Common Tube Amp Troubleshooting Scenarios

Written by
Andy Johnson
Published on
January 22, 2021 11:09:08 AM PST January 22, 2021 11:09:08 AM PSTnd, January 22, 2021 11:09:08 AM PST
In the world of tube amps, it seems like any number of issues can arise. However, there are a few broad problems that will occur in just about any amp at one point or another. These are problems that almost every tube amp owner has encountered over time: Tubes not lighting up, fuse(s) blowing, and bad ground connections.   
Today’s article will present these broad problematic scenarios and provide numerous troubleshooting techniques for each. We will not be providing detailed breakdowns of how to complete each troubleshooting procedure, but rather this article should serve as a basic skeleton for those with some degree of technical knowledge/experience, and should help pin-point the root of these problems. But before we start…
**AMPLIFIERS CONTAIN LETHAL VOLTAGES** As always, take your time and work safely. If you are unsure about which circuit is where, and what voltages and/or grounds you need to check, take the time to do thorough research.  The web has a limitless supply of recommendations and schematics for you to review before you start your troubleshooting adventure.  If you are still unsure of what you need to be looking at, take your amplifier to a qualified tech.  There are many things that need to go right, and just one thing going wrong can shut down the amp. Be safe and smart about any type of troubleshooting! Okay now let’s dive in!

SCENARIO 1: TUBES AREN’T LIGHTING UP


Let’s say we have our amp assembled to the best of our ability, or we are servicing an amp that has no tube glow.  Where do we start?  We need to test voltages beginning where the power comes into the amp, and working our way down the line. Step 1: Test The FuseLet’s grab our multimeter and get ready to test the “mains” fuse. There is generally only one fuse for the whole amp in vintage Fender-style amps, but there are usually two fuses in Marshall-style amps: one is for high-tension (usually labeled “HT” or “B+”), and the other is “mains,” which is the 120v feeding your power transformer.Locate the fuse and do a quick visual inspection. Is it blown or even missing? If so, replace the fuse with one of the proper value and check to see if your tubes light up. If the fuse is present and not determined blown via your visual inspection, use your multimeter to check the fuse for continuity.  To do this, turn your multimeter to the beeper continuity setting.  If your meter does not have this setting, then you’ll need to test for resistance. Once on the right setting, hold one probe on each of the metal contacts on either end of the fuse. If on the beeper setting, you should hear a beep if continuity is good.  If it’s bad, no beep! If testing for resistance, a good reading is 0ohms, a bad reading is no reading at all.If you’re able to determine that the fuse is “bad,” this typically means that something caused the fuse to blow, as you will very rarely have a “bad” fuse straight out of the box. At this point, you will need to look over the entire amp to see if there is shorting somewhere in the amp.Step 2: Visual InspectionWhen it comes to a blown fuse, the usual suspects are two pins on a tube socket being shorted together, a tube that has developed an internal short (after many hours of play time), a bad component such as a filter cap or missing (broken) ground, a failing power or output transformer, or general incorrect wiring. So before anything else, it is worth taking this time to visually inspect the tube socket connections, and if possible, find a copy of the amp’s wiring diagram/schematic so you can visually compare the document to the physical wiring of the amp to make sure nothing obvious has been improperly wired. If everything checks out, you can move on to the next phase of troubleshooting...Step 3: Testing Wall VoltageUsing your meter you’ll want to start at the cord and check for wall voltage.  For this, you’ll turn your multimeter to the AC Voltage setting, and place one probe at the switch, and the other at the fuse. Then you should check from the fuse to ground. Both of the aforementioned tests should have roughly a 120v reading. If you are not seeing voltage here, there is most likely a problem with the power coming from the wall or you could have a bad power cable.Step 4: Testing Power Transformer Secondary VoltagesIf the above test checks out okay, then you can move on to testing the transformer secondaries.  To do this, you’ll need to remove all of the tubes from the amp. Have a copy of the proper transformer diagram on hand to make sure your voltage readings are in line with the requirements for your particular transformer. Check the filament secondary for the tube rectifier (if equipped) as well as your output and preamp tube sockets.  Are you showing 6.3 vac at the further most preamp socket?  If you’re showing correct filament voltage there, then it’s more than likely fine across all of your sockets but it is best practice to check them all anyway. If you are not showing voltage, you should work backwards to the transformer to determine what the issue is.  It’s not uncommon to break a filament line on a vintage amp when installing new tubes.  Keep in mind, the pins inside the socket are supposed to move. If you are getting bad readings at this point, you will most likely have to replace the amp’s power transformer.Step 5: Checking Tubes By Process Of InclusionIf the amp powers on with no tubes in it and doesn’t blow the fuse (and the above voltage tests are on point), then you’ll need to check the tubes by reinserting them into the amp one by one. Start with the preamp tubes first, then the rectifier tube.  After installing each tube, turn the amp on and wait a minute or so to see if the fuse blows.  If it does, then more than likely, it’s a tube issue.  If all is well throughout the reintroduction of preamp tubes, install the rectifier tube, make sure all of your filaments are working, and let the amp sit for a couple of minutes.  Next, place the amp in standby (if equipped). If your amp does not utilize a standby switch, then connect a speaker (or load) to the output jack.  Now install your power tubes one at a time and see if they come on.  This will tell you if there is a bad tube in your amp.  Essentially, you are breaking down the filament voltages bit by bit, isolating the issue, determining if this is an amp issue or a tube issue. If at any point during this process you find that the inclusion of a certain tube causes the amp to malfunction, you will need to take a closer look at that particular tube socket. If no problem can be identified within the tube socket, it’s time to test the tube itself by removing it from the equation and replacing it with a known “good” tube.  

