Echo / Elk Amplifier Service Notes

     This page contains technical and historical information on Echo and Elk amplifiers. It is intended for owners of Echo and Elk amplifiers who may need some assistance in servicing and refurbishing their units. Working with vacuum electronics poses some risk and therefore the following warnings and disclaimers must be posted.

1. Vacuum tube amplifiers operate at sufficiently high voltages and currents which are potentially lethal to human life. Please be aware of this, especially when probing a unit that is turned on and operating. 
2. Replacing discrete internal components such as resistors, capacitors, etc may require the use of a soldering iron. Soldering irons operate at temperature levels high enough to melt lead and cause burns to human tissue and electronics. Please be aware of this when handling this tool.
3. Soldering may cause exposure to lead which has been shown to be harmful to humans and the environment. Please be aware of this when coming in contact with lead, either directly through the skin or through inhalation of vapors.
4. If you do not have sufficient experience in working with electronics then it is strongly recommended to have professional service personnel examine your equipment.
5. The authors of this blog shall not be held liable for any harm or damages caused to human life and equipment. 

Now, with all that said, let's move on.

General Layout of Amplifier Circuitry

     These amps were built in the 1960s and the 1970s and followed the general manufacturing standards of the time. The circuits were built using discrete components that were individually hand wired and soldered from point to point. There were no reference designators for components (i.e. R1, C7, etc) on the board. In order to locate a component it is necessary to start with a major feature such as a pin on a vacuum tube socket that seems to close to the component and then follow the wiring according to the schematic towards the component of interest. Working through a schematic and a design like this may seem tedious by today's standards but it forces a familiarity with the circuit and with the layout as a whole.

Echo-Sonic Internal Electronics

     Be aware that these amps could be 40+ years old. Even if the amp saw zero hours of service the wiring and the components may have degraded. The insulation on the wiring and the components could have dried. Component values may have drifted. Contacts may have oxidized. This page will cover some of these topics. It is also encouraged to search the internet for other vacuum tube diagnostic help. There are more resources available than can be practically listed here. 

     Final note: consider that you may not be the second, third, or even fourth owner of your vintage amp. The amp could have been owned by several people during its lifetime and the amp may have been serviced or modified from its original design. Tread carefully and go slow. 

Grounding

     Good grounding is important in any electric circuit and guitar amps are no exception. There are two grounding topics to consider. First, there is grounding for the power and second is grounding for the circuit itself.

     If your amp has a two prong cord then one of the first things to do is replace it with a three prong cord. The Line and Neutral feeds into the amp as the two prong cord did before. Connect the ground line to the chassis. Sometimes you may find that it is easier to connect it to the transformer case which is already connected to the chassis. 

This is not necessarily the cleanest example of power ground but the path is still visible. Note the green wire connected to the chassis in the center of the picture. The line and neutral wires connect to the fuse and the switch

     The circuit also needs good grounding. Grounding the circuit is achieved by connecting to the chassis through the shortest possible path. Be aware that in some cases it is possible to produce a ground loop where the ground in one part of the chassis is at a slightly different potential than another part. This could happen when a localized area of the chassis is grounding a large potential. A localized distribution of current is produced that is diffused through the chassis. Ground loops can be avoided by connecting the various ground points on a chassis together. This will equalize the ground potentials across the chassis.

Vacuum Tubes

     Vacuum tubes are the life and blood of a tube amp. If the amp is sounding a bit thin with poor frequency response and lots of hiss then a new set of tubes may be in order. Echo and Elk amps were populated with vacuum tubes made by Matsushita, which is better known today as Panasonic. These tubes can still be found on eBay and other places in various conditions. Newly manufactured tubes can also be used in their place. There are many name brands to choose from such as JJ Electric from the Slovak Republic, Electro Harmonix from Russia, and Ruby from China. 

Original Matsushita vacuum tubes from an Elk Echo-Sonic. From L to R: 12AX7, 6AQ8, 6CA4, and 6BQ5.

     Picking a combination of tubes is a personal choice. Some prefer vintage NOS tubes and other people prefer newly manufactured tubes. Different tubes will produce different tonal qualities. This is neither "good" nor "bad" as much as it is a matter of taste. As long as the same type of tubes are used then all should be well. Be aware that swapping in a different set of tubes may require some tweaks to bias and gain circuitry. 

     The amps used what would be considered for the most to be standard tubes today: 12AX7 tubes, 6CA7 tubes, 6BQ5 tubes, etc. However, there are some designs that use the occasional odd tubes. For example, some designs use a 6AQ8 in the reverb circuitry and 7189 in the output stage. Suitable replacements exist but it may be necessary to dig around on the internet a bit to find them. Continuing with the 6AQ8 example, it has been found that Electro Harmonix makes these tubes today. The 7189 replacement can be found in a Sovtek EL84M or a Soviet made 6P14P-EV.

