What exactly is bias? Bias is prejudice in favor of or against … oh wait, wrong kind of bias. I think he wanted me to write about bias in a tube amplifier, which is far less polarizing.
Bias, as defined in the RCA Radiotron Designers Handbook, is “voltage applied to the grid [of a tube] to obtain a desired operating point.” Well, that is the most basic explanation, but for the most part it is good enough and pertains to the majority of tube output stages in our favorite tube guitar amps.
Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly biased.
Besides “applying” a voltage to a vacuum tube, however, biasing can occur in another way as well. There are quite a few amplifiers, such as a Vox AC15 and AC30, any of my Budda and EAST designs, and even most of the early, low-wattage amplifiers of the tweed era that use what’s known as a “cathode bias” design. This is where the current flowing through the tube (which attains the aforementioned “desired operating point”) is not set by the voltage “applied” to the grid of the tube, but is instead set by the resistor in the cathode leg of the tube. It’s a bit more complicated than that, but the result is an amplifier whose output stage is “self-biasing.”
Most amplification devices, including transistors and even preamp tubes, need to be “biased” in order to perform properly, but this type of biasing is fixed in the design parameters of the circuit. In the case of the preamp tubes in your guitar amp, bias is based on the value of the cathode resistor, among other things. But that’s enough design theory for today. Let’s get back to the core task of biasing the output tubes in most guitar amplifiers.
First, the bias voltages you see listed on many schematics, such as 52V on a black-panel Fender Twin Reverb or 51V on a Marshall 100W Super Lead schematic, are merely approximations of the voltages that should be expected in that area of the circuit. Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly biased. Tube bias is also dependent on the high voltage (or B+) applied to the plate of the output tube, which can vary within tolerances of the transformers as well as in the AC line voltage fed to the amp. (This is why amps can sometimes sound better in one room or club than others.)
But even more important to understand is that tubes produced in different factories across the globe will bias up differently! What I mean by this is, if you properly bias a set of output tubes—let’s say 6L6s made in Russia—and then you swap them out with a set made in China, in the same amplifier without changing the setting of the bias control, the end result will almost always be a different bias reading. This is why it’s always best to have checked and reset the bias whenever output tubes are replaced. Now, how do we do that?
The Preferred Method
Fig. 1
There are several different ways to measure output-tube bias current at idle. The safest method is to use what is commonly called a bias probe (Fig. 1). This is a device that is inserted between an output tube and its socket. (I typically make my own bias probes, but if you simply search “bias probe” online, you’ll find plenty to choose from. If you already own a multimeter, you can simply purchase the probes, but there are also options to purchase a full system with either a digital or analog meter, should you need it.) This device breaks the connection between the cathode (which is the metallic electrode from which electrons are emitted into the tube) of the tube and its ground connection, and inserts a small value resistor in between. It then allows the voltage across the resistor to be read. The resistor is typically 1 ohm and the resulting voltage drop across it is in millivolts (mV), so no chance of shock here. This provides a true and accurate measurement of the actual current flowing through one tube. Then, you set your bias and you’re done!
But even more important to understand is that tubes produced in different factories across the globe will bias up differently!
Ah, but wait! How do you set your bias? Let’s learn a bit more. Most tube amplifiers, if they are not cathode-biased designs, have some way to adjust the output-tube bias. One longstanding exception to this are most Mesa/Boogie amps. The bias voltage in these amps is not adjustable, which is why Mesa suggests only purchasing their tubes for their amps, because they are designed to fall within the acceptable bias range for their amps. This adds a certain degree of confidence for owner servicing, although, of course, it limits your options.
Let’s take a look, however, at a typical Fender or Marshall bias control. Most older Fenders have a pot with a slot for a screwdriver mounted to the chassis in the area of the power or mains transformer, while most older Marshalls have their bias pot mounted on the circuit board. (You might want to go online to look at schematics for your amp to help you find it.) Either way, this is where you’ll make your adjustment.
