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Reactive or Resistive, Part 2

There continue to be a number of people who vocally insist that the THD Hot Plate is not a reactive device. I can only think of two reasons why someone would make this claim:

1. They don’t understand what the word “Reactive” means 2. They don’t understand how a Hot Plate functions

Let’s look at the first possibility. Reactance (the noun on which the adjective “reactive” is based), is defined by Wikipedia (an acceptable source for this purpose, as it’s easy for the skeptical to verify) as follows: “In electric and electronic systems, reactance is the opposition of a circuit element to the flow of current due to that element's inductance or capacitance. Greater reactance leads to smaller currents for the same voltage applied. Reactance is similar to electric resistance in this respect, but differs in that reactance does not lead to dissipation of electrical energy as heat. Instead, energy is stored in the reactance, and later returned to the circuit whereas a resistance continuously loses energy.”

This definition is incomplete, but it’s a good start, and will work just fine for the purpose of this post.

If you understand this definition, you see that it is impossible to build a purely reactive attenuator, as it would not be capable of reducing the amount of energy sent to the speaker, thus defeating the entire purpose of an attenuator. Along the same lines, it would be impossible to make a purely reactive load, as it would have no way of getting rid of the energy. An attenuator or a load has to have a resistive character, at least in part, in order to dissipate electrical energy as heat or light.

This leads us to the 2nd possibility, that the person doesn’t know how a THD Hot Plate works. The Hot Plate uses a series & parallel network of inductive resistors to provide the baseline reactance and to dissipate most of the energy. If we were using only non-inductive resistors, and we didn’t have the capacitors and inductor that we use, then it could be argued that it is purely resistive, but I don’t know any attenuator that uses only non-inductive resistors.

Secondly, the Deep switch adds a significant inductive reactance to the network, and, in the process, also gives a low-frequency bypass around some of the series portion of the dividing network both allowing the speaker to get more bass energy and simultaneously allowing the amplifier to “see” more reflected reactance from the speaker’s own voice coil and the motion of the cone (in the bass range), thus also changing the damping of the circuit in the bass frequency range.

Thirdly, the Bright switch adds a significant capacitive reactance to the network, and, in the process, also gives a high-frequency bypass around some of the series portion of the dividing network both allowing the speaker to get more treble energy and simultaneously allowing the amplifier to “see” more reflected reactance from the speaker’s voice coil and the motion of the cone (in the treble range), thus also changing the damping of the circuit in the treble frequency range.

This doesn’t even address the more basic question of “Is a reactive attenuator or load better than a purely resistive one?” That’s a whole other kettle of fish that I hope to go into at some future time. The answer is not straightforward.

If this doesn’t set the record straight, I don’t know what will.

Reactive or Resistive?

We keep getting asked if the Hot Plate attenuator is resistive or reactive.

There is a lot of misinformation floating out there, including a recent "article" on Reverb claiming that the THD Hot Plate is a resistive attenuator. I both phoned and sent emails to correct the writer of the article, but never got a call or email in return.

People seem to assume that reactive attenuators are something new, which they are not. We’ve been building them since 1994. Many assume that since the Hot Plate has been around for so long, it must be a primarily resistive unit like the Scholz Power Soak and the Altair PW-5. (For the trivia minded, the PW stood for "power waster".)

The Hot Plate is fully reactive in three different “modes", as opposed to one mode like all the other attenuators claiming to be reactive. The bright, deep and light bulb switches let you select exactly how reactive it is, and in which frequency ranges and dynamic ranges the reactance is most prevalent, but no matter how you set it, it is always reactive in all attenuation modes and in the “Load” setting as well. It’s the only attenuator which does this.

Some attenuators like the Marshall Power Brake (no longer produced) use transformers instead of networks of resistors, capacitors and inductors, to do the dropping. The problem with this entire approach is that the only way that a transformer can reduce the power getting to the speaker is by creating an impedance mismatch, which is never good for the amp, nor does it ever sound or “feel” particularly good. With a Hot Plate, if you have an 8 ohm amp, an 8 ohm Hot Plate and an 8 ohm speaker, no matter how you set it, the amp is always “seeing” an 8 ohm load. It is the only attenuator that does this, which is why there are 5 versions. In each version, every single capacitor, resistor and inductor is optimized for that impedance.

