This Lisajous pattern means, "Your amp is angry!"
In our little online world we run into folks who work with musical instrument amps in a variety of capacities. Some are just hobbyists, many are learning, some are professional electronic technicians and so on.
Building something is not that tricky if you can follow directions carefully and solder well. But where the stuff hits the fan is when it comes time to troubleshoot something. Anything from 'why is it sometimes making this noise?" to "why does it blow fuses" to just plain going up in smoke.
It is often much easier to fix a commercial device than a homebrew or DIY clone. Why? Because the commercial device actually WORKED at some point.
The DIY could have multiple fatal (or non-fatal) errors and may never have worked.
And to be crystal clear, I didn't develop any of these things, I learned them from smarter & more experienced folks along the way. Hopefully something here is helpful to you.
The key to fixing anything is roughly the same:
What are your first principals? What makes it work (when it works)
Take measurements, compare measured and expected
Devise tests and compare measured and expected
aka:
Identify the issue
Isolate the issue
Resolve the issue.
When I fixed cars, the first question is usually 'is it getting gas?" followed by 'is it getting spark?'. These are the first principles. If you have fuel and a spark, something is going to happen. If the car just turns over and wont start, you are probably missing one or the other.
If there is a noise in the chassis, the first questions will focus on when does it happen (Turning? Accelerating? Braking? At a specific speed? Just over bumps?) and then does it vary with speed? (is it part of the driveline?) You get the idea.
If you're fixing a piece of electronic gear, component failures will generally lead to voltages that are off. How do we know they are off? Even if we don't have a schematic handy, we should understand the basic principles and some handy generalities.
The Basics
First is Ohms Law. Volts = Amps X Resistance. You should become familiar with this in all of its forms. This one formula can let you essentially reverse engineer an amp assuming you have a couple other pieces of data:
An average triode stage such as a 12AX7 draws about 1.5mA of current. This is ballpark, but close enough to troubleshoot with.
Your power tubes will draw whatever idle current they have been biased at. Try not to think about bias voltage (what is usually on the schematics) but the current that is passing through that device.
An amp circuit will *always* make some low level of hiss or hum. If nothing at all is coming out the speaker, you need to start there - with the speaker, speaker leads and output jack before you do anything else.
Hum comes in 2 flavors in your amp circuit; 60Hz AC is normally used to operate the filaments so filament circuit issues will have a different sound than the 120Hz hum that comes from your power supply. 120Hz is roughly B2 on your keyboard. 60Hz is roughly B1 (123.47Hz and 61.74 are the actual notes) These are close enough to learn to hear the difference. It matters because they have two very different sources.
Now, with just those generalities and Ohms law, you can dive into all sorts of issues and resolve them fairly quickly.
We know the control grid on a tube (any signal tube) will have 0 volts (or usually *should* - but not always - a direct coupled cathode follower will not), the plate will have some high DC voltage reading and the cathode will be at 0 for fixed bias tubes and at some low voltage for cathode biased tubes. Typically 25V or less. So the plate has high voltage, the grid is often (but not always) at 0 and the cathode will be at some low voltage. That's a good start.
Measuring backwards
Now if you measure 440V at the first power supply node, (probably your output transformer center tap), you should expect to see close to that at the power tube plates. The voltage will drop based on the primary resistance and current (since we're talking about DC here) so if your primary is 180 ohms and one side is 80 ohms and the power tube on that side is biased at 35mA, you should expect to see a voltage drop of V = I * R, .035A * 80 Ohms = 2.8V. Not much drop.
The screens will be a little lower after going through a choke (common) or resistor (less common).
The phase inverter plates will be a little lower still, and there will be a measurable drop across their plate resistors. And so on.
If you take a typical schematic and just start with the first power supply node - just pick a voltage - say 450, you should be able to estimate the rest pretty easily. Just add the currents of the downstream devices and figure out your voltage drops with Ohm's law. If they are significantly off, you have probably found at least one of your issues.
For a 60's style 'fender reverb amp', a deluxe reverb or larger, you have 6 pre-amp tubes with 2 sections each, so 12 x 1.5 (we're just estimating for ballpark "ok-ness" here) so you've got something like 18mA of current going there in total. Your power tubes are probably 20mA (6V6) or 35mA (6L6) X the number of tubes (2 or 4). And now you can figure out your ballpark voltage drops at each B+ node. If you measure 300 and were expecting 400, go see why. Being off a 10 or even 20% can be 'normal' sometimes. So a 300V node could be 270 - 330V and be fine. (+/- 10% off)
Safety First!
