Amp Tech Thread / Ask a tech Q

[Lumpy]

Pissy the guitar has a strat-style 5 way pickup selector. Mongo probably not smart enough to know how to rewire his guitar. I might bring my cheap hollowbody to the space (Oscar Schmidt!), and bring the cheap Washburn home and open her up.

Danny if a Super Hardon would work, then I will give my MXR Micro Amp a try. Supposed to have 50db of gain, something like that. I will throw some other gain pedals in there and screw around with pedal order. The Rat clone was getting a little muddy though.

I do suspect a pickup replacement would be good. So I want high output pickups? Because the crummy vintage pickups which scream seem to be low output (if resistance is the same as output). I see GFS has three different lipstick pickups, roughly 4/6/8 ohms (?) or is it measured another way. 8 for the bridge, 6 for neck, 4 for middle. Maybe I should get a couple of those to replace my cheap single coils, and a P90 for the bridge?

[Danny G]

FML

The SVT-II is still fucked.

I'm done with it. You want to be an 85 pound door stop? Go right the fuck ahead. I'm done with you.


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[VOLVO)))]

Ship to me. I wouldn't let it sit if I were you. How did you manage to kill it Big poppa?

[Danny G]

It has an inherent design flaw that causes some of them to crackle, like a scratchy volume pot, but its none of the pots.

Took it to best SVT tech in Austin and he was stumped.

Someone on this forum was talking about them and mentioned a friend pulled his apart and apparently solved the problem.

Got mine out of storage, pulled it apart and immediately noticed a burned solder trace from an electro cap and a fried resistor at other end of the board.

Tracked down schematic, thought I found value of resistor. Replaced it and re-established the connection of the burned trace.

Fired it up to bias. It crackled a little as per problem immediately but that stopped once I got the voltage where it should be.

Made noise for a lil bit and sounded fine. But noticed bias voltage slowly climbing despite turning it down repeatedly.

Crackled some more. Then stopped making noise altogether unless dimed. And even that sounded quiet and thru a shitty cable.

Bias still climbing, started to smell funny but no smoke.

Shut it down for good.

Seeing if I can sell as is. I don't have the money to fix it, but was a great amp when it worked.


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[VOLVO)))]

Let me ask my guy, ill see if I can give you a couple extra paths to follow.

[Danny G]

I may have gotten that resistor value wrong and just fried a bunch of other shit on it...


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[VOLVO)))]

Oh boy, get to tracing the circuit.

[VOLVO)))]

If i had the money to take it off your hands, i would

150$ + buying 6550s + shipping would be too much of an investment for me, not because i want to sell it, but because im broke. Haha.

[Danny G]

That's the thing. I'm self taught and know what I'm looking at on a schematic, but translating the schematic into the parts layout is a connection my brain has yet to make.

I was going more by the evidence that I had in identifying the resistor (1/2 watt by size, first band is black, rest burnt to a crisp) than by what's in what part of the circuit.

The only likely candidate I could identify on the chicken scratch/hieroglyphics/schematic was a 1/2w 10 ohm resistor.

Took my chances. I may have chosen correctly. But I don't know cause the amp's fucked regardless...


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[VOLVO)))]

I usually pick all the parts around it, then discern the value. Don't give up on it yet, it may be really easy.

[Danny G]

I get confused because the parts next to it on the board aren't necessarily anywhere near it on the schem.


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[VOLVO)))]

Hmmm... Follow traces, son!

[Mr. Foxen]

Rip that shit out and build an amp on turrets in the chassis with the iron.

[Danny G]

Hmmm... Follow traces, son!

o_O How did you know I'm on acid right now?

Heh (I'm not... Sigh)


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[Danny G]

That, and there are traces on both sides of the board. It's like this thing was built by Ford


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[VOLVO)))]

You can do it, papi. I promise. Just sit and stare at it for a bit.

[Danny G]

That's what she said.


