Amp Tech Thread / Ask a tech Q

[bass sic]

Fuck! The one I have is Nl-2 +1. Looks like I'll be at the GC tomorrow. See, this place is beyond useful.

[Hemisaurus]

Why you wanna fuck me all up with equations and shit. What I got was a speakon to 1/4 plug. Now I'll have to see if it's for mono mode. I've read thru that manual a few times, but a lot of it is foreign to me.

Dude, I didn't have time to make it nice and pretty, there's no equations. Just there's 3 major types of Speakon Cable. 1+, 1-, 2+ and 2- are what Neutrik chose to label the terminals.

The standard cable the signal is carried on the 1+ and 1- terminals


The bridged cable the signal is carried on the 1+ and 2+ terminals


For biamp, the lows are on the 1+ and 1- terminals and the highs on the 2+ and 2- terminals

[bass sic]

OK, one more Dumbass question so I don't fuck this up. Having the proper speakon connector in hand, a +2 -2 do I use regular instrument cable or speaker cable? I ask because the fully assembled one I bought had the biggest cable and biggest 1/4 jack I've ever seen. I plan to hack a inst cable and install the speakon on the end.

[Hemisaurus]

OK, one more Dumbass question so I don't fuck this up. Having the proper speakon connector in hand, a +2 -2 do I use regular instrument cable or speaker cable? I ask because the fully assembled one I bought had the biggest cable and biggest 1/4 jack I've ever seen. I plan to hack a inst cable and install the speakon on the end.

Yeah, you want to use a speaker cable, you don't want to put > 1000W bridged through an instrument cable 

So hack the end of a speaker cable, it should be pretty simple, those Neutrik's use screw terminals, so you just strip the ends and screw them in, no solder required. Do try and use a fairly hefty 1/4", you'll be carrying a lot of current

Power = Amps ^2 x Resistance

So if you're pushing a 4 ohm cab

1200 = Amps ^2 x 4
Amps ^2 = 1200/4
Amps = SQRT (300)
Amps = 17.3A

Yeah that's math, don't worry about it, just realise your household outlets carry only 15A, so use a nice hefty cable

[bass sic]

Sweet. It all makes better sense once I had parts in hand. Done, and thanks for the help.

[inductorguitars]

Herbie - is there any methodology hints and tricks for tracing out a pcb to draw out a schematic?

My Dimarizo Very Metal pedal doesn't seem to have any schematics online so I'd do the world a favour.

[Hemisaurus]

Herbie - is there any methodology hints and tricks for tracing out a pcb to draw out a schematic?

My Dimarizo Very Metal pedal doesn't seem to have any schematics online so I'd do the world a favour.

Pick an end and work from it? At least you have the unit in front of you, I'm going to try and trace a Solasound Sustain from a bunch of pics (see amp guts thread).

According to this link http://www.freestompboxes.org/viewtopic.php?f=1&t=8713&p=95198&hilit=dimarzio+very+metal#p93103 it's a RAT, and there are a ton of RAT schematics out there, maybe you can just check that's how it is, and mark one up.

[inductorguitars]

Ah I see. I started tracing/drawing but got lost. The Rat schematics I have saved were the newer ones and look nothing like the Very metal. The older rat's (Rat I?) look like my drawings.

[RAGER]

Ok  here's one for ya Hemi.  My 69 Super Bass is redplating v4 and v5 and blowing the 4 amp Mains fuse.
also when I have channel II volume up around 7 or 8, it wants to feedback like micrphonics. i haven't even opened it up yet.  I have several other amps I can use.  I'm pretty capable at fixing just about anything.  Fixing things is what I do for a living.  I know my way around amps a bit having done several repairs already.  Curious to see what direction you would take first.  Thanks.

[Hemisaurus]

Redplating is usually leaky coupling caps in my experience, let me grab a schematic later and look for particular culprits, but whateve cap couples those valves to the previous section.

[Hemisaurus]

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/jcm800_superbass_100w_1992.pdf

Check if C17 is passing DC voltage.

Channel II a two channel SuperBass? Microphonics == preamp tube, get a new tube swap out each preamp tube 'til you find the dodgy one, or replace them all.

Oh and you might find once you fix C17, others start going as well, might be time for a full recap.

[RAGER]

Well i think you need to change your thought process straight off.  This is a 1969 early metal panel Super Bass.  Think Plexi.  Not a JCM 800 1992.  And yes, all 4 holers are 2 channel. (I-2 inputs,II-2 inputs) just not switchable.  I know you know this .

But I think you're prolly right about the coupling cap.  that's my first inclination.