Another aspect of this discussion that is worth mentioning is that the absolute safest load for any amplifier is one that is 100% resistive with no reactive element at all. The problem with a purely resistive load is that they don’t sound very good at all. Guitar players are used to the hearing the amp as it responds to the inductance of the speaker’s voice coils, the motion of the cones (causing a huge momentary change in the load), and often the effect of the compression of the air in the cabinet on the cone/voice coil motion, and thus on the load. So, more reactance is not necessarily a good thing, especially when it goes beyond mimicking the reactance of a speaker cabinet.

Over the past 26 years, we’ve sold more than 100,000 Hot Plates, making it the best-selling attenuator ever built. It’s also the only attenuator made that is recommended by other manufacturers. The Weber is recommended only by Weber. The Rivera is recommended only by Rivera. The THD Hot Plate is recommended my Marshall, Fender, HiWatt, Soldano, Bogner, Diezel, MESA/Boogie, and dozens of other amp companies. And, over 30 years, we’ve only ever had two people come forward, claiming that their amps were damaged by a Hot Plate. One was using a 4-ohm Hot Plate on a Marshall set to 16 ohms, which caused the output transformer to fail, just as it would have if he had run into a 4 ohm cabinet. The other had just had the output transformer in his Vox AC30 replaced when he started using a Hot Plate on his amp with the new transformer. He claimed that the Hot Plate had blown his new transformer. We paid to have the amp sent to us for investigation. It turned out that his "tech" had replaced his 30-watt AC30 output transformer with a 15-watt Fender Tweed Deluxe output transformer, which could not possibly take the output from a quad of EL84 tubes and was less than 1/4 the size of the original output transformer.

We hope this helps to dispel some of these myths.

Vintage Vox AC50 Bias Requirements (A Bit of a Rant)

Somewhere around 2002 I was doing some work with Bob Dylan and his band in preparation for a tour. They bought a THD BiValve-30 head and a pair of our 2x12 speaker cabinets for Bob to use as his stage rig. I helped the band out in rehearsals with a few technical things and, in the process became friends with their two main techs, Tom Morrongiello and Yuek Wong, both great road techs and wonderful people.

At one point in this process, while they were on tour, Yuek phoned me, asking how to properly bias a 1960s Vox AC50 that Charlie Sexton had on tour. I told him that, even though the schematic recommended 47mA per tube, it was happiest at 25ma. (There’s that number again.) Yuek responded that my advice was dramatically different from what he was being told by all the other techs he had spoken to. I recommended that he simply try my numbers and let his ears make the decision. Instead he decided to call every tech he knew. He called Ken Fischer, Blackie Pagano, Tom Mitchell, Harry Kolbe, Aspen Pittman (not an amp tech, but a tube expert), Gerald Weber, Mark Sampson, and a host of other techs in the US and Europe. All of them told him that it was a class-A amp, and needed to be biased to numbers as low as 120mA per tube to as high as 175mA per tube, depending upon who he asked. This is the range where he was biasing it (because the majority must be correct, no?), and still the amp sounded awful. His conclusion was that the amp must be broken in some way.

Again, Yuek called me, and asked how my opinion on biasing the AC50 could be so different from everyone else’s. My answer was not subtle: First, it’s not an opinion, it’s a fact. Second, for a few years, one of my main amps was an early 1966 AC50, and I loved it! (I only sold it because someone offered me a ridiculous amount of cash for it.) Third, the AC50 is not, and never was, a class-A amplifier; it is a class-AB amplifier. I said, “I know how bad and arrogant this sounds, but I am right and they wrong.”

I reminded him that a pair of EL34 power tubes in class-A will only put out about 30 watts max, and only if they are using a huge power supply and enormous output transformer designed for class-A operation. (A 50-watt class-A output transformer will be about the size of a 150-watt class-AB output transformer.)

The AC50 is called an AC50 because it is designed to put out 50 watts RMS. I said: “If you bias it as they are recommending, it will have no highs, the bass will be tubby and weak, and it will put out about 10 watts maximum, and a bad-sounding 10 watts at that.” He agreed that it sounded exactly the way I was describing. I was finally getting through to him.

Again, I advised him to bias it to 25mA per tube and give it a listen. He admitted that the amp would not even go that low with its current bias supply, and he didn’t know how to rebuild the supply to give that range. As they were in LA, and as round-trip airfares from Seattle to LA were $99 at the time, I offered to fly down and sort it out for them. I added that if I was wrong, then the amp needed to be repaired, because something else was wrong with it, and I would repair it for free and pay for my own flight. But, if I was right, I would like them to pay for my flight and my work. Yuek agreed.

