone and one volume..like a p-bass. been like this for years with no issues. I run it through an ampeg rocket bass amp at practice. plug strait in..no fx boxes whatsoever. also next to that is a peavey kb-300 keyboard amp that I use for vocals. I plug a sm-58 strait in. both amps are plugged into the same power strip.

'weird thing'.. as I was fucking around trying to figure out the source of the hum, by changing instrument cables [didn't work], at one point I plugged the bass into the peavey. still had the hum. I turned the peavey off, shut the mike off, had the ampeg on and my bass plugged into it, when I went to plug the mike cable back into the peavey, I could hear 'crackling' coming out of the bass amp as I touched the mike jack to the metal input on the peavey. how is that even possible?? remember..the mike and pa were both off. the only common denominator was them both plugged into the same power strip.

wtf is going on..help please..     

[The Shocker]

Try a different power strip?

[mortlock]

I did. didn't make a diff.
I can safely say its not my bass as I tried it at home on a different amp. no issues..

[jibberish]

capacitive energy storage discharging maybe?

long shot: verify polarity of everything plugged in to that strip.  some devices are not polarized but the COMMON wire on any power cord has a distinguishing mark like a stripe or a textured line or something. if you were facing a wall socket with ground down, the neutral/common would go into the left prong. if anything is 2 wire, check that that is properly plugged in

you for sure have a  loop in the mix whether or not there is more going on. the question is whether it is a common loop or a ground loop. you will get tickled from a common loop like if you touch the back of your wrist to the edge of something metal and you get a 20-30v tickle. also a seriously floating ground can give you a tickle. a potential actually exists across physical material of the earth ground and if you pipe the ground from a remote location via low resistance wiring an then ground on location, there can be a potential difference(ac voltage) which then flows current and makes noise. 60 cycle hum is no coincidence. that is the power grid in your shit.

fwiw, a person is a big capacitor+resistor and we certainly affect the circuit we are plugged in to.  we are good noise filters when we attach our bad RC filter selves to a feisty circuit and are excellent rabbit ears accessories for pulling in that really weak squirrely station. tv tuner is a variable capacitor amongst other stuff.

[mortlock]

does that mean I could be in danger of being electrocuted?? I haven't felt any tickles for what its worth..

[mortlock]

also why all the sudden?? same set up and it wasn't happening before..

[RacerX]

does that mean I could be in danger of being electrocuted??

Absolutely.

You gotta drive a spike into the dirt to ground that sucker.

[mortlock]

if I get out of the same power strip and go to another outlet, will that fix it??

[jibberish]

if you still have any direct wired death amps yes because the hot wire is now attached to like the transformer frame and if you touch that frame and ground = ZAPPO.
if you actually still have one(I did my kustom 30w combo and have to do an old bose mixer yet)take a cut off computer cord and rewire the amp. just attach the ground and the common to where only the common once was. this always ties the common to ground and doesn't permit a common loop from a floating common-ground potential.

modern 2 wire stuff like wall warts is 2 wire because it is transformer isolated. that means that the transformer frame floats or is sealed away from any contact, then only the 2 wires from the secondary coil, which is electrically isolated from the rest go out. this then is like a battery since the energy has been inducted through the transformer core, not directly wired.

still try to plug those in correctly because ripple is out of phase still. out of phase flows current and makes 60 cycle hum.

same setup, wasn't happening before: something changed. maybe subtle like plugged in wrong or maybe something failed somewhere. have to look around and meditate heh. GL

edit: my kustom combo has a 2 position power switch that reverses the polarity of the input or internal power, w/e to get rid of hum, just thought of that. so polarity fubars did/do exist

[mortlock]

I serious apreesh the words of wisdom..thanks.

[RacerX]

I'm assuming you have the reissue Rocket. Unless you have an old-school, '60s vintage Rocket, chances are good that it has a 3 pronged, grounded power cord. If you ARE using an oldie, jibberish's wisdom jibberish could possibly apply.

[jibberish]

just more musings on that noise:

I wonder if the power switch only cuts the hot wire out?

maybe power supply filter caps bleed out via the ground or common when the mic jack momentarily shorted or completed some path as it was plugged in.

you also could have created RFI which picked up in the pickups or bass amp circuit

edit: dunwich may have some insight since he lives inside of amps. there may be common things that happen at certain times that he is aware of or w/e

[dunwichamps]

serious question..aside from demonic possession, I need an answer to my issue.

so here goes. I am experiencing an unusual static-y hum from my bass only when volume knob is turned to the 'on' position. if I touch the strings or any metal on the bass [bridge, volume/tone knob, tuners] it pretty much goes away. I haven't changed anything in my set up and this is a new problem.

im thinking its a ground issue but it gets weird.. bare with me.