[Hemisaurus]

Well if I can't see a schematic labelled Super Bass that's not a 1992, show me a schematic and I can give you a better answer

The reason I was asking on the two channel ones, where it's two volumes and a common tone stack, the jacks feed two separate tube stages, I can't recall whether it's two parts of the same 12AX7 or two different 12AX7's, and then they are mixed for the tone stack. Anyhoo, if the microphony is only on channel 2, then the tube that's unique to that stage is the one with problems.

Like I said though, seriously consider a more thorough recap, I fixed a bad coupling cap on one of my amps, and a few hours later it started redplating somewhere else, so do keep an eye on it.

[Hemisaurus]

Actually it's STILL called a 1992, thanks for being consistent Marshall.




and the two channels are two parts of the same 12AX7, so it's probably V1 that is bad.

and my comment on the coupling cap holds, the one that feeds both V4 and V5 (the two that are redplating), the .1, thanks Unicord for not labelling any of the components, bet your parts lists were a barrel of laughs.

Have you ever noticed how some schematics are just much easier to read than others? Despite being split into two sections, the Marshall, to me, is clearer than the Unicord, Peavey is one of the worst offenders of unintelligible schematics, because they draw the schematics separated into boards.

[justinhedrick]

hey hemi, i have a relatively simple (i hope) question:

how do you figure how where to set pickup height? is it a personal preference? i got my guitar back from the tech last week, but he jacked w/ the pickup height and now it doesn't seem like my amp is as loud . . . should i just crank that bastard back up? or does he know something i don't???

[VOLVO)))]

It's a matter of preference. The further the pickup from the strings = the less output. All of my bridge pickups are pretty damn close to the strings, as I enjoy maximum output. You just have to play with the height until you get the bridge/neck at an even output, and go from there, whether you want more or less output, raise/lower accordingly.

[justinhedrick]

It's a matter of preference. The further the pickup from the strings = the less output. All of my bridge pickups are pretty damn close to the strings, as I enjoy maximum output. You just have to play with the height until you get the bridge/neck at an even output, and go from there, whether you want more or less output, raise/lower accordingly.

right on. man, i thought something was wrong with my amp . . . i'll mess with that over the course of the next week or so. luckily we don't have any shows/practices scheduled so i can get all my shizz in order before the next one.

[inductorguitars]

<rant>
I'm borrowing an Ampeg B-15n portaflex '63. First I thought the grill was rattling, now I'm not so sure. It sounds more like a torn speaker, but only on high volume settings. I even taped up the handle, since it hangs in the cab.

I'm thinking one of three things
1) power tubes are shot
2) a bad solder joint in the power section
3) needs a recone or new speaker

I'll be testing with a different cab soon...

Any other suggestions?

[Hemisaurus]

Well I can't hear it, so it's hard to tell 

I think the new cab will probably help, I'm always finding loosened mountings on speakers, rattling baffle boards, and that kind of thing. A few cabs have had an arsenal of extra hardware added to keep them rock solid.

Don't sit the amp on the cab if you can help it.

[inductorguitars]

Don't sit the amp on the cab if you can help it.

can't help it it's a fliptop. 

I figured it out after a bunch of experimenting. The handle is vibrating side to side  instead of back and forth when I taped it up.

It still has the Original Ampeg tubes made by Sylvania. Still sounds good but a retube is probably in order soon.

[Hemisaurus]

Even the fliptops you should be able to release the catches and lift it off.

[inductorguitars]

Even the fliptops you should be able to release the catches and lift it off.

Yep but I'd have to seal it off somehow. (Don't really want to go scrouning for plywood)

[Hemisaurus]

It's not like the fliptop makes it airtight, if you're worried a piece of scrap wood on top and a couple of phonebooks holding it down would have much the same effect

Or turn the cab upside down, let the floor do the work.

[VOLVO)))]

Sigh, 10.66 ohms, 6x10 cab, run amp at 8 or 16? Sunn 610L with Celestions sounds like shiitttt. Dunno why, either.

6 16 ohm speakers. sigh sigh sigh sigh

fucking annoying. I don't want to sell this cab, but I hate this cab.

[justinhedrick]

Sigh, 10.66 ohms, 6x10 cab, run amp at 8 or 16? Sunn 610L with Celestions sounds like shiitttt. Dunno why, either.

6 16 ohm speakers. sigh sigh sigh sigh

fucking annoying. I don't want to sell this cab, but I hate this cab.

8. i think it is too far away from 16 but close enough to 8.

it sounds like shit? really?