Two days later, I arrived at the venue and, without even listening to the amp (because running it at that bias is very bad for the transformers, the filter caps and the tubes), I performed a basic voltage test on the amp, warmed up my soldering iron, rebuilt the bias supply and biased it for 25mA per tube. When I was done, I plugged in to it and played a bit. The amp sounded glorious. It was loud and clear, tight, detailed, and with the touch-sensitive midrange punch that I always loved about the AC50 when hit hard. Yuek was at the same time thrilled and in disbelief. How could so many famous techs be so completely wrong?

A few minutes later Charlie Sexton came onto the stage and Yuek had him try the amp. Charlie lit up like a Christmas tree with joy at how fantastic it sounded how rewarding and responsive it felt to play. He thanked Yuek profusely and asked what he had done to bring it to life. Yuek, being a gentleman, said “Thank Andy, not me. All I did was let him bias it his way.” I got a huge smile and hearty thanks from Charlie (another gentleman), and all was well. I turned down their offer to pay my airfare and work.

The crew was apologetic that there was no ticket to offer me, as the show was completely sold out, but they did give me a stage pass for the night. Again, they apologized that there would not be much room at the stage sides either. As it was Los Angeles, the number of friends of the band attending would be huge, but I was assured that I would at least have room to stand, and perhaps to sit.

As fate would have it, I watched the show from stage left, seated on a huge flight case, squeezed between Kate Hudson on my right and Johnny Depp on my left. At first, I thought I might chat with them. I knew Kate’s then husband Chris Robinson from my time with the Black Crowes, when I helped them get their gear in order while rehearsing for the “Three Snakes and One Charm” tour. My conversation starter with Johnny would have been his friendship with my brother Gray (a visual effects supervisor), who had worked with him on a few of his films. In the end, I decided that conversing with them would only have boosted my ego, and would have likely been awkward and annoying to them, so I demurred and sat in silence. It was a good evening.

If there is a moral here, it is that the truth is the truth, regardless of who says otherwise, regardless of how loudly they shout it and how many of them are preaching it. If you are right, stick to your story and don’t be bullied by anyone or any group of people. I’m not trying to say that I am always right, but I make sure that I am before I stick my neck out.

Why Filter Caps Need to be Replaced Before They Fail

One very good question that was raised recently by a few people is why it is important to replace filter capacitors when they appear to be functioning. In other words, why not wait until a filter capacitor fails before you have it replaced?

First a little bit of basic background information. Without getting too technical, filter capacitors are critically important components in the power supply of any tube amplifier. They are used to reduce the 120 Hz (100 Hz in countries with 50 Hz mains voltage) AC ripple from the rectifiers into smooth DC voltage by storing energy when the voltage rises and releasing it when it drops, thus smoothing the rectified DC voltage into relatively even DC. Ripple from the power supply that is applied to the plates of single-ended (non-push-pull) stages in an amplifier will turn up as hum in the output. In push-pull stages with properly matched sides, the ripple voltage on the plates of the tubes cancels out in. This is part of why high levels of capacitance are not always necessary in the first stage of most higher-powered tube amplifier power supplies.

As aluminum electrolytic capacitors (the type used in 99.9% of tube amplifier power supplies) age, their ability to store and release energy reduces though aging of the aluminum conductors, the electrolyte solution and the coating on the aluminum. This also translates into increased heat generated within the capacitor itself and will eventually result in either an internal short in the capacitor, which can damage the power transformer, rectifiers, filter chokes and/or divider resistors; or in the capacitor either exploding like a firecracker or squirting hot corrosive fluid into the chassis of the amplifier, damaging everything it touches. Neither of these are good scenarios, as they both can be much more expensive than replacing filter capacitors.

How long a set of filter capacitors should be expected to last before catastrophically failing is variable, and depends upon amount of use and environmental factors, mostly ambient temperature. Even if these two factors are tracked (and who does that?), it can be very difficult to predict when the capacitors will fail.