To get started, you’ll most likely need to pull the chassis and place it in a stable work environment. Insert the bias probe device between one of the tubes and the socket (Fig. 2). Make sure all the volume controls are set to zero, turn the amp on, and let the tubes warm up. It’s also good to try to have a load on the speaker jack—whether a speaker or an appropriate resistor or load box. This is not 100 percent necessary for just setting the bias to a particular number, but sound checking is one of the ways I like to make the final adjustments, so being able to connect the speaker to the chassis while it’s on the bench is certainly a necessity for me.
Now, where to set the numbers? There are certainly more than a few opinions floating around on the interwebs about what optimal bias settings are. Some engineering types will tout 50 percent maximum plate dissipation or 70 percent maximum dissipation, and while it may look good or make sense on paper, I’ve heard the result of guitar amplifiers designed by the book to optimal specifications … and to me they sound, well, less than optimal. It may work in the hi-fi world, where perfect sound reproduction is the goal, but guitar amplifiers are in the sound production business, so it’s a bit different. (In the most basic terms, maximum plate dissipation is the amount of power the plate of the tube is designed to deliver.)
Different types of output tubes have their own acceptable range of bias current. There are so many variables at play that there is no “correct” number. The plate voltage in the amplifier, the output transformer’s primary impedance, and the country of origin of a tube all factor into how it interacts with the voltage and output transformer to define what the optimal bias current will be. Below are the average ranges for some typical octal output tubes:
• 6L6: 25–35 mA
• EL34: 30–40 mA
• 6V6: 18–25 mA
• 6550: 35–45 mA
• KT66: 30–40 mA
Fig. 3
These should be the ranges in which these tubes will perform and sound the best, and they can be accurately measured with a digital multimeter. The best way for you to decide what setting is best for you is a combination of the reading on the meter and your ears! Using the bias control, set the bias to somewhere in the ranges given above (Fig. 3) and play the amp. Note: Some amps will act funny and develop horrible noises (parasitic oscillations) when a bias probe is in place while the amp is being played. If this is the case, you’ll need to remove the bias probe each time you play the amp.)
Move the setting a couple mA in one direction or the other and play again. Don’t expect extreme changes; that’s not what we’re looking for. Listen for subtle differences. Is one setting a little more or less harsh? Is the bottom end too soft or flubby? Is the amp as clean as you want it? Sometimes these little subtleties are what make one amp sound and feel better than another!
Most older Fenders have a pot with a slot for a screwdriver mounted to the chassis in the area of the power or mains transformer, while most older Marshalls have their bias pot mounted on the circuit board.
Also, you should be doing this at the volume you would typically use onstage or in the studio. You may not notice much change if your volume is at 1, but you want to optimize the amp for the way you will be using it.
Eyes Wide Open
Fig. 4
Knowing the ballpark bias numbers is good, and adding your ears is even better, but I also like to see what I’m hearing, so I always incorporate an oscilloscope when I’m setting the bias on an amp. I mentioned crossover distortion above, and when it comes to setting up amps for today’s pedal-hungry players, I find that setting the bias to where there is just a hint of crossover distortion at full output is what works best. Fig. 4 is what that looks like on the oscilloscope. This keeps the amp very clean and makes most pedal users happy.
By the way, here’s a mini primer in crossover distortion. In a push-pull output stage, which is found in most amplifiers with two or more output tubes, each tube (or pair of tubes) is responsible for amplifying at least half of the audio signal. If the tubes are not biased properly, one tube (or pair) will stop amplifying before the other tube (or pair) start amplifying. This will create crossover distortion. Proper biasing will allow the two halves to interact correctly. It’s like a nice firm handshake between both halves.
Beware These Old-School Methods
Let’s look at a couple popular methods that I do not recommend, but are worth discussing because they are, nonetheless, common. The first is: With the amp off and output tubes removed, use a multimeter to measure the resistance of each half of the primary side of the output transformer. This would typically be from the center tap to each side of the primary winding.
