Amp Tech Thread / Ask a tech Q

[Hemisaurus]

IIRC there's some known problem with the switching jacks carrying too much current or voltage on the effects loop of some Marshalls, dunno if that's what's happening here though..

That could be, maybe they used crappy jacks?

I did read this:

Nature of the beast due to the design. This is one of those that was placed AFTER the master volume, which is the absolute wrong place to install a loop within an amp circuit.

So the "proper" way to set it up would be to start with setting the input level on the FX unit to the 12:00 position, set the master on the amp to give you sufficient drive signal into the FX unit, then use the FX unit's output level knob as your "master volume". You may have to play with the amp's master setting and the FX unit's input level setting to get the right balance, but the end objective is to be able to use the FX unit's output level knob as a substitute for your amp's master volume.

Kind of a stupid way to do shit, but unfortunately due to Marshall's inferior design this would be about the only way to make this setup work without lots of noise/hiss.

hence my suggestion of just moving the position of the master volume in the circuit.

[AgentofOblivion]

Hmm, interesting.  Are there any "Do NOT do this..." that I need to know so that I don't damage anything?

[Hemisaurus]

That's hard to answer, what were you thinking of doing?

[Mr. Foxen]

Who wants to help me fix an oscilloscope then?

[Hemisaurus]

What kind, what's it doing?

[Mr. Foxen]

Unspecific dead power supply, according to the guy who gave it to me, he just bought another one as he had better shit to do than fix this:

[Hemisaurus]

Looks of an era, where it won't be a switch mode supply. I'd suspect it needs a cap job. They used the same style of multi-can caps in scopes as they did in tube amps. Go in, find the power supply, and fix

[Mr. Foxen]

Kinda what I was hoping.

[Hemisaurus]

I will say, multican caps are a bit of a bugger to remove from PCB's. Be extremely careful to get all traces of solder off the lugs or you'll end up tearing up the PCB. Solder sucker, followed by solder wick, if you have flux, use it.

[Mr. Foxen]

I found a neat trick of getting some silicon fuel hose for model cars and putting it over the end of the solder sucker, so you have a flexible tip that seals over a board well.

[mortlock]

seriously..id never flex a board into a secondary circuit without amping up the voltage to at least match if not surpass the primary discharge level of negitive energy.

[Instant Dan]

The amp rating for my Randall RG100SC


But the outlet cord I am using says

10A 250V

Am I damaging it?

[Hemisaurus]

The amp rating for my Randall RG100SC


But the outlet cord I am using says

10A 250V

Am I damaging it?

No, each is a maximum rating, the amp will draw the current it needs, the cord can handle the needs of the amp, and more, so you are fine.

[Cursed71]

I recently aquired a Laney Klipp that is in great shape.  The previous owner had it serviced recently, with fresh tubes and caps.  I played it for 5 mins, and it blew a fuse.  I took it to a tech friend for a once over, and even retubed and biased it.  He said everything looked great under the hood, and there were some nice upgrades, and jammed with it at full volume with no problems. I got it back to the space, played it for a few mins and again blew a fuse!  What's up with this thing?  I was running it at 16 ohms into a 16 ohm cab.  I was plugged into the klipp channel with the vol at 4 boosted with a catalinbread boost pedal.  Id be grateful for any advice.

[Hemisaurus]

So what is different between you playing it at your friends and at your space? Different cab?

Not knowing what else may be different I'd suspect either your cab is not what you think it is, maybe someone changed the drivers, or one of them is blown, or that the power in your practice space is suspect. Or it could be something mundane like a bad speaker cable.

So think, what changed between tech friend and practice space?

[Cursed71]

Yeah, I did check all of that.
  After the klipp went out I played the rest of practice with my other amp.  Both cabs are orange (16 ohms 4 V30 speakers) and I ran both of them no problem with a 120 watt Matamp at 8 ohms.  My plan was to run both amps with a radial aby box.  I had them both at 16 ohms with each amp running one cab with the split signal.

[Hemisaurus]

Well different amps react differently to different loads. Were you using a 16 ohm cab at your tech friends? Are you stacking it differently? Putting it on it's side? Which fuse is it?

[Cursed71]

I'll check it out tonight at practice.  There are two fuses in the back.  The one that blew is on the right when looking at the back of the amp.

[Mr. Foxen]

It has top bollock Partridge trannies, so it should be pretty consistent with different impedance loads, but if one is wrong the high HT will make it a bit sensitive to stuff like brief open circuits and such. What valves are in there, don't get away with much on them. Good clean of the valve bases is sort of necessary also.

[Hemisaurus]

It is weird that it runs A-OK at your friends, and not in your practice space, that would imply the problem is outside the amp, not in it.

Unless, it's some weird transformer issue.

If he was willing I would ask your friend if you can take whatever setup he used, and run it at your practice space, just move it all, cables, cabs, etc. Or I suppose alternatively, take the exact setup of your practice space, round to your friend, and he can examine it all, I guess in retrospect that makes more sense. He can find any bad cables, bad drivers, etc.

Fixing things is so much easier when it's all on the bench in front of you

[Cursed71]

Thanks for all of the suggestions.  I will get to the bottom of this!

[Hemisaurus]

Let us know what it turns out to be

[Pissy]

I'm trying to think of a good way to hook up three taps from the OT for the different impedance options. Currently these Garnets that I have are hard wired to the output jack and in the event I want to hook up to a different impedance cab, I gotta open up the head and swap wires.

Any insight into a good triple throw switch to do this that can handle the current?  I don't think a typical toggle is a good option here as it would likely be subject to inadvertant movement without me knowing it during transport.

[Hemisaurus]

The easiest way, I've seen done in high power amps, is separate jacks, one for each impedance tap.



You can buy impedance selector switches, Nick could get you one from cedist.com.
http://www.cedist.com/search/node/impedance%20select%20switch

[liquidsmoke]

Experiencing a mild burning smell today in the music room. Running a couple of new pedals but when I stick my nose near them they don't seem to be the source, nor is my amp or cabs  Nothing is burning in the kitchen or elsewhere in the house or outside. The air conditioner is off. I noticed a similar smell 6 or 7 years ago and couldn't locate the offending device then either. Weird.