So, the safest route is to have the entire set of electrolytic capacitors replaced every 20 to 30 years. When doing so, it is also a good idea to replace not only the high-voltage supply capacitors, but also the capacitors in the bias supply and the electrolytic capacitors in the cathode stages of the circuit. While the cathode capacitors seldom fail catastrophically, it’s just good practice to replace them at the same time.

There are a few other questions that come up when replacing filter capacitors, which I think would best be addressed in future posts, just to keep these from becoming too lengthy. One is what kind of capacitors to replace with. Another is how to mount capacitors that are physically different from the ones being replaced. A final item to be addressed later has to do with customers who prefer the sound of worn-out filter capacitors, and no longer like their amps when the caps are replaced with new ones.

Circuit Board Humor, Part 2

Here is another bit of circuit board humor from our Hot Plate boards in the 1990s. Some people took it seriously and wanted to know which components kept track of the number of notes played. Others thought it discriminated against speed metal players.

Topic Suggestions?

Are there any specific topics you would like to see addressed in this blog? I've got many things already in mind, but your suggestions will help me add to that list, and to prioritize it. Please either post your suggestions as comments or send a message. Thanks!

Filament Issues in an Ampeg SVT Classic

A customer recently brought me an Ampeg SVT Classic that was sounding weak and lifeless.

Though I am not especially fond of working on them, I do have a lot of experience with SVT heads, especially the earliest version with 6146B power tubes, and the second version with 6550s. I have gotten quite adept at patching the holes burnt in their circuit boards when they arc.

For a few weeks in 1992, I had to do this almost every night while helping-out on Keith Richards “Main Offender” recording sessions at a studio called “The Site” in Marin County, California. All of Keith’s and Charley Drayton’s bass tracks were recorded with a 1969 SVT using 6146B tubes. Every night while recording, they would blow at least one 6146B, which would, in turn, create a hole or two in the circuit board of the amp.

So, each day, while the band slept, I would replace the offending tube(s), patch the hole(s) with FR4 (the fiberglass/epoxy material the boards are made of), replace the burnt traces with carefully bent copper wire, glue them in place with epoxy and insulate them with RTV, so that the amp was ready for that night’s recording. My patched areas never burnt again, but there was plenty of original area still, to die a fiery death. Madness, pure madness.

I should mention that I was not permitted to modify the circuit for greater reliability, as they liked the way it sounded, and were afraid it would change the sound if I made my recommended changes to keep it from blowing up so spectacularly.

But I digress.

Back to the SVT Classic on my bench. The owner said that it would sometimes sound OK, but would get much weaker over the course of the night, and that sometimes it would be weak sounding as soon as it was turned on. First off, I made sure that the recommended updates to the circuit had been made, which they had at the factory.

Initially I suspected mis-biased tubes, but the real culprit was the connection between the filament wires from the power transformer and the power tube board itself. Ampeg’s designers chose to use push-on connectors rather than solder, in the interest of easier assembly and repairs. The connectors had clear silicone boots for electrical insulation, and the boots were badly discolored and burnt from the heat generated by the resistance in the connection.

I measured the filament voltage on both sides of the connectors. I got a steady 6.3 VAC on the transformer side. On the board side though, due to the resistance in the connection, the voltage started at 6.0 VAC and, over the course of an hour, slowly dropped to 5.1 VAC. All this voltage loss was due to the resistance in the female connectors’ friction contact with the tabs on the board. No wonder the amp was sounding lifeless!

I measured the filament voltage on both sides of the connectors. I got a steady 6.3 VAC on the transformer side. On the board side though, due to the resistance in the connection, the voltage started at 6.0 VAC and, over the course of an hour, slowly dropped to 5.1 VAC. All this voltage loss was due to the resistance in the connection itself. No wonder the amp was sounding lifeless!

Power loss due to a resistive contact is a vicious cycle. A little bit of resistance in the contact causes a little bit of heat to develop. As the connection heats up, its resistance increases, causing still more resistance, and so it continues. Eventually it can get hot enough to burn the board, melt the wire’s insulation, or melt the solder, causing the tab to fall out of the board altogether. Additionally, the heat increases the electron activity in the metal, making it more reactive, causing accelerated oxidation, which, in turn increases resistance, and adds to this whole nasty mess.

My solution was to remove the push-on connectors from the wires, strip the insulation back, tin the wires appropriately, slide on two lengths of thick 3-1 heat shrink tubing, and then solder the wires firmly to the tabs, making sure to have good, flowing, filleted solder joints for maximum current flow and minimum resistance.