In the most basic terms, a transformer is a bunch of wire wound around a steel core. On the primary side of an output transformer, the center tap is the electrical “middle” of this long length of wire. This is typically where the high voltage is applied. The ends of this length of wire are connected to the plates of the tube, thereby applying the high voltage to the tubes. As an example, typically in most Fender amps, the center tap is red, and the ends of the primary windings are blue and brown.
Fig. 5
Next, install the output tubes, turn the amp on, and measure the voltage drop across each half of the output transformer with the amp at idle in operational mode (Fig. 5). Voltage divided by resistance will give you the DC current through the tubes. For example, 1.17V / 15.8R = 0.074, or 74 mA. The numbers I used here were actual measurements in one side (one half) of a 100W amp using four output tubes (two per side). So, divide the 74 mA by two, and you get an average of 37 mA per tube.
Next, you can try the shunt method. This requires a multimeter that can read DC current in milliamps (mA). Connect one meter lead to the center tap of the output transformer and the other lead to the output transformer’s primary side. Typically, in most amps using octal tubes (6L6, 6V6, EL34, 6550, KT88, etc.), this will be pin 3 on any output tube socket. Turn the amp on and, in operating mode at idle (i.e., volume off), measure the current across that half of the output transformer. For example, if your measurement is 72 mA and it’s an amp that utilizes four output tubes, the current measured is for two of those tubes, so once again divide by two to arrive at 36 mA per tube.
I’ve heard the result of guitar amplifiers designed by the book to optimal specifications … and to me they sound, well, less than optimal.
Both of those methods are very old school and still in practice, but I wouldn’t use either for two reasons: 1) I don’t believe they’re very accurate, and 2) they’re dangerous! You’re probing around inside the high voltage area of the amp, and one slip will either take out a fuse, take out a tube, take out your meter, or, worse case, let you know exactly what 450V DC feels like! So, although these methods are used, let’s just say no here.
Some Personal Insights
I’d also like to add a little personal experience to this procedure, based on decades in the biz. Back in the day, when I began servicing and modifying gear, guitarists were regularly playing 50- and 100-watt amps. (Everybody looked at me like I had three heads when I came out with the 18-watt Budda Twinmaster, but that’s a whole other story.) There were some overdrive and distortion pedals around (now all vintage), but certainly not the pedal proliferation we have now, so players were pretty much guitar, cable, amp … go! In these situations, I would most times run the tubes with a pretty hot bias so the amp would be fatter and overdrive a bit earlier and easier, as a decent percentage of the overdrive was developed by pushing the output tubes. As time went on, output attenuators became more popular, so amps could be pushed hard, but at more manageable volume levels. That was still a good scenario for a hotter bias of the output tubes in high-power amps. Eventually, players started playing lower-power amps, so they could open them up and get great output-tube distortion at lesser volumes. The problem is that hotter-biased low-power amps tend to get mushy and have less definition when pushed hard, so a more moderate bias setting is preferred here—just enough so there is no crossover distortion. Move up to today’s scenario and you’ll find that almost all overdrive and/or distortion is typically coming from a pedal. In that case, an amp is nothing more than an amplification device for pedals.
So, that’s what I’ve learned about tube-biasing from my decades of experience. But the bottom line is, there is no absolute right or wrong settings when it comes to biasing an amp. Keep your ears open and go with what sounds best to you.
Hello Amp Man readers. This month I’ve picked two interesting questions, and since the first references a Fender Super Reverb, I’m going to answer both based on that amp. So read on!
Question #1
Okay, let’s get started with question #1. As far as “would I want a wire connected between a Super Reverb and my ears?” Well, that’s for you to decide, but if you choose “yes,” here’s how to do it.
Fig. 1 is a simple circuit I came up with to allow you to play your amp through your headphones without waking the kids or disturbing your neighbors at 2 a.m. So you can understand what’s happening, I’ll go through it and describe the function of each component.
Fig. 1: This diagram illustrates the modifications required to add a headphone out to a Fender Super Reverb.
