PDF Electronics
 

Signal Path Schematic	PSU and Bypass System Schematic

1st Stage:

The first stage is pretty simple. The idea behind the design of this stage was to just have it act like a voltage amplifier. Just boost the signal some so it can be passed on to the next stage. A boost switch is present, that would be the 100mF cap from cathode to ground. It acts as a full boost and adds a nice bit of grit and added gain to your sound. A presence control has also been added to tame highs.

2nd Stage:

This stage is as simple as they get. The purpose of this stage is simply boost the signal more. We want to boost and increase the gain of the signal because it will soon hit a passive network of filters for shaping tone. This will cause a signal loss/gain so it is important the signal has adequate strength prior to the tone control. The gain of the stage is maximized with the cathode tied right to ground, this is called a mu-follower. Vk and Vg are quite close so the signal could get clipped quite easily. This doesn't really happen though, the unit is actually pretty clean. Boosting the incoming signal with a boost pedal really hits the tubes hard and they clip.

3rd Stage:

After exiting from the plate of the second stage's triode and going through a coupling cap the signal sees a tone network. The tone section consists of a tone and body control. The tone control acts in a similar fashion as most tone controls cutting the high end. Now normally this tone control has a large mid cut. It should be noted the tone control is based of that of the Big Muff's. With the added body control though the frequency response can go from a large mid cut to a flat response. This tone network is not my own idea unfortunately. Credit goes to Jack Orman at AMZ. Read more about the tone section here.

After exiting the tone network the signal sees a gain pot which is just a variable voltage divider that "dumps" portions of your signal to ground depending how the control is set. The idea behind the gain controls is the same as that of a master volume amp. Set the gain control high then set the volume low to get distorted tones at low volumes. Really push the tubes and then limit them at the very end basically. The gain control then leads up to another triode which is set up in a standard fashion as the other stages. A 680nF cathode bypass cap is added though. It boosts the high to middle range frequencies. This keeps the sound clear and prevents things from getting muddy.

4th Stage:

The final stage is just like the second stage. It's purpose is to boost the signal. Even though the signal was boosted quite a bit prior to the tone network there is still a loose of signal gain. The final stage helps boost the signal back up and even above where it was in terms of voltage gain. Again cathode is tied right to ground for the most gain possible. After the final coupling cap the signal sees one final variable voltage divider which is the volume. This control sets the overall output level of the pre-amp. The wiper of the pot (labeled OUT) goes to the output jack.

Here is where I am at now...

Bypass System:

"The Hood" also will feature a really neat bypass system. It will be true bypass but instead of a mechanical DPDT or 3PDT switch it will use a DPDT 12V DC relay. This will allow external switching from a tiny little control/stomp box that you can hook to the back of "The Hood". This will allow one to bypass "The Hood" from one side of the stage if its on the total opposite. More than likely though Ill be bypassing it from one end of my basement to the other though, ha.

The relay is powered by the 12V DC that can be tapped off from the heaters. The system will be "anti-pop" and will also have a nice bright LED indicator.

As of right now I am still experimenting and finalizing the design on the relay bypass system on my bread board.

 

Finalized and Complete!

Finally after weeks of trying to figure out why there was hum in my PSU I figured it out. I had AC signal leaking into my DC voltage (over 1V!!!). After fixing that I still had some hum. The supply also needed to be regulated so I used a 24V+ regulator. Yeah now the B+ isn't as high as originally intended but regulated was the only way I could remove the hum. In the future I will come up with better ideas for power distribution and a more solid grounding scheme. Overall I am very happy though, my first 100% original design. Works flawlessly and sounds am amazing.

Relay system took a bit of time to figure out. In the end R.G. Keen helped me a lot and showed me a way of noiseless switching using a single BJT to slow down the voltage coming to the relay's coil to avoid popping and clicking. There is a tiny tiny bit of capacitance between the two coils. This can cause a discharge when the relay is activated and can cause a nasty little non musical pop. As well as switching between bypass and the pre amp the relay also turns on a bright red LED mounted on the front panel. The LED only turns on when the signal goes through the pre-amp.  The bypass is done remotely by a small footswitch that connects to the back of "The Hood" with a standard 1/4" guitar cable.

Unit is pretty quiet. The wiring is really messy compared to most of my work but it was hard cramming so much stuff into an enclosure of this size. Also heat isn't a big issue. Both the 24V+ regulator and the 12V+ regulator are only dissipating 12V or less. There is lots of air holes for air flow, both regulators are also very well heat sunk to the enclosure. Originally I had issues with an exploding 12V+ regulator, it was dissipating over 22V!!! I thought regulators were supposed to shut themselves down though when they overheat, then not start back up until they are cool?

Pictures

 

The Sound:

So how does it sound? Killer if I do say so myself!

The knobs are all very interactive, epically the volume and gain control. "The Hood" is capable of having tons of headroom, on the other hand it gets nice and thick and dirty with the right settings. The clean/dirty switch adds a nice bit of grit and bite when you need it. The "pullable" pot adds a nice tubey crunch sound. I love the tone controls, the body and tone provide endless combinations of sounds. The presence is subtle yet allows you to dial in just the right amount of high end, perfect when you need to tame that distorted tone to something a bit softer.

I love playing with "The Hood" using my 75W solid state amp. When you step on the bypass footswitch you instantly get hit by that "tube tone". So warm and juicy with amazing dynamics, very touch sensitive.

 

Soundclips:

All clips using a Telecaster I built into a crappy Randall solid state amp (the crappy SS amp makes the unit sound better and shows the difference). Used the middle position on the pickup selector, both bridge and neck pickups. The EQ on the amp had bass flat, mid slightly scooped and treble boosted a wee bit.

Amp has a lame mushy sounding Celestion in it.

Recorded using a Shure SM57 using a Line6 Toneport as the recording interface. The Toneport's modeling and effects was bypassed, no effects or mixing added after the fact...

Still very new to recording and haven't quite got the hang of it yet, o well hope the clip sound decent...

Clips broken into four parts
1. Dry signal, true bypassing "The Hood"
2. "The Hood" engaged, showing its cleaner tone and how the dynamics of ones playing affects the sound.
3. Slightly dirty sound, light drive. Somewhat crunchy?
4. Heavier sounding distortion than part 3.
 

Click here
(may need to your pop-up blocker if you have one)