I measured the voltage at the tabs and at the tube sockets, and they both remained steady at 6.3 VAC for over an hour, with no drop in the voltage. Success!

Then it was a matter of pushing the heat shrink tubing over the tabs, shrinking them with a heat gun, and putting the amp back together.

It is worth pointing out that the push-on connectors that Ampeg used were rated for well in excess of the current that they were seeing. These connectors were rated by their manufacturer for 15 amperes of continuous current and 30 amperes of intermittent current. So, any recent engineering school graduate would feel that they were doing good, robust design work by specifying these in the amplifier. This is a good example of why there is no substitute for adult supervision in the design process. Someone who has been working on amplifiers for a half century would look at that and say that while the connector did not technically fail, it did create a serious problem for the amp it was used in.

Resistance in low-current connections is seldom a problem, but in high-current, low-voltage connections like the filaments of an SVT, where they carry 9.6 amperes, resistance in a killer. A loss of 1.2 volts in a 200-volt supply would usually be insignificant, but a loss of 1.2 volts in this 6.3-volt supply was very much a serious problem.

Avoiding Shorts Before They Happen, Part 1

A good bit of my work is preventative in nature. Customers often bring me amplifiers before a tour so that I can increase the chance that the amp will complete the tour without incident. This requires a sharp eye for potential problems.

In this example, we are looking at a Marshall JCM 2000 power transformer that has already been replaced by another tech before it came to me. You can see by the corrosion on the chassis that there was a serious arcing incident, which vaporized the protective zinc plating on the steel chassis, leaving the steel bare and prone to rust.

The fact that this took out the power transformer also tells us that the incorrect value fuse was in the mains fuse holder. If the correct value fuse was used, the fuse would have blown before the transformer was damaged, as the wire in the filament winding is huge (almost coat-hanger size), and would not easily melt.

The other tech correctly surmised that the push-on connectors on the power transformer had become resistive, generating heat and melting the header and wire insulation, so when they replaced the transformer, they wisely soldered the filament wires to the transformer tabs, creating a much better, dramatically lower-resistance connection, and eliminating the chance of that exact thing happening again.

What he or she missed are the three smaller wires leaving the header between the tabs. (These wires are for the low voltage supply that power the transistor switching circuit and op amps in the unit.) When it came to me, these were pressed tightly against the bare metal of the vertical tabs protruding up from the header, and had become a potential shorting hazard. In time, the PVC insulation would have worn away, and the wires would have shorted, likely causing another power transformer failure, as the windings on this secondary are rather fine.

My preferred solution for these situations is to add small lengths of silicone coated woven fiberglass sleeving over the wires as they exit the transformer. This material is extremely resistant to both heat and abrasion. To hold it in place, I used a small amount of RTV silicone adhesive. These will never short to their neighbors on the transformer header. (Though in the photo, it looks like the upper white wire is touching the bare tab sticking out to the left of the header, it is, in fact, over ¼” away from it, and cannot touch it.)

Marshall JCM 2000 Grounding Issue Fixed

Today we are addressing an issue that plagues all the Marshall JCM 2000 series amplifiers (and a few other Marshalls of the same era). It is a problem of very poor grounding of the circuit boards. This poor grounding causes all sorts of symptoms, such as unnecessarily high noise-floor (especially high-frequency hiss), excess 50/60 cycle hum, tubby bass response, signal level changing from day to day and over the course of a day, and in extreme cases, inconsistent switching of channels.

Why does this happen? The four circuit boards that carry audio signal all derive their grounds through a horrid and haphazard mess of crimp connections, push-on connections and rivet connections. These are iffy at best, and extremely bad if there is high humidity or, worse yet, saltwater in the air.

I know that the engineers (likely fresh out of university) thought that they were using best practices, and doing what they were taught to do in school, but real-world experience, along with in-depth study of materials sciences, has shown that this is a recipe for disaster. (It does make for quick, cheap and easy manufacturing, though!)

If you follow the ground path from any of these four boards to the actual chassis ground (which is a DC voltage ground, an AC signal ground, and a shield), you see that the board’s ground tab is soldered to the board’s copper ground trace (good), but it then has a friction connector which is crimped (again friction) to the wire. At the other end of the wire is another crimp connection to another friction connector, which goes to a 2 to 1 friction connector, which in turn connects to yet another 2 to 1 friction connector, which is pop-riveted to the chassis. So, counting all that, the ground path for each board has 6 unreliable connections per board, in series with each other, before it gets to the chassis. That’s a total of 24 potential sources of trouble in an all-important part or the circuit!