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[liquidsmoke]

From the Dual Showman manual


"an additional 8 ohm speaker can be connected to the EXT. SERIES jack for a total load of 16 ohms, set the IMP. SEL. switch accordingly"

Pretty damn cool. Was this just a Fender gimmick or does it actually work and not fry anything? I don't think I've ever owned a head that could power 3 cabs in such a way. Assuming you had the right ohm cabs, 16/16/8 for 16 or 8/8/4 for 8, would this setup send equal juice to each speaker assuming you were using identical speakers in all of the cabs? I'm thinking about getting another 2x12 for more volume because it would be a lot cheaper than starting over.


edit: this was me being hopeful. Why would the amp split the signal equally 3 ways with 3 cabs? That's not how they work. It would be a 2 way series split of course.

[dk419]

So.... Tube Biasing question: I have a Green Amp from 2005/06 era. I've always taken excellent care of it, and regularly re-tubed when needed. I was playing a show last week when my guitar dropped out altogether. At first glance, the power indicator light was not on, I got no sound at all, and the tubes were still glowing like normal. Nothing looked off with the tubes. After checking the fuses, we discovered that the HT fuse (1.25A/250V fast blo 5x20) was blown, and the power fuse (6.3A/250V Slo-Blo) was not blown, but looked a little cooked in the center.

Aside from the fuses being quite the task to find (and I ordered a 5 pack of each fuse), it was suggested to me that I should replace the power tubes just in case. A failing power tube can be a cause of a blown HT fuse apparently...

So, a couple of questions...

1) Is a power tube issue most likely the main reason that I could have blown the HT Fuse, or could old age and not having a standby switch be a possibility? What else could it be?

2) If I'm gonna retube (EL34's by the way), my tech showed me how to do it pretty much for the biasing. Each tube has its own trim pot to adjust the bias. I'm pretty sure that he and Joel @ Electric Amp recommended between "032 and 035" Not sure what that means, but IO guess I'm supposed to put one of the leads of a DMM to a green resistor at any othe tubes, and the other lead grounded on the chassis somewhere...



I have a call in to my tech, but I believe he is away for a bit. I'm gonna do this myself, unless I figure out the tubes and fuses don't fix the problem.

What should I be setting the DMM to? mA? Any advice?

[RAGER]

My advice is; if you are asking these kind of questions then you should have a tech do this. No shit, you can kill yourself with the voltages in your amp. No exaggeration. Also have your tech check your speaker cab for a blown speaker.

[RacerX]

I wouldn't put much stock in Joel's opinion—he's an insurance salesman. Ask John McNeece.

[Mr. Foxen]

Green one looks like the one to measure across. Might be connected up to test points for bias. Read the code, check its 1ohm. Chances are HT fuse going is one dud power valve, and with individual bias, you only need to replace the dud one. Replacing whole sets of output valves is to give free valves to your tech.

[dk419]

Update: Got the fuses, as well as new power tubes and even preamp tubes. Why not? Anyway... installed fuses and tubes. I got 3 of the tubes biased all nice and happy, but one was not being cooperative. Didn't have the range of adjustment that the other 3 did. Also, it was glowing a bit too much compared to the rest. Super hot spot! I switched the tubes around, but figured what the result would be. Same shit. So one tube position is funky. Meanwhile... that plastic burning smell that was present when I blew the fuse came back. I sniffed around the amp, and unfortunately it seemed to be coming from the output transformer. Shit!

And so... it's going to the tech on Wednesday. Time to replace shit and tune it up. It's from 2006, and although it hasn't been on the road for that amount of time, it's gotten its fair share of use and abuse. I gotta say... Joel called it back in '06 about the output transformer and all the other shit. Damn! After this, it better be all set. Until then, gonna run the V4 and see if I can blow that one up.

[jibberish]

does anyone see any reason that the power tubes can't be pulled if an amp is going to be used for just its pre and effects out?  it looks like the output xformer is out of the circuit,but double checking doesn't hurt[the amp so much] heh.

[Pissy]

no danger.  do it.