Never ever probe with both hands in a live chassis. Buy the alligator clips for your DMM probes, clip the negative to the chassis securely and then probe using your right hand. Why? Because you never want to make a path through your heart, and it is on the left. Really safety conscious folks will have you put your left arm behind your back while you probe - which is a good idea too. FWIW I do this because otherwise I get sloppy and touch the chassis with my left or do something equally dumb. You can never eliminate mistakes, but you can reduce the number of opportunities to make them.
Make sure your power supply caps have been discharged. Do this however you prefer, but do it.
If the amp is plugged in and powered off for some reason, make sure you are keeping track of the fact. There will be live & dangerous voltage on the on/off switch and surrounding components. It is easy to get sloppy so I always either unplug them or I use the Variac to cut power (shutting it off instead). That way I can also glance up at the volt and current meters and confirm nothing is energized. Try to work under the assumption that the chassis is always live and unplug it before starting any repairs.
Going along with #1, never work barefoot and never ever work barefoot on concrete. It's not a weird edge case; a lot of us probably work in the basement or the garage. Concrete conducts really well and having bare (possibly sweaty) feet makes you an attractive ground. You do not want to be an attractive ground. Wear rubber soled shoes or sneakers.
Where do I start?
If it is your amp, just dive in. If it is for a client, listen to them, empathize with them, and then ignore them. 9 times out of 10 they will have a diagnosis or theory that is well intentioned but wrong. Just clear you mind and be methodical.
I usually start with voltage readings, working my way from the output stage backwards to the input. Why? Because the output stage has a lot more current, gets hotter and does harder work. It breaks more. It is also where your B+ "starts" - where it is highest. You can put the rest of the amp in context from that. If it is off at the start, well now you know where to begin.
I also do some really basic tests to see if I can narrow things down
Do the knobs affect the sound (or lack of sound)?
* if you know where the volume knob is in the circuit this can potentially tell you where the sound is originating. Maybe the bass knob makes the hum louder. Same idea.
Does the issue occur on all channels or just one? (eg is it in the pre-amp or power amp?)
* again, we're trying to ID where (roughly) this thing is coming from. Pre-amp issues in a 'classic ' fender reverb amp will just affect that channel. If both channels are affected the issue is probably after the mixer.
Does plugging into a different jack change anything?
* (how about the FX loop?) Jacks go bad from too much use or not enough, and can cause all kinds of fun noises. If you have a signal generator you can try injecting a sine wave into the phase inverter input and see if you can drive the amp to full power out. Or use the FX return jack. This lets you test the power amp separately from the pre-amp.
What does the amp do with nothing plugged in?
* is this possibly a guitar or cable problem? User error? <cough>
You do not need all kinds of fancy equipment. It's impressive in pictures but not needed day to day usually. With good basics and a little creativity you can fix most issues with a voltmeter. And if you think like a shop owner, you will want to fix things this way because it is often faster. Every device you need to buy, maintain, calibrate, hook up and then interpret.... is time & money.
When I made studio calls, I just brought a DMM, a chopstick, freeze spray and some basic hand tools. (and popular spare parts)
We're not re-engineering stuff, we're just trying to make it 'not broken'. Resist scope creep unless you are being hired to do a restoration or something (different story).
Some really common scenarios:
If your cathode voltage is much higher than normal you probably are not passing current through the tube. The tube may be dead or the cathode connection to ground may be at fault.
Ditto Plate voltage that is either too high, or not different between the supply side of the plate resistor and the plate side. Ohms law says you should have a voltage drop. If we take our "guestimate" of 1.5mA as common for a single pre-amp triode section and 100k plate resistors as "standard", we should be looking for a drop of about 150V. (.0015A * 100,000 Ohms) So if the plate is at 200V, the supply would be 350V - ish. Keep in mind that 10-20% tolerance is pretty normal here. If the plate and the supply are both at 350V, either the tube is dead or the cathode circuit isn't getting to ground.
If your voltages are reasonable but you are not getting a good signal out the speaker you can use the circuit disturbance test as a first check. Simply touch the control grid of each tube with your voltmeter probe or other insulated probe (but the end should be metal), working from output tubes back to the 1st stage. You should get a pretty good "pop" when you touch it. The stage where you don't has an issue. You can often work quite fast this way. eg; if you touch the output tube control grid and nothing happens, you probably need to look at the output stage. If the issue is more subtle, you can use a scope - or if you don't have one handy, lift one end of the coupling cap at that stage and try injecting a signal. A leaky coupler will skew the bias and cause distortion under signal. Don't bother testing the thing, just replace it. Caps are cheap, time is expensive. You can check voltages in 5 minutes, you can do a quick circuit disturbance test in about the same. You can often have a basic amp diagnosed in under 30 minutes in many cases, just being disciplined and focused on how you attack it. I did this in front of the store manager at Sam Ash in-person, working with just a voltmeter and a phillips head in the store. He rolled in a broken silver face twin reverb, and I had it diagnosed before I lost his attention. Ken Fisher was my inspiration for finding a fast and simple methodology. He was a legend at finding faults fast, probably thanks to his days working the line at Ampeg. Like any other activity, the more you do it, the better you get at it. Your technique will get better, your solutions will get better, even your guesses will get better.