     In some cases it may be possible to replace a vacuum tube with a solid state equivalent. Search around the internet and you will find some solid state replacements for the most common vacuum tubes. These solid state replacements pack modern silicon to fit into a legacy vacuum tube pin-out. Rectifier tubes are the easiest to design a substitute for. The design simply uses modern diodes. Pre-amp and power output stage tube substitutes typically pack a FET with some support circuitry to make the linear device mimic the nonlinear behavior found in a traditional tube.

     Be sure to check the vacuum tube socket contacts. It is important that the contact between the vacuum tube and the socket be clean. Increased contact resistance may change the performance of the amp. Corroded or loose electrical contacts may produce sound filled with static, pops, and hiss.  

Capacitors

     If vacuum tubes are the heart and blood of an amp then the capacitors are the veins and arteries. Echo and Elk amps were populated with many oil filled capacitors. The oil dries out over time and then they can no longer hold a charge according to their value. Symptoms of dry capacitors include hiss, poor tone, and a loss of functionality (i.e. like in a resonant tremolo circuit). The only solution is swap out the old capacitors with new units. Today's capacitors are made from a variety of materials and come in almost countless combinations of capacitance and Voltage rating. Finding replacements is not difficult at all. Some of the best pricing can be found at large online electronic component stores such as Digikey and Mouser. Be sure to check on eBay and other sites as well.

Original JCP / Zeus branded 600V capacitors sorted by capacitance value. From Top to Bottom: 0.005uF, 0.02uF, 0.03uF, 0.1uF, and 0.2uF. A modern 400V, 47uF electrolytic is shown for comparison.

     Today's capacitors are notably smaller than the units from before. Don't be surprised to find a tiny ceramic capacitor replacing what was once a small brick of an oil filled unit. This could free up space in the amp and make future work easier. You can use the reduction in size to add extra capacitance in places like the power supply circuit. Just be careful not to induce extra inductance when using electrolytics. Large value electrolytic capacitors still require a certain amount of surface area which is coiled into the package. This coil can add some inductive behavior to a circuit. It may be better to connect a few medium sized capacitors in parallel instead. The capacitance will add like resistors in series (goes up) and the inductance will add like resistors in parallel (goes down). 

     Be aware that capacitors have two voltage ratings. The first is a maximum voltage and the second is an operating Voltage. Capacitors like to stay conditioned with a certain amount of charge on them. Operating far below their operating value is like running an engine that is too cold. Don't use a 600V capacitor for a 5V signal. 

Resistors

     The most common brand of resistors used in Echo and Elk amps is Atlas. They are one of the least likely source of any issues unless they were damaged due to a failure of a neighboring component. Leave them be and all will be well. If it is necessary to replace one then any modern metal film, carbon film, wire wound, etc will work just fine. Metal film seems to be the preferred type due to its precision and stability.

     Some would make an argument that the original Atlas resistors have a small amount of inductance due to their construction. Look closely and it can be seen that the resistor is made of a hollow ceramic tube with a resistive deposit on the exterior. 

A gathering of original Atlas branded hollow tube resistors. Also visible are freshly installed ceramic capacitors that replaced the original oil filled units.

     The deposit spirals around the ceramic tube a certain number of times. This makes it into an inductor of sorts. Now, in all reality, any inductance would be negligible and it would probably have almost no impact on tone. It's just something to think about.

Potentiometers

     One of the most common signs of a worn or dirty potentiometer is a scratchy static filled sound when the volume and control knobs are turned. This happens because of poor contact between the internal wiper and the resistive element inside the potentiometer. Sometimes the wiper and/or the resistive element is worn thin or the metal surfaces are corroded. Perhaps the entire mechanism is dirty from outside elements seeping. Grains of dirty are preventing good contact.

     Replacing a potentiometer could be difficult due to the intricate wiring and connections to the part. It is possible in some cases to clean the potentiometer. Several good commercial cleaners are available. This would avoid the need to desolder and snip connections. However, in some cases, the potentiometer may be beyond rescue and it's time for a replacement. Both log and linear taper potentiometers were used in the amps. Check the back of the potentiometer. There will be a letter designation which indicates the type of taper.

Two potentiometers are shown installed side by side. The unit on the left is a tone control knob and the unit on the right is a volume control knob. Note the letter designations appended to the potentiometer value.

     The letter A denotes a logarithmic (or "log" or "audio") taper and the letter B denotes a linear taper.

     Be sure to use the same taper style when replacing the potentiometer, especially for the volume control. Volume controls typically use a log taper. Installing a linear taper would theoretically work too but manipulating the knob for the right volume level would be difficult. A log taper control would allow lots of rotational adjustment for a small amount of adjustment in the "low" positions. Using a linear taper potentiometer would result in very dramatic changes with minimal physical adjustment. The control would be very sensitive.