Jack 1 is the input from your amplifier’s speaker output, at left in Fig. 1. The resistance value should be as close to the output impedance of your amp as possible. For a Super Reverb, that would be 2 ohms. If it’s not possible to get the exact resistance, it’s okay to go up in value, but not down. This resistor will be replacing the speakers in your amp, which need to be disconnected, so it will be absorbing your amp’s full output power. I recommend that the resistor’s power-handling capability be at least double your amp’s output power. A Super is rated at about 40 to 45 watts, so I recommend at least a 100-watt resistor here. Also, if you’re using the large, gold anodized aluminum resistors, they get very hot and need to be mounted to properly dissipate the heat. If you’re building this inside a Bud box or something similar, mount the resistor to the box and be sure to install feet on the bottom of the box so that the heat doesn’t damage what the box is sitting on—like possibly your amplifier. It would also be a good idea to vent the box.
Next, the signal goes to the volume pot. A 1k-ohm, half-watt or higher, linear pot works fine here. A 22-ohm 1-watt resistor gets connected to the pot’s counter-clockwise arm to provide a little isolation from the amp chassis and add a bit of a signal drop.
Then the signal from the pot’s wiper gets connected to the headphone output jack through a 100-ohm resistor. For testing, I used a set of Tascam headphones with a 32-ohm impedance, which is pretty standard today, and the 100-ohm resistor worked fine and provided plenty of volume. If you need a bit more level, decrease the value of this resistor. Also, the output jack should be insulated from the chassis because we’re trying to maintain a bit of isolation from input to output. A typical “British-style” jack works fine here. Just make sure it’s stereo and don’t forget to connect the tip and ring connections together. The last component you see is a 5 µF 50V non-polarized cap. Since a resistive load on an amplifier causes it to react differently from a speaker load, amps tend to sound more “spiky” with a resistive load, so this capacitor helps smooth out some of the brittleness of the sound in the headphones.
There you have it—a way to play your amp silently.
Question #2
Dear Premier Guitar,
I’ve grown to appreciate your DIY pieces, and they’re well written to the targeted reader. I would like to get your take on the possibility of using the reverb “send and return” loop as an effects loop. It seems easy: An adapter cable changes the RCA plugs to 1/4". The cables go to your effects pedals and return, instead of the reverb can. The cool part is the reverb control would now mix the wet and dry signals. Will this work? Can you use “Y” cables and a switch to include the reverb as well? Are the impedances so far out to lunch that it’ll never work? Is that why I’ve never heard of anyone doing this?
Wysong Perabula
Question #2 asks about the possibility of using the amp’s reverb circuitry as an effects loop. In essence, it already is an effects loop, but it’s optimized for use with the reverb tank, which is far different from an effects pedal. We can, however, get it to work as a pedal loop.
Fig. 2: This schematic shows how a reverb circuit can be turned into an effects loop. Unfortunately, it’s at the price of the reverb—but there are pedals for that.
The biggest difference between a normal effects loop and a reverb circuit is basically in the “send” department. A typical Fender-style reverb tank has very low input impedance, and this requires a substantial level to drive it. This level is far too hot to feed into any effects pedal, so we must first tame the beast. Looking at Fig. 2, R1 serves as a load on the reverb drive transformer. Next R2 and R3 form a voltage divider to reduce the signal to an acceptable level for an effects device. Since the reverb drive circuit is actually a small amplifier output stage using its own little output transformer, removing the inductive load from the reverb tank and replacing it with the 100-ohm resistive load is similar to removing the speaker load from an amplifier and replacing it with a resistive load. Things get a little “spiky.” To compensate for this, we add the .001 µF capacitor—identified as C1—to smooth out the sound.
Now we have a signal that will be a better match for a pedal. Regarding the effects return, the effects can be connected directly to the reverb return (out) of the amplifier, either by 1/4" to RCA cable or, if you’d like to contain this all in a project box, simply connect the loop return jack directly to the reverb return jack, as most pedals should be able to comfortably drive its 220k input impedance. The loop can now be used for your time-based effects (delay, reverb, chorus, flange, phase, etc.). The reverb knob will mix in the amount of effect, so set your effects for 100 percent wet, if possible, to minimize any phase cancellation problems. The footswitch will now turn the effects on and off as well. As far as including the reverb tank back into the circuit, the output impedance is too low and would more than likely attenuate the output of the effects devices.
