Separate names with a comma.
Discussion in 'Amps and Cabs [BG]' started by Brennan, Sep 9, 2022.
Yea, I was thinking of going that route. Good advice
Just general cleaning and removing the bad AC cable. Years of gunk. I’m more concerned with the circuit board potentially being compromised.
I would not worry about the circuit board being compromised. Those old boards are easy to work on, not like the light and small traced stuff of today. A good tech can figure out how things should be and craft a fix where the board is burnt if need be. The guys are right though, there is something significant that has failed elsewhere in the amp. That resistor smoking was a result of some other failure. And no, a crappy extension cord is very unlikely going to be the cause of an amplifier's internal failure.
Thanks, could a ac surge cause that?
Do you know how to work with capacitors? If not STOP! First, Read up on discharging capacitors.
You can certainly repair it yourself, if you have a variac, scope w/ an octopus, good meter, dummy load, cap discharger, tone generator… and the right schematic for that model.
Seriously, old amps that have not been turned on in a while should always be brought up slow on a Variac to limit the chance that one failed component would cause damage to other components. (If something if starting to give off a tiny little bit of smoke at 10 volts, you can bet it would be melted or on fire at a 120.)
Indeed. That amp took a big hit, whether voltage spike, tube short, lightning, something. Don't plug it in until you've had a qualified tech check it out.
What’s an octopus?
When using a Variac, always monitor the current to PREVENT additional damage.
An octopus is an eight legged sea creature, no idea where that comes in when fixing amps though, inspiration perhaps.
Op resistors burn when there is too much load placed on them, ie something between the resistor and ground not the resistor and a positive charge.
Bottom line is you need a competent tech with the proper tools for this. It's not a matter of swapping parts that might be suspect or that someone on the internet recommends, it's a matter of having the skills and tools to properly diagnose what is needed.
If it were large enough, and long enough.
If you weren't there when the surge happened then there is no point in asking. Is it likely that the typical AC power surge could have done this? NO. For a "surge" to do this kind of damage to your amp, pretty much a lot of other equipment in a lot of other homes or business would have been damaged too.
If your next questions are about a direct lightning strikes, solar mass ejections or large EMP's, then stop. Whatever unlikely external factors you can come up with that you wonder may have caused this does not matter at this point. What matters is getting it fixed by someone who knows what they are doing.
No one here can diagnose what's wrong with your amp in a forum. Everyone has done their best to give you all the free advice you need. Take what you saved on that advice, add that to the money you haven't spent yet chasing guesses, and get it to a qualified tech.
Don't even bother with the AC cord. There are right and wrong ways to do that as well. If you get it wrong, then you'll have to pay the tech twice. Once to undo it, and again to redo it properly.
I agree, another component, probably a cap, failed and caused this, but resistors can and do fail all on their own. There are more than a couple of failure modes. Seen it many times.
AKA, curve trace component tester.
And watching for smoke is an easy way to monitor current.
Here is a QSL article about octopuses… (yes that is the correct pluralization of the singular.)
They’re extremely intelligent and those that use vintage amplifiers (rocktopi) are very capable of repairing their own gear due to their high intellect and their ability to hold a lot of tools at once. That’s why they’re handy to have around for doing amp repairs. I’m here to help….
OP, it might seem like everybody’s ganging up on you when they tell you not to try this yourself but it’s really not meant maliciously. Tube amps operate with internal DC voltages that are very high. Hundreds of volts. If you’re not sure - really, really sure - what you’re doing in there the amp can kill you. No fooling.
What’s worse is that the banks of capacitors in these amps can store those hundreds of volts for several days in some cases. That means you can unplug the amp (to make it “safe”) and it can STILL kill you! Seriously; be very careful.
I’m a big fan of DIY. I’ve been working on my own tube amps for 25 years or more but I also studied electrical engineering and I started with simpler, less dangerous things. If you want to learn to work on your own tube amps I totally recommend it but don’t start here. Work your way up. A pedal, a kit…. Something to get your feet wet without putting yourself (and possibly your new vintage amp) in great danger.
Resistors do not fail on their own burned like that.
The failure of resistors is very uncommon and almost always mechanical in nature. On our products, we see a resistor failure less than 1 in 1 million, all axial resistors are tested on our auto-insertion equipment before being inserted into the PCB during manufacture. Since there are only 5 or 6 axial resistor manufacturers of these types of resistors representing ~95% of resistors manufactured, and we use 4 of them, our results will be the same as everybody else’s.
I do see resistors fail because the designer didn’t do their calculations right, or simply screwed up, but that’s not a resistor failure, it’s a design failure.
In my 45 years in electrical engineering, I have never heard the term octopus used.
[edit: it appears that the term octopus is a home brew or DIY name for a DIY type of curve tracer emulator rather than what the industry uses, which is why I never heard it. ]
Hint… if you monitor the current rather than looking for smoke, you can avoid a whole lot of damage.
Regarding the smoke, yes I agree 100%.
I was only making a joke.
In fact, for anyone who might have an old variac, you can get a digital ammeter on Amazon for about $15 and permanently attach it to that old variac to monitor draw while using it.
Resistors can drift out of tolerance. That is a gradual process that can take decades. Resistors do not spontaneously combust. When the smoke gets out of a resistor, it's because it had help.
Old Garage-Banderyou and agedhorse have got it right. Ive looked at every low impedance path possible for both of the 22k 1/2 watt resistors. so with a single point failure, this can only happen if one of the 20 mfd 500 volt caps shorted. the 22k resistors are voltage dividers. one for both the preamps through 470k and 220k 1/2 watt and the other for the phase inverter through 2 47k resistors 1/2 watt. as agedhorse earlier hinted at. there is 445 v on the first filter cap rated at 40mfd 500vdc then a 1k resistor and another 40mfd 500vdc cap with 425vdc on it tied to the top of both 22k resistors. you can see all of those resistors in the photo provided with at least no catastrophic damage. of course someone could pour a bucket of pennies or dropped a screwdriver in it, etc.
old electrolytics are famous for shorting I would bet if only that were fixed the others would blow soon enough
No. This schematic is all you need.