SCENARIO 2: BLOWN FUSE

Some of the concepts outlined for this section will closely resemble those in the above section regarding how to diagnose no glow on your tubes. Additionally, we will break down this section into two sub-scenarios: diagnosing on a brand new amp build, and diagnosing on an older amp that has worked in the past. New Build Step 1: Removing The TubesThe easiest way to identify which “side” of the amp (AC or DC) this fuse blowing problem is coming from, is to remove all of the tubes and try to replicate the issue.  Before anything else, go ahead and remove the old blown fuse and replace it with a fuse of the appropriate value. Next, you’ll need to remove all of your amp’s tubes and turn the amp on. Wait a second and see if the fuse blows again. If the fuse blows with all of the tubes removed, you can turn your focus towards the AC side of the amp (skip to ‘Older Build Step 2’ scenario below). New Build Step 2: Checking Tubes By Process Of InclusionIf the fuse does not blow, it is time to start adding the tubes back in one by one. Install the first preamp tube and allow it to heat up (about 10 to 20 seconds) before moving on to the next tube. If the fuse blows while you're installing a particular tube, then it points in the direction you need to look. You can double-check by installing a known good tube in the same location and trying to replicate the issue. If the new tube fixes the issue, the problem is the tube itself; however, if you try a new tube and the fuse still blows, the issue is more than likely something in the circuit pertaining to that particular tube.  If it doesn’t blow, then it’s more than likely the first tube that was installed.  Now, moving down the line, install the rectifier (if applicable), and so on and so forth, until all of the tubes have been installed and the problem has been isolated. If this process does not point directly to a problematic tube, it is time to test your B+ Voltages…New Build Step 3: Testing B+ VoltagesYour amp’s B+ voltage is the main DC voltage coming straight off of the rectifier that is used to supply the amp.  This can be tested at the rectifier, at the stand-by switch (if equipped), at the output tube plates (usually pin 3 for octals), or at the main feed for the filter supply.  To begin testing, you’ll need to have the amp’s schematic/wiring diagram or a list of test voltages available so you know what values you are testing for.  Start at the rectifier, then move to the standby switch, the output tube plates, and then finally (if those tests don’t point to an issue) the filter supply.  As you move down the filter supply, the voltage is dropped via resistors wired in series for the screen supply, preamp plate supply, etc.  Again, you will need to know what the correct voltages should be in order to accurately test for this. Test one point in the filter supply, make sure everything looks good, and then move on to the next point.   If you measure a voltage in the power supply and it looks strange, you’ll need to check your grounds, as this could be where the problem is located. There could be multiple grounds for the power supply, and even components laying right next to each other may have 2 different grounds.Older Builds Step 1: Removing The TubesOn amps that have worked perfectly in the past, there is probably not a wiring issue considering that the amp would have been working all this time until now, so testing is a little easier.To start, pull all the tubes and replace the fuse (just like you did in the above scenario). Turn the amp on, wait a few seconds for it to warm up a bit, and if it doesn’t blow, the issue is more than likely just a bad tube. At this point simply perform the tube test by inclusion like we did in the above scenario to isolate which tube is causing the issue.  Once found, replace the tube with a known working tube and you should be all set.Older Builds Step 2: Visual Inspection And Power Transformer TestingIf your fuse still blows with no tubes installed, you will need to gain access to the chassis internals.First, look over the amp, does anything look burnt or smell weird?  If not, disconnect all of the secondaries from the transformer to the circuit.  Now replace the fuse with a new fuse, and turn the amp on.  Does it still blow the fuse?  If so, it’s more than likely a shorted power transformer.  If not, then it’s a circuit related component that you will need to trace down by connecting one circuit at a time.  Check your rectifier filament (if equipped), then the 6.3 filament, and then your bias supply and HV supply for the rectifier.  This breaks the circuit down into 4 parts, which gives you the ability to isolate the circuit wherein the problem lies. 

