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Swapping and Substituting Octal Base Output Tubes

Changing output tubes in your amp (for reasons other than repairing it) allows you to discover whether you can get your amp’s sound closer to your goals. Things like output level, breakup threshold, and overall tone can be affected. Keep in mind that these changes can be subtle - so subtle in fact that you might even imagine changes that aren’t really there.


Swapping 9-pin dual-triode preamp tubes is a comparatively benign process; you can’t do much damage unless you greatly exceed the heater-cathode maximum voltage spec, in which case all you’re likely to lose is the tube. With power tubes, the potential for disaster is much greater. Most other tube types that fit the socket are incompatible, and the results can be expensive or even deadly. So if you don’t want to turn your bank account into a black hole, your amp into a boat anchor, or your body into a cadaver, please pay close attention to this info and any manufacturer’s or other warnings regarding working on high voltage equipment like your tube amplifier. If in doubt, take it to a tech. If you’re not in doubt, then do a quick reality check and ask yourself whether you should be.


As with rolling preamp tubes, work with a helper so you can test your ability to detect differences. This is important because if you can’t detect any differences, then you’ve just saved yourself a bunch of time and money. It’s even more valuable to have someone help you with power tube rolling in particular because it takes longer for output tubes to cool down and be swapped out, by which time your brain will have forgotten what the last ones sounded like. That’s not an insult - it’s the bitter reality.


It’s also too easy to get fooled by your expectations. You might replace a pair of EL34s with a pair of 6CA7s in an attempt to give your amp more of an American sound, and be totally convinced that you can actually hear that difference, despite the fact that those two tube types are identical in many cases. Bottom line: always tube-roll with a friend, having them change the tubes (or not) without telling you which ones are in the amp each time you try it. If you can’t consistently tell which tubes are in the amp by ear, then your experiment is over. This outcome is better than wasting your time and chasing your tail. 


Can you just go ahead and try a different output tube type in your amp? While it’s sometimes possible, it’s more likely that there will be one or more incompatibilities that will first require some measurements and possibly some modifications to your amp. And in some cases it’s a definite no-go. So don’t let the fact that different tube types will fit the same 8-pin socket tempt you to try a swap until you’ve done your research. 


If you want to cut to the chase, I’ve created a fairly dense color-coded chart below to help you check whether one tube type could be safely plugged into an amp designed for a different one. However, it might save you time and money in the long run to read through the rest of this info.  


What kinds of potential incompatibilities are there?


First and foremost, bias. You should always adjust the bias any time you change output tubes, even with the same type. You definitely need to adjust the bias when rolling output tubes. I will do another video about biasing tubes, but for now please remember that if you don't have the tools and know-how to adjust bias properly and safely, then someone else should be doing this for you. Measuring bias usually means going inside the amp where there can be lethal voltages even after the amp has been turned off and unplugged! Even a non-lethal shock can cause a secondary injury, like cutting yourself as you jerk your hand away, or knocking the amp onto your foot.


Most non-EL84 tube amps have what’s commonly called ‘fixed bias’ on the output tubes, which often means that it’s adjustable, confusingly. On the other hand, the amps with output tubes that are cathode-biased - like the Vox AC30 - are rarely adjustable (although Auburn amps with cathode bias usually are). Cathode biased high-power output tubes are generally only found in esoteric, specialist and audiophile amps. So if we're talking about musical instrument amps using any of the tubes in the chart, then we’re usually looking at bias that can be adjusted. One exception is many Ampeg amps, whose fixed bias truly is fixed, unless the amp has been modified.


Bias adjustments are generally a job for a tech, but if your amp has been fitted with an inline-type bias checker and the adjustment trimmer is easily accessible, then you can probably make the adjustment yourself. Just be careful and follow safe practices. 


Here are the other incompatibilities you can run into when substituting different types of output tubes: 


Physical size (S): overall height and diameter, retainer type


It may seem obvious that the tube has to fit inside the amp to be usable, but don’t forget about the need for the retainer or cover to fit as well. Also keep in mind that some tube types may have come in a different size and/or shape than they do now, so in rare instances you can run into problems even when substituting tubes of the same type.


Pinout (P): which tube elements (plate, cathode, grids etc.) are connected to which pins. 


Some pin assignments are almost universal, like the plate connection to pin 3, the cathode to pin 8, and the filament connections to pins 2 and 7. Things get sticky when it comes to the grids. Some output tubes are tetrodes, meaning they have two grids: the control grid and the screen grid. Pentodes have three, with the addition of a suppressor grid. Sometimes this is internally connected to the cathode, and sometimes it’s connected to its own pin - generally pin 1. Your amp was wired for the tubes shown on the tube chart glued inside its cabinet or in the manual, unless it was modified, which hopefully the tech would have indicated on the tube chart, although you can’t count on this. The wiring of the output tube sockets in your amp is therefore key to whether a different tube type will work. Some manufacturers - Ampeg in particular - often wired their output tube sockets to be as forgiving as possible with tube type swaps, but others, like Fender, would use unused socket terminals for mounting components. A common example is Fender’s former habit of mounting control grid resistors between pins 1 and 5, and screen grid resistors between pins 4 and 6. They could do this because pin 1 isn’t used by most output tubes, and pin 6 is used only on tubes that Fender never used, so those terminals made convenient attachment points for the resistors. They’re also the reason you can’t just put a set of EL34’s into your stock Twin Reverb.  


Filament current (F): the amount of current consumed by the tube’s filament. 


This current is essentially constant, regardless of how hard the amp is being pushed. Filament current requirements can vary widely between tube types, and this is an issue because manufacturers generally don’t allow for any extra filament current draw when they’re designing their amps, as this would make the amp heavier and more expensive to build. One exception is Ampeg amps, which often have enough extra iron to accommodate a modest increase in filament draw. In general though, I would recommend against increasing the load on an amp’s filament circuit to the point where the measured filament supply voltage is lowered or the power transformer gets noticeably warmer to the touch. Not only is a power transformer expensive to replace, a meltdown there can be a serious safety hazard.  