Well, there you have it. Enjoy the experimentation!
Photo 2
Anyway, the problem here is the 7355 is rated at a maximum plate dissipation of 18 watts, while the typical 6L6/5881 is rated for a maximum of 30 watts. Unfortunately, tube data is not provided in any sort of standard format, so attempting to compare tube specs on an apples-to-apples basis is difficult at best, but tube data suggests output from a pair of 6L6 tubes can be in the 50-plus watt range, which is typical, and that the output from a pair of 7355s can be 40 watts, which is past the amp’s stated design maximum of 36 watts (18 watts each).
Photo 3
That said, maybe they were hoping to design an 80-watt amp with a better frequency response. Again, who knows? But a quick look at these tubes (Photo 2), which are no bigger than 6V6 tubes, screams “good luck!” On top of that, Fender was following up the blonde 1961 Showman (model 6G14) and 1962 Twin (model 6G8), which were 80 watts and had a quad of 5881s and a huge chunk of iron for an output transformer (#45268) (Photo 3). This four-output-tube amp is using undersized tubes and a redesigned output transformer that is half the size (#125A18A) (Photo 4). That doesn’t quite sound like a recipe for success to me.
Photo 4
The plot thickens. And there are even more questions. According to the tube chart, this is a Showman amp, model AA-763 (Photo 5), but not according to schematics. In fact, no schematic that I could find reflects the use of 7355 output tubes or the 125A18A output transformer. Furthermore, the name “Showman-Amp”—as it appears on the faceplate—has typically denoted a single-speaker cabinet, while the name “Dual-Showman” designated a two-speaker cabinet. Apparently not so here, because the owner claims his dual 15” cabinet is the stock, matching cabinet for this particular head.
WARNING:
All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.This normally would be easy to discern, as the blackface Showman and Dual Showman amps this model quickly morphed into utilized specific output transformers depending on the speaker load. The Showman used the 125A30A for an 8-ohm load and the Dual Showman used the 125A29A for a 4-ohm load. There is, however, a bit of information gleaned from researching more transformer history that may fill in the blank here. There was a blonde Fender Twin-Amp manufactured for a very short time as well. It’s possibly even more rare and short-lived than this Showman on my workbench, and was built with 7591 output tubes and utilized the same transformers as this Showman. Since the specs of the 7591 and 7355 output tubes are relatively close and the Twin-Amp combo is, of course, a 2x12 with a total load of 4 ohms, one may assume the output impedance of the A18A transformer to be 4 ohms and that the 2x15 cabinet could indeed be its mate. Boy, that was a long way home!
Photo 5
Anyway, this also provided me with some useful service information regarding output tube replacement/substitution, because they did need to be replaced, since most OEM tubes by this time have been run ragged and are probably as microphonic as the day is long. The first inclination would be to replace the 7355s with a typical 6L6, but that’s probably not a good idea. Not only would the sockets need to be rewired, but primary impedance of the output transformer is not a great match and the filaments of the 6L6 draw more current and would put additional load on an old mains transformer that, theoretically, is already running on more wall voltage than it was originally designed for. The better solution would be the 7591s, as they are currently manufactured and available. The 7355s were obviously produced with unobtainium, since they’re virtually unavailable in the NOS market and no one has done a reproduction. The 7591s, in my opinion, are also a better sounding tube, so they make a great replacement for the 7355s.
