Tube Amp Build #2 Part 2 (The Circuit Design)

This amp has gone through quite a few changes. The reading might be interesting to see why I made certain choices, but the schematics here are for reference ONLY. They work, but they need work. Check on my tube amp page for the latest schematics.

[Brackets note changes and experiences after real-world testing had begun. Some things had to be ironed out during the perfection/acceptance process.]

Designing a tube amp circuit isn’t really that difficult, but it can be frustrating. The hardest part that I experienced was designing the output stage. I never really read anything that helped me understand why I did what I did. I’ll explain this when we get there. Also note that everything is designed to be within 10% of the expected value. Some things could be changed after other values are figured, but with tolerances of components falling between 5% and 25%, what’s the point in trying to get any closer on paper? Testing the actual circuit is the only true measure of its performance.

Power Supply Design

The first step that I took was to design, or at least have some idea of a power supply. I first looked at transformers in the $50 price range. I chose the Hammond 269EX because it is a 190-0-190. After rectification this can be as high as 190/.707= 268V. I used Duncan’s PSU Designer II to estimate my power supply. I concluded that 250 volts would be the nominal voltage.

[(6-20-2010) The power supply as first designed used 50 ohm resistors in the DC smoothing stage. I believe that the peak-to-peak voltage bumps caused the preamp and power amp to have hummmmmm. This will be later verified later in the week when I receive my parts order. Stay tuned.

(8-9-2010) After much trouble with hum, I opted to neglect the 6.3v windings of the Hammond power transformer. I installed a 12.6V Radio Shack transformer, rectified it, and regulated it with two LM315 adjustable voltage regulators. This defeated a lot of hum. I installed some heat sinking to keep them from overheating.]

I have to admit that I backtracked to the 250 volt range. I originally only had a 170 peak voltage power supply design since I had a $20 transformer, but the EL84 output tube’s datasheet gave the specs for 250 volts, so I assume that following the EL84′s datasheet would be the best design. As seen in the schematic, I am using a solid-state rectifier. This saves on cost since rectifier tubes call for heater voltages and of course, a $15+ rectifier tube. The downside to this decision is that the standby switch must always be switched off when turning the amplifier on. Also, the capacitors must also be rated for max voltage in the event that the switch is accidentally engaged prior to heater warm-up.

Preamp Stage 1 Design

I designed the bias points of the preamp from my own idea, although there are mixtures and assumptions taken from other designs. I chose to design the preamp using the 12ax7 datasheet and plotting a load line. I used the theory document from AX84* in figuring out what I needed. I made points on the Ep-Ip chart at 250V and at 2mA. Then I connected these two points with a line. This is the load line. Then I chose a bias point at -1 volt. Next, I drew a line straight down from the intersection of the load line and the bias point. This is the voltage dropped by the tube: 130V. I chose the bypass capacitor (C6) and coupling capacitor (C1) just because that’s what the AX84* project used. They are both chosen based on a frequency cutoff with which they both fall below the human threshold of hearing. The 1M volume pot and 1M resistor was also chosen because of the AX84* design. The 1M resistor and 1M pot act as a voltage divider. The 1M resistor keeps the voltage from the previous stage from becoming horrible sounding. I’m sure it could be less, but I chose to stick with a large value.

[(8-9-2010) Real world testing of V1.2 was successful, but didn’t sound right. The amplifier’s distortion is a little harsh. I believe the breakup is happening in the first preamp stage. V1.3 will be edited/modified to show the change in the first stage cathode bypass circuit. I am opting to use a similar route to AX84*. They used a 2.7k resistor.

(8-12-2010) The circuit is finished!!! I’m very pleased with the sound as laid out in V1.3′s schematic.

(9-6-2010) I’m having issues with the tone and distortion when playing with different guitars. I’m no longer happy with my design. I am going to revise the schematic later as I experiment. I think that the AX84 project will help me determine where I went wrong. The amp has too much highs and sometimes the distortion is dirty.

(9-11-2010) I’ve upgraded the schematic to V1.4, and so far I think I like it. I’ve only played it with my Telecaster for a few minutes. Tomorrow morning I’ll be testing it for at least an hour at the time to see what happens. V1.3 tended to start breaking up after a while of continuous playing. I’m not sure if it was the output tube not having a stiff enough grid 2 voltage. I’ll soon find out how the upgrades went. I’ll be playing the amp with a 1970 Strat with staggered AlNiCo pickups. It has a lot of low end, and I’m thinking this is part of the culprit. The amp just couldn’t reproduce the lows. We shall see.

Preamp Stage 2 Design

The stage 2 preamp was designed in the same manner as stage 1, except that I just copied stage 1 as stage 2. The only change was that I added a 1uF cathode bypass capacitor, again because of the AX84 project. [afterthought: Originally, stage 1 was almost an exact copy of stage 2, but in V1.3, some things were changed.]

Tone Stack

For the tone stack it would look like I copied the one used in the AX84*, but this is the Marshall tone stack from Duncan’s Tone Stack Calculator 1.3. The only change I made was to change the original 220k treble pot to 250k, since a 220k was too hard to attain. It appears that the AX84 project* did the same thing.

Output Stage

The output stage was designed simply by using the values given in the EL84 datasheet. Of course, some items were changed to easier to find values. C12 was copied from the AX84 project*.

References: *http://www.ax84.com/classicprojects.html

Schematic:

Version 1.2 [6-20-2010] – For reference only.
Version 1.3 [8-12-2010] – [Full of mistakes!!! Changed stage 1 preamp and added DC heater power supply]
Version 1.4 [9-11-2010] – Changed the output stage adding a resistor and cap to grid 2. Changed the 1M resistor from stage 1 to stage to back to the 470k. Increased the cathode bypass cap in preamp stage 1 to 1uF. Also increased the first coupling cap to .022uF. I also tried to make sure it said EL84 instead of EL34. I know that these are two different species of tubes. [I believe this amp is done :)]

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About robbie

I am an electronics enthusiest and a ham radio operator (W1RCP). I like to play with electronics. It's fun and educational. I looked forward to working in the engineering field in the future. I have a BS in Electronics Engineering Technology from DeVry University. I also have an Associate's degree in Marketing Management from Moultrie Tech, and a diploma in Electronics from MTC.

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