Maximum plate voltage (V): how much voltage the tube can handle before going pyrotechnic.


This specification alone is a very good reason to become familiar with reading tube data sheets, which are freely available online. It’s also important to note that this specification can vary between tube manufacturers and when the tube was made. For example, old 6V6GT’s could handle a maximum of 315 volts, whereas the current JJ 6V6S can be operated up to 500 volts. And amp designed for EL34’s or KT88’s that pushes them anywhere near their 800 volt limit would turn 6L6s into a spectacular but brief light show. 


Load impedance (Z): the optimum impedance the plates should ‘see’ for proper operation.


This spec is less of a safety concern as tubes can operate into a range of loads, albeit with altered performance characteristics - principally output power and the balance between 2nd and 3rd harmonic distortion. This is why it’s especially important not to draw conclusions about the sound of a tube type based on its performance in an amp that wasn’t designed for it. Also be very wary of descriptions of the sounds of the various tube types, even from their manufacturers. Much of this is purely subjective folklore that gets repurposed as marketing hype. If you’re doing blind changes using a helper, trying to identify tubes by ear without knowing what’s inside your amp each time, you’ll soon find your own truth and be able to make informed decisions, or look elsewhere in your signal chain for places to get the sound you’re after. But back to impedance: if the tube you want to try is otherwise compatible with your amp but has an optimum load that’s close to twice or half that of the tubes currently in your amp, then you should change your speaker impedance, either with the switch (if there is one) or by changing the speaker load itself. Here’s a very common example. Say you have a Fender Hot Rod Deluxe running the stock 6L6s and you want to try a pair of new JJ 6V6Ss in there. You check the chart below and see that the only things you have to watch for are the maximum plate voltage and the load impedance. You can see from JJ’s datasheet that their 6V6S is good to 500 volts - the same as a 6L6 - so no problem there. But the 6L6 wants to see a plate-to-plate load of about 5k, and the 6V6 is looking for a load of about 10k. Because the load on the tubes is the speaker load reflected to them via the output transformer, and because that amp doesn’t have an impedance selector switch, you need to apply a load to the main speaker jack that is twice as light as the 8 ohms that the amp normally sees. In other words, you need to plug a 16 ohm speaker (or two 8-ohm speakers in series) into that jack when you're running 6V6s. Some people skip this step and claim to like the sound, but it’s worth noting that they’re not hearing the 6V6s operating under comparable conditions as the original tubes. They’ll also be getting less life out of them.


What if you wanted to try a pair of 6550s - the tubes used in Ampeg SVTs and some Marshalls - in that same Hot Rod Deluxe? Looking at the chart, if they’ll fit the amp physically, the only problem is the filament current, but it’s a deal breaker in this case. Those original 6L6s draw a total of 1.8 amps from the filament supply. On the other hand, the 6550s would try to pull a whopping 3.2A - definitely more than that amp’s power transformer could supply.


Can you stick a pair of 6L6s into an amp designed for 6V6s, like a Fender Princeton, and get more power? No - you can’t. Remember that the power isn’t created by the output tubes; they only modulate the power created by the power supply. There’s no real point to putting a higher-rated tube into a stock 6V6 amp, and the filament supply probably wouldn’t handle it anyway. However, if you beefed up the output and power transformers in that Princeton and tinkered with a few other things, you would be able to run 6L6s in it. You’d also essentially have the equivalent of the first Mesa Boogie.       


The chart below doesn’t tell the whole story, but it offers a useful reference. Find the output tubes your amp was designed for (or modified for) in the left column, then look across that row to the other tube types to see whether they might work in your amp and what kind of incompatibility problems you might run into. Notice that you can sometimes run into problems replacing tubes with the same type as the originals due to differences between old and current versions. 


Anything marked in red on the chart is a definite no-go: you won’t be able to use those tubes in your amp without significantly modifying it. Orange indicates something a tech will have to look into to make sure the change won’t damage your amp or the new tubes. Modifications may be required, and measurements definitely will be. Yellow indicates a warning - something requiring further investigation or keeping an eye on when making the swap. Will the tubes and their retainers fit? Are the replacement tubes rated for a high enough plate voltage? Will they perform acceptably into the non-standard load? Things like that. The green marks indicate a tube swap you should be able to make without any problems, assuming that the bias is immediately adjusted correctly.


Here’s a chart showing the relevant data for all the tubes from the compatibility chart, so you can see exactly where the discrepancies are. Please note that this information is based on the original versions of all these tube types. Current production models often differ in size and plate voltage ratings. Always check the applicable data sheets and schematic for your amp first.


A couple of final notes: please keep in mind that you can’t rely on online sound clips or other people’s subjective descriptions of how a particular tube type sounds because these will always be different from you playing through your amp, which is what you’re hoping to improve. If you can, borrow tubes from another player and get them to help you make the tests. Or purchase tubes from a vendor who will allow you to return them if you don’t like them.


One problem with output tube swaps is that the tubes hit a price point that’s dangerously manageable for many people. This is why the music gear market is saturated with things in the $100-500 range, like pickups and pedals. It’s easier to part with a few hundred dollars and forgive yourself for not making a rational, informed purchase decision than it is to wait until you can be sure of exactly what you need to achieve your goals. At the end of the day, if your prime focus is to make music, you owe it to yourself to avoid spending time and money on distractions. That being said, experimenting with output tube types can be informative and helpful, so if you’re confident in your mission and your ability to undertake it safely, then go for it!

Rob Robson

Auburn Amplifiers Kft.


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