If the amp is blowing fuses right away, pull the tubes and try again. (to eliminate them) If you blow the fuse with no tubes in the amp, disconnect the power transformer secondaries and try again. We are devising tests here to help isolate the issue
1 - remove the devices that short the most and draw the most current (the tubes)
2 - isolate the power supply as far towards the start as we can. If the PT is ok, then you can start adding secondaries back one at a time. (bias, filament, B+) and see how it goes.
3 - The B+ supply can have a short due to a bad filter cap (not uncommon) The filament supply can have a short from a power tube that shorted and arced. But by isolating things you can ID the area you need to measure and repair.
If you pull the power tubes and everything is ok, you have ID'd the area of the issue and now you can look at things like leaky coupling caps, bias settings, bad tubes and so on.
For powering up unknown gear, I have used a metered Variac forever; I have never even owned a lightbulb limiter. (since the mid 1990s at least) Both work fine but with incandescent bulbs going away, I would recommend setting up a metered Variac. It's super easy to use. I go into it HERE
Check your grounds!
Clip your negative lead to ground and probe all of your ground connections and see how many ohms they indicate. Ideally they are all zero or some really low fractional amount. If any are over an ohm or two, dig in and see if it's real or just rust under the probe etc. Fix/re-flow/etc. as needed.
Hum
Hum that starts and increases is nearly always something pulling more and more current - and you should shut the device down ASAP. This is different than say the buzz you get from bad grounds, badly grounded guitars and so on. This is that HUMMMMM that you will quickly learn to pick out - it's the sound of tubes red-plating, of resistors about to burn or caps about to pop, and of fuses getting ready to blow. Hopefully.
Noise/Static
This is often a ground issue. Learn to differentiate this from power supply hum (120Hz). A bad pre-amp tube can cause all kinds of noises too. It may test fine; don't hesitate to swap in a known good tube.
If you have a distorted or weak signal, the usual suspects are:
Tubes (hey, that is why they are in sockets!)
Plate resistors (they can drift up or even go open)
Coupling caps (can leak or go open which screws up the bias etc.)
Ground connections!
Pots
Speaker damage (easy to test - just hook up your shop cabinet)
If nothing at all comes out and your speaker, leads, and jack are all fine, and your output tube voltages all look ok, it is possible you have a bad output transformer. But this should always be the end of the list - not the beginning. It is pretty uncommon.
Vic, the gent at Stanton TV in Mamaroneck, NY who generously let me watch as he fixed my old Fenders would do things like re-flow the grounds in the amps just like you might clean the pots. It was just normal maintenance and could help eliminate issues without even having to t-shoot the thing. Bad grounds can cause a raft of oddball issues.
Vic also kept an oven mitt by his bench so he could swap in new pre-amp tubes while the amp was on (put on standby though so it wouldn't POP! loudly) and he used an electric drill to take the amps apart. Very simple techniques to save time, but very effective. He fixed everything from microwave ovens and vacuums to synths, hifi gear and old guitar amps. Time is money, and he was very fast.
Testing your output transformer
People make some really common mistakes here, so lets see if we can clear that up a bit.
DON'T TAKE DCR MEASUREMENTS EXCEPT TO SEE IF A WINDING IS "OPEN"
Differences in DCR across a winding are usually meaningless and if you do not have a "known good" set of measurements you cannot infer much of anything.
If you have an LC meter, you can place a resistor of a known value (it does not have to be 8 ohms... it can be 47, or 100... but 4-18-16 will make it much simpler) connect it across the secondary, keeping in mind that secondary imp value. Now measure the primary impedance with your meter, and then correct for the difference in resistor value. This will show you what is going on with the transformer, if the primary is way off and/or one side is a lot different.
If you do not have an LC meter, go buy one. They are cheap. Seriously. I use THIS ONE
Ok, you can also use a signal generator (or even a Variac in a pinch but I don't like this solution because the voltages can get high by accident) to put a 400hz - 1kHz sine wave into the secondary and turn it up until you measure 1V rms across the secondary with your DMM. Now measure the voltage across the primary. Since the voltage ratio also equals your turns ratio, that is your turns ratio, and you can use it to calculate the impedances, and again, see if you are in the ballpark. So if you inject a signal at 1V into the secondary and you get 20V across the primary (plate to plate), your turns ratio is 20:1. The square of that, 400:1, times the load imp (8 ohms) will give you your primary impedance as seen by the tubes of 3,200 ohms. And again you can measure the sides of the primary to the center tap and see if they are way off. By WAY, I mean like one side is 20 ohms and the other is 150 or something. This is the kind of thing you'll see sometimes with shorted windings. Open is obviously bad as well.