I run a stock fender jazz bass with the only mod being both pickups wired together with one tone and one volume..like a p-bass. been like this for years with no issues. I run it through an ampeg rocket bass amp at practice. plug strait in..no fx boxes whatsoever. also next to that is a peavey kb-300 keyboard amp that I use for vocals. I plug a sm-58 strait in. both amps are plugged into the same power strip.

'weird thing'.. as I was fucking around trying to figure out the source of the hum, by changing instrument cables [didn't work], at one point I plugged the bass into the peavey. still had the hum. I turned the peavey off, shut the mike off, had the ampeg on and my bass plugged into it, when I went to plug the mike cable back into the peavey, I could hear 'crackling' coming out of the bass amp as I touched the mike jack to the metal input on the peavey. how is that even possible?? remember..the mike and pa were both off. the only common denominator was them both plugged into the same power strip.


wtf is going on..help please..     

grounding issue, your body is acting as a better ground for the hum then the actual ground connection is

[beerrhino]

I have an SMF head that blew a power transformer about 6 months ago. The tech took it out and sent it to Mercury Magnetics and they cloned it. It worked fine for the next 6 months and then blew the power transformer again. It was sent back to mercury and they said it was a "slow burn" and not really their fault but replaced it under warranty anyway.
When the replacement was installed the tech told me that the secondary on the pt was regularly getting 1 amp of current and was rated at about 500ma, it was probably going to burn out again in about another 6 months. There are no faults or problems in the circuit, everything matches the schematic. They are talking about adding a big resistor before the secondary to knock the current down to a safe level, does anybody have any experience with this?
What I would like to know is is this a problem inherent to this model? They weren't made for very long, could this be why? Any other insight into this problem would be appreciated.

[Jake]

Not technically an Amp Tech question, but some pretty smart fellas in this thread might be able to offer their opinion on something my caveman brain cooked up.

I've been thinking about introducing some 15" speakers into my guitar setup since dropping down to an A tuning. Can someone tell me why a speaker cab with this configuration of 12s and 15s would be a bad (or not so terrible) idea? I'm not trying to get maximum driver/bass efficiency or super massive bass tones that will cross into a bassist's spectrum, but I do like the idea of really moving some air. Thoughts?

[The Shocker]

Ha, I had a dream last night about designing a speaker cab with two 12's and TEN 8's.  No knowledge about speaker design, so have no idea how it would sound, just seemed cool in the dream. 

[Mr. Foxen]

Mixing speakers is unpredictable because they won't operate in the same phase across the frequency spectrum. Means random mid peaks and cancellations. Plus the sound varies in more complicated ways around the room, instead of just being treble beamy.

[jibberish]

I have an SMF head that blew a power transformer about 6 months ago. The tech took it out and sent it to Mercury Magnetics and they cloned it. It worked fine for the next 6 months and then blew the power transformer again. It was sent back to mercury and they said it was a "slow burn" and not really their fault but replaced it under warranty anyway.
When the replacement was installed the tech told me that the secondary on the pt was regularly getting 1 amp of current and was rated at about 500ma, it was probably going to burn out again in about another 6 months. There are no faults or problems in the circuit, everything matches the schematic. They are talking about adding a big resistor before the secondary to knock the current down to a safe level, does anybody have any experience with this?
What I would like to know is is this a problem inherent to this model? They weren't made for very long, could this be why? Any other insight into this problem would be appreciated.

PT secondary as in : the main power supply for all the tubes secondary?
if so, this is telling you that the entire amp circuit needs an amp of juice to function.
the only way to kill current with a resistor is to series and increase the overall resistance to limit current. the only problem is that you also eat half your voltage in the resistor drop. this is dissipated as pure heat.  so doing some math...what is the secondary voltage? 400V..(round number) so your amp needs 400 watts to run (I*V=W=power) doesn't seem too unreasonable, esp if the secondary is really even less like w/e 300v.
anyway, you are going to have to drop 200 volts at .5 amp in that resistor = 100watt dissipation
also, what is going to happen to your amp circuit with only half power and current limiting?

can you get a 1 amp rated PT?  that is the cheap(er) one isn't it vs the output one?

[beerrhino]

I don't know about cheaper, jibberish, the first one was more than $500 w/labor.  I just don't understand why the amp came with a transformer that can only handle 500ma if the circuit draws 1amp

[Mr. Foxen]

Sounds like they established that the first transformer was wrong for the amp. Then replaced it with same.

[RacerX]

^
Actually, yeah.

Sounds like beerrhino'll need a burlier tranny next time round.

[beerrhino]

The first transformer was the original transformer from the factory.  I sounds like it was originally built with the incorrect transformer.