SCENARIO 3: BAD GROUND

One day, you sit down to play your favorite amp and all of a sudden you start getting a loud hum. Or, let’s say, you’re building a new amp kit. You get your amp kit assembled and wired up, you turn it on after testing, and it has an annoying oscillation and/or hum. What’s the deal?!To track down issues like this, you’re going to grab your trusty multimeter, turn it to the ‘beeper continuity’ setting and start troubleshooting (if you do not have a beeper continuity setting, you’ll test resistance).Step 1: Test The Power CordStart right at the power cord or IEC inlet and make sure your cable actually has a ground.  Measure continuity from the ground plug prong to the chassis ground.  Don’t ever assume that the cord is good, especially on vintage amps that have a 3 prong grounded cord, as the ground prong can be disconnected internally and still look fine on the exterior; there could even be an actual break in the wire if it’s pulled hard enough at any point in its lifetime.  If you test this connection, and you’re sure you have a good plug and grounded cord, it’s time to move on to the power supply. Step 2: Test The Power SupplyProceed with the following continuity checks after removing the tubes from the amplifier.  This will isolate different circuits in the amp. Inspect the power transformer to ensure the necessary center taps for the secondaries are grounded properly (sometimes the HV and/or filament secondaries will have these).  Also, look at the output transformer secondary ground.  Make sure it has a solid ground; you would be amazed how many people forget this and automatically think the transformer is “bad.”Your power supply has multiple grounds associated with it.  Some are joined together and some are separate. The main filter caps (usually electrolytic polarized), depending on the amp, will sometimes have the first and second stage filtering grounded together.  Locate the negative side of the caps and check for continuity to the chassis ground.  Next, locate the third stage filtering and do the same thing; make sure you have continuity to the main chassis ground.  Also, be sure to check the jumper or bus wire(s) are connected where they should be.  The same goes for any other filtering in the power supply. Some tube amps have a solid state rectifier supply ground reference either built onto a main board or on a separate SS rectifier board. Step 3: Test Bias SupplyIf steps one and two check out fine, you’ll need to look at your bias supply.  This can sometimes be integrated onto the main board (in amps like Vox and Marshall), but is usually separate on most Fenders, with its own board being near the pilot lamp (sometimes integrated with a SS rectifier).  This will have one ground coming off the filter cap(s).   While you are looking at that filter cap, make sure any jumpers on the cap to resistor and/or other caps are good as well. Step 4: Test The BoardNow we move on to the board: the signal processing part of the amp.  Your preamp tubes will usually have a cathode resistor that goes to ground, which is sometimes run in parallel with a bypass cap. In many cases, two cathodes will share the same resistor, which also goes to ground.  These grounds are almost always separate from the main power supply, and are either grounded on a brass strip (under the pots of a Fender), soldered directly to ground, or they could even have their own ring terminal.  When you are looking at these grounds, especially ones soldered directly to the chassis, move it around slightly to ensure it is in fact grounded. Sometimes connections will look grounded when they aren’t.  Proof, again, is in the continuity meter.   Step 5: Test The Pots and JacksAt this point it may seem like we’ve tested every component in the entire amp, but we haven’t! So, if you’re still not finding answers, start testing out the pots. Sometimes one leg of the pot will be grounded. These grounded legs are sometimes bent down and soldered directly to the pot case.  Sometimes they have a jumper going from pot to pot and ultimately grounded at the input jack or a ground bus somewhere.  Keep in mind that 99% of all pots in vintage gear are automatically grounded to the chassis when installed, so an extra ground bus along the back is redundant, but it does provide a good attachment ground for cathodes. Try to physically move/wiggle any of these solder joints on the pots to make sure they are good. And don’t be scared to get carried away with the continuity tests here!Next up, we want to look at the input and output jacks. These connections should be relatively easy to see, but it’s best practise to check the continuity anyway.  If you are using a J12A switching jack that automatically grounds the signal when the jack is not in use (to eliminate unwanted noise), make sure the switch has continuity to ground.  If the switch part is not touching the tip contactor even by a fraction of a mm, it will create noise and hum.Step 6: Test The Extra StuffBy now it is likely that you will have identified the problem. But in some cases, maybe not! The last thing to look at are any “extras” on the amp (if applicable), like your reverb driver, effects loop, oscillator ground, tremolo, and the ground switch itself (if it is still using the 2-pronged cord).Give a visual inspection of these areas, do some wiggle tests, and definitely test for continuity.As you can see, there are many grounds across even the simplest amp designs, and it only takes one bad/missing ground connection to result in a noisy or non-functioning amp.
We hope you’ve gained some knowledge from this article. It’s never fun when something goes wrong with your amplifier, and there are typically so many things to test that the process of troubleshooting can be daunting.  But with the right tools and a little bit of insight, there are a great number of problems one can solve on their own. Thanks for tuning in -- we’ll see you next time!