The fix for this is to drill out the pop rivet which connects the last 2 to 1 ground tab to the chassis, disconnect all of the push-on connectors, clip off the push-on connectors from the wires, strip the wires, tin the ends, and properly solder one end of each ground wire to the ground tab on each board. Solder the other ends of these four wires to a nice star-lug grounding lock-washer and screw that lug very tightly to the chassis using a lock nut on the other side, so that it does not rattle loose over time.

This reduces the ground path to two solder connections and one very good lug, with its teeth tightly dug into the chassis, which, in my opinion, is what Marshall should have done in the first place.

I am not taking a poke just at Marshall. Many manufacturers are guilty of this kind of assembly practice, but I’ve had a few JCM 2000s come across my bench recently, so this one is fresh in my mind. I do this upgrade to every amp that comes in with this grounding technique, whether it is showing symptoms of poor grounding yet or not, because eventually, it will.

Wire Tinning and Why It Matters

Let’s address a very prevalent problem in amplifier construction and repair, which is also the cause of many failures: wire tinning.

I know that the tinning of wires is not something that most people give much thought to, but in my 50 years of working on amplifiers, I have seen thousands of amplifiers that have either failed or lost the beauty of their sound because of incorrect wire tinning.

For those who are not familiar with the term, tinning is the process of pre-applying solder to a wire or component lead so that it is easier to get a good solder joint when it is soldered to either the board or the component that it is meant to go to. Solid-core wires are not as critical and not prone to the problems I am going to focus on here, but they have their own issues that I may address in a later article. Also, stranded wire that is pre-tinned as a unit so that it acts like solid-core wire and is very stiff (like Belden’s Uni-Strand) is not very common, and has its issues. Here we are concerned about either stranded bare copper insulated wire, or stranded copper wire where the individual strands are tinned.

It is good and common practice to apply flux-core solder to the exposed portion of a stripped wire end. The problem is that when most people do this, they only solder a portion of the exposed end of the wire, so that the solder stops before the insulation begins. Some people do this on purpose to keep from melting the insulation.

Most people do this haphazardly, because they are not aware of the dangers of doing so. When the tinning ends before the insulation starts, you create a stiff portion (the tinned section) then a flexible portion (the untinned-uninsulated section) and then the stiffer insulated section. When any stress or motion is applied to the wire, such as vibration from being moved, or from loud playing, this bare portion sees most of the movement, and is always the first to break. Even if it does not break all the way through, many of the copper strands will break, increasing resistance and decreasing skin-effect at this point.

A much better practice, and the method that we have always used at THD both in production of our products and in repair and restoration of other amplifiers, is to tin the wire so that the molten solder wicks up underneath the insulation a little bit, thus the tinned section of the wire extends into the insulation. When the tinning solder runs up underneath the insulation like this, you have no weak section at the end of the insulation, thus no area prone to wire fatigue and breaking.

In a production situation, wire tinning is usually done in batches of pre-cut, pre-stripped wire that is dipped first in liquid flux, then in a solder pot. Under these circumstances, it is very easy to get excellent tinning results that wick well up underneath the insulation without any risk of melting the insulation, as the thermal mass of the solder pot is so high. Naturally, all of this assumes that you have the correct solder alloy, the correct flux in the correct dilution, and the correct solder pot temperature.

If you are using at least 105 degree C PVC wire, or Teflon wire, melting won’t be a problem unless you are using too small a tip on your iron or too low wattage an iron. (More overheating damage is done with soldering irons of low wattage than high wattage, and with irons with very fine tips than irons with very thick, broad tips. This is because of poor heat transfer, so the iron must be on the wire or component for too long.)

Another potential cause of poor wicking is either an inappropriate solder alloy or flux in the core, or sometimes too little flux in the core.

For both production and repair, we like to use Hakko soldering stations. We switched from Weller to Hakko around 1989 and have never looked back for our regular soldering needs. The current version of what we use is the Hakko FX-888D digital soldering station, which is usually available around $100. We always recommend using the largest tip that you can fit into the space. For 95% of our work, we use ¼” wide chisel tips, like the Hakko T18-S3 tip.

