This is the final installment in our look at building a custom DIY amp for the electric guitar.  (Here are links to Part 1 and Part 2)

If you’ve been following along, let’s recap. Part 1 was about the design of the amp and a list of design goals for the project. Part 2 dealt with tools, construction methods and basic assembly. In this final installment I’m going to look at commissioning the amp and doing some performance testing. I’ll also close the loop by going back to the design goals that were established in part one, and seeing if the final product met those goals.

While the project has not been a how-to guide, I do hope that you will get enough insight to decide if building your own amp is a project that you might want to try.

So what do I mean when I talk about commissioning an amp? Simply put, this is the activity related to the initial start-up and evaluation of the amp. It consists mainly of a visual inspection and taking readings of the critical voltages in the circuit to make sure the amp will run safely and be able to meet the design goals of the project.

Important measurements will include filament voltages and high voltage DC supplies feeding the different parts of the circuit. Once we know that everything is solid and safe the amp will be examined and listened to. Listening is one of the most important activities because an amp can look good on paper but have sonic problems that don’t show up on basic test equipment. Noise is a perfect example of something that you won’t find using a multi-meter. But noise can make you aware of a problem that can be located using an oscilloscope.

To do these tests safely and accurately, it’s very important you use a good quality meter or meters, and also follow good practices for personal safety while testing. Voltages up to 400 volts DC are present in the amp while running and may remain in the amp after it’s shut down and unplugged. I will show you the test points and assemble a table of test results that can be used to determine the power output using a variety of output tubes and rectifier tubes.

Commissioning:

Let’s have a look at figure 1 to see where all the critical test points are.

FIGURE 1

Figure-1

When we are commissioning the amp these are the critical points to measure voltage. When I start an amp up for the first time I always have a load on the output. You can use a speaker or a high power resistor with the correct load resistance. I like to start by powering up with no tubes installed except the rectifier. This allow you to check the grounds, ac line supply, heater voltage and high voltage DC.

Remember that this is now a device with potentially lethal voltage exposed. I generally reference all voltages to ground. Attach the ground lead of your test meter to the power supply main ground point. Now you can check all AC and DC voltages using one hand to hold your meter probe. This reduces the chance of a deadly shock should your hand accidentally contact high voltage. An even better way is to plug the amp into a ground fault interrupter. Should you accidentally touch a live component the breaker will trip and kill the power.

So let’s start with the power supply as seen in figure 2.

FIGURE 2

When you switch on the amp you should see the pilot light come on indicating that the transformer is energized and the 6.3 volt supply is putting out voltage. If not, start at the AC supply fuse and see if there’s power coming in. Be sure to check both tabs on the fuse holder. If a fuse has blown on start-up stop immediately and unplug the amp. A blown fuse indicates a major fault and you should give the amp a detailed visual inspection, looking for mis-wired components, bare wires touching the chassis or shorted connections at sockets. If that doesn’t provide a solution you really need a person with technical chops to find and correct the fault.

Assuming the problem is fixed, you want to check the 6.3 volt supply for balance. This is really important for noise elimination. Measure each side of the 6.3V supply to ground at each of the 100 ohm balance resistors. Half of the supply should appear on each resistor. Since this is an AC line run in two strings, the AC noise should cancel itself. This is much like a humbucking pickup. If there’s an imbalance the noise will not be fully cancelled and you could get hum.

The next thing we’ll measure is the DC supply. Measure the B+ at the cap labelled 32µF – 2. That’s the first point where power is drawn to feed the output transformer. If there’s no power at this point there will be no high voltage anywhere in the amp. Finding a problem here is easy. You work your way back to the rectifier tube. So, measure the voltage at 32 uf – 1. If there’s power there you know that the problem is in the choke between the two 32 uf caps. If there was no power there you would go back to pin 8 on the rectifier tube socket and see if DC is being produced. If not you check the 5 volt ac line that powers the rectifier filament. If there’s power there, the rectifier tube is bad. It’s all a matter of being methodical and checking one thing at a time.

Once we have B+ voltage it is carried to the power transformer and output tube plate at pin 3. Additionally the voltage is connected to a chain of voltage nodes that supply the rest of the amp. The first node is an 18k dropping resistor and 22uf 1. This point is the supply node for the screen grid of the output tube. The voltage then is dropped with a 10K resistor and joins 22uf 2 to form the node that supply’s the preamp and tone control. The high voltage circuit is designed to increase filtering and decrease voltage at each node. This ensures that as you progress to the more noise sensitive sections in the front end of the circuit any residual AC will be eliminated and the voltages will be reduced to useable levels.

FIGURE 3

If you look at figure 3 you can see the components for the tone section. The high voltage comes from 22uf 2 and feeds the 100k plate resistor. In turn, the voltage is then transferred to the 100k slope resistor in the tone stack.

If there’s voltage at these two points proceed to the pre-amp as seen in figure 4.

FIGURE 4

You can see that another line is coming from 22uf 2 to supply both the preamp plate and the screen grid. Remember, this is a pentode. The wiring for this tube is more like the wiring for a power amp tube. The voltage value will be the same as that feeding the tone stack.

These are all the points I check before adding tubes for the preamp, tone controls and power tube. The values will all be noticeably lower when all the tubes are installed as the power supply will be loaded. At this point you would power down the amp and install the tubes.

Tuning and Tweaking (TNT)

Now that the amp has been checked over and the power supply operation confirmed, we can add the tubes and repeat the testing at all the voltage points I’ve shown.