You can have high voltage issues - the OT can look and measure fine but under load it all goes to heck. This is less common, but easily tested either with a insulation tester (give it 1kV or 1.5kV) or by swapping in a spare OT.
Just to help clarify #2, lets say you measure 180 ohms across the primary, and one side is 100 ohms and the other is 80. OMG what a disaster? Isn't that 20% difference? Well, no. Why not? Because you are conflating DCR with Impedance, and they are not the same thing. Everyone does this at some point so don't feel too bad. Resistance is DC, Impedance is AC.
If your primary impedance in the circuit is 4,000 ohms for example, each tube is working into 2,000 ohms (lets assume this is class A for now) 20 ohms out of 2,000 is 1%. That DCR variance is meaningless against the AC impedance. Like Run DMC warned us, "It's Tricky!"
Even if it is class B where each tube works into 1,000 ohms (because the other tube is in cutoff), that 20 ohm DCR is still only 2% of the impedance.
Probably still a lot smaller than the mis-match on the tubes themselves.
Flow charts get unwieldy, at least for me, so I made an outline instead to try and compartmentalize some common scenarios. This is not everything that can happen, but it probably covers 80% or more of what typically goes wrong. There is always more.
Do you have a schematic?
Is it s simple (SE, vintage or otherwise straight forward circuit?) - draw it out yourself! It is great practice, and most of these things have more in common than not.
The more you do this, the better you get and the faster you can work on things.
If it is modern or complex, send it to a pro unless you have a schematic. Even then, some are wrong from the factory. You really need to have a good grasp of what makes sense and what doesn't.
What are symptoms?
Think carefully about what they are and why they might occur. (why does it work when it works?)
Visual inspection
Look at the chassis. Has it had any obvious prior repairs, upgrades or modifications?
Are there any obvious bad/cold solder joints or sketchy wiring?
Are there any burnt or broken components? Tap things, push on things.
Are any electrolytic caps bubbling or bulging on the ends?
No sound
is there hum or hiss in the speaker
no? - check speaker (open?) speaker wiring, speaker jack, OT secondary wiring
yes? - circuit disturbance test on power tube grids or PI input (anything?) work backwards towards the input and find out where you lose your signal.
Hum
60 or 120?
All the time or just under certain notes you play or?
Constant level or increasing? (if it is getting louder, power the device off ASAP)
Static-type noises / odd hums etc.
Check grounds & grounding
look for cold solder joints
Is there shielded cable in the chassis? Is it wired properly?
Is there a cel phone on the amp, or is it plugged into a circuit with a dimmer?
Buzzing on certain notes
This can be a speaker/cabinet resonance issue but also can be a power supply issue.
Squealing feedback
Check phase of feedback loop (if there is one) could be OT secondary, OT primary or phase of grid leads from phase inverter. Anywhere inside the negative feedback look where phase could be reversed could cause this.
Bad pre-amp tubes can do this. Try pulling tubes one at a time to see if it stops. Start with first pre-amp stage and work back to phase inverter. Why? The first stage gets amplified the most. Any noise here just gets worse.
Thumping low frequency instability
Filter caps - this is called "motor boating" happens in solid state amps as well.
Typically gets worse as you bring up the volume (level) or may not happen under the volume comes up
Misc Rattles: (pick one based on what you think is going on)
Remove the chassis from the cabinet (eliminate tubes and or other microphonic components)
Remove the speaker and baffle from the cabinet
Remove the speaker from the baffle (isolate speaker to see if speaker is rubbing or if cabinet has a rattle)
Tap on tubes with a chopstick/wooden dowel/etc. check for microphonic tube
rattles are usually power tubes, ringing is more likely pre-amp tubes
References:
Ohms Law Chart I stole from Bruce Egnater (thank you!!)
Useful applications:
Q: I had a 40W amp on the bench that has 8 ohm out. What are the volts and amps I can expect at full power?
A: Using the P = I^2 * R here, we can say 40W = I^2 * 8, so 5 = I^2, and I = 2.236A. Which means voltage would be V = I * R, or 2.236 * 8 or 17.88 volts. You could use other forms of course.
Using this you can see why running a 16 ohm load is a bit easier on the amp - you get the same 40W, but more of that is voltage vs amperage, and current is what stresses the wire on the secondary. At 16 Ohms you only have 1.58A. The same 40W amp with a 2 ohm tap is pumping out 4.47A; 2.83X as much current.
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