For wire solder, we have always been lead-free. I never thought it was a good idea to poison yourself, your employees and the environment just to save a few pennies. Also, lead-bearing solder creates noisier solder joints than most lead-free solders. Our go-to solder for all board and assembly work is Kester 24-7068-1402, which is an alloy of 96.5% tin, 3% silver and 0.5% copper with a 3.3% core of Kester 48 activated rosin flux. It’s great, but a bit expensive at roughly $70 per 1-pound spool.

Biasing Tube Guitar Amplifiers

A customer, who owns a THD Flexi-50, recently asked me why our recommended bias is so much lower than the "commonly accepted 70% of maximum dissipation."

The following is my rather verbose response:

Let’s start out by stating that I am a proper amplifier designer, not a hobbyist who cuts and pastes from other designs, and gleefully repeats whatever he reads on the internet, as if it were fact. I’ve been at this for 50 years and have consulted for everyone under the sun in terms of sound, reliability, manufacturability, and just about every aspect of analog audio circuit design you can think of. I do amp repairs and restorations for dozens of multi-platinum rock bands, and I’ve often gotten that work because they see that the only amps they have that almost never need repair are their THDs. I’ve also helped Philips, Fluke and Hewlett-Packard design and build better test equipment based on my experience. I don’t even take proper textbooks as gospel, preferring to actually measure and analyze real-world situations, often proving PhD engineers and university department heads wrong in their assumptions.

Whoever started the myth that standard push-pull class AB1 guitar amplifiers (eg. Fender, Marshall, HiWatt, etc...) should be biased to 70% of maximum allowable tube dissipation should have sugar put in their gas tank. This is totally wrong in every way. There is no magic percentage of maximum dissipation to which one should bias, but most class AB1 audio amplifiers are happiest in the 35% to 40% range. (For the record, 70% is what is generally recommended for portable class B2 radio transmitters, and is close to what a class A1 amp wants.) It all depends upon the impedance of the output transformer primary, the load line of the output transformer, the impedance of the screen supply, and a few other, more esoteric factors that aren’t really worth getting into here.

People tend to repeat something that they read somewhere, as if it were truth, and it gets repeated and repeated until 95% of the hits on the internet are recommending something that was wrong to begin with. At times I find it frustrating.

That 70% factor also goes out the window when dealing with current production tubes, which are not designed to proper specs, but rather are made by reverse-engineering well-made old tubes and making countless assumptions as to why things were done the way they were. For instance, few of the Russian power tubes in production will work in a class-A amplifier. They tested their “designs” in Marshall 50-watt heads. If they worked for a few hundred hours, they gave the go-ahead to production to make 100,000 of them. Everyone who really understood how to design power tubes has been dead for 40 years, and their expertise died with them. Most people don’t spend their hard-earned money on NOS tubes because they sound better (some do, some don’t), they usually buy NOS tubes because they will sound good for 10 years of daily use as opposed to 1 year of daily use with current production stuff.

The purpose of biasing a class AB1 push-pull amplifier is to eliminate crossover distortion in the signal, plain and simple. This can be done with a distortion analyzer, but it does not differentiate crossover distortion from other types of distortion, so an oscilloscope is the best way of determining when crossover distortion has gone. When a tube is biased too cold, the signal from one tube (or tubes on one side) stops flowing before the signal from the other tube (or tubes on the other side) starts flowing, and this “gap” is called crossover distortion, as it occurs around the zero-crossing of the signal voltage. You bias a push-pull audio output section by first making it too cold on purpose, applying signal that is well below clipping, usually around 25% to 50% of clipping, and slowly increase the bias current (making the bias voltage less negative) until the gap at zero crossing is gone. When you have reached this point, you have attained the maximum dynamic range possible from the amplifier without the (very ugly-sounding) crossover distortion.

I got curious about where this point fell, as it could be a little bit ambiguous exactly where the crossover distortion “disappeared”, so I started writing down the no-signal (quiescent) dc current in the tubes where I was sure that it was gone. I documented this in roughly 10,000 amplifiers between 1969 and 1999. What I found was that, in high-powered class-AB1 amplifiers with properly designed output stages that ran their tubes near their voltage limits and had relatively linear load lines in the output transformers, the crossover distortion was sometimes gone by the time the bias was set to 21ma quiescent current per tube, often gone by 23ma, and always gone by 25ma quiescent current per tube.