For the B+ you want to see something between 325 and 400 volts DC. This will provide a useful plate voltage for any output tube you might want to install. The screen grid voltage is often very close to the plate voltage and that’s ok. You don’t want it to be higher than the plate voltage. In that condition the amplification of the tube is negatively affected. I use a 1k screen grid resistor to limit screen current, but the setting of the screen voltage is set using the resistor and capacitor at its voltage node. In this case it was the 18k resistor and 22uf cap.

For the tone controls you want to see something between 180 volts and 300 volts. The tone stack in this amp uses a cathode follower circuit so the plate voltage will be high, in the 300 volt range +/- 20 volts. In the preamp the single supply line feeds the plate resistor and screen resistor. The voltages on the preamp components are very important in the reduction of noise and microphonics.

This design uses an EF86 pentode as the preamp. The EF86 tube is notorious for microphonics and this is due to its high gain and super large plate. I’ve found that limiting the power to the plate resistor helps big time. The tube has huge gain so starving it a bit still leaves tons of gain and gives you a useable tube. Not many designers use the EF86 and when I check their designs I usually see 180 volts to 225 volts on the plate. This may be where the microphonics come from. At any rate, you are best to get a few EF86’s and test each one for noise in the circuit under operating conditions.

The screen grid has a big effect on the operation of the preamp and I like to have it running with about 20% less voltage than the plate. This seems to make the screen more efficient and help control secondary plate emissions. In this design I have the plate running at 105 volts and the screen at 86 volts. That is really low for a classic 12AX7 preamp but just fine for the EF86. You can see in figure 4 that there is a 220k plate resistor. This is a common value used in many designs and almost standard for an EF86. If you look at Gibson amp schematics you’ll see it was used often for a 12AX7 preamp. The screen grid resistor is very often a 1 Meg value. When I was testing the amp I found that the screen voltage was closer to the plate voltage than I wanted. After some experimenting I found that 1.5 Meg would give me the desired screen voltage. I didn’t have that resistor so I connected a 500 ohm resistor with the same wattage rating and placed it in series with the 1 Meg. I put a piece of shrink wrap around the entire thing.

That’s about all we can do without plugging in an instrument, so it’s time to get cracking and have a listen. The tube set I installed was a NOS Tesla EF86 tube, the tone controls were running off a JJ ECC803-s Gold and the output tube was a JJ 6550. The rectifier is a Sovtek 5AR4 (GZ34).

The first thing I picked up was a hum in the amp. There’s really no reason for it because my grounds, filtering and voltages all look perfect. When trying to locate a hum it’s best to simply pull tubes and see if it goes away. This is a quick way to troubleshoot a problematic section. I pulled the preamp and tone stack tubes and still had a hum. That told me that the problem was coming from the output section. I checked for ac voltage on my dc supply and it was good, so what’s causing the hum?

In fixed biased amps the cathode is connected directly to ground. In a cathode biased amp the cathode is connected to a resistor and cap that are grounded. This generates a voltage drop on the cathode to produce the required bias voltage. The cathode is essentially a tube with a filament inside of it. When the filament heats the cathode, it can release electrons to flow to the plate. Remember, inside the tube voltage flows from a low potential to a high potential.

The heater is running at about 6.3 volts referenced to ground. The cathode is generating a voltage because of the cathode bias resistor. This is very common and almost universal in small tubes like the 12AX7. In that tube the cathode voltage is small, usually less than 5 volts. In a cathode biased output tube things are different. The cathode voltage is now higher than the voltage of its internal filament and current begins to flow from the filament to the cathode and induces hum.

There is a simple way to fix this and you’ll see it often in high fidelity audio equipment. Our filament supply is balanced and tied to ground. Ground is zero volts. But we don’t have to reference the filament supply to ground. Instead, we can reference it to another DC voltage. Since we want our filament voltage to always be higher than our cathode voltage, we simply disconnect our filament balance point from ground and run a wire from this point to the cathode. Now the 6.3 AC volt filament is riding on top of the DC cathode voltage. Now that the filament voltage is higher than the cathode voltage there is no current flow from filament to cathode and the hum is killed. If you look at figure 5, our schematic, you can see how the filament supply is connect to the cathode.

FIGURE 5

As a wrap up for this project I connected the amp to an 8 ohm load and input a 150 millivolt at 1 KHz and made all sorts of adjustments to the tone controls. I repeated this using a number of different output tubes. Regardless of what you might expect, all these combinations yielded between 5 and 8 watts RMS output into the load. It’s safe to assume that the relative peak outputs would be between 10 and 15 watts, about twice the RMS output.

Ep – Plate voltage   Es – Screen voltage   Rs – Screen resistor   Is – Screen current Ek – Cathode voltage

Rk – Cathode resistor   Ip – Plate current   Dissipation – Plate dissipation at 1 KHz 100mv input

That doesn’t sound like much power, but this is very efficient power, much like a true high fidelity audio amp. The sound it produces is detailed and punchy, no one will believe you when you tell them how much power it produces. I have a 25 watt solid state practice amp for my bass and this amp just buries it.

The tone from this amp is amazing and unlike many single-ended guitar amps it is dead quiet. Operation is quiet and cool. The power transformer is not hot at all, just a bit warm. Because of the expanded tone controls there’s a lot tweaking available. Until you reach half volume the amp is strong and clear with silky highs, and has really deep, solid lows. Bass and treble controls work really well but I think the midrange could be more effective so I plan on playing with that in the future.

There’s also good news for those that like pedals. This amp loves them and with a good boost into the front end you won’t believe that you’re not using a 40 watt amp. Trust me.

I really hope you enjoyed this brief look at what amp building is all about. It’s a great hobby and if you’re interested I encourage you to take on a modest project and get your feet wet. If you get the bug, building your own tube gear can be a rewarding pastime.

Thanks for following along with my project and be sure to come back for informative articles at www.thetubestore.com.