Photo 6
There will, unfortunately, be a couple modifications necessary. (And, as always, unplug the amp and drain the power supply caps before servicing any amp. If you don’t know how, find someone that does!) First, the 470-ohm screen grid resistor and associated wiring needs to be disconnected from pin 4 of every socket (Photo 6). Then stand the resistor, still connected to pin 8, up in the air and reconnect the wires to the top of the resistor that were disconnected from that socket. Do this for all four sockets and resistors. Be sure they won’t come in contact with the inside of the cabinet once the head is reassembled. You may need to bend them down to be sure of this. Next, there may be a change in the bias supply voltage necessary, as the 7591s require less negative voltage. Unfortunately, you cannot measure the bias current using a standard octal socket bias-measuring device, but to measure the current, connect a 1-ohm resistor between pin 5 and the ground on the output tube socket, and measure across it. It’s a 1:1 ratio, so a reading of 30 mV is equal to 30 mA of current.
Well, there you have the story of a rare beast, although we may never know why it exists.
’Till next time…
The owner said this amp was sitting for a while and he wanted it serviced and brought back to its glorious self. He also mentioned that one of the output tubes was either missing or looked bad, so he installed a new quad of output tubes and turned it on, but it didn’t sound right and was making noises. As some of you may know from reading my columns through the years, that was a bad thing to do, but it was also good information for me to have.
WARNING:
All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.I, as well as many others in the tube-amp service industry, have always said, “If an amp has been sitting idle for years, it’s not a good idea to just plug it in and turn it on.” The electrolytic capacitors in the amp tend to dry out, and if there’s any hope or desire of keeping the amp original, as well as in service, the caps should be brought up slowly over time with a Variac to allow them to “reform.” Not doing this can compromise the performance of the caps or possibly leave you with a mess that’s a chore to clean up after one decides to vent or explode. Luckily neither of these things occurred, but I made the decision that the original caps should be replaced for this Titan to become a usable, reliable amp.
Photo 2 — The amp arrived in original condition except for the prior replacement of one output tube socket.
Let’s get into the servicing. As you can see in Photo 2, the amp is pretty darn original. The only real change to this point had been the replacement of one output tube socket. Things looked good for the most part, except for a couple small problems. The screen grid resistor on one of the output tube sockets had been fried to a crisp and needed to be replaced. This is obviously where the missing or bad output tube was located, but no worries. The amp was originally built using 1/2-watt screen resistors, but, since we had to replace one, I upgraded them all to much more reliable 5-watt versions. There was also one signal capacitor in the phase inverter section that had broken away from its connection on one side (Photo 3). Luckily there was enough lead remaining on the capacitor that I could attach an extension, properly re-attach it to the connection, and keep the part original.
Photo 3 — A signal capacitor in the phase inverter section had broken away from its connection on one side. There was enough lead remaining to attach an extension.
So, I repaired the signal cap and upgraded the screen grid resistors. Then it was on to the filter caps, and I’d like to impart one very important piece of information here that has come from decades of experience. A fact I’ve learned that may be one of the most important to consider not only in building amps but also in repairing them is that “ground” is not just ground.
Photo 4 — When installing new caps, be sure to use the same negative connection used with the original caps, even if replacing them with a different style cap.
What do I mean by this? Connecting a component that needs a ground connection to just any location on the chassis or to any electrical connection to ground does not mean the unit will function optimally. In many instances, this will cause a low-level hum that should not exist. Proper ground location is crucial for the best performance, and in most amplifiers, that has been optimized during the design and development process. For this reason, whenever I replace filter capacitors, I make sure to use the same negative connection used with the original caps, even if I’m replacing them with a different style cap, as I did for this install. I chose to use the original wires from the multi-caps and attach them to the replacement discrete caps, which I mounted in the same locations (Photo 4).
Photo 5 — The amp’s original two-prong power cord had a hole for a ground wire that was manually attached to a wall outlet’s center screw for grounding.
There you have it. A little piece of someone’s childhood music store experience is back up and running, hopefully to produce more memories. Until next time…
Hello Ask Amp Man fans. While I’ve been answering your questions for many years now, the last few columns have been about interesting amps I’ve come across, and this column follows that path. I still very much enjoy servicing amps when possible—and occasionally a really cool and unusual piece will cross my bench. Over the years, I’ve seen plenty of vintage Marshall, Fender, Ampeg, Vox, Hiwatt, and Gibson amps. They’re all cool, of course, but sometimes there’s a genuinely unusual one. This month’s featured SVT was one of those. The customer hadn’t played the amp for years and wanted it brought up to spec.