One surprising thing is that it didn’t seem to matter at all what the tube type was. This number held true for 6L6, EL34, 6550, KT66, KT77, KT88, KT90, 7027A, 6CA7, 7591, 7868, USA, German, Dutch, British, French, it just didn’t seem to matter what the tube was, or who made it, as long as it was in this family of tubes that could deliver roughly 50 watts from a pair.

So, in order to make life easier for my techs, and to take some of the “guesswork” out of the biasing process, I determined that with all Marshall and HiWatt 50-watt and 100-watt amps, Fender 40, 50, 80 and 100-watt amps, and similar circuits, that they were to measure cathode current with no signal, and set the bias so that the tubes drew 25ma per tube with no signal. Then they had to do a listening test, to make sure that all was indeed well. On a few occasions the amps sounded bad at that rating, and when that happened, I took over to try to figure out why. Sometimes the two tubes were not matched, sometimes the output transformer was damaged on one side, sometimes the output transformer had been replaced with one that was not correct for the amplifier, sometimes the screen supply had too little or too much impedance. But, when things were as they should be, 25ma always gave a clean signal with no crossover distortion.

So, if your amplifier has maximum dynamic range at 25ma quiescent current per tube, what happens if you run it hotter, like 30ma or 40ma? Good question. First off, the noise floor comes up and, the tubes run physically hotter, which reduces tube life, the power and output transformers run hotter (due to the extra current running through them, even with no signal), increasing resistance in the windings, reducing overall output levels, and shortening the life of the transformers. As you can likely guess, the filter caps don’t last as long, either. I do know a few people who claim to prefer the sound of an amp biased to 40ma per tube, but these people almost always run full out, maximum distortion, 100% of the time, and never ride that expressive slope where clean gives way to “fat”, “fat” gives way to “crunch”, and “crunch” gives way to overdrive.

Another side of this is that the power supply and output stage can only deliver so much power before they just can’t deliver any more, and this is the maximum distorted full output. If you bias the output stage hotter than it needs to be, you are reducing the ratio of clean output to maximum overdrive output, and robbing the amplifier of dynamic range. Think of it like the idle in your car engine. If your engine redlines at 8000 RPM and will idle smoothly at 400 RPM, you have a ratio of 20 to 1 in terms of “dynamic range” when it comes to acceleration and general driving characteristics. You always want your idle as low as you can get it and still have it running smoothly. You can set the idle at 2000 RPM, but you will only have a dynamic range of 4 to 1, and will be shifting gears a LOT more often to get to speed. Also, you’ll be burning a lot more fuel and fixing your car a lot more often.

Biasing your amplifier to 70% of maximum plate dissipation is like setting your car’s idle to 5600 RPM with a redline of 8000 RPM. You’ll be rebuilding your engine (or replacing power tubes, filter caps and transformers) a lot more often.

I have a lot of customers who have amps worked on (under warranty) by other companies (as I only do warranty work on THD amplifiers), and then bring them to me for biasing, as everyone else seems to bias them too hot, and then their tubes don’t last very long, their amps sound overly compressed and “mushy”, and they hum too much.

It is perfectly safe for your Flexi-50 if you want to try running your bias a little bit hotter. I would not recommend going above 40ma per tube, though.

Circuit Board Humor, Part 1

Since our first products, we have used spare space on circuit boards and chassis to add little quips and sayings that we find either funny or poignant. This is from the upper side of a 1995 revision of the Hot Plate circuit board. My ex-wife didn’t think it was funny. (She and I are still on good terms, though.)

It’s not often that you see 4 Soldano heads in one place! A customer brought them to us to be modified so that the master-volume control could be bypassed with a footswitch, like it is in the THD Flexi-50. After discussing our method with Mike Soldano, and getting his blessing, we did the work on all 4 of these beauties. Now the owner can bypass the masters and control the stock boost function with a stereo footswitch cable and adapter, and a standard THD Flexi-50 footswitch. There is no click or pop at all, and no degradation to the signal path, as the switching is done with special gold-contact relays made specifically for audio signal path.

We continue to offer tube amplifier repair, restoration and modification services, as we have for the past 33 years. Clients regularly send us amps to work on from all over the world. If you have an amp that needs our expertise, please get in touch either through Tumblr messaging or email: [email protected]

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