At first glance, it looked like a typical Saint Louis Music-era SVT (Photo 1). It was dressed in black, maybe late-’80s or early-’90s, and possibly an offshore-manufactured unit. But then I saw a commemorative plaque (Photo 2) attached to the rear grate. What could this be? Time to do some digging.
Photo 2
In disassembling the amp, I noticed the transformers looked to be old-school, U.S.-made and not the later-period SVT offshore transformers, which to me are pretty recognizable. The lower power-amp chassis also looked to be of original U.S. manufacture. It had the same multi-pin chassis connector and same tube location and callout as classic SVTs. It looked from a time warp to the late-’60s/early-’70s—not a typical SLM-era Ampeg. So, it was time for some research.
The first thing I sought to learn about was the plaque on the rear. Surely “1987 SVT Limited Edition 019 of 500” means something! Turns out it does. This amp is part of a production run known as “Skunk Works” and was, indeed, built by SLM.
Here’s the bloodline: Ampeg was sold to the Magnavox Corporation in 1971. Magnavox then sold Ampeg to Music Technologies Inc. in 1980. SLM bought the rights to the Ampeg name in 1986 and took possession of any Ampeg inventory left at MTI. That inventory contained enough of the old major parts (transformers, chassis, etc.) to build 500 SVTs! These amps were built in-house by the Skunk Works crew at SLM. They were supposedly the first SVTs made in the U.S. since Magnavox had the brand. SLM built and covered all the wooden cabinets and used some stock typical of their ’80s/’90s SVTs for the vinyl, knobs, grille cloth, etc.—which is why, at first glance, they look to be standard SLM-era products.
Photo 3
That perception is quickly put to rest once these units are disassembled. The layout and circuit boards look pretty much identical to the ’70s units I’ve serviced over the years (Photo 3 and Photo 4). Now that I knew what an interesting piece I had on the bench, it was time to see what condition it was in.
Photo 4
Since the customer hadn’t used the amp for years, the first procedure was to bring the amp up slowly over the course of the day to re-form the filter caps. So, I pulled the chassis, removed all the tubes, checked the mains fuse’s condition and value, hooked the amp to a Variac, and started to bring it up slowly while monitoring the DC voltage. In cases like this, during the course of the day I’ll bring up the Variac until the voltages are close to the voltage rating of the capacitors. If the amp doesn’t blow the fuse, we can at least be sure that none of the filter caps are shorted.
Next, I discharged the caps and checked the components known to fail. There are six 5-ohm, 5-watt resistors on the power amp circuit board (Photo 5). Each is attached to the plate of one of the 6550 output tubes and is actually used as a safeguard. If one of the output tubes shorts out, the resistor will open up instead of the amp blowing a fuse, allowing the amp to continue to function through the gig. Pretty ingenious, but there is no indicator to let you know that a tube has blown, so unless you’re very familiar with the feel and response of the amp, you may be running with only five working output tubes for quite a while. The only downside to this feature is that the amp will need to be disassembled and serviced for replacement of the bad resistor before the bad tube is replaced.
Photo 5
Anyway, these resistors checked out fine, so it was time to clean all the jacks, pots and sockets, install a new set of tubes, and see if the amp behaved. I chose the new reissue Tung-Sol tubes because, if memory serves, some of the mid-’60s SVT amps came stock with the original Tung-Sol 6550s. That was a bit of a nod to the past—plus, these tubes sound nice!
I always set up and test amps prior to reinstalling the chassis in the head shell, in case there are any problems. With all new tubes installed, I fired up the amp and set the bias with the very friendly biasing scheme on these amps. Test points and adjustment pots on the front of the power amp chassis, along with instructions, allow for proper biasing, even when the chassis is mounted in the head box. That’s pretty slick. The amp biased up properly and the output on the scope looked great. It was time to put all the screws back in and send this historically significant amp back to its owner—